The basis for the compounding of surfactants and classification

The combination of surfactants with each other or with other compounds is called compounding. In the solubilization application of surfactants, if a suitable compatibility can be selected, the solubilization ability can be greatly increased and the amount of surfactants can be reduced.

 

Compatibility with neutral inorganic salts: Adding soluble neutral inorganic salts to the ionic surfactant solution increases the solubilization amount of hydrocarbon solubilizers. On the contrary, the amount of solubilization of polar substances is reduced.

 

Compatibility with organic additives: Generally, fatty alcohols with carbon atoms below 12 have a better effect. Some polyols such as fructose, xylose and sorbitol have similar effects. On the contrary, some short-chain alcohols not only cannot form mixed micelles with surfactants, but may also damage the formation of micelles, such as C1-C6 alcohols. Polar organics such as urea, N-methylacetamide, ethylene glycol, etc. 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 can adsorb surfactant molecules, reduce the number of free surfactant molecules in the solution, and the critical micelle concentration Therefore rise.

 

Basis for compounding

 

The purpose of surfactant compounding is to achieve an additive synergistic effect, that is, a synergistic effect. That is, different types of surfactants are artificially mixed, and the performance of the resulting mixture is better than that of the original single component, which is the effect of "1+1>2".

 

For example, sodium lauryl sulfate mixed with a small amount of dodecyl alcohol, lauryl alcohol amine and other substances can improve its foaming, washing, surface tension reduction, emulsification and other properties in the detergent formulation.

 

The compounding of surfactants can produce additive effects, and has been applied to actual production, but its basic theoretical research is still only a matter of recent years, and the results can be used to predict the additive and synergistic behavior of surfactants. Guidance in order to get the best compounding effect. But its research is still in its infancy, mainly focusing on the two-component compound system.

 

In the compounding system, the interaction between surfactant molecules of different types and structures determines the performance and compounding effect of the entire system. Therefore, mastering the interaction between surfactant molecules is the basis for studying surfactant compounding.

 

Interaction parameters between surfactant molecules

 

The β value of most mixed systems is negative, that is, the two surfactant molecules attract 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.

Effect of surfactant ion type

 

The size of the interaction between different types of surfactant molecules is different, and the order of magnitude is anionic-cationic>anionic-amphoteric>ionic-polyoxyethylene nonionic>betaine amphoteric-cationic>betaine amphoteric- Polyoxyethylene non-ionic type> polyoxyethylene non-ionic type-polyoxyethylene non-ionic type.

 

Since the probability of additive synergism increases with the increase of the interaction force between the two surfactant molecules, the most likely to produce additive synergism with anionic surfactants is anion-cation and anion-zwitterion Surfactant compound system. The cationic-polyoxyethylene type non-ionic and anionic-anionic complex systems can only have a synergistic effect when the two surfactants have specific structures.

 

Influence of hydrophobic groups

 

As the length of the hydrophobic base carbon chain of the surfactant increases, β will become more negative. When the carbon chain lengths of the two surfactants are equal, the interaction parameters between the molecules in the mixed monolayer are the largest and the attraction is the strongest. The β value in mixed micelles increases as the sum of carbon chain length increases.

 

Influence of medium pH

 

Examples of amphoteric surfactants in aqueous solutions vary with the pH 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, the amphoteric surfactant gradually turns into an electrically neutral molecule, even anions, and the interaction force with the anionic surfactant decreases.

 

For the same reason, the amphoteric surfactant itself is low in basicity and has poor ability to obtain protons, and its interaction with anionic surfactants is also low.

 

Effect of adding inorganic electrolyte

 

The addition of inorganic electrolytes will reduce the intermolecular interaction force in the mixed system of ionic surfactants and polyoxyethylene nonionic surfactants, which indicates that there is an electrostatic force between the two types of surfactant molecules.

 

Influence of temperature

 

Normally, in the range of 10-40°C, the intermolecular force decreases with increasing temperature.

 

The meaning of the interaction parameter β

 

The interaction parameter β is affected 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 after the two surfactants are mixed. If there is an additive synergistic effect, the molar ratio when the two produces the maximum additive effect And the nature of the system. This is the meaning of introducing the interaction parameter β.

