What are surfactants. Surfactants: general information
In relation to the non-polar phase (gas, hydrocarbon, non-polar surface of a solid body), a hydrocarbon radical is possessed, which is pushed out of the polar medium. In an aqueous solution of a surfactant, an adsorption surfactant is formed at the boundary with hydrocarbon radicals oriented towards . As it becomes saturated, the (ions) of the surfactant, condensing in the surface layer, are located perpendicular to the surface (normal orientation).
Depending on the state of surfactants in solution, truly soluble (molecularly dispersed) and colloidal surfactants are conditionally distinguished. The conditionality of such a division is that the same surfactant can belong to both groups, depending on the conditions and chem. the nature (polarity) of the solvent. Both groups of surfactants are adsorbed at phase boundaries, i.e., they exhibit in solutions, while only colloidal surfactants exhibit bulk properties associated with the formation of a colloidal (micellar) phase. These groups of surfactants differ in the value of a dimensionless quantity, which is called. hydrophilic-lipophilic balance (HLB) and is determined by the ratio:
where is the affinity (free energy of interaction) of the non-polar part of the surfactant to the hydrocarbon (b is a dimensionless parameter depending on the nature of the surfactant, is the free energy of interaction per one CH 2 group, v is the number of CH 2 groups in the hydrocarbon radical), a -affinity of the polar group for . For colloidal surfactants (b + or , where the indices m correspond to the minimum affinity values at which the colloidal properties of the surfactant begin to manifest themselves. The minimum number of carbons in the radical for different types colloidal surfactants lies in the range of 8-12, i.e. colloidal surfactants have a fairly large hydrocarbon radical. At the same time, colloidal surfactants must also have true solubility in , i.e. the polarity of the hydrophilic group must also be sufficiently high. This corresponds to the condition:
In the beginning. 60s 20th century D. Davis developed the HLB scale with values from 0 to 40. Surfactants with lipophilic properties have low HLB values, those with hydrophilic properties have high ones. Each group included in the surfactant is assigned a group number. When these numbers are added together, HLB is obtained by the formula:
HLB = hydrophilic group numbers + 4 hydrophobic group numbers + 7.
Although the concept of HLB is rather formal, it allows one to determine the areas of application of surfactants. So, for the formation of water / oil, HLB lies in the range of 3-6, oil / water yes-8-16, for wetting agents - 7-9, for agents - 13-15.
Amphoteric (ampholytic) surfactants contain a hydrophilic radical and a hydrophobic part capable of being an acceptor or donor, depending on the pH of the solution. Typically, these surfactants include one or more basic and acidic groups, and may also contain a nonionic polyglycol group. Depending on the pH value, they exhibit the properties of cationic or anionic surfactants. At certain pH values, naz. , Surfactants exist in the form of zwitterions. The ionization constants of acidic and basic groups of truly soluble amphoteric surfactants are very low, but cation-oriented and anion-oriented zwitterions are most common. The cationic group is usually a primary, secondary or tertiary ammonium group, residue or. In principle, instead of N m. b. S, P, As, etc. The anionic groups are carboxyl, sulfonate, sulfoether or phosphate groups.
According to chem. structure and some similarity of properties, ampholytic surfactants are divided into 5 main. groups: 1) alkylaminocarboxylic acids RNH (CH 2) n COOH; the alkyl radical is usually normal (straight-chain), but if it is located between the amine group and the carboxyl group, it sometimes has a branched character. The same group includes alkylamino-phenylcarboxylic acids RNHC 6 H 4 COOH; alkylaminocarboxylic acids with a primary, secondary or tertiary amino group RCH (NH 2) COOH, RCH (NHR) COOH, R(CH 3)NCH 2 COOH; with an intermediate hydroxyl, ether, ester, amide or sulfoamide group; substances with two or more amino and amido groups, with several amino and hydroxyl groups.
2) Alkyl betaines are the most important group of zwitterionic surfactants. They can be divided into 5 bases. groups: a) alkylbetaines -C-alkylbetaines RCH COO - and N-alkylbetaines RN + (CH 3) 2 CH 2 COO - ; b) sulfite-, sul-pho-, sulfate- and phosphate betaines RN + (CH 3) 2 CH 2 CH 2 RN + (CH 3) 2 CH 2 CH 2, RC 6 H 4 CH 2 N + (CH 3) 2 CH 2 CH 2 RN + (CH 3) 2 CH 2 CH(OH)CH 2 OP; c) amidobetaines RCONH(CH 2) 3 N + (CH 3) 2 COO - ; d) ethoxylated RN + [(C 2 H 4 O) p H] [(C 2 H 4 O) g H] CH 2 COO - ; e) other zwitterionic surfactants.
3) Derivatives of alkylimidazolines, in which anionic and cationic groups have approximately the same ionization constants (formulas VII and VIII), where R is alkyl C 7 -C 17, R "-H, Na, CH 2 COOM (M-metal). By structure and methods of synthesis, betaine surfactants are isolated, including carboxy-, sulfo-, sulfate- or sulfoester group [formula IX; R "= (CH 2) n COO - , (CH 2) 3 , CH 2 CH(OH)CH 2] and other ("non-betaine") imidazoline surfactants [formula X; R " \u003d CH 2 COONa, (CH 2) 2 N (CH 2 COOH) 2, (CH 2) 2 N \u003d \u003d CHC 6 H 4 SO 3 H, (CH 2) 2 OSO 3 H]. The balance of ionized groups provides these compounds have good colloid-chemical and sanitary-hygienic properties.
4) Alkylaminoalkane sulfonates and sulfates (AAAC 1 and AAAC 2 respectively). Anionic reference. substances easily pass into the zwitterionic form, which makes it possible to isolate them in their pure form. The ionization constant of the acid group is much greater than the basic group, so they are used in alkaline environment. However, in the case of several main groups and in the presence of other hydrophilic groups next to the acid group, these substances are similar in properties and areas of application to ampholytic surfactants and have a bactericidal effect. Depending on the ionization constants, AAAC 1 RN (R ") -R: - SO 3 M, AAAC 2 RN (R") -R: - OSO 3 M, derivatives of aromatic amino sulfonic acids RR "N-Ar-SO 3 M, aminosulfonates with N in heterocycles (formula XI); aminophosphates, aminophosphonates and other amine-containing compounds of the type RR"R:P(O)(OH) 2 , RR"R""OP(O)(OH) 2 , where R and R" - long and short hydrocarbon radicals, R: - short divalent radical; conn. RN(CH 2 CH 2 SO 3 Na) 2 . Their difference is a good ability to disperse calcium and resistance to.
5) Polymeric ampholytic surfactants: natural (proteins, nucleic acids, etc.); modified natural (oligomeric hydrolysates, sulfatyr. chitin); step products, fatty acids; derivatives obtained by introducing carboxyl and diethanolaminoethyl groups; synthetic, which combine structural features all the above groups of amphoteric surfactants (see, for example, formulas XII-XVI).
The use of surfactants. World production of surfactants is 2-3 kg per capita per year. Approximately 50% of the produced surfactants are used for (detergents and cleaners, cosmetics), the rest is in industry and with. x-ve. Simultaneously with the annual increase in the production of surfactants, the ratio between their use in everyday life and industry is changing in favor of industry.
The use of surfactants is determined by their structure of adsorption layers and bulk properties of solutions. Surfactants of both groups (truly soluble and colloidal) are used as dispersants in drilling of hard rocks (hardness reducers), to improve, lower and wear, the intensity of oil recovery, etc. etc. an important aspect of the use of surfactants is the formation and destruction,. Surfactants are widely used for regulation and stability with a liquid dispersion medium (aqueous and organic). Micellar systems are widely used, which are formed by surfactants in both aqueous and non- aqueous aquatic environment, for which it is not the surface activity of surfactants and the properties of their adsorption that are important. layers, and bulk properties: pronounced anomalies with an increase in surfactant up to the formation, for example, in an aqueous medium, crystallization. structures of solid or solid-like structures (in based on petroleum oils).
Surfactants are used in more than 100 industries National economy. Most of the produced surfactants are used as part of the environment, in the production of fabrics and products based on synthetic. and natural fibers. Large consumers of surfactants include petroleum, chemical. industry, construction industry. materials and a number of others. The most important applications of surfactants:
Drilling with mud and reversible water/oil. To regulate the aggregative stability and rheological characteristics of solutions, high-molecular water-soluble surfactants, polyacrylamide, etc. are used, calcium is added to nature. and synthetic. fatty acids (C 16 -C 18 and above), alkylaromatic. , alkylamines, alkylamidoamines, alkylimidazolines;
Enhanced oil recovery through micellar flooding (ethoxylated and alkylaromatic sulfonates);
Antioxidant, extreme pressure and other additives in the production of miners. oils (synthetic soaps of fatty acids, petroleum, hydroxyethyl alcohols) and plastic. lubricants (derivatives, arylamines, alkyl and aryl phosphates);
Regulation of iron and manganese (soaps of natural and synthetic fatty acids, higher aliphatic amines), rare (alkylarsonic and alkylphosphonic acids, alkylaromatic sulfonates);
Emulsion, production and other vinyl (carboxymethylcellulose, poly, synthetic fatty acids, alkyl sulfates, and alkylphenols);
Chemical production. fibers (hydroxyethyl and amides, and, higher and acids);
Mechanical restoration
Chemistry of Surfactants
Adsorption of surfactants at interfaces
Surface- active substances are characterized by a pronounced ability to be adsorbed on surfaces and at interphase boundaries. The term "interface" is usually referred to the boundary between two immiscible phases, the term "surface" indicates that one of the phases is a gas, usually air. Thus, there are five different interfaces:
solid - steam
solid - liquid
solid - solid
liquid - liquid
The driving force of surfactant adsorption on surfaces and at interfaces is the decrease in the free energy of the interface. The interfacial free energy per unit area is equal to the work that must be done to increase the surface. Instead of the term "interfacial free energy per unit area", the term "interfacial tension" is often used. So, surface tension water is equivalent to the specific free energy of the boundary between water and air. If the surface is covered with surfactant molecules, the surface tension decreases. The denser the packing of surfactant molecules on the surface, the greater the decrease in surface tension.
