The combination of surfactants with each other or with other compounds is called compounding. In the solubilization application of surfactants, if suitable compatibility can be selected, the solubilization capacity can be greatly increased and the amount of surfactants can be reduced.
Compatibility with neutral inorganic salt: adding soluble neutral inorganic salt to the ionic surfactant solution increases the solubilization of hydrocarbon solutes. On the contrary, the solubilization amount of polar substances decreases.
Compatibility with organic additives: generally, fatty alcohols with carbon atoms below 12 have better effects. Some polyols such as sugar, xylose and sorbitol have similar effects. On the contrary, some short chain alcohols, such as C1-C6 alcohols, can not form mixed micelles with surfactants, but may also destroy the formation of micelles. Polar organic compounds such as urea, N-methylacetamide and ethylene glycol all increase the critical micelle concentration of surfactants.
Compatibility with water-soluble polymers: water-soluble polymers such as gelatin, polyvinyl alcohol, polyethylene glycol and povidone adsorb surfactant molecules, reduce the number of free surfactant molecules in solution, and thus increase the critical micelle concentration.
Compounding basis
The purpose of surfactant compounding is to achieve additive and synergistic effect, that is, synergistic effect. That is, different types of surfactants are artificially mixed, and the performance of the mixture obtained is better than that of the original single component, that is, the effect of "1 + 1 > 2" as it is commonly called.
For example, sodium dodecyl sulfate is mixed with a small amount of dodecyl alcohol, dodecyl alcohol amine and other substances, which can improve its foaming, washing, reducing surface tension, emulsifying and other properties in detergent formulations.
The compounding of surfactants can produce additive effect, which has been applied to actual production. However, the research on its basic theory is only a matter of recent years. The results can provide guidance for predicting the additive and synergistic behavior of surfactants, so as to obtain the best compounding effect. However, its research is still at the initial stage, mainly focusing on the two-component composite system.
In the compounding system, the interaction between surfactants of different types and structures determines the performance and compounding effect of the whole system. Therefore, mastering the interaction between surfactants is the basis for studying surfactant compounding.
Interaction parameters between surfactant molecules
The two basic properties of surfactants are the surface adsorption of surfactants and the formation of micelles. Therefore, the generation of addition and synergy will first change the surface tension and critical micelle concentration of the system. In general, when two surfactants produce compounding effect, the critical micelle concentration of the mixed system is not equal to the average of the two critical micelle concentrations, but smaller than the critical micelle concentration of any one of the surfactants used alone. The reason for this is the interaction between surfactant molecules.
The two surfactants used in combination will form a mixed monolayer adsorption layer on the surface and a mixed micelle inside the solution. Both mixed monolayer and mixed micelle have interactions between the two surfactants. The form and size of the interaction can be determined by the intermolecular interaction parameters β express.
Interaction parameters between surfactant molecules β The value is related to the free energy of mixing the two surfactants, β A negative value indicates that the two molecules attract each other; β When the value is positive, it means that the two molecules are mutually exclusive;; β When the value is close to 0, it indicates that there is almost no interaction between the two molecules, which is almost ideal mixing. Many scholars have found through a large number of experiments and calculations β The value is generally between - 2 (weak repulsion) and - 40 (strong attraction).
Factors affecting intermolecular interaction parameters
Most mixed systems β The value is negative, which means that the two surfactants are attracted to each other. This attraction mainly comes from the electrostatic attraction between molecules, which is closely related to the molecular structure of surfactants, and is affected by external factors such as temperature and electrolyte.
Influence of surfactant ion type
The interaction between different types of surfactants is different, and the order of size is anion cation > anion amphoteric > ionic polyoxyethylene nonionic > betaine amphoteric cationic > betaine amphoteric polyoxyethylene nonionic > polyoxyethylene nonionic.
Since the probability of addition and synergism increases with the increase of the intermolecular interaction force of the two surfactants, the anionic cationic and anionic zwitterionic surfactant complex systems have the greatest possibility of addition and synergism with anionic surfactants. However, the addition and synergism of cationic polyoxyethylene nonionic and anionic anionic complex systems can only occur when the two surfactants have specific structures.
Influence of hydrophobic groups
With the increase of the length of the hydrophobic carbon chain of the surfactant, β It will become more negative. When the carbon chain lengths of the two surfactants are equal, the interaction parameters between molecules in the mixed monolayer are the largest and the attraction is the strongest. While in mixed micelles β The value increases with the increase of the sum of the carbon chain lengths.
Influence of medium pH value
The example types of amphoteric surfactants in aqueous solutions vary with the pH value of the medium. When the pH value of the solution is lower than the isoelectric point, it exists in the form of cations and interacts with anionic surfactants through cations. Therefore, when the alkalinity or pH value of the medium increases, amphoteric surfactants gradually change into electrically neutral molecules, even negative ions, and the interaction force with anionic surfactants decreases.
For the same reason, amphoteric surfactants have low alkalinity and poor proton acquisition ability, so their interaction with anionic surfactants is also low.
Effect of adding inorganic electrolyte
Tianjia of inorganic electrolyte will reduce the intermolecular interaction force in the mixed system of ionic surfactant and polyoxyethylene nonionic surfactant, which indicates that there is an electrostatic force between the molecules of these two types of surfactants.
