The use of surfactants. Olloidal surfactants. Surfactants. Natural and synthetic. Their advantages and disadvantages
All soluble substances according to their ability to be adsorbed at the liquid-air interface can be divided into two groups: surfactants and surface-inactive substances. Surfactants (surfactants) able to accumulate in the surface layer. This phenomenon is called positive adsorption. Positive adsorption occurs if: - the surface tension of the solute is less surface tension solvent (in this case, the free energy of the surface decreases); - the solubility of the substance is relatively low. Surfactants include fatty acids with a sufficiently large hydrocarbon radical, salts of these acids (soaps), sulfonic acids and their salts, high molecular weight alcohols, amines. A characteristic feature of the structure of all surfactants is their amphiphilicity, i.e. that they consist of two parts - a polar group and a non-polar hydrocarbon radical. The polar group determines the affinity of the substance for water, and the hydrophobic hydrocarbon radical is the cause of reduced solubility in water. Surface Inactive Substances tend to escape from the surface into the bulk of the liquid. This phenomenon is called negative adsorption. They have good solubility in water and higher surface tension. Surface-inactive substances include all inorganic electrolytes - acids, alkalis, salts. Substances whose surface tension is equal to the surface tension of the solvent are evenly distributed between the surface layer and the volume of the solution. Sugar is one of these substances. The surface activity of a substance depends not only on its nature, but also on the nature of the solvent. Water has a high surface tension, and therefore, in relation to it, many substances exhibit surface activity. Alcohol has a much lower surface tension than water. Therefore, some water surfactants are alcohol inactive. Many surfactants with amphiphilicity can form both true and colloidal solutions. For such systems, there is a reversible transition and the corresponding thermodynamic equilibrium true solution ↔ sol ↔ gel. Systems in which such transitions are observed include aqueous solutions of soaps, tannins (tannins), some dyes, etc. Solutions of colloidal surfactants are formed spontaneously, at low concentrations they are molecular, and with an increase in concentration, micelles from amphiphilic surfactant molecules appear in them. , which are in equilibrium with a molecular solution of constant and low concentration. The appearance of micelles in solution occurs when a certain concentration is reached, called critical micelle concentration (CMC) . A change in the solution structure during CMC leads to a sharp change in the physicochemical properties of the solution. The mechanism of micellization is related to the mechanism of surfactant adsorption: the interaction forces between water molecules are greater than between water molecules and surfactants; Surfactant molecules are first pushed out of the water into the surface layer, where they are adsorbed and oriented by hydrocarbon chains into a nonpolar medium. Then, with increasing concentration, surfactant molecules are pushed out by water molecules into micelles, and, trying to find a favorable orientation, they turn their nonpolar hydrocarbon chains to each other - spherical micelles appear. With an increase in the surfactant concentration in the solution, spherical micelles are rearranged into rod-shaped and then lamellar ones. Micellization leads to an increase in the viscosity of the system up to the loss of fluidity - the sol turns into a gel. Spontaneously formed systems with molecular and colloidally dissolved parts in equilibrium are called lyophilic colloidal systems . Lyophilic colloidal systems are characterized by the phenomenon solubilization , which consists in the ability of water-insoluble organic substances to dissolve in the hydrocarbon part of surfactant micelles. As a result, almost transparent, thermodynamically equilibrium solutions are formed. The solubility of organic substances increases with an increase in the surfactant molecular weight, surfactant concentration, in the presence of electrolytes that promote micellization. Organic substances with a small molecular weight and containing polar groups are more easily solubilized. The phenomenon of solubilization is extremely important for carrying out the polymerization of unsaturated hydrocarbons in emulsions in order to obtain synthetic latexes or synthetic rubbers. Reverse solubilization – the dissolution of water in oils in the presence of the corresponding colloidal surfactants dissolved in oil is of great importance in the food industry, in particular in margarine production. Colloidal surfactants are of great practical importance. They are used, for example: - as stabilizers of dispersed systems; - to change the nature of the surface (hydrophobization or hydrophilization); - to reduce the strength during crushing; - are one of the main components of lubricants; - as components detergents.