Surfactants can adsorb at any of the five interfaces listed above. Examples of various interphase boundaries and systems in which such boundaries play an important role are given in Table. one.
In many composite products, several types of interfacial boundaries are present at the same time. Examples are water-based paints and paper dyes. From the point of view of colloidal chemistry, they are extremely complex systems in which there are both solid-liquid and liquid-liquid interfaces. In addition, at the stage of application of these systems, foaming with the appearance of the liquid-gas interface is often observed. All interfacial boundaries are stabilized by surfactants and the total interfacial surface is huge. Interfacial boundaries oil-water and solid body - water existing in one liter of paint, you can cover the surface of several football fields.
The tendency of surfactants to accumulate at interphase boundaries is their fundamental property. In principle, the stronger this ability, the higher the effectiveness of the surfactant. The degree of surfactant concentration on the surface depends on the structure of their molecules and on the nature of the contacting phases. Therefore, there is no universal effective surfactant suitable for all systems. The choice of a suitable surfactant is determined by the functions that it must perform in a given system. An effective surfactant should have low solubility in liquid phases. Some surfactants are insoluble in water and in non-polar liquids and are localized only at interphase boundaries. Such substances are difficult to work with, but they are very effective in reducing interfacial tension.
A surfactant is capable of reducing surface or interfacial tension up to a certain limit. Typically, this limit is reached when micelle formation begins in the solution. From the data given in table. 2 shows how effectively surfactants reduce surface and interfacial tension. The indicated values of surface and interfacial tension are typical for conventional soft liquids. detergents. By specially selecting surfactants, it is possible to achieve ultra-low interfacial tension, i.e. values of the order of 10 -3 mN/m or less. An example of systems with ultra-low interfacial tension is a three-phase system consisting of a microemulsion in equilibrium with an excess of water and oil phases. Microemulsions are of interest for enhanced oil recovery.
Table 2. Typical values of surface and interfacial tension
Aggregation of surfactants in solution
As noted above, the fundamental property of surfactants is the ability to be adsorbed at interphase boundaries. Another important property of surfactants is that their molecules tend to form aggregates - the so-called micelles. Free or unassociated surfactant molecules are often referred to in the literature as monomers, although this term is unfortunate because it is predominantly used to designate blocks that form polymer molecules. Micellization can be considered as an alternative mechanism to adsorption at interphase boundaries, leading to the elimination of the contact of hydrophobic groups with water, as a result of which the free energy of the system decreases. This is an extremely important phenomenon, since the properties of surfactants are determined by the form - micellar or molecular - they are present in the system. Only molecularly dissolved surfactants lower surface and interfacial tension; in addition, dynamic phenomena are regulated by the concentration of the truly dissolved surfactant.
Micelles can be considered as reservoirs of molecular surfactant. Depending on the size and structure of a surfactant molecule, the rate of exchange of surfactant molecules between a micelle and a solution can vary in magnitude within several orders of magnitude.
In water, micelles appear already at very low concentrations. The concentration at which micelles begin to form is called the critical micelle concentration; this is one of the most important characteristics of surfactants. Thus, CMC = 1 mM means that the concentration of molecularly dissolved surfactant will never exceed this value, regardless of the amount of surfactant introduced into the solution.
Amphiphilic properties of surfactant molecules
The term "amphiphile" is often used as a synonym for surfactants. The term comes from the Greek word amphi, meaning "both". Its use is due to the fact that the molecules of all surfactants consist of at least two parts, one of which is soluble in liquid, and the other is insoluble. If the liquid is water, one speaks of the hydrophilic and hydrophobic parts of the molecule, respectively. The hydrophilic part is usually referred to as the polar group or "head" and the hydrophobic part as the radical or "tail".
In a micelle, hydrophobic groups are inside the aggregate, while polar groups are directed towards the solvent. Therefore, a micelle is a polar aggregate, highly soluble in water, and does not itself have a noticeable surface activity. When a surfactant is adsorbed from an aqueous solution on a hydrophobic surface, the surfactant molecule is usually oriented with its hydrophobic part towards the surface, and with its polar group towards water. In this case, the interfacial surface becomes hydrophilic, as a result, the interfacial tension decreases. Adsorption on hydrophilic surfaces often results in more complex aggregates surfactant molecules.
The hydrophobic part of the surfactant molecule can be linear or branched. The polar group is usually but not always attached to the end of the alkyl chain, which usually contains 8 to 18 carbon atoms. The degree of chain branching, the position of the polar group, and the chain length are the most important parameters that determine the physicochemical properties of surfactants.
The polar group of surfactants can be ionic or nonionic, which largely determines the properties of surfactants. This makes it possible to classify surfactants into ionic and non-ionic. The size of the polar group of a nonionic surfactant can vary over a wide range. In ionic surfactants, the size of the polar group is more or less constant. It should be emphasized that the physicochemical properties of surfactants in solution are determined by the ratio of the sizes of the hydrophobic and polar groups, and not by their absolute sizes.
Rice. one. Schematic representation surfactant molecules
Typically, a surfactant contains only one polar group. Recently, there has been a marked interest in dimeric surfactants containing two hydrophobic tails and two polar groups connected by a short bridge. Such substances have not yet been found. practical application, however, they have interesting physicochemical properties. These surfactants effectively reduce surface tension and have very low CMC values. For comparison: the CMC of a conventional cationic surfactant - dodecyltrimethylammonium bromide - is 16 mM, and the CMC of the corresponding dimeric surfactant with two carbon atoms in the bridge connecting the monomers is 0.9 mm. The difference in CMC values for conventional and dimeric surfactants can be of great practical importance. A typical example of a dimeric surfactant is shown in fig. 2.
Rice. 2. Dimeric surfactant
Ineffective surfactants that can adsorb on the surface but do not form micelles are used as additives in many surfactant compositions. Such surfactants are classified as hydrotropic substances; they destroy the ordered packing of conventional surfactants. For example, the introduction of a hydrotrope can prevent the formation of highly viscous liquid crystalline phases, the occurrence of which often creates significant difficulties in the formulation of surfactant compositions. Xylene sulfonate and cumene sulfonate are typical representatives of hydrotropic substances used in compositions of many detergents. Short chain alkyl phosphates are widely used as hydrotropes in compositions based on long chain ethoxylated alcohols.
Natural surfactants
Surfactants of natural origin primarily include polar lipids. They are widely distributed in living organisms. In biological systems, surfactants perform essentially the same functions as synthetic surfactants in technical systems.
For example, they help the body overcome the problem of the solubility of poorly soluble substances, they are emulsifiers and dispersants, as well as surface modifiers, etc. Many interesting examples can be given that characterize the role of surfactants in biological systems. Thus, bile salts are extremely effective solubilizers of the hydrophobic components of the blood mixture.
Rice. 3. Examples of polar lipids
phospholipids are packed into ordered bilayers like liquid surfactant crystals and cell membranes are composed of such structures. On fig. 3 shows examples of the most common polar lipids. A striking example of a natural surfactant, which is obtained directly, without chemical procedures, from natural sources, is lecithin. Lecithin is extracted from sources rich in phospholipids
Some microorganisms efficiently produce natural surfactants. Both high molecular weight surfactants, such as lipopolysaccharides, and low molecular weight polar lipids can be obtained in good yields, especially if the microorganisms are cultivated on a water-insoluble substrate. On fig. Figure 4 shows the structure of a low molecular weight acylated carbohydrate, a glycolipid based on trehalose, whose high surface activity has already been proven. Such derivatives and some other surfactants produced by yeast have recently attracted great interest. A lot of effort has already been spent on improving existing fermentation processes and on developing new ways of cultivating microorganisms. Despite the progress made, the commercial use of such surfactants is still limited due to their high cost.
Rice. 4. Surfactant glycolipid based on trehalose, obtained during the fermentation process
Petrochemistry and chemistry vegetable oils as sources of raw materials for the production of surfactants
V last years there is a trend towards the use of "green" surfactants, especially in everyday life. The term "natural surfactant" refers to the natural source of the substance. However, none of the surfactants used today in significant volumes can be considered natural in the full sense. With a few exceptions, all surfactants are produced by organic synthesis, often under very harsh conditions, when by-products are inevitably formed. For example, monoglycerides are widely distributed in nature, but surfactants marketed as monoglycerides are obtained during the industrial hydrolysis of triglyceride oils at temperatures above 200 °C, which leads to the formation of by-products - di- and tri-derivatives of glycerol. Alkyl glucosides are extremely common in living organisms, but this class of surfactants, often referred to as APGs, are prepared using multi-stage chemical processes, and they certainly cannot be considered natural.
In order to correctly assess the origin of surfactants, it is useful to divide them into two classes depending on the raw materials from which they are obtained: oleochemical and petrochemical surfactants. Oleochemical surfactants are produced from renewable raw materials, usually vegetable oils. Petrochemical surfactants are made from small "building blocks" such as ethylene, which is obtained from the cracking of petroleum. Often, vegetable oils and petrochemical products serve as raw materials for surfactants at the same time. Ethoxylated fatty acids are one of many examples.
Sometimes oleochemical and petrochemical processing methods result in identical products. For example, normal alcohols with C10-C14 hydrocarbon radicals, commonly used to introduce hydrophobic groups in the synthesis of nonionic surfactants and anionic surfactants, are obtained either by hydrogenation of methyl esters of the corresponding fatty acids, or by the polymerization of ethylene with triethylaluminum as a catalyst. In both cases, unbranched aliphatic alcohols are obtained, which differ little in the composition of homologues, since it is determined by the distillation process. Both methods of production are widespread.
The production of surfactants using vegetable oils as raw materials does not always provide less toxic and less environmentally harmful surfactants than petrochemical production. However, given the carbon dioxide cycle, chemical production based on renewable raw materials is always preferable.
