The use of surfactants with each other or with other compounds is called compounding. In the solubilizing application of surfactants, if appropriate compounding can be selected, the solubilizing capacity can be greatly increased and the amount of surfactant can be reduced.
Compounding with neutral inorganic salts: Adding soluble neutral inorganic salts to ionic surfactant solutions increases the solubilizing capacity of hydrocarbon solubilizers. On the contrary, the solubilization of polar substances decreases.
Compatibility with organic additives: generally to the carbon atom in 12 or less fatty alcohols have better results. Some polyols such as fructose, xylose, sorbitol, etc. also have similar effects. On the contrary, 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 adsorption effect on surfactant molecules, reducing the number of free surfactant molecules in the solution, and the critical micelle concentration is therefore increased.
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Basis of compounding
The purpose of surfactant compounding is to achieve additive and synergistic effects, i.e. synergistic effect. That is, different types of surfactants are artificially mixed, and the performance of the mixture is better than that of the original single component, which is usually called “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 performance in detergent formulations such as foaming, washing, reducing surface tension, emulsification and so on.
The compounding of surfactants can produce additive effect, which has been applied to the actual production, but the research on its basic theory is still only in recent years, and the results can provide guidance for predicting the additive and synergistic behaviors of surfactants in order to get the best 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.
Interaction parameters between surfactant molecules
The two most basic properties of surfactants are surface adsorption of surfactants and micelle formation. Therefore, the generation of additive and synergistic effects 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 the critical micelle concentrations of the two surfactants, but is less than the critical micelle concentration of either surfactant used 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 β value of the interaction parameter between surfactant molecules is related to the free energy of mixing of the 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 intermolecular interaction parameters
Most of the mixed systems have a negative β value, i.e., the two surfactant molecules are interacting with each other in an attractive manner. This attraction mainly originates from the intermolecular electrostatic attraction, which is closely related to the structure of surfactant molecules and is affected by external factors such as temperature and electrolyte.
Influence of surfactant ion type
Different types of surfactant molecules interact with each other in different sizes, the size of the order of 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. Cationic-polyoxyethylene nonionic and anionic-anionic complex systems can only occur when the two surfactants have specific structures.
Influence of hydrophobic groups
The β becomes more negative with the increase of the carbon chain length of the hydrophobic group of surfactant, and the intermolecular interaction parameter in the mixed monolayer is the largest and the attraction is the strongest when the carbon chain lengths of the two surfactants are equal. In contrast, the value of β in the mixed micelles increases with the increase of the sum of the 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 as a cation and interacts with anionic surfactants through cations. Thus when the alkalinity or pH of the medium increases, the amphoteric surfactant gradually transforms into an electrically neutral molecule, or even a negative ion, and the interaction force with the anionic surfactant decreases.
For the same reason, amphoteric surfactants, which are themselves less alkaline and have a poor ability to acquire protons, also interact less with anionic surfactants.
Effect 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 indicates the existence of electrostatic forces between the molecules of these two types of surfactants.
Effect of temperature
Normally, in the range of 10-40°C, the intermolecular interaction force decreases with increasing temperature.
Significance of the interaction parameter β
The interaction parameter β’s is influenced by many factors. After understanding the meaning of the parameter and the factors affecting it, it is necessary to further use it to determine whether there is a compounding effect after mixing two surfactants, and if there is an additive and synergistic effect, the two produce the maximum additive and synergistic effect of the molar ratio and the nature of the system and how. This is the significance of introducing the interaction parameter β.
Judgement of producing additive and synergistic effect
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 good performance can reduce the surface tension of solution to a very low degree and form micelles at a low concentration.
Reduction of surface tension
In the context of surface tension reduction, additive synergism 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 to use either surfactant in the complex system alone. If this concentration is higher than that required for either surfactant, a negative additive and 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, it is called the production of positive additive and synergistic effect; if the critical micelle concentration of the mixture is higher than that of any one of the single components, it is said to produce a negative additive and synergistic effect.
Comprehensive consideration
Will reduce the surface tension and the formation of mixed micelles combined, positive plus and synergistic refers to the two surfactant complex system in the mixed micelles of the critical micellar concentration of the surface tension is lower than any one of the surfactant in its critical micellar concentration of the surface tension, on the contrary, it will produce a negative plus and synergistic effect.
It can be seen that the introduction of 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. And can be calculated and judged by the relevant formula whether the two surfactants produce the compounding effect after mixing, and can further find out the composition of the compounding system, that is, the compounding ratio of the two surfactants when the maximum additive effect is produced, which provides theoretical guidance for the application of surfactant compounding.
Compounding system of surfactants
In addition to reducing surface tension and micelle formation, surfactants have many important roles in practical applications, such as washing, foaming, solubilizing 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 practical applications, i.e., the additive and synergistic effects are often related to the reduction of surface tension or the formation of micelles to a certain extent.
Anion-anion surfactant compounding system
This kind of compounding system, if produce additive and synergistic effect, will make the surface tension lower, make the detergency, decontamination, and wettability and emulsification are improved, while the Kraft point will be reduced. However, it should be pointed out that the compounding of this system will produce additive and synergistic effects only when it has a specific structure.
Anionic-Cationic Surfactant Compounding System
Anionic – cationic surfactant intermolecular interaction force is stronger, their compounding system in reducing surface tension, the formation of mixed micelles have shown strong additive and synergistic effect, in the wetting properties, foam stabilization properties and emulsification properties, etc. There is also a greater improvement in the wetting properties. 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 the surface activity, so the choice of surfactant species should be carefully.
Anionic-amphoteric surfactant compounding system
This system of two surfactant molecules with the role of the medium of the acidity and alkalinity of the foam height of the existence of the maximum value to reduce surface tension nature also appears to be the maximum plus and synergistic effect.
Anionic – nonionic surfactant compounding system
This type of complex system may either increase or decrease the solubilizing effect of 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
Adding 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 between the ionic groups of the cationic surfactant and the polar polyoxyethylene groups of the nonionic surfactant.
Nonionic-nonionic surfactant complex system
Most of the polyoxyethylene nonionic surfactants themselves are mixtures, the nature of which is quite different from that of a single substance. Usually, when two nonionic surfactants with the same hydrophobic group and similar number of ethylene oxide additions are mixed, it is nearly an ideal solution, and it is easy to form mixed micelles, and the hydrophilicity of its mixture is equal to the average value of the two substances, and when there is a big difference in the number and hydrophilicity of the two surfactants, the hydrophilicity of the mixture is higher than that of the two substances. When the difference between the number of ethylene oxide additions and hydrophilicity of the two surfactants 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 the 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 be more and more extensive and play a greater role in various fields of the national economy.
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