The use of surfactants with each other or with other compounds is called compounding. In the solubilizing application of surfactants, the solubilizing capacity can be greatly increased and the amount of surfactant can be reduced if a suitable compound can be selected.
Compatibility with neutral inorganic salts: The addition of soluble neutral inorganic salts to ionic surfactant solutions 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 fructose, xylose, sorbitol, etc. have similar effects. In contrast, some short-chain alcohols not only can not form mixed micelles with surfactants, but also may destroy the formation of micelles, such as C1 ~ C6 alcohols. Polar organic substances such as urea, N-methylacetamide, ethylene glycol, etc. all elevate the critical micelle concentration of surfactants.
Compatibility with water-soluble macromolecules: water-soluble macromolecules such as gelatin, polyvinyl alcohol, polyethylene glycol and polyvinyl ketone have an adsorption effect on surfactant molecules, which reduces the number of free surfactant molecules in the solution, and the critical micellar concentration is thus increased.
Basis for compounding
The purpose of surfactant compounding is to achieve additive and synergistic effects, i.e. synergistic effect. That is, the different types of surfactants artificially mixed, the performance of the mixture than the original single component performance is more excellent, which is often referred to as “1 +1 > 2” effect.
For example, sodium dodecyl sulfate mixed with a small amount of dodecanol, dodecanoyl alcohol amine and other substances can improve its foaming, washing, lower surface tension, emulsification and other properties in detergent formulations.
The compounding of surfactants can produce additive and synergistic effects, which have been applied to the actual production, but its basic theoretical aspects of the research is still only in recent years, and the results can provide guidance for the prediction of additive and synergistic behaviors of surfactants in order to get the optimal compounding effect. However, the research is still in the primary stage, mainly focusing on the two-component compounding system.
In the compounding system, the interactions between surfactant molecules of different types and structures determine the performance of the whole system and the compounding effect, so mastering the interactions between surfactant molecules is the basis for the study of surfactant compounding.
Surfactant intermolecular interaction parameters
The two most fundamental properties of surfactants are surface adsorption of surfactants and micelle formation. Therefore, the generation of additive synergism will first change the surface tension and critical micelle concentration of the system. Generally, when two surfactants are compounded, the critical micelle concentration of the mixed system is not equal to the average of their critical micelle concentrations, 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 combination form a mixed monomolecular adsorbent layer on the surface and mixed micelles inside the solution. Whether it is a mixed monomolecular adsorption layer or mixed micelles, there are interactions between the two surfactant molecules. The form and size of the interaction can be expressed by the intermolecular interaction parameter β.
The interaction parameter β between surfactant molecules is related to the free energy of mixing of two surfactants, and the negative value of β indicates that the two molecules are attracted to each other; the positive value of β indicates that the two molecules are repulsed by each other; the value of β close to 0 indicates that the two molecules have almost no interaction with each other, which is close to an ideal mixing. Many scholars through a large number of experiments and calculations found that the β value is generally between -2 (weak repulsion) and -40 (strong attraction).
Factors affecting the parameters of intermolecular interactions
Most of the mixed systems have negative β-values, that is, the two surfactant molecules are mutually attracted to each other’s role. This attraction mainly comes from the intermolecular electrostatic attraction, which is closely related to the molecular structure of surfactants and affected by external factors such as temperature and electrolyte.
Effect of surfactant ion type
The size of intermolecular interactions between different types of surfactants is different, and the order of size is anionic-cationic > anionic-amphoteric > ionic-polyoxyethylene nonionic > betaine amphoteric-cationic > betaine amphoteric-polyoxyethylene nonionic > polyoxyethylene nonionic-polyoxyethylene nonionic.
Since the probability of addition and synergism increases with the increase of the interaction force between the two surfactant molecules, the anion-cation and anion-amphoteric surfactant complex systems have the highest probability of addition and synergism with anionic surfactants. Therefore, the greatest possibility of addition and synergism with anionic surfactants is anionic-cationic and anionic-amphoteric surfactant complexes, while cationic-polyoxyethylene nonionic and anionic-anionic complexes can only occur when the two surfactants have specific structures.
Effect of hydrophobic groups
The β becomes more negative with the increase in the length of the hydrophobic carbon chain of the surfactant, and the interaction parameter between the molecules in the mixed monolayer is the largest and the strongest attraction when the carbon chain lengths of the two surfactants are equal. In contrast, the value of β in the mixed micelles increases with the sum of carbon chain lengths.
Influence of medium PH value
The types of examples of amphoteric surfactants in aqueous solution vary with the pH of the medium. When the solution pH is below the isoelectric point, it exists in the form of cations, through which it interacts with anionic surfactants. Therefore, when the alkalinity or pH of the medium increases, the amphoteric surfactant gradually transforms into electrically neutral molecules, or even negative ions, and the interaction force with the anionic surfactant decreases.
For the same reason, amphoteric surfactants, which are inherently less basic and have poor access to protons, have lower interactions with anionic surfactants.
Effects of adding inorganic electrolytes
The tianga of inorganic electrolytes decreases the intermolecular interaction force in the mixed system of ionic surfactants and polyoxyethylene-based nonionic surfactants, which suggests the existence of electrostatic forces between the molecules of these two types of surfactants.
Effect of temperature
Typically, the intermolecular forces decrease with increasing temperature in the range of 10-40 °C.