Alcohols with long linear hydrophobic radicals are often referred to as fatty alcohols, regardless of how they were prepared. Alcohols with branched hydrocarbon radicals are also of great importance as raw materials for the production of surfactants. They are produced only by synthetic means; among them, the most widespread is the so-called oxo process, in which, as a result of the reaction of an olefin with carbon monoxide and hydrogen, an aldehyde is obtained, which is then reduced to an alcohol in the process of catalytic hydrogenation. As a result, a mixture of branched and normal alcohols is obtained, the ratio between which can be controlled to a certain extent by selecting a catalyst and reaction conditions. Commercial "oxoalcohols" are mixtures of straight and branched alcohols with specific alkyl chain lengths. Various ways obtaining primary long-chain alcohols are schematically shown in fig. 5.
Rice. 5. Various ways of obtaining primary alcohols as raw materials for the production of surfactants.
From left to right: Ziegleranatta polymerization of ethylene; recovery of methyl esters of fatty acids; hydroformylation of higher olefins.
Classification of surfactants by polar groups
The first classification of surfactants is based on the charge of the polar group. It is generally accepted to subdivide surfactants into anionic, cationic, nonionic, and zwitterionic. Molecules of surfactants included in the last group contain at normal conditions both charges: anionic and cationic. In the literature, they are often referred to as "amphoteric" surfactants, but this term is not always correct and should not be used as a synonym for the term "zwitterionic" surfactant. An amphoteric surfactant is a substance that, depending on the pH of the solution, can be cationic, zwitterionic, or anionic. good example amphoteric organic substances are simple amino acids. Most of the so-called zwitterionic surfactants have similar properties. However, some zwitterionic surfactants retain one of their charges over a wide pH range, such as compounds containing a cationic quaternary ammonium group. Thus, a surfactant containing a carboxylate and a quaternary ammonium group will be zwitterionic down to very low pH values, but will not be amphoteric.
Most ionic surfactants are monovalent, but there are also important representatives of divalent anionic surfactants. The physicochemical properties of ionic surfactants are affected by the nature of the counterion. In most cases, anionic surfactants use the sodium ion as the counterion, while other cations, such as lithium, potassium, calcium or protonated amine ions, are used as such only for special purposes. The counterions for cationic surfactants are usually halide ions or methyl sulfate ions.
The hydrophobic groups of surfactants are usually represented by hydrocarbon radicals, as well as polydimethylsiloxane or fluorocarbon groups. Surfactants of the last two types are especially effective in non-aqueous media.
For a small number of surfactants, there is some uncertainty in the classification. For example, surfactants containing amine oxides are sometimes referred to as zwitterionic, sometimes as cationic, and even as nonionic surfactants. The charge of the molecules of these substances depends on the pH of the aqueous phase; it can be said that in the neutral state they carry anionic and cationic charges or are nonionic dipole molecules. Ethoxylated fatty amines containing an amino nitrogen atom and a polyoxyethylene chain can be included in the class of cationic or nonionic surfactants. The non-ionic character of such surfactants prevails in the case of very long polyoxyethylene chains, while at short or medium lengths of polyoxyethylene chains, the physicochemical properties, as a rule, correspond to cationic surfactants. Surfactants containing an anionic group in the molecule, such as sulfate, phosphate or carboxylate, and polyoxyethylene chains are also very common. Such surfactants, such as sulfoesters, etc., usually contain short polyoxyethylene chains, and therefore are always considered as anionic surfactants.
Anionic surfactants
The polar groups in anionic surfactants are usually carboxylate, sulfate, sulfonate, and phosphate groups. On fig. Figure 6 shows the molecular structures of the most common surfactants of this class.
Anionic surfactants are used in much larger volumes than other types of surfactants. According to a rough estimate, the world production of surfactants is 10 million tons per year, of which 60% is accounted for by anionic surfactants.
Rice. 6. Structures of some typical anionic surfactants
The main reason for the popularity of these surfactants is the simplicity and low cost of production. Anionic surfactants are part of most detergents, and surfactants with alkyl or alkylaryl groups containing 12-18 carbon atoms in the hydrophobic chain have the best detergent effect.
Na + , K + , NH4 + , Ca 2+ ions and various protonated alkylamines usually act as counterions. Sodium and potassium ions increase the surfactant solubility in water, while calcium and magnesium ions increase the surfactant solubility in the oil phase. Protonated amines and alkanolamines provide surfactant solubility in both phases.
Soaps also make up a huge class of surfactants. They are produced by saponification of natural oils and fats. Soaps are commonly referred to as alkali metal salts of carboxylic acids obtained from animal fats or vegetable oils. Solid soaps, as a rule, contain fatty acids, which are obtained from tall, palm and coconut oils. When used in optimal conditions Soaps are ideal surfactants. Their main drawback is their sensitivity to hard water, which determined the need for the creation of synthetic surfactants. A very specific use is found for the lithium salt of a fatty acid, namely lithium 12-hydroxystearate, which is used as the main component of lubricants.
Alkylbenzenesulfonates are a group of synthetic surfactants, which are considered to be the main "workhorses". They are widely used in household detergents, as well as in a wide variety of industries. They are obtained in the process of sulfonation of alkylbenzenes. In large-scale synthesis, sulfur trioxide is most often used as the sulphurizing agent, but other substances such as sulfuric acid, oleum, chlorosulfonic, or amidosulfonic acids can also be used. In some cases, they are even more preferable. Industrial synthesis is carried out in a continuous process using a film apparatus with a free-flowing film. At the first stage of the process, pyrosulfonic acid is formed, which slowly and spontaneously reacts further, forming sulfonic acid.
The sulfonic acid is then neutralized with caustic soda to form the alkylbenzenesulfonate salt. Due to the large volume of alkyl substituents, almost exclusively n-sulfonic acids are formed. In the scheme above, R is an alkyl group, typically containing 12 carbon atoms. Initially, branched alkylbenzenes were used as an intermediate in the synthesis of surfactants, but at present they have been almost completely replaced by linear derivatives; therefore, such surfactants are called linear alkylbenzenesulfonates. The rejection of branched derivatives and their replacement by linear ones is mainly due to their faster biodegradation. Alkylbenzenes, in turn, are obtained by alkylation of benzene with n-alkenes or alkyl chlorides using HF or AICI3 as catalysts. The reaction produces a mixture of isomers with a phenyl group attached to one of the non-terminal positions in the alkyl chain.
Another type of sulfonate surfactant used in detergents are paraffin and α-olefin sulfonates, the latter often referred to as AOS. In general, the resulting surfactants are complex mixtures of substances that differ in their physicochemical properties. Paraffin sulfonates, or n-alkane secondary sulfonates, are mainly produced in Europe. They are obtained, as a rule, by sulfoxidation of paraffinic hydrocarbons with sulfur dioxide and oxygen when irradiated with ultraviolet light. In an older process, which, however, is still in use, paraffin sulfonates are obtained by the sulfochlorination reaction. Both processes are radical reactions, and since the secondary carbon atoms form more stable free radicals than the primary carbon atoms, the sulfo group is randomly introduced to any non-terminal carbon atom of the alkane chain. A mixture of C14-C17 hydrocarbons, sometimes called "Euro-fraction", is the most common hydrophobic feedstock, and the final products in this case are very complex mixtures of isomers and homologues.
Sulfonates of a-olefins are obtained by the reaction of linear a-olefins with sulfur trioxide; the result is a mixture of alkene sulfonates, 3- and 4-hydroxyalkane sulfonates, and some disulfonates and other substances. Two olefin fractions are mainly used as feedstock: C12-C16 and C16-C18. The ratio of alkenesulfonates to hydroxyalkanesulfonates is to some extent controlled by the ratio of the amounts of SO3 and olefins introduced into the reaction mixture: the higher this ratio, the more alkenesulfonic acid is formed. The formation of hydroxyalkane sulfonic acid occurs through an intermediate cyclic sultone, which is then cleaved with alkali. Sultone is toxic, so it is important that its concentration in the final product be very low. The acquisition scheme can be written as follows:
Sodium disulfosuccinate is an alkylsulfonate surfactant widely used in surface chemistry studies. This surfactant, due to its bulky hydrophobic group, is especially suitable for obtaining water-in-oil microemulsions.
Isethionate surfactants with the general formula R-COOC^C^SO^Na* are esters of fatty acids and salts of isethionic acid. They are among the mildest surfactants and are used in cosmetic formulations.
Sulfonate surfactants, obtained by sulfonation of lignin, petroleum fractions, alkylnaphthalenes, or other cheap hydrocarbon fractions, are widely used industrially as dispersants, emulsifiers, demulsifiers, defoamers, wetting agents, etc.
Sulfated sirts and ethoxylated alcohols constitute another important group of anionic surfactants that have found wide application in detergents. These are sulfuric acid monoesters in which the ester bond is very labile and breaks relatively easily at low pH as a result of autocatalytic hydrolysis. Linear and branched alcohols with 8 to 16 carbon atoms are used as raw materials for this type of surfactant. When using a linear alcohol with 12 carbon atoms, sulfuric acid dodecyl ester is obtained, and after neutralization with caustic soda, sodium dodecyl sulfate is formed - the most important surfactant of this type. Ethoxylated alcohols commonly used as intermediates are aliphatic alcohols with two or three oxyethylene units. The process is similar to the sulfonation discussed above. In industrial production, sulfur trioxide is used as a reagent,
and similarly to sulfonation, the reaction proceeds through the formation of pyrosulfate as an intermediate:
The synthesis of sulfate esters of ethoxylated alcohols is carried out in a similar way. The reaction is usually accompanied by the formation of a noticeable amount of 1,4-dioxane. Since dioxane is toxic, it must be freed from distillation. Such surfactants are commonly referred to as ethoxylated alkyl sulfates. They have good foaming properties, low toxicity to the skin and eyes and therefore are used in dish detergents and shampoos.
Ethoxylated alcohols can be converted to carboxylates, i.e., ethoxylated alkyl carboxylates. Traditionally this has been done using sodium monochloroacetate:
The Williamson reaction generally proceeds in low yield. Newer synthesis methods are based on the oxidation of ethoxylated alcohols with oxygen or hydrogen peroxide in an alkaline medium using platinum or palladium as a catalyst. In this reaction, the conversion of ethoxylates occurs in good yield, but oxidative degradation of the polyoxyethylene chain is also possible. Ethoxylated alkyl carboxylates are used in personal care products or as co-surfactants in various liquid detergent formulations. Like ethoxylated alkyl sulfates, ethoxylated alkyl carboxylates are stable in very hard water. Both types of surfactants also have good dispersing properties of calcium soaps, which is very important for surfactants used in personal care products. The dispersibility of calcium soaps is usually expressed as the amount of surfactant required to disperse calcium soap prepared from 100 g of sodium oleta in water with a hardness equivalent to 0.0333% CaCO3.