As surfactant monomers are added to the solution, the surface or interfacial tension will decrease until the surfactant concentration reaches the so-called critical micelle concentration. In addition, the aggregate structure is dictated by the polarity of the solvent in which the surfactant is dissolved. For example, in aqueous solution the polar head groups of the micelle will be oriented outward towards the aqueous phase, and the hydrophobic tails will bind in the core of the micelle.
In contrast, in oil, the polar head groups will associate at the center of the micelle, while the hydrophobic tails will be oriented outward. While many common cellular components are surface active, such as fatty acids and phospholipids, a large number of microorganisms produce certain biosurfactant molecules that have unique structures. The earliest interest in biosurfactants resulted from their antibiotic activity. Of further interest was the observation that biosurfactants were produced in response to the presence of hydrophobic substrates, indicating their potential use in oil waste treatment and oil recovery.
Soaps and synthetic detergents
Ordinary soaps are salts of monobasic carboxylic acids with a long hydrocarbon chain (C 15 - C 22). For technical purposes, sodium salts of palmitic, stearic and unsaturated oleic acids are of particular importance; less important are potassium and ammonium salts, liquid in normal conditions. Salts of divalent and trivalent cations (Ca +2, Mg +2, Al +3, etc.) are insoluble in water, but form colloidal solutions in hydrocarbon media and are used in mineral oil greases, as well as to stabilize emulsions of the second kind. The salts of naphthenic acids contained in soap naphtha, a product obtained during the purification of kerosene and solar oil, also have a washing effect. Detergents include salts of sulfonic acids containing a sulfo group as an active group (alkylsulfonates C n H 2n + 1 SO 3 Me and alkylarylsulfonates C n H 2n + 1 C 6 H 4 SO 3 Me, where Me is a monovalent cation of sodium, potassium, ammonium ). As detergents, alkyl sulfates are also widely used - esters of sulfuric acid with higher alcohols, as well as their salts C n H 2n + 1 - O - SO 3 Me. Since sulfonic acids are strong acids, not only their salts with monovalent cations, but also salts with polyvalent cations, as well as the sulfonic acids themselves, are fairly well soluble in water. This is an important advantage of sulfonic soaps over conventional soaps, which allows them to be used as detergents in hard and acidic water. In salts of carboxylic and sulfonic acids, the carrier of surface-active properties is the anion. It is the anion that is adsorbed on the surface, as a result of which the surface becomes negatively charged. Such surfactants are called anionic . If the carrier of surface-active properties is a cation, then surfactants are called cationic . Such soaps include salts of tetrasubstituted ammonium bases, for example cetyltrimethylammonium chloride C 16 H 33 (CH 3) 3 NCl, octadecylammonium chloride C 18 H 37 NH 2 HCl, pyridine compounds substituted at the nitrogen atom, for example cetylpyridinium chloride, etc. There are also non-ionic soaps unable to dissociate in solution. Their molecules usually consist of a long hydrocarbon chain with several polar but non-ionic groups at the end, which makes these soaps soluble. Such groups are hydroxyl or ether groups. An example of such compounds is the reaction product of a high molecular weight alcohol or alkylphenol with several molecules of ethylene oxide. C n H 2n+1 (OCH 2 CH 2) m OH. The washing action of soaps is associated with a number of different effects: - Soaps lower the surface tension of the solution, thereby improving the wetting of the fabric with the washing liquid. This contributes to the penetration of liquid into such capillaries of contaminated tissue, into which pure water cannot penetrate. - Soap molecules create a well-hydrated adsorption layer on the fiber surface and particles of solid and liquid contaminants, which causes the occurrence of disjoining pressure. This contributes to the separation of contaminant particles from the surface of the fiber. - Adsorption films on the surface of the particles give these particles a high aggregative stability and prevent them from sticking to the surface of the fiber elsewhere. - In the presence of soap, foam is formed, which contributes to the mechanical removal of contaminants. - If the dirt particles are oily in nature, they can be solubilized in soapy solutions.
Interest in biosurfactants remains high as it is considered environmentally compatible. Biosurfactants are substances produced by microorganisms with a tendency to accumulate at interfaces, especially at liquid-air interfaces. These compounds inhibit the growth of pathogens by avoiding adhesion of microorganisms along epithelial cells.