The significance of the interaction parameter β
The interaction parameter β is affected by many factors. After understanding the meaning of this parameter and the factors affecting it, it is necessary to further use it to determine whether there is a compounding effect after mixing between two surfactants, and if there is an additive and synergistic effect, the molar ratio of the two produces the maximum additive and synergistic effect, and the nature of the system and how. This is the significance of introducing the interaction parameter β.
Criteria for the generation of additive and synergistic effects
The most basic property of surfactants is to reduce surface tension and form micelles, and the main measure of surfactant activity is to examine the degree of reduction of surface tension of the solution and the size of the critical micelle concentration. Generally speaking, surfactants with excellent performance can make the surface tension of the solution drop to a very low degree and form micelles at a low concentration.
Reduced surface tension
In the context of surface tension reduction, an additive synergistic effect means that the sum of the concentrations of the two surfactants required to reduce the surface tension of a solution to a certain level is lower than the concentration required for either surfactant in the compounded system alone. If this concentration is higher than that required for either surfactant, a negative additive synergistic effect has occurred.
Formation of mixed micelles
When the aqueous solution of the compounding system to form mixed micelles of the critical micelle concentration is lower than the critical micelle concentration of any one of the single surfactant, that is, known as the production of positive additive and synergistic effect; if the critical micelle concentration of the mixture is higher than any one of the single component, then it is said to produce a negative additive and synergistic effect.
a round-up
Combining the reduction of surface tension and the formation of mixed micelles, positive additive and synergistic means that the surface tension of a complex system of two surfactants at the critical micelle concentration of a mixed micelle is lower than the surface tension of either surfactant at its critical micelle concentration, and the opposite produces a negative additive and synergistic effect.
It can be seen that the introduction of the intermolecular interaction parameter can qualitatively understand the interaction between the two surfactant molecules, whether they are attracted to each other or repelled by each other, and how strong or weak the interaction force is. It can also be calculated and judged whether the two surfactants produce the compounding effect after mixing by the relevant formulae, and the composition of the compounding system, i.e., the compounding ratio of the two surfactants, when the maximum additive effect is produced can be further found, which provides a theoretical guidance for the application of surfactant compounding.
Compounding systems for surfactants
In addition to the reduction of surface tension and the formation of micelles, in practical applications, surfactants have many important roles, such as washing, foaming, solubilization and wetting. There is still no mature theoretical guidance on the additive and synergistic effects in these aspects, but some experiences have been summarized in the practical application, i.e., additive and synergistic effects are often associated with the reduction of surface tension or the formation of micelles to a certain extent.
Anionic-anionic surfactant compounding system
This type of compounding system, if it produces an additive and synergistic effect, will lead to a lower surface tension, so that the detergency, detergency, and wetting and emulsifying properties are improved, while the Kraft point will be reduced. However, it should be pointed out that the compounding of this system will only produce additive and synergistic effects when it has a specific structure.
Anionic-Cationic Surfactant Compounding System
Anionic – cationic surfactant intermolecular interaction force is stronger, their complex system in the reduction of surface tension, the formation of mixed micelles have shown a strong plus and synergistic effect, in the wetting properties, foam stabilization properties and emulsification properties, etc. have also been greatly improved. At present, this kind of compounding system has been widely used in the soft and antistatic treatment of fibers and fabrics, foam and emulsion stabilization.
However, it should be noted that when these two types of surfactants are compounded, it is easy to generate insoluble salts precipitated from the solution, thus losing surface activity, so the choice of surfactant species should be carefully.
Anionic-amphoteric surfactant complex system
The mode of action of the two surfactant molecules in this system is related to the acidity and alkalinity of the medium, and there is a maximum in the foam height and a maximum additive and synergistic effect in the nature of reducing surface tension.
Anionic-Nonionic Surfactant Compounding System
Such complexed systems may either increase or decrease the solubilizing effect of the micelles. The appearance of different solubilizing effects is related to the interaction of the two surfactant molecules and the form of the mixed micelles. It is generally believed that when the hydrocarbon chain of nonionic surfactants is longer and the number of ethylene oxide additions is smaller, it is easy to form mixed micelles when compounded with anionic surfactants. When the hydrocarbon chain is shorter and the number of ethylene oxide addition is larger, it is easy to form two types of micelles, anion-rich surfactants and nonion-rich surfactants, and they coexist in solution.
Cationic nonionic surfactant compounding system
The addition of a nonionic surfactant to a cationic surfactant solution results in a significant reduction in the critical micelle concentration. It is the result of the interaction of the ionic groups of the cationic surfactant with the polar polyoxyethylene groups of the nonionic surfactant.
Nonionic-Nonionic Surfactant Compounding System
Most of the polyoxyethylene nonionic surfactant itself is a mixture, its properties and a single substance has a big difference, usually the same hydrophobic group, the number of ethylene oxide addition of two nonionic surfactants similar to the mixing of the ideal solution, it is easy to form a mixture of micelles, and the hydrophilicity of its mixture is equal to the average value of the two substances, when the number of the two surfactants of the ethylene oxide addition and the hydrophilicity of the larger difference, the hydrophilicity of the mixture is higher than the average value of the two. When the difference between the number of ethylene oxide addition and hydrophilicity is large, the hydrophilicity of the mixture is higher than the average value of the two substances, and the oil-soluble varieties may be solubilized in the micelles of water-soluble surfactants.
In short, when compounding produces additive and synergistic effects, the various properties of surfactants will be improved and enhanced. With the deepening of the theory in this area, the application will become more and more extensive and play a greater role in various fields of the national economy.
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