Phosphate-containing anionic surfactants, such as alkyl phosphates or ethoxylated alkyl phosphates, are prepared by treating fatty alcohols or ethoxylated alcohols with a phosphorylating agent; usually phosphorus pentoxide P4O10 is used for this. As a result of the reaction, a mixture of mono- and diesters is obtained phosphoric acid, and the relative proportions of these substances are controlled by the ratio of the reagents and the amount of water in the reaction mixture:
Key facts about anionic surfactants
1. Anionic surfactants - the most common class of surfactants.
2. Generally, anionic surfactants are incompatible with cationic surfactants.
3. They are sensitive to hard water, and the sensitivity decreases in the series carboxylates > phosphates > sulfates - sulfonates.
4. The introduction of a short polyoxyethylene chain between the anionic group and the hydrocarbon radical significantly increases the resistance of anionic surfactants to salts.
5. The introduction of a short polyoxypropylene chain between the anionic group and the hydrocarbon radical increases the surfactant solubility in organic media, but at the same time can lead to a decrease in the surfactant biodegradation rate.
6. Sulfate surfactants are rapidly hydrolyzed in acidic media as a result of autocatalytic hydrolysis. Surfactants of other types are stable in not too harsh conditions.
All commercial phosphate surfactants contain mono- and diesters of phosphoric acid, and the relative content of these components varies widely depending on the manufacturer. Since the physicochemical properties of alkyl phosphate surfactants depend on the ratio of various esters, alkyl phosphates from different manufacturers less interchangeable than other types of surfactants. Phosphorus oxychloride POCI3 can be used as a phosphorylating agent for the production of alkyl phosphate surfactants. In this case, a mixture of mono- and diesters of phosphoric acid is also formed.
Phosphate surfactants are used in the metalworking industry where they have proven to be more suitable than other surfactants due to their anti-corrosion properties. They are also used as emulsifiers in plant protection formulations. The most important information about anionic surfactants is summarized in Table. 3.
Non-ionic surfactants
Nonionic surfactants contain either polyester or polyhydroxyl fragments as polar groups. The vast majority of non-ionic surfactants contain oxyethylene groups obtained by polymerization of ethylene oxide. Strictly speaking, the prefix "poly" is used incorrectly. Surfactants with 5 to 10 oxyethylene units in the polar chain are most common, but some surfactants, such as dispersants, often contain longer oxyethylene chains. Ethoxylation is usually carried out in alkaline media. Any substance containing active hydrogen can be ethoxylated. Usually, aliphatic alcohols, alkylphenols, fatty acids, and aliphatic amines serve as starting materials for the production of oxyethylene surfactants. Esters, such as triglyceride oils, can be ethoxylated in a process in which alkaline hydrolysis of the ester bond occurs in a single reactor, followed by ethoxylation of the acid and alcohol formed and their partial condensation. Ethoxylated castor oil, used in animal feed, is an important example of a triglyceride-based surfactant.
Examples of polyhydroxy surfactants are sucrose and sorbitol esters, alkyl glucosides and polyglycerides. The latter are actually a combination of surfactants, derivatives polyhydric alcohols and polyesters. Surfactants based on polyhydric alcohols can be ethoxylated. The best known examples are fatty acid esters of sorbitol and the corresponding ethoxylated products. The five-membered ring structure of sorbitan is formed by the dehydration of sorbitol during the manufacturing process. Surfactants based on sorbitan can be eaten, so they are widely used in the production of food and drugs. Acetylene glycols are surfactants containing an acetylene bond localized in the center and hydroxyl groups at adjacent carbon atoms. Such substances constitute a special group of hydroxyl surfactants used as antifoam agents, especially in the production of coatings.
On fig. Figure 7 shows the molecular structures of the most common non-ionic surfactants. As will be shown below, surfactants based on oxyethylene are represented by a wide range of compounds, much wider than surfactants of other classes. Fatty acid ethoxylates give particularly complex mixtures with a high content of by-products. Most important type non-ionic surfactants are ethoxylated aliphatic alcohols. They are used in liquid and powder detergents and are widely used in industry. In particular, they are used as stabilizers for oil-in-water emulsions. Ethoxylated alcohols can be considered resistant to hydrolysis in a wide pH range: from 3 to 11. They are slowly oxidized in air, and the oxidation products in more irritate the skin than the original surfactants. In this book, ethoxylated fatty alcohols are designated as C t E p, where m is the number of carbon atoms in the alkyl chain of the alcohol, and n is the number of oxyethylene units in the surfactant molecule. Some general and most important properties of non-ionic surfactants are given in Table. 4.
Table 4. The most important information about non-ionic surfactants
Rice. 7. Structures of some typical non-ionic surfactants
In ethoxylated surfactants, it is possible to introduce with great accuracy a given number of oxyethylene groups attached to a specific hydrophobic residue, for example, to an aliphatic alcohol residue. However, the process ends with the formation of products with a wide distribution along the chain length. If all hydroxyl groups of the initial alcohol and the resulting glycol ethers have the same reactivity, a set of oligomers is obtained that obeys the Poisson distribution. Since the initial alcohol is somewhat less acidic than the resulting glycol ethers, its deprotonation is less favorable, so there is less chance of interaction with ethylene oxide.
Rice. 8. Ethoxylation of fatty alcohols catalyzed by bases
As a result, a significant amount of unreacted alcohol remains in the reaction mixture along with reaction products containing more oxyethylene groups. A lot of research efforts have been spent on developing methods for obtaining a product with a narrower distribution of homologues. The choice of ethoxylation catalyst also affects the product distribution. The use of alkaline earth metal hydroxides, such as Ba2 and Sr2, leads to products with a much narrower distribution than when using KOH, apparently due to the mechanism of coordination. The use of Lewis acids, such as SnCU and BF3, also makes it possible to obtain ethoxylated surfactants with a narrower distribution. However, ethoxylation in an acid medium has a significant drawback: 1,4-dioxane is formed as a by-product in large quantities. On fig. 9 shows the homologue distribution of ethoxylated alcohol using KOH, Lewis acid, and strontium hydroxide as catalysts.
Rice. 9. Typical distributions of homologues for the reaction products of a fatty alcohol with 4 mol of ethylene oxide in the presence of various catalysts
Ethoxylates with a narrow homologous distribution are gaining an increasing share of the market. The advantages of such ethoxylates are undoubted.
As can be seen from Table. 4, non-ionic surfactants containing polyoxyethylene chains have inverse relationship solubility in water on temperature. As the temperature rises, the system separates into two phases. The temperature at which this happens is called the cloud point because the solution becomes cloudy. The cloud point depends on the length of the hydrophobic part and the number of oxyethylene groups in the surfactant molecules and can be determined with great accuracy. In the production of polyoxyethylene surfactants, cloud point determination is used to control the degree of ethoxylation. Since the turbidity of the system can vary with increasing surfactant concentration, in the standard test method, the cloud point is determined by heating a 1% aqueous solution of surfactants above the cloud point and then recording the clearing temperature of the solution while slowly cooling the sample. Surfactants with long polyoxyethylene chains may have a cloud point above 100°C. For such substances, the cloud point is determined in electrolyte solutions, since most salts lower the cloud point.
Ethoxylated triglycerides, such as castor oil ethoxylates, have a strong market position and are often referred to as semi-natural surfactants. In recent years, there has been a sharp increase in interest in ethoxylated fatty acid methyl esters, which are obtained from the corresponding methyl ester by carrying out the ethoxylation process using a special catalyst, such as hydrotalcite. Methyl ester ethoxylates have a number of advantages over ethoxylated alcohols because they are more water soluble. Surfactants combining high water solubility with high surface activity are needed in a wide variety of compositions.
Ethoxylated alcohols, in which the terminal hydroxyl groups are replaced by a methyl or ethyl ether group, have their own market niche. Such non-ionic surfactants with “closed” ends are produced by O-alkylation of the ethoxylate by reaction with an alkyl chloride or dialkyl sulfate, or by hydrogenation of the corresponding acetal. Compared to ethoxylates of normal alcohols, surfactants with "closed" ends are more resistant to the action of alkalis and oxidizing agents. They are also characterized by an extremely low foaming capacity.
Cationic surfactants
Most cationic surfactants contain a nitrogen atom that carries a positive charge, i.e., they are amines or quaternary ammonium compounds. Amines exhibit surfactant properties only in the protonated state; therefore, they cannot be used at high pH. In contrast, quaternary ammonium compounds are insensitive to changes in pH. Amines are also more sensitive to the action of multiply charged anions. As already mentioned, ethoxylated amines have the properties of nonionic and cationic surfactants, and the longer the oxyethylene chain, the more pronounced the properties of nonionic surfactants in such compounds.
On fig. 10. shows some typical cationic surfactants. Quaternary ammonium compounds with ester groups are new class environmentally friendly surfactants, displacing the corresponding di-alkyl derivatives in the processes of softening fabrics.
Rice. 10. Structures of some typical cationic surfactants
The synthesis of non-ether quaternary ammonium surfactants proceeds through the formation of nitrile compounds. The fatty acid reacts with ammonia at elevated temperature to give the corresponding nitrile. This reaction proceeds through the formation of an intermediate amide. The nitrile is then hydrogenated to a primary amine in the presence of a catalyst:
Secondary amines are obtained either directly from the nitrile or in two steps from a primary amine. In a one-step process that appears to involve the formation of an imine intermediate, ammonia is continuously removed from the reaction mixture to facilitate the formation of the secondary amine:
Primary amines are converted by cyanoethylation into long-chain 1,3-diamines:
Primary or secondary long chain amines can be methylated and converted to tertiary amines, for example by reaction with formaldehyde under reducing conditions:
Ethylene oxide can also be used as an alkylating agent for converting primary and secondary amines to tertiary amines of the type R-CH2N2 and 2NCH 2 CH 2 OH.