These substances include various chemical structures such as glycolipids, lipopeptides, polysaccharide-protein complexes, phospholipids, fatty acids, and neutral lipids. In addition to this inhibitory ability, biosurfactants also help the binding of lactobacilli to collagen on cells. Biosurfactants show various advantages over chemical surfactants: they have low toxicity, high biodegradability and high efficiency at extreme temperatures or pH values.
Surfactants
1. Surfactants include substances that can reduce surface tension, tk. σ surf< σ жидкости. Поверхностно-активными по отношению к воде являются вещества менее полярные, чем вода (спирты, амины, жирные кислоты, мыла, белки и др.)
2. For them dσ / dС< 0, т.е. с увеличением концентрации ПАВ поверхностное натяжение раствора уменьшается. Графически эта зависимость изображается кривой - изотермой поверхностного натяжения (Рис. 4). Из графика видно, что для ПАВ характерно резкое снижение σ даже при малых концентрациях. По мере роста концентрации ПАВ график становится более пологим и, наконец, переходит в горизонтальную прямую. Это означает, что поверхностное натяжение достигло своего минимального значения. При этих условиях на поверхности жидкости образуется сплошной мономолекулярный слой ПАВ и дальнейшая адсорбция уже невозможна.
Surfactants, also known as surfactants, are those substances that preferentially adsorb at air-liquid, liquid-liquid, or liquid-solid interfaces. The surface activity of a solute is related to a particular solvent. Surfactant molecules contain at least two separate parts, a part that strongly interacts with the solvent, a lyophilic part and another fragment of the lyophobic part, the interaction of which with the solvent is less than its interaction with molecules of a structure similar to its own.
Rice. 4 Surface tension isotherm for surfactants
3. In 1878, the American scientist J. Gibbs derived an equation relating the amount of adsorption of a substance (G) with its ability to change the surface tension of a solution (dσ / dС)
Where C is the concentration, mol/l
R is the universal gas constant, equal to 8.32 J/mol K
This part of the molecule imparts solubility to the molecule, while the lyophobic part restricts or even prevents solution. In oily water systems, the hydrophobic group is usually a single or branched chain hydrocarbon containing 8-18 carbon atoms, the solubility of the molecule decreases as the chain length increases.
The number, originally proposed by Davis, is an empirical and convenient measure widely used for composition. Group numbers are shown in Table 1. Anionic surfactants are the most common and are used primarily as detergents. Cations that are positively charged have greater bacterial affinity and are used in medical applications and cosmetics. They are now easily adsorbed to negatively charged textile fibers and are effective softeners and conditioners.
Т – absolute temperature, K
dσ / dС is the change in surface tension with concentration at a constant surface area.
4. It follows from the Gibbs equation that for dσ / dС< 0 величина Г >0. They say that surfactants are characterized by positive adsorption, when the surface concentration of surfactant molecules is greater than in the volume.
5. Surfactant molecules have an amphiphilic structure: they contain polar and non-polar groups. Such atomic groups as -COOH, -OH, -NH 2 , -NO 2 , -SO 2 OH, etc. have polar properties. They are capable of hydration and are hydrophilic. The non-polar part of surfactant molecules is a hydrophobic hydrocarbon chain or an aromatic radical. Thus, alcohols, amines, fatty acids, soaps, proteins, etc. are surfactants with respect to water.
Nonionic surfactants are increasingly being used as dispersants in the aqueous paint industry, in emulsion technology and in the rheological behavior of pastes, cuttings and drilling fluids. Interfacial Phenomena, Academic Press, New York. Systematic analysis of surface active agents, chemical analysis, vol. 12, 2nd ed. John Wylie & Sons. Lowering liquids or two-phase systems.
The structural commonality of most surfactants is the presence of at least one hydrophobic and at least one hydrophilic region. Because of the differences in chain length in many surfactants, one often speaks of a hydrophilic "head" and a hydrophobic "tail".
6. Due to the amphiphilic structure of surfactants, their molecules spontaneously form an oriented monolayer on the phase interface: polar groups of molecules are located in the aqueous (polar) phase, and hydrophobic radicals are in the less polar phase. The reason for this orientation is that the energy of interaction of water molecules with each other is greater than with the hydrophobic parts of surfactant molecules: En 2 o - n 2 o > En 2 o - surfactant. For the image of surfactant molecules, conventions. A straight or wavy line denotes a hydrocarbon radical, and a circle denotes a polar group. Schematically, the orientation of surfactant molecules can be depicted as follows (Fig. 5).
