The use of surfactants in combination with each other or with other compounds is called compounding. In the application of surfactant solubilization, if the appropriate compatibility can be selected, the solubilization ability can be greatly increased and the amount of surfactant can be reduced.
Compatibility with neutral inorganic salt: Adding soluble neutral inorganic salt to ionic surfactant solution increases the solubilization of hydrocarbon solubilizers. On the contrary, the solubilization of polar substances decreases.
Compatibility with organic additives: Generally, fatty alcohols with carbon atoms below 12 have better results. Some polyols such as sugar, xylose, sorbitol, etc., also have similar effects. In contrast, some short-chain alcohols not only cannot form mixed micelles with surfactants, but also may destroy the formation of micelles, such as C1~C6 alcohols. Polar organics 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 have adsorption effects on surfactant molecules, reducing the number of free surfactant molecules in the solution, and thus increasing the critical micelle concentration.
Compounding basis
The purpose of surfactant compounding is to achieve additive and synergistic effect, that is, synergistic effect. That is, the different types of surfactants are artificially mixed, and the performance of the mixture obtained is better than that of the original single component, which is usually referred to as the "1+1> 2" effect.
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, emulsification and other properties in the detergent formula.
The additive effect of surfactants has been applied in practical production, but its basic theory research is still only a few years, the results can provide guidance for predicting the additive and synergistic behavior of surfactants in order to get the best compounding effect. However, its research is still in the initial stage, mainly focusing on the two-component complex system.
In the compound system, the interaction between different types and structures of surfactant molecules determines the performance and the compound effect of the whole system, so understanding the interaction between surfactant molecules is the basis of studying the surfactant compound.
Interaction parameters between surfactant molecules
The two most basic properties of surfactants are surface adsorption and micelle formation. Therefore, the addition and synergism will first change the surface tension and critical micelle concentration of the system. In general, when the two surfactants have a complex effect, the critical micelle concentration of the mixed system is not equal to the average of the critical micelle concentration of the two surfactants, but is less than the critical micelle concentration of either surfactant alone. The reason for this is the interaction between surfactant molecules.
The two surfactants used in the combination will form a mixed monomolecular adsorption layer on the surface and a mixed micelle inside the solution. There are interactions between the two surfactants, whether they are mixed monolayer or mixed micelle. The form and size of the interaction can be expressed by the intermolecular interaction parameter β.
The interaction parameter β is related to the free energy of the two surfactants. The negative value of β indicates that the two molecules attract each other. When the beta value is positive, it means that the two molecules repel each other. ; When the beta value is close to 0, it indicates that there is almost no interaction between the two molecules, which is close to ideal mixing. Through a large number of experiments and calculations, many scholars have found that the beta value is generally between -2 (weak repulsion) and -40 (strong attraction).
Factors affecting the parameters of intermolecular interactions
The beta value of most mixed systems is negative, that is, the two surfactant molecules are attracted to each other. This attraction mainly comes from the electrostatic attraction between molecules, is closely related to the molecular structure of surfactants, and is affected by external factors such as temperature and electrolyte.
Effect of surfactant ion type
The interaction between different types of surfactant molecules is different in order of anion-cationic type > anion-amphoteric type > ionic-polyoxyethylene nonionic type > betaine amphoteric type > betaine amphoteric type - polyoxyethylene nonionic type > polyoxyethylene nonionic type - polyoxyethylene nonionic type.
Since the probability of addition and synergism increases with the increase of the interaction force between the two surfactants, the most probable addition and synergism systems with anionic surfactants are anion-cationic and anion-zphoteric surfactants. The addition and synergism of cation-polyoxyethylene type non-ionic and anion-anionic complex systems can occur only when the two surfactants have specific structures.
The influence of hydrophobic groups
With the increase of the length of the hydrophobic carbon chain of the surfactant, the β will become more negative. When the length of the carbon chain of the two surfactants is equal, the interaction parameters between the molecules in the mixed monolayer are the largest and the attraction is the strongest. The β value of the mixed micelle increases with the total length of the carbon chain.
Influence of medium PH value
The types of examples of amphoteric surfactants in aqueous solutions vary with the PH of the medium. When the PH value of the solution is below the isoelectric point, it exists in the form of cations and interacts with anionic surfactants through cations. Therefore, when the alkaline or PH value of the medium increases, the amphoteric surfactants gradually transform into electrically neutral molecules, or even negative ions, and the interaction force with the anionic surfactants decreases.
For the same reason, amphoteric surfactants themselves are less alkaline and have a poor ability to obtain protons, so their interaction with anionic surfactants is also low.
Effect of adding inorganic electrolytes
The addition of inorganic electrolyte can reduce the interaction force between ionic surfactant and polyoxyethylene non-ionic surfactant in the mixed system, which indicates that there is electrostatic force between the two surfactants.