Quaternary ammonium compounds are usually prepared from tertiary amines by reaction with a suitable alkylating agent, such as methyl chloride or bromide or dimethyl sulfate, the choice of reagent determining the counterion of the surfactant:
Quaternary ammonium surfactants containing ester groups are obtained by esterification of a fatty acid with an amino alcohol followed by N-alkylation as described above. An example is the reaction of triethanolamine, taken as an amino alcohol, and dimethyl sulfate as a methylating agent:
Sulfoxonium surfactants are obtained by oxidation of a sulfonium salt with hydrogen peroxide. Industrial use of non-nitrogen cationic surfactants is low because these substances rarely offer advantages over cheaper nitrogen-containing surfactants. Phosphonium surfactants with one sufficiently long alkyl chain and three methyl groups have found application as biocides.
Most surfaces - metals, minerals, plastics, fibers, cell membranes, etc. - are negatively charged. The main use of cationic surfactants is associated with their ability to adsorb on negatively charged surfaces. Some examples are given in table. 5, and the most important characteristics of cationic surfactants - in table. 6.
Table 5. Application of cationic surfactants due to their adsorption on surfaces
Table 6. Main characteristics of cationic surfactants
Zwitterionic surfactants
Zwitterionic surfactants contain two oppositely charged groups in their molecules. The positive charge is almost always provided by the ammonium group, and the negatively charged groups can be varied; most often, the negative charge is provided by the carboxylate ion. Such surfactants are often referred to as amphoteric, but, as noted above, these terms are not identical. The charges of an amphoteric surfactant change depending on the pH, and when going from acidic to alkaline pH, the type of surfactant changes from cationic through zwitterionic to anionic. Neither acidic nor basic groups carry a permanent charge, and such a surfactant becomes a zwitterion only in a certain pH range.
A change in the charge with a change in the pH of an amphoteric surfactant naturally affects its properties such as foaming and wetting abilities, detergent action, i.e., the main properties of the surfactant turn out to be dependent on pH. At the isoelectric point, the physicochemical properties of such surfactants are similar to those of non-ionic surfactants. Below and above the isoelectric point, there is a gradual shift towards the cationic or anionic character of the surfactant, respectively. Surfactants with sulfate or sulfonate groups providing a negative charge of the molecules remain zwitterionic down to very low pH values due to the very low pKa values of monoalkylsulfuric and alkylsulfonic acids.
Typical representatives of zwitterionic surfactants are N-alkyl derivatives of simple amino acids, betaine 2 NCH 2 COOH), aminopropionic acid). Such surfactants are not obtained from amino acids, but by the reaction of long-chain amines with sodium chloroacetate or acrylic acid derivatives, thus forming structures with one or two carbon atoms, respectively, between the nitrogen and the carboxylate group. As an example, below is a reaction scheme for obtaining a typical surfactant - a derivative of betaine from alkyl dimethyl amine and sodium monochloroacetate:
Amidobetaine derivatives are obtained similarly, starting from amidoamine.
Another type of zwitterionic surfactants, commonly referred to as imidazolines, are synthesized by reacting a fatty acid with aminoethylethanolamine followed by treatment with chloroacetate. The nomenclature of this type of surfactant proved to be somewhat confusing as the final product was thought to contain an imidazole ring, but it was later found that the five-membered ring was cleaved in the second step of the synthesis. A typical sequence of reactions is:
Zivitterionic surfactants are characterized by very good dermatological properties, do not irritate the eyes and are therefore often used in shampoos and cosmetics. Since the total charge of the molecules of such surfactants is equal to zero, they, like non-ionic surfactants, do not lose their properties in compositions with a high electrolyte content. Traditionally, zwitterionic surfactants have been used in alkaline cleaners. On fig. 11 gives examples of typical zwitterionic surfactants, and in table. 7 summarizes the basic information about surfactants of this class. As already mentioned, amine oxide surfactants, or, more correctly, N-oxides of tertiary amines, are sometimes classified as zwitterionic, sometimes as nonionic, and sometimes as cationic surfactants. Formally, they have separated charges on their nitrogen and oxygen atoms and generally behave as non-electrolytes, but at low pH or in the presence of an anionic surfactant, they accept a proton to form a conjugated cationic acid. 1:1-valence salt is formed between the anionic surfactant and the protonated amine oxide; such salts have high surface activity. Amine oxides are obtained by oxidation with hydrogen peroxide of the corresponding tertiary amine.
Rice. 11. Structures of some typical zwitterionic surfactants
Dermatological effect of surfactants
Dermatological effect of surfactants generates serious problems and is the subject of many contemporary research. The main dermatological problems in the workplace are related to the contact of unprotected skin with surfactant solutions, which are used as a variety of cleaning agents, as well as cutting fluids, oil emulsions for rolling, etc. Usually the effect is reduced to skin irritation of varying severity, less often allergic reactions occur. Skin irritation is caused by direct exposure to surfactants, and allergic reactions are activated by by-products present in surfactant compositions. A well-known example of a severe allergic reaction is the so-called "margarine disease" discovered in the Netherlands in the 1960s. The disease was found to be caused by a by-product found in a new surfactant that was added to margarine products to reduce splatter during frying. This surfactant allows you to keep water droplets in a finely dispersed state. The sensitizing agent in the composition of the surfactant turned out to be a substance with pronounced electrophilic properties. When reacting with nucleophilic groups of proteins, they are denatured, and the body perceives this substance as a foreign antigen. The surfactant used in the production of margarine contained a significant amount of an unreacted intermediate product, in which, when ingested, apparently, the cycle was opened when interacting with amine or thiol groups of proteins.
The physiological effect of surfactants on the skin is being investigated by various dermatological and biophysical methods, starting from the surface of the skin and the stratum corneum with its protective functions to a deeper layer of basal cells. Individual sensitivity or susceptibility of the skin is recorded by tactile sensations and experience. Skin-friendly surfactants include polyhydric alcohol surfactants, zwitterionic surfactants, and isethionates. These surfactants are often used in cosmetics.
In the homologous series of surfactants, a maximum irritating effect on the skin is usually observed at a certain length of the hydrophobic radical. For example, in a comparative study of alkyl glucosides containing 8,10, 12,14 and 16 carbon atoms in alkyl radicals, the maximum effect on the skin was found for C12 derivatives. The same maximum effect was found in the study of the biocidal activity of surfactants. Apparently, these data reflect the fact that the biological reactions caused by the action of surfactants on the mucosal membrane or bacterial surfaces, respectively, are due to high surface activity and a high concentration of molecularly dissolved surfactant. Since an increase in the chain length of the hydrophobic "tail" of the surfactant leads to an increase in its surface activity and to a decrease in the CMC, i.e., to a decrease in the molecularly dissolved surfactant, an extremum appears in the homologous series at a certain length of the hydrocarbon chain.
Comparatively mild surfactants include alcohol ethoxylates, but they are inferior in terms of skin softness to non-ionic surfactants based on polyhydric alcohols, for example, alkyl glucosides. Recent studies have shown that the dermatological effects of alcohol ethoxylates are caused not by the surfactant itself, but by the products of its oxidation occurring during storage. It was found that all ethoxylated products undergo autoxidation with the formation of hydroperoxides on methylene groups associated with ether oxygen in polyoxyethylene chains. Such hydroperoxides are not very stable, making their isolation difficult. However, the hydroperoxide with the UN group at the second carbon atom of the hydrophobic "tail" is relatively stable and was isolated in an amount of ~1% after storage of α-alkyl ethoxylate for a year. This substance is highly irritating to the skin. Another oxidation product with a similar effect on the skin is aldehyde, shown below. This aldehyde is unstable, and its further oxidation leads to the breaking of the polyoxyethylene chain and the formation of formaldehyde and other products. Both aldehydes are irritating to the skin and eyes:
To monitor the autoxidation of ethoxylated alcohols, it is useful to measure the change in cloud point over time. On fig. 12 shows examples of such a test for C12E5 and C12E6. It can be seen from the figure that both nonionic surfactants show a sharp decrease in the cloud point when stored at 40°C due to autoxidation.
Anionic surfactants, as a rule, affect the skin to a greater extent than non-ionic surfactants. So, sodium dodecyl sulfate, used in some personal care products, has a relatively high toxicity to the skin. Ethers of alkyl sulfates
Rice. 12. Dependences of the cloud point on the storage time of 1% solutions of surfactants.
These nonionic surfactants did not contain any homologue impurities. The measurements were carried out at two temperatures. Sodium is a milder surfactant compared to sodium alkyl sulfates, for this reason, ether derivatives are more often used in products for hand wash utensils. An important role for such products is played by the good foaming properties of these surfactants. The superior dermatological performance of alkyl sulfate esters compared to alkyl sulfates has been the main reason for the interest in ethoxylates with the narrow distribution of homologues discussed above. When sulfating such ethoxylates as an intermediate, the content of aggressive alkyl sulfates is noticeably lower than when using standard ethoxylates with a wide homologue distribution.
The effect of surfactants on the skin is often determined using the modified Dühring chamber test. On fig. 13 shows typical results of such a test for sodium dodecyl sulfate, decyl glucoside, and mixtures thereof. The irritating effect on the skin decreases almost linearly with an increase in the mixture of glucoside surfactants. In other cases, even small additions of a mild surfactant can cause a very significant improvement in the dermatological properties of the compositions: such a synergistic effect is possibly associated with a strong decrease in the CMC of the composition due to the formation of mixed micelles. Some amphoteric surfactants are extremely effective at reducing skin irritation caused by contact with anionic surfactants, such as alkyl sulfate esters. The effect can be associated with the protonation of carboxyl
Rice. 13. Test for the magnitude of the relative irritant.
The measurements were carried out using a modified Dühring chamber. Courtesy of Wiley-VCH
groups of betaine surfactant, which turns into a cationic surfactant with subsequent packaging into mixed micelles with anionic surfactants. Thus, the gain in the energy of formation of mixed micelles leads to the protonation of the carboxyl group of the betaine surfactant even at pH values much higher than pAG a.