During adsorption at the interface, the region with low affinity for the surrounding phase protrudes beyond the interface, and the region with higher affinity coincides with the bulk phase. Surfactants are most commonly used in water as the surrounding phase so that the hydrophobic group is directed outward.
Ampholytic surfactants: compounds having an anionic and cationic group; often carboxylate and quaternary amino groups. Non-ionic surfactants: Compounds with non-ionic polar groups such as alcohol, ether or ethoxylate. Quaternary amines. . Surface tension is due to the fact that for liquid molecules, being in the bulk phase is energetically preferable compared to being at the interface or interface. In contrast, surface surfactants are more preferred in their structure at the interface, so that in the presence of surfactants, the work force required to form the surface is reduced.
Rice. 5 Surfactant orientation at the liquid-gas interface
7. The ability of surfactants to reduce surface tension is quantified by surface activity q = - dσ/dС. In the homologous series, there are clear patterns in the change surface activity: it increases as the length of the hydrocarbon radical increases and depends on the non-polarity of the substance. At the end of the 19th century, Duclos and Traube, on the basis of a large experimental material, formulated the rule: with an increase in the length of the hydrocarbon chain by the -CH 2 group, the surface activity increases by 3 - 3.5 times. In other words, an arithmetic increase in chain length leads to an exponential increase in surface activity. On fig. Figure 6 shows surface tension isotherms for a number of acids. It can be seen from the graph that q 1 When the surface is completely filled with molecules of surfactants in the bulk phase, aggregates are formed, the so-called. They are used wherever it is necessary to improve the contact between different phases or their mixing: during cleaning and coating during emulsification, dispersion or flooding. By measuring the effect of adding a surfactant on the wetting of hard surfaces, we measure it. If the detached oil droplets and dirt particles were not suspended in the detergent solution in a stable and highly dispersed state, they would tend or coalesce into aggregates large enough to be re-deposited onto the cleaned surface. When washing fabrics and similar materials, small droplets of oil or small, deflocculated dirt particles are more easily carried through gaps in the material than relatively large ones. Therefore, the action of the detergent in keeping the dirt in a finely dispersed state is important to prevent the detachable dirt from being retained by the cloth. Rice. 6 Isotherms of surface tension of some acids: 1. CH 3 COOH - acetic acid, 2. CH 3 CH 2 COOH - propionic acid, 3. CH 3 (CH 2) 2 COOH - butyric acid, 4. CH 3 (CH 2) 3 COOH - isovaleric acid. The rule is valid for aqueous solutions and applies to hydrocarbon media. Indeed, the longer the hydrocarbon chain, the more non-polar the substance, the more its molecules are pushed by water to the surface, because E n 2 o-n 2 o > E n 2 o-surfactant. The Duclos-Traube rule was the theoretical basis for the synthesis of modern detergents. To be used as detergents, soaps and detergents must have certain chemical structures: their molecules must contain a hydrophobic moiety, such as a long chain carbon group or, for example, alkylbenzene. This hydrophilic part makes the molecule soluble in water. In general, the hydrophobic part of the molecule attaches to the solid or fiber and soil, while the hydrophilic part attaches to water. There are four groups of surfactants. Which produce electrically negative colloidal ions in solution. which produce electrically positive ions in solution. which produce electrically neutral colloidal particles in solution. or amphoteric, detergents that are capable of acting as anionic or cationic detergents in solution depending on the pH of the solution. Soap was the first detergent. In a strictly chemical sense, any compound formed by the reaction of a water-insoluble fatty acid with an organic or a can be called a soap. 8. In accordance with the Duclos-Traube rule, adsorption increases with chain lengthening in the homologous series, but for all members of the series, the adsorption value tends to the same limiting value Г ∞, called limiting adsorption (Fig. 7). Rice. 7 A series of adsorption isotherms at the solution-gas interface for the homologous series of surfactants. 