Influence of temperature
Generally, in the range of 10-40 ℃, the intermolecular force decreases with the increase of temperature.
Interaction parameters β Meaning of
Interaction parameters β Is affected by many factors. After understanding the meaning and influencing factors of this parameter, we need to further use it to judge whether there is a compounding effect after mixing the two surfactants. If there is an additive synergism, what is the molar ratio when the two produce the maximum additive effect and the nature of the system. This introduces the interaction parameter β Meaning of.
Criteria for generating additive and synergistic effects
The most basic properties of surfactants are to reduce the surface tension and form micelles. Measuring the activity of surfactants is mainly to investigate the degree of reduction of the surface tension of their solutions and the size of the critical micelle concentration. In general, surfactants with excellent performance can reduce the surface tension of solutions to a very low level and form micelles at a relatively low concentration.
Reduce surface tension
In terms of reducing the surface tension, the additive synergism means that when the surface tension of the solution is reduced to a certain extent, the sum of the concentrations of the two surfactants required is lower than the concentration required for using any one of the surfactants in the complex system alone. If this concentration is higher than the concentration required for any one of the surfactants, it means that negative addition and synergism have occurred.
Forming mixed micelles
When the critical micelle concentration of the mixed micelles formed by the aqueous solution of the complex system is lower than the critical micelle concentration of any single surfactant, it is said that positive addition and synergism occur; If the critical micelle concentration of the mixture is higher than that of any single component, it is said that negative addition and synergism occur.
Comprehensive consideration
From the perspective of reducing surface tension and forming mixed micelles, positive addition and synergism means that the surface tension of the composite system of two surfactants at the critical micelle concentration of mixed micelles is lower than that of any one of them at the critical micelle concentration, and on the contrary, negative addition and synergism will occur.
It can be seen from this that after introducing the intermolecular interaction parameter, the interaction between the molecules of the two surfactants can be qualitatively understood, whether they attract or repel each other, and how strong the force is. It can also calculate and judge whether the two surfactants produce compounding effect after mixing through relevant formulas, and further determine the composition of the compounding system when the maximum additive effect occurs, that is, the compounding ratio of the two surfactants, which provides theoretical guidance for the application of surfactant compounding.
Complex system of surfactant
In addition to reducing surface tension and micelle formation, surfactants have many important functions in practical applications, such as washing, foaming, solubilization and wetting. There is still no mature theoretical guidance for the addition and synergism in these aspects, but some experience has been summarized in practical application, that is, there is a certain relationship between the addition and synergism and the reduction of surface tension or the formation of micelles.
Anionic anionic surfactant complex system
If this kind of composite system has additive and synergistic effects, it will lower the surface tension, improve the washability, decontamination, wettability and emulsibility, and reduce the Kraft characteristics. However, it should be pointed out that the combination of this system will only produce additive and synergistic effects if it has a specific structure.
Anionic cationic surfactant complex system
The interaction force between anionic and cationic surfactants is strong, and their composite system shows strong addition and synergy in reducing surface tension and forming mixed micelles, and also has great improvement in wettability, foam stability and emulsification performance. At present, this kind of composite system has been widely used in the softening and antistatic treatment of fibers and fabrics, and the stabilization of foam and lotion.
However, it should be noted that when these two types of surfactants are mixed, insoluble salts are easy to form and precipitate from the solution, thus losing surface activity. Therefore, the varieties of surfactants should be carefully selected.
Anionic amphoteric surfactant complex system
The action mode of the two surfactant molecules in this system is related to the acidity and alkalinity of the medium. There is a maximum value in the height of foam, and there is also a maximum addition and synergy in the property of reducing surface tension.
Anionic nonionic surfactant complex system
This kind of compounding system may increase or decrease the solubilization of micelles. The appearance of different solubilization effects is related to the interaction of the two surfactant molecules and the form of mixed micelles. It is generally believed that when the hydrocarbon chain of non-ionic surfactants is long and the addition number of ethylene oxide is small, it is easy to form mixed micelles with anionic surfactants. However, when the hydrocarbon chain is short and the addition number of ethylene oxide is large, it is easy to form two kinds of micelles, i.e. anionic surfactant rich micelles and nonionic surfactant rich micelles, which coexist in solution.
Compound system of cationic and nonionic surfactants
The critical micelle concentration can be significantly reduced by adding non-ionic surfactant to cationic surfactant solution. It is the result of the interaction between the ionic group of the cationic surfactant and the polar polyoxyvinyl group of the non-ionic surfactant.
Nonionic nonionic surfactant complex system
Most polyoxyethylene nonionic surfactants are mixtures themselves, and their properties are quite different from those of a single substance. Generally, when two nonionic surfactants with the same hydrophobic group and similar ethylene oxide addition number are mixed, they will be almost ideal solutions and easily form mixed micelles. The hydrophilicity of the mixture is equivalent to the average value of the two substances. When the ethylene oxide addition number and hydrophilicity of the two surfactants differ greatly, The hydrophilicity of the mixture is higher than the average of the two, and the oil-soluble varieties may be solubilized in the micelles of water-soluble surfactants.