Environmental impact of surfactants
Despite the fact that the concern about the impact of surfactants on the environment was legalized more than 20 years ago, only very recently this factor has become the main requirement that determines the possibility of using surfactants in various compositions. A huge amount of surfactants used in everyday life and industry passes into wastewater. Biodegradation rate in factory settling tanks Wastewater determines the amount of surfactants released into the environment. Two parameters - the rate of biodegradation and the degree of toxicity in the aquatic environment - determine the potential impact of surfactants on the environment. The Organization for Economic Cooperation and Development has developed rules and guidelines regarding
Aquatic toxicity
Biodegradability
bioaccumulation
Aquatic toxicity
Aquatic toxicity is measured on fish, daphnia or algae. Toxicity is expressed as LC50 or EC50, where LC and EC are the lethal and effective concentrations of the surfactant, respectively. Surfactant concentrations below 1 mg/l, leading to the death of half of the individuals within 96 hours in the test on fish and algae and during the 48-hour test on daphnia, indicate the toxicity of surfactants in the aquatic environment. Environmentally friendly surfactants should have appropriate values above 10 mg/l.
Biodegradability
Biodegradation is a process carried out in nature by bacteria. As a result of a series of enzymatic reactions, the surfactant molecule eventually turns into carbon dioxide, water and oxides of other elements. If a product is not naturally biodegradable, it is persistent and accumulates in the environment. The rate of biodegradation depends on the type of surfactant and ranges from 1-2 hours for fatty acids and 1-2 days for linear alkyl benzene sulfonates to several months for branched alkyl benzene sulfonates.
When determining the biodegradation of surfactants, it must be remembered that the rate depends on many factors: surfactant concentration, solution pH and temperature. Temperature has a particularly strong influence. Degradation rates of chemicals in factory sewage lagoons vary by a factor of up to five times in northern Europe depending on the time of year.
Two criteria are important for determining biodegradation: primary degradation and end products. Primary degradation of the surfactant is associated with loss of surface activity. For example, ester surfactants can quickly decompose into alcohol and acid, which do not have high surface activity. This criterion is of interest in special cases, for example when deciding whether products will accumulate in the environment, causing foaming of water bodies.
From an environmental point of view, the end products of biodegradation are more important. There are many methods for conducting biodegradability tests. Among them, the most popular was the modified Sturm test. This test determines the conversion of surfactant to carbon dioxide over time. The determination is carried out in closed vessels, into which the sediment from the wastewater of the factory sump is introduced. Surfactants are introduced into one series of vessels, the other series remains without surfactants. Measure the amount of gas released as a function of time. The difference recorded in these two series makes it possible to evaluate the biodegradation of surfactants. For most surfactants, an induction period of biodegradation was found, followed by a steep rise in the gas evolution curve, after which the dependence reaches a plateau. A typical test result and the criteria that must be met are shown in fig. 14.
Rice. 14. Criteria for passing the biodegradability test and a typical surfactant biodegradation kinetic curve.
Note the long induction period before decomposition begins
Bioaccumulation
Hydrophobic organic compounds accumulate in the environment, since all biodegradation processes require a certain water environment. Bioaccumulation can be measured directly in fish but is more commonly calculated from model experiments. To do this, measure the distribution of the component between two liquid phases - octanol and water - and use the logarithmic value lgP. A surfactant is considered bioaccumulative at 1^ 0 kt/water > 3. Most surfactants are characterized by lgP values< 3, поэтому биоаккумулирование не рассматривается как опасная экологическая проблема.
LogP values are known and collected for many surfactants, they can be used to assess the hydrophilicity of surfactants: the lower the logP value, the higher the hydrophilicity of the surfactant. The hydrophilicity of surfactants is useful to consider when compiling compositions. Often, a more well-known standard based on the concept of hydrophilic-lipophilic balance is used for the same purpose. HLB is widely used for the selection of emulsifiers. There is an inverse relationship between lgP and HLB: the higher the HLB value, the lower the lgP value. The concept of the critical packing parameter provides another way to assess the hydrophilicity of surfactants.
Surfactant marking
According to the recommendations of the Organization for Economic Cooperation and Development, the labeling of surfactants should include the values of toxicity in the aquatic environment and biodegradability. Rice. 15 illustrates this procedure. Properties based on biodegradability and toxicity in aquatic
Rice. 15. Ecological classification of surfactants
environment. The shaded areas of the diagram correspond to the permissible values of environmental indicators
Most of the surfactants commonly used today lie in the frontier region. This explains the desire of researchers to replace them with components whose properties would allow substances to be located on this diagram "higher and to the right."
Aquatic toxicity, biodegradability and bioaccumulation tests certainly provide a complete picture of the impact of surfactants on the environment. There are other parameters that are useful to determine the environmental impact of surfactants. In addition, the final product often contains a mixture of surfactant or a mixture of surfactant and polymer. Since it is well known that the physicochemical behavior of surfactants in such mixtures is very different from the behavior in an individual solution, it can be expected that the biological effects will be different. To get a complete picture, environmental tests must be carried out for all final products.
Table 8. Additional important factors that are useful to consider along with aquatic toxicity, biodegradability and bioaccumulation to assess the ecotoxicity of surfactants
Biodegradation rate and structure of surfactant molecules
Let us consider some parameters that affect the rate of surfactant biodegradation. First of all, it is necessary that the surfactant be sufficiently soluble in water. Too lipophilic surfactants, such as fluorinated derivatives, accumulate in the lipid tissues of the body and are very slowly destroyed. As mentioned above, many surfactants are sufficiently soluble in water, so the bioaccumulation of the original surfactant does not pose a big threat. However, the initial stage of biodegradation may end with the formation of intermediate products that are sparingly soluble in water. A well-known example is the class of ethoxylated alkylphenols, which are degraded by oxidative cleavage from the hydroxyl terminus of the polyoxyethylene chain. In this case, ethoxylated alkyl phenols with polar groups are formed from only a few oxyethylene fragments. Such compounds are very lipophilic and decompose extremely slowly. A study of fish exposed to ethoxylated nonylphenols showed high level accumulation of nonylphenols with two and three oxyethylene groups in lipid tissues. These data served as one of the reasons for strict control over the effect of this class of surfactants on the environment. Fatty alcohol ethoxylates appear to be degraded by various mechanisms, so lipophilic metabolites are not formed in appreciable amounts.
Along with solubility in water, it is necessary that the surfactant molecule contain bonds that are easily destroyed in the processes of enzymatic catalysis. Most chemical bonds are completely destroyed in nature, but it is important that the rate of destruction is high enough to prevent the surfactant and its metabolites from entering the environment in unacceptable quantities. To increase the rate of biodegradation, it is now common practice to introduce weak bonds into the structure of surfactant molecules. In principle, such easily broken bonds can be arranged randomly in the surfactant molecule, but for the convenience of synthesis they are usually introduced between the hydrophobic “tail” and the polar group. Typical examples of such bonds are ester and amide bonds, the destruction of which is catalyzed by esterases/lipases and peptidases/acylases, respectively. One might think that ester bonds in non-ionic surfactants are a source of environmental problems, since enzymes that cleave ether bonds are not very common in nature. However, it is not. Under aerobic conditions, hydroperoxides are formed in the a-position with respect to the ether bond, and the destruction of molecules occurs with the formation of aldehydes and acids.
The third factor that must be taken into account in addition to solubility in water and the presence of destructible bonds is the branching of the nonpolar part of the surfactant molecule. High branching of hydrocarbon "tails" often leads to a decrease in the rate of biodegradation. This is probably due to steric hindrances created by the side groups for the surfactant molecule to reach the active site of the enzyme. But the picture is not entirely clear. Some types of branching seem to be more dangerous than others, which is probably due to the specific features of a particular enzyme. Methyl side groups are less of a problem than longer alkyl side chains. But if the surfactant molecule contains many methyl branches in a row, as in poly derivatives, such compounds become dangerous from an environmental point of view. Convincing proof of the significance of the linearity of alkyl chains is the difference in the rates of biodegradation of alkylbenzenesulfonates with linear and branched chains. As already mentioned, branched alkylbenzene sulfonates, derivatives of tetra-1,2-propylene as the alkyl chain, were used before as the main components in household detergent formulations. They are cheap, effective as surfactants and chemically stable, but too stable when viewed from an environmental point of view. When in the 1960s and 1970s arose ecological problems, these surfactants were quickly replaced by analogues with linear alkyl chains. Linear alkylbenzenesulfonates decompose satisfactorily under aerobic conditions. However, their anaerobic biodegradation rate is relatively slow, so researchers are making efforts to overcome this difficulty as well. It has been established that the ability of surfactants to biodegrade strongly depends on the position of the branching in the hydrocarbon chain. Branching at carbon atoms two atoms away from a cleavable bond is less detrimental to biodegradation than branching at a carbon atom separated by one atom from a cleavable bond. All of the above is important because oxoalcohols, which are widely used as raw materials for the synthesis of surfactants, contain a large amount of 2-alkyl branches. It has been established that the length of the 2-alkyl side chain has practically no effect on the rate of biodegradation. However, a lot of work is still required to learn how to predict the biodegradation of surfactants based on chemical formula its molecules.
Protection environment as an incentive to search for new safe surfactants
All major types of surfactants have existed around us for decades. Their production methods are optimized and their physicochemical properties are relatively well studied. Along with the constant challenge of finding ways to make the production of existing surfactants cheaper, the development of surfactant chemistry in recent years has been influenced by a market with an ever-increasing demand for "green" products. Today, two distinct trends in research aimed at the synthesis of new surfactants can be noted: 1) the synthesis of surfactants from natural building blocks, 2) the synthesis of surfactants with breaking bonds. Below we briefly review how the production of surfactants from natural building blocks is developing.
polar groups
Two types of natural products have been studied for use as polar groups of surfactants - carbohydrates and amino acids. Surfactants can be obtained using organic and enzymatic synthesis or a combination of both methods of synthesis. The greatest efforts have been directed towards obtaining surfactants with polar groups from carbohydrates.