1 - for the lowest member of the series, 3 - for the highest member of the series In practice, however, soap refers mainly to those water-soluble soaps that result from the interaction between fatty acids and alkali metals. In some cases, however, fatty acid salts with ammonia or with triethanolamine are also used, as in shaving preparations. According to the Phoenicians, it was prepared from goat and ashen in 600 BC. and sometimes used it as an item of barter with the Gauls. Soap was widely known in the Roman Empire; whether the Romans learned of its use and manufacture from the ancient Mediterranean peoples or from the inhabitants of Britain is not known. The importance of soap for washing and cleaning does not appear to have been recognized until the 2nd century; the Greek physician Galen mentions it as a and as a means of cleansing the body. Previously, soap was used as. The presence of G ∞ is proof of the existence of a monomolecular surfactant layer on the liquid surface. At low concentrations in a region far from saturation, the hydrocarbon chains pushed into the air "float" on the surface of the water, while the polar group is immersed in water. The interaction between surfactant molecules is insignificant, monolayers are called gaseous (Fig. 8a). With increasing concentration, the number of molecules in the surface layer increases, the chains rise and, in the limit, acquire a vertical position (Fig. 8b). The works written by the 8th century Arab scholar Jabir ibn Hayyan repeatedly mention soap as a cleansing agent. In Europe, the production of soap in the Middle Ages was concentrated first in Marseille, later in Genoa, then in Venice. Leo sent Lady von Schleinitz a package of soap from Italy, he accompanied it with a detailed description of how to use the mysterious product. The Celts used animal fats containing a percentage of free fatty acids. The presence of free fatty acids certainly helped start the process. This method probably prevailed until the end of the Middle Ages, when slaked lime was used to caustify alkali metal carbonate. Thanks to this process, chemically neutral fats can be easily saponified with caustic liquor. Rice. 8 Scheme of the formation of a monomolecular layer With this orientation, a change in the chain length does not change the area occupied by a molecule in the surface layer, and, therefore, does not change the number of molecules per unit surface, which is proportional to Г ∞ . Such monolayers are called condensed. It is known that the use of solubilizers is in principle associated with a number of problems and inconveniences. In addition, the strongest solvents are harmful or even dangerous to humans and the environment and can damage the cleaned surface. Our soil release agents overcome many of the disadvantages of conventional solvents and can replace virtually all of them. They are absolutely safe for all materials, do not cause corrosion and corrosion of metals, do not pollute the environment. They easily remove all types of dirt. Surfactants make the water more liquid, which allows our products to penetrate the dirt layer. Molecules of surfactants include microparticles of oils, separating them from each other and promoting degreasing. The same removes mold and mildew from surfaces without residual and unpleasant chlorine odor. One of the challenges for the future is to create products that are less dangerous, less polluting and require less energy. One of these products is an enzyme. 9. According to the ability of molecules to dissociate into ions, surfactants are divided into two large classes: ionic (dissociating) and nonionic (non-dissociating). Ionic surfactants, in turn, are classified into 1) anionic, giving a surface-active anion upon dissociation: soap RCOOMe (Me - K +, Na +, NH 4 +), sulfonic acids, their salts and other compounds; 2) cationic, forming a surface-active cation upon dissociation: salts of amines, quaternary ammonium bases, alkyl-pyridine compounds; 3) amphoteric, capable of exhibiting anionic properties (in an alkaline environment) or cationic properties (in an acidic environment), depending on pH: alkylamino acids, etc. As an example of anionic surfactants used in medicine, one can cite sodium lauryl sulfate - Na +, cationic surfactants - cetyltrimethylammonium bromide + Br -. The amphoteric ones include alkyldiaminoethylglycine hydrochloride HCl. Cationic and anionic surfactants are used in surgery as antiseptics. For example, quaternary ammonium compounds are approximately 300 times more effective than phenol in terms of destructive action against microorganisms. When the length of the alkyl radical is from C 8 to C 14, surfactants have a pronounced antiphage activity. The antimicrobial effect of surfactants is associated with their effect during adsorption on the permeability of cell membranes, as well as the inhibitory effect on the enzyme systems of microorganisms. Nonionic surfactants are obtained by reacting higher alcohols, acids or phenols with ethylene oxide molecules. Compounds of the type R(OSH 2 CH 2) m OH are obtained. The longer the oxyethylene chain, the more pronounced the hydrophilic properties. They are widely used in pharmacy as span and tween stabilizers (esters of fatty acids, sorbitol or ethoxylated sorbitol)