Effect of temperature
Normally, in the range of 10-40 ° C, the intermolecular force decreases with the increase of temperature.
The significance of the interaction parameter β
The interaction parameter β is influenced by many factors. After understanding the meaning and influencing factors of this parameter, it is necessary to further use it to judge whether there is a compounding effect between the two surfactants after mixing, if there is an addition and synergism, the molar ratio of the two to produce the maximum additive effect and the nature of the system. This is the significance of introducing the interaction parameter β.
The criterion for producing additive and synergistic effects
The most basic property of surfactant is to reduce the surface tension and form micelles, and to measure the activity of surfactant is mainly to investigate the degree of surface tension reduction and the critical micelle concentration. Under normal circumstances, surfactants with good performance can reduce the surface tension of the solution to a very low degree and form micelles at a low concentration.
Reduce surface tension
In terms of reducing the surface tension, addition and synergism is the reduction of the surface tension of the solution to a point where the sum of the concentrations of the two surfactants required is lower than the concentration required for the use of either of the surfactants in the compound system alone. If this concentration is higher than that required for any of the surfactants, a negative addition and synergistic effect has been produced.
Form a mixed micelle
When the critical micelle concentration of the mixed micelle formed by the aqueous solution of the composite system is lower than that of any single surfactant, it is called positive addition and synergism. If the critical micelle concentration of the mixture is higher than that of any single component, negative addition and synergism are said to occur.
Comprehensive consideration
Combining the reduction of surface tension and the formation of mixed micelles, positive addition and synergism mean that the surface tension of the two surfactants at the critical micelle concentration of the mixed micelle is lower than that of any of the surfactants at their critical micelle concentration, and on the contrary, negative addition and synergism will occur.
It can be seen that after the introduction of intermolecular interaction parameters, the interaction between the two surfactants can be qualitatively understood, whether they attract each other or repel each other, and how strong the force is. In addition, the composition of the compounding system, that is, the compounding ratio of the two surfactants, can be further obtained by calculating and judging whether the compounding effect occurs after the mixing of the two surfactants by relevant formulas, which provides theoretical guidance for the application of surfactant compounding.
Compound system of surfactants
In addition to reducing surface tension and micelle formation, surfactants have many important functions in practical applications, such as washing, foaming, solubilizing and wetting. There is still no mature theoretical guidance on addition and synergism in these aspects, but some experience has been summarized in practical application, that is, there is a certain relationship between addition and synergism and the reduction of surface tension or the formation of micelles.
Anion-anionic surfactant complex system
The addition and synergistic effect of such a compound system will reduce the surface tension, improve the washing, decontamination, wettability and emulsification, and reduce the Clough characteristics. However, it should be pointed out that the compounding of this system will only produce addition and synergism when it has a specific structure.
Anion-cationic surfactant complex system
The interaction force between anionic and cationic surfactants is strong, and their composite systems show strong additive and synergistic effects on reducing surface tension and formation of mixed micelles, and also have great improvement in wettability, foam stability and emulsification properties. At present, this kind of composite system has been widely used in the soft and antistatic treatment of fibers and fabrics, and the stability of foams and emulsions.
However, it should be noted that when these two types of surfactants are combined, it is easy to generate insoluble salt precipitation from the solution, thus losing surface activity, so the variety of surfactant should be carefully selected.
Anion-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, the foam height has the maximum value, and the surface tension reduction property also has the maximum effect and synergistic effect.
Anion-nonionic surfactant complex system
Such combination systems may either increase or decrease the solubilization of micelles. The different solubilization effects are related to the interaction of the two surfactant molecules and the form of the mixed micelle. It is generally believed that when the nonionic surfactant has a long hydrocarbon chain and a small ethylene oxide addition number, 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, anion-rich surfactant and non-ionic surfactant, which coexist in the solution.
Cationic nonionic surfactant complex system
The critical micelle concentration can be significantly reduced by adding non-ionic surfactant to cationic surfactant solution. It is the result of interaction between ionic groups of cationic surfactants and polar polyoxyethylene groups of non-ionic surfactants.
Nonionic - nonionic surfactant complex system
Most polyoxyethylene non-ionic surfactants themselves are mixtures, and their properties are quite different from a single substance. Usually, when two kinds of non-ionic surfactants with the same hydrophobic base and similar ethylene oxide addition number are mixed, they are almost ideal solutions and easy to form mixed micelles. The hydrophilicity of the mixture is equivalent to the average value of these two substances. The hydrophilicity of the mixture is higher than the average of the two surfactants when the ethylene oxide addition number and hydrophilicity of the two surfactants are different, and the oil-soluble variety may be more soluble in the water-soluble surfactant micelles.
In a word, the properties of surfactants can be improved and enhanced when the combination produces addition and synergism. With the deepening of this theory, it will be applied more and more widely and play a greater role in various fields of national economy.