In the past few years, research has focused on three classes of surfactants in which the polar groups are carbohydrate or polyhydric alcohol - these are alkyl polyglucosides, alkyl glucamides and sugar esters. The molecular structures of these three types of surfactants are shown in Figs. sixteen.
Rice. 16. The structure of some typical surfactants, derivatives of polyhydric alcohols
At present, there is a great interest in the study of alkyl glucosides in connection with the prospects for their wide use. Such surfactants are synthesized by direct reaction of glucose with fatty alcohol using a large excess of alcohol to minimize carbohydrate oligomerization. An alternative method is the transacetalization of short-chain alkyl glucosides when interacting with long-chain alcohols. Both processes use an acid as a catalyst. The feedstock is glucose or fractions of hydrolyzed starch. Rice. 17 illustrates such a synthesis. Alkyl glucosides can be obtained by enzymatic synthesis with P-glucosidase, as a result of the reaction, only the P-anomer is obtained in low yields. The corresponding a-anomer can easily be obtained by hydrolysis of the racemate catalyzed by p-glucosidase. a, p-Mixture, which is a product of organic synthesis, differs significantly from pure enantiomers obtained by biocatalytic means. The p-anomer of n-octylglucoside has found application as a surfactant in biochemical research.
Alkyl glucosides are stable at high pH and sensitive to low pH, where they are hydrolyzed to carbohydrate and fatty alcohol. The carbohydrate part of the surfactant molecule is more soluble in water and less soluble in hydrocarbons than the corresponding polyoxyethylene blocks; thus, alkyl polyglucosides and other surfactants derived from polyhydric alcohols are more lipophobic than polyoxyethylene surfactants. This makes the physicochemical behavior of alkyl glucoside surfactants in oil-water systems quite different from that of conventional non-ionic surfactants. Moreover, alkylglucoside surfactants do not show a pronounced inverse temperature dependence of solubility characteristic of nonionic surfactants. And this leads to large differences in the properties of solutions of the two considered types of surfactants. The main attraction of alkyl glucoside surfactants lies in their favorable ecological properties: they are characterized by high speeds
Rice. 17. Methods for obtaining alkylglucoside surfactants
biodegradable and non-toxic to water bodies. In addition, these substances, being mild surfactants, do not have undesirable effects on the skin and eyes, which makes this class of surfactants very attractive for personal care products, although they have found a wide range of other technological applications.
Alkyglucamides are commercially important products. The product consumed in large quantities in detergents is N-dodecano-yl-K-methylglucamine, i.e., a C12 derivative. It is made from glucose, methylamine, hydrogen, and methyl laurate in two steps. The physicochemical properties and other characteristics of this class of surfactants are similar to those of alkyl glucosides. However, while alkyl glucosides are extremely resistant to alkalis and labile to acids, alkyl glucamides are also resistant to alkalis, but relatively resistant to acids.
Esters of glucose can be obtained either by enzymatic synthesis using a lipase catalyst, or by organic synthesis. With the optimal choice of enzyme, the bioorganic pathway can lead to esterification almost exclusively at the 6-position of the carbohydrate residue. Organic synthesis requires the use of protecting groups to provide the necessary selectivity. Selective enzymatic synthesis of esters of other carbohydrates without prior protection of the groups is difficult. At the same time, based on the acetals of carbohydrates and fatty acids, it is possible to obtain carbohydrate monoesters with a good yield, and after the stage of deprotection from part of the starting materials, mono- and di-derivatives can be obtained. All carbohydrate esters are very labile in alkaline conditions and very stable in acidic conditions. Decay products are natural products. Thus, carbohydrate esters are an ideal candidate for food surfactants, these substances seem to undergo rapid biodegradation, regardless of the size of the carbohydrate group and regardless of the length of the acyl chain. Sulfonated esters of carbohydrates have also been obtained. It turned out that such anionic surfactants undergo biodegradation at lower rates.
Surfactants based on polyhydric alcohols have many attractive properties: they are soft to the skin, non-toxic to water bodies, they biodegrade at high rates, they are easy to work with, and they are tolerant of high electrolyte concentrations. Some characteristic properties of surfactants based on polyhydric alcohols are shown in Table 9.
Hydrocarbon radicals
Fatty acids have been primarily and for a long time used as natural sources of hydrophobic parts of surfactants, for example, to obtain ethoxylated fatty acids and esters of sorbitan and fatty acids. Recently, fatty acids have been used to obtain ethoxylated derivatives of monoethanolamide.
Table 9. Characteristics of surfactants based on polyhydric alcohols
| 1. | Aerobic and anaerobic biodegradation occur rapidly. |
| 2. | Low toxicity in the aquatic environment. |
| 3. | Hydroxyl groups are highly lipophobic. In the same time |
| Surfactants with sufficiently long hydrocarbon tails have more | |
| shoy hydrophobicity. Because of this, such surfactants exhibit a pronounced | |
| nuyu propensity to be localized at the water-oil interface. | |
| 4. | The effect of temperature on the behavior of solutions is insignificant and counter- |
| false to the effect of temperature on ethoxylated surfactants. When compiling | |
| mixtures of surfactants from derivatives of polyhydric alcohols and | |
| ethoxylated surfactants, compositions of non-ionic | |
| surfactants whose phase behavior does not depend on temperature |
which in turn is obtained by aminolysis of fatty acid methyl esters with ethanolamine. Ethoxylated fatty acid amides are of interest as an alternative to fatty alcohol ethoxylates for the following reasons: 1) they are easily biodegraded to form fatty acids and poly-aminated ends, 2) the amide bond in the surfactant molecule facilitates the packaging of surfactants due to the formation of hydrogen bonds, 3) double bonds in fatty acid chains the chains are stored in the product.
It is known that double bonds in hydrocarbon tails increase the CMC due to an increase in the hydrophilicity of the surfactant, and also due to an increase in the chain volume, which makes it difficult to pack into dense aggregates and form hydrogen bonds between amide groups. The presence of one double bond has little effect on the area cross section molecules on the surface. Two double bonds significantly increase this area. Fatty acid amide ethoxylates with double bonds in the hydrocarbon "tail" are of interest as polymerizable surfactants.
Sterols are another class of natural substances that are of interest as a source of hydrophobic blocks for the production of surfactants. A characteristic feature of surfactants based on sterols is the presence of a large hydrophobic group of natural origin, which, due to the almost flat structure of four cycles, promotes dense packing in the surface layers. Phytosterols are called sterols of plant origin, at present they are already widely used as raw materials for the production of surfactants. Their structure is similar to that of cholesterol, which is an example of animal sterols. Sterols contain a secondary hydroxyl group that can be ethoxylated. However, the alcohol group is sterically protected and the reaction with ethylene oxide does not proceed directly. The optimal procedure is as follows: first carry out ethoxylation with a Lewis acid as a catalyst, then stop the reaction after introducing 3-5 moles of ethylene oxide and continue the reaction again, but using KOH as an initiator.
Ethoxylated sterols are large, rigid molecules; therefore, it takes a long time for the formation of an equilibrium surface layer, and it takes several hours to achieve an extremely low surface tension. It is also possible that the long time to reach equilibrium is associated with the exchange reactions of sterol ethoxylates with different lengths of the polyoxyethylene chain at the water-air interface and slow conformational changes of molecules on the surface. The structure of sterols is rigid, and the time required to achieve the optimal conformation in the surface layer can be very long. Sterol-based surfactants are of interest as solubilizers and emulsifiers in the production of drugs and cosmetics.
Surfactants are compounds that affect the amount of surface tension. In the process of interaction of liquid molecules, cohesive forces are formed between them. This force will be different in the surface and inner (deep) layers. Considering the state of the liquid, it is easy to establish that the particles that are directed into the system, with different sides surrounded by the same molecules that affect them. The resultant of all forces that act on such a molecule is zero. Therefore, liquids have the smallest surface area for a given volume. This is clearly manifested in the spherical shape of the droplets. The presence of impurities of various compounds in liquids determines the magnitude of surface tension.
The structure of surfactant molecules
Particles of fatty acids and alcohols consist of two parts that have different properties, so these compounds are very often called amphiphilic structures. One part of the molecule is represented by a hydrocarbon chain, and the other by various functional groups (amino group, hydroxyl, carboxyl, sulfo group). The longer the hydrocarbon chain, the stronger the particles will be expressed, the weaker they will interact with water.
Surfactants of organic origin: proteins, soaps, alcohols, ketones, aldehydes, tannins, ketones, etc. Surface-inactive substances do not affect surface tension (starch, glucose, fructose).
Nonionic surfactants (NSA) are high molecular weight biocompounds that do not form ions in water. These substances enter water bodies together with industrial (chemical, textile, household (use of various household) effluents, as well as with sewage from agricultural land (herbicides, fungicides, insecticides, and also folios as emulsifiers).
Surfactants: harm and benefit
The surface tension has great value for intestinal absorption. For example, fats, as well as lipids, enter the alimentary tract in the form of droplets. The latter are emulsified in the small intestine with the help of bile acids. Only after that these fats are hydrolyzed by lipolytic enzymes. Soaps (surfactants) are often added to insecticides to increase effectiveness. The manipulation allows insecticides to better interact with the surface of the body of insects. However, surfactants have not only positive, but also negative effects on the body. For example, shampoo contains very harmful foaming agents (surfactants), such as sodium and ammonium lauryl sulfate, ammonium and sodium laureth sulfate. There is an opinion that these components have a carcinogenic effect.
Surfactants have a polar (asymmetric) molecular structure, are able to adsorb at the interface between two media and reduce the free surface energy of the system. Quite minor additions of surfactants can change the surface properties of the particles and give the material new qualities. The action of surfactants is based on the phenomenon of adsorption, which simultaneously leads to one or two opposite effects: a decrease in the interaction between particles and stabilization of the interface between them due to the formation of an interfacial layer. Most surfactants are characterized by a linear structure of molecules, the length of which significantly exceeds the transverse dimensions (Fig. 15). Molecular radicals consist of groups that are related in their properties to solvent molecules, and of functional groups with properties that are sharply different from them. These are polar hydrophilic groups, having pronounced valence bonds and having a certain effect on wetting, lubricating and other actions associated with the concept of surface activity . In this case, the stock of free energy decreases with the release of heat as a result of adsorption. Hydrophilic groups at the ends of non-polar hydrocarbon chains can be hydroxyl - OH, carboxyl - COOH, amino - NH 2, sulfo - SO and other strongly interacting groups. Functional groups are hydrophobic hydrocarbon radicals characterized by secondary valence bonds. Hydrophobic interactions exist independently of intermolecular forces, being an additional factor contributing to the convergence, "sticking together" of non-polar groups or molecules. The adsorption monomolecular layer of surfactant molecules is oriented by the free ends of hydrocarbon chains from
the surface of the particles and makes it non-wettable, hydrophobic.
The effectiveness of a particular surfactant additive depends on the physicochemical properties of the material. A surfactant that has an effect in one chemical system may have no effect or the opposite effect in another. In this case, the surfactant concentration is very important, which determines the degree of saturation of the adsorption layer. Sometimes high-molecular compounds exhibit an action similar to surfactants, although they do not change the surface tension of water, such as polyvinyl alcohol, cellulose derivatives, starch, and even biopolymers (protein compounds). The effect of surfactants can be exerted by electrolytes and substances insoluble in water. Therefore, it is very difficult to define the concept of "surfactant". In a broad sense, this concept refers to any substance that, in small quantities, noticeably changes the surface properties of the dispersed system.
The classification of surfactants is very diverse and in some cases contradictory. Several attempts have been made to classify according to different criteria. According to Rebinder, all surfactants are divided into four groups according to the mechanism of action:
- wetting agents, defoamers and foaming agents, i.e. active at the liquid-gas interface. They can reduce the surface tension of water from 0.07 to 0.03–0.05 J/m2;
– dispersants, peptizers;
– stabilizers, adsorption plasticizers and thinners (viscosity reducers);
- detergents that have all the properties of surfactants.
Abroad, the classification of surfactants according to their functional purpose is widely used: thinners, wetting agents, dispersants, deflocculants, foaming agents and defoamers, emulsifiers, and stabilizers of disperse systems. Binders, plasticizers and lubricants are also released.
According to the chemical structure, surfactants are classified depending on the nature of hydrophilic groups and hydrophobic radicals. Radicals are divided into two groups - ionic and nonionic, the first can be anionic and cationic.
Nonionic surfactants contain non-ionizable end groups with a high affinity for the dispersion medium (water), which usually include oxygen, nitrogen, and sulfur atoms. Anionic surfactants are compounds in which a long hydrocarbon chain of molecules with a low affinity for the dispersion medium is part of the anion formed in an aqueous solution. For example, COOH is a carboxyl group, SO 3 H is a sulfo group, OSO 3 H is an ether group, H 2 SO 4, etc. Anionic surfactants include salts of carboxylic acids, alkyl sulfates, alkyl sulfonates, etc. Cationic substances form in aqueous solutions cations containing a long hydrocarbon radical. For example, 1-, 2-, 3- and 4-substituted ammonium, etc. Examples of such substances can be amine salts, ammonium bases, etc. Sometimes a third group of surfactants is distinguished, which includes amphoteric electrolytes and ampholytic substances, which, depending on by the nature of the dispersed phase, they can exhibit both acidic and basic properties. Ampholytes are insoluble in water, but active in non-aqueous media, such as oleic acid in hydrocarbons.
Japanese researchers propose a classification of surfactants according to their physicochemical properties: molecular weight, molecular structure, chemical activity, etc. Gel-like shells on solid particles arising due to surfactants as a result of different orientations of polar and non-polar groups can cause various effects: liquefaction; stabilization; dispersion; defoaming; binding, plasticizing and lubricating action.
A surfactant has a positive effect only at a certain concentration. There are very different opinions on the issue of the optimal amount of surfactants to be introduced. P. A. Rebinder points out that for particles
1–10 µm, the required amount of surfactant should be 0.1–0.5%. Other sources give values of 0.05–1% or more for different fineness. For ferrites, it was found that for the formation of a monomolecular layer during dry grinding of surfactants, it is necessary to take at the rate of 0.25 mg per 1 m 2 of the specific surface of the initial product; for wet grinding - 0.15–0.20 mg / m 2. Practice shows that the concentration of surfactants in each case should be selected experimentally.
In the technology of ceramic SEMs, four areas of application of surfactants can be distinguished, which make it possible to intensify physical and chemical changes and transformations in materials and control them during synthesis:
- intensification of the processes of fine grinding of powders to increase the dispersion of the material and reduce the grinding time when the specified dispersion is achieved;
– regulation of the properties of physical and chemical disperse systems (suspensions, slurries, pastes) in technological processes. Here, the processes of liquefaction (or a decrease in viscosity with an increase in fluidity without a decrease in moisture content), stabilization of rheological characteristics, defoaming in dispersed systems, etc. are important;
– control of flame formation processes when spraying suspensions upon obtaining the specified dimensions, shape and dispersion of the spray plume;
– an increase in the plasticity of molding masses, especially those obtained under the influence of elevated temperatures, and the density of manufactured blanks as a result of the introduction of a complex of binders, plasticizers and lubricants.
Surfactants (surfactants) are chemical substances, which are able to concentrate at the phase boundaries and reduce the surface (interfacial) tension. Surfactants are used in pharmaceutical and cosmetic products, in the production of shampoos and foaming agents.
Chemical structure of surfactants
A surfactant molecule consists of a hydrophobic hydrocarbon radical and a hydrophilic polar (functional) group, i.e. the molecule is amphiphilic, as a result of which it has a high adsorption capacity. For example, in a water/oil emulsion, at the phase boundary, the hydrophilic group of the surfactant molecule is oriented towards water, and the hydrocarbon part is towards oil. At the same time, the interfacial tension decreases, which ensures the stabilization of oil droplets in water.
The detergent action of surfactants is based on the fact that the surface-active ingredients of lotions, shampoos, soaps are adsorbed on the surface of such contaminants as fat and solid particles, envelop and facilitate their transfer into the washing solution. Surfactants facilitate the spreading of water or products based on them over the skin surface by reducing interfacial tension.
Types of Surfactants
The classification of surfactants is based on the division, depending on the nature of the polar group: non-ionic, which do not dissociate into ions in water, and ionic, which, depending on the charge formed during dissociation in water, are divided into: anionic, cationic, amphoteric.
Anionic surfactants, when dissolved in water, form negatively charged ions with a long hydrocarbon chain (organic anions) and an ordinary cation. Anionic surfactant emulsifiers are very effective:
- when creating oil/water emulsions;
- when dispersing a number of powdered materials;
- when used in foaming detergents to provide high foaming in hard water.
An example of an anionic surfactant that is often used in cosmetic formulations such as detergents is sodium lauryl ethoxysulfate (INCI nomenclature "Sodium Laureth Sulfate"). It is obtained by sulfation of saturated or unsaturated primary higher alcohols, followed by neutralization with sodium hydroxide, ammonia or triethanolamine. It is often produced in the form of a pasty mass containing up to 70% of the main substance.
Cationic surfactants, when dissolved in water, form positively charged ions (organic cations) and a low molecular weight anion. Cationic surfactants include salts of fatty amines and quaternary ammonium bases. Cationic emulsifiers are less effective than anionic emulsifiers, since they reduce surface tension to a lesser extent. But they exhibit bactericidal activity by interacting with cellular proteins of bacteria. Cationic surfactants are actively used in hair care products (shampoos, conditioners, hair conditioners). Aliphatic cationic surfactants with one and two hydrocarbon tails are good antistatic agents and are used in hair cosmetics.
Amphoteric surfactants, depending on the pH of the medium, behave in an alkaline environment as anionic or in an acidic environment as cationic. Their molecules contain functional groups that can have both negative and positive charges. Such surfactants are well compatible with cationic and anionic ones. Amphoteric surfactants are dermatologically gentle on the skin, which is why they are often used in “no tears” baby shampoos and products for sensitive skin. So, for example, in combination with an anionic surfactant sodium lauryl sulfate, almost completely soften its dermatological rigidity. Amphoteric surfactants have good foaming properties.
Betaines are one of the varieties of amphoteric surfactants. They belong to soft and high-foam surfactants. The amphoteric surfactant cocamidopropyl betaine (Dehiton / Betadet) is included in the composition of cosmetics in the production of shampoos, gels and cream gels, liquid soap, cleansing bath foams. This surfactant contributes to the compatibility of the cosmetic product with the skin, while improving the viscosity and foaming of this product. Thus, Dechiton, especially in children's foaming products, is an emollient component and contributes to the safety of using a detergent.
Non-ionic (non-ionic) surfactants are surfactants that do not form ions when dissolved in water. They, in comparison with anionic ones, having a weaker foaming ability, have a milder effect on the skin. Such surfactants are often used as emulsifiers, dispersants, solubilizers, as well as co-surfactants, foam stabilizers, wetting agents, etc. Fatty acid diethanolamides can be cited as an example of a nonionic surfactant. They are used in the production of shampoos and foaming detergents in an amount of up to 3% as a refatting agent, foam stabilizer and thickener.
In Russian-made shampoos to achieve the necessary consumer properties and quality improvement, various combinations of surfactants are used depending on the purpose of the cosmetic product.
Surfactants used in the cosmetic industry must comply with the Unified Sanitary and Epidemiological and hygiene requirements to goods subject to sanitary and epidemiological supervision (control).
Benefits of using surfactants:
- lead to the stabilization of the dispersed system, make it impossible for the particles of the dispersed phase to stick together and coagulate;
- facilitate the process of dispersion and obtaining cosmetic compositions;
- improve the wettability and spreadability of cosmetic substances on the skin;
- provide stability of reverse emulsions;
- as part of foaming detergents, they improve their foaming and increase the stability of the foam during use.
Literature
Surfactants and compositions. Directory. Edited by M.Yu. Pletnev 2002. - p.40-44.
Fundamentals of cosmetic chemistry. Basic provisions and modern ingredients. Ed. Puchkova T.V. 2011, pp.122-133.
Explanatory dictionary of cosmetics and perfumery v.1 Finished products 2nd ed. 2004. p.20.