 

Criteria for additive effect

 

The most basic property of surfactants is to reduce surface tension and form micelles. The measurement of surfactant activity is mainly to investigate the degree of decrease in surface tension of the solution and the size of the critical micelle concentration. In general, surfactants with excellent properties can reduce the surface tension of the solution to a very low level and form micelles at a lower concentration.

Reduce surface tension

 

In terms of reducing surface tension, additive synergistic effect means that when the surface tension of the solution is reduced to a certain level, the sum of the concentration of the two surfactants required is lower than any one of the surfactants in the compound system when used alone The required concentration. If this concentration is higher than the required concentration of any one of the surfactants, it means that a negative additive synergistic effect has been produced.

 

Mixed micelles

 

When the critical micelle concentration of the mixed micelle formed by the aqueous solution of the compound system is lower than the critical micelle concentration of any single surfactant, it is called positive additive and synergistic effect; if the critical micelle concentration of the mixture is more than The high of any single component is said to produce negative additive and synergistic effects.

 

Considering

 

Integrating the reduction of surface tension and the formation of mixed micelles, positive additive and synergistic effect means that the surface tension of the composite system of two surfactants at the critical micelle concentration of the mixed micelle is lower than any of them. The surface tension of the agent at its critical micelle concentration, on the contrary, produces a negative additive and synergistic effect.

 

It can be seen from this that after introducing the intermolecular interaction parameters, it is possible to qualitatively understand the interaction between the two surfactant molecules, whether they are mutually attractive or mutually repelling, and how strong or weak the force is. It can be calculated and judged by related formulas whether the two surfactants have a compounding effect after being mixed, and the composition of the compounding system when the maximum additive effect is produced can be further calculated, that is, the compounding ratio of the two surfactants. This provides theoretical guidance for the application of surfactant compounding.

 

Surfactant compound system

 

In addition to reducing the surface tension and the formation of micelles, in practical applications, surfactants also have many important functions, such as washing, foaming, solubilization and wetting. There is still no mature theoretical guidance for the additive and synergistic effects in these aspects, but some experience has been summed up in practical applications, that is, the additive and synergistic effects are often related to the reduction of surface tension or the formation of micelles.

 

Anionic-anionic surfactant compound system

 

If this type of compound system produces an additive synergistic effect, the surface tension will be lowered, and the washing performance, detergency, wettability and emulsification will be improved, but the Kraff characteristic will be reduced. But it needs to be pointed out that the combination of this system will only produce additive and synergistic effects when it has a specific structure.

 

Anionic-cationic surfactant compound system

 

Anionic-cationic surfactants have a strong interaction force between molecules, and their compound systems have shown strong additive and synergistic effects in terms of reducing surface tension and the formation of mixed micelles, and are effective in wetting performance and foam stabilization. Performance and emulsification performance have also been greatly improved. At present, this type of compound system has been widely used in the softening and antistatic treatment of fibers and fabrics, and the stabilization of foams and emulsions.

 

However, it should be noted that when these two types of surfactants are compounded, insoluble salts are likely to be precipitated out of the solution, thereby losing surface activity. Therefore, the types of surfactants should be carefully selected.

 

Anionic-Amphoteric Surfactant Compound System

 

The mode of action of the two surfactant molecules in this system is related to the acidity and alkalinity of the medium. The foam height has a maximum value, and the surface tension reduction property also has the maximum additive effect.

 

Anionic-nonionic surfactant compound system

 

This type of compound 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 nonionic surfactant has a longer hydrocarbon chain and a small addition number of ethylene oxide, it is easy to form mixed micelles when compounded with an anionic surfactant. When the hydrocarbon chain is shorter and the ethylene oxide addition number is larger, it is easy to form two types of micelles, anionic-rich surfactants and non-ionic surfactants, which coexist in the solution.

 

Cationic nonionic surfactant compound system

 

Adding non-ionic surfactants to the cationic surfactant solution can significantly reduce the critical micelle concentration. It is the result of the interaction between the ionic groups of cationic surfactants and the polar polyoxyethylene groups of nonionic surfactants.

 

Non-ionic-non-ionic surfactant compound system

 

Most polyoxyethylene nonionic surfactants are mixtures themselves, and their properties are quite different from a single substance. Usually, when two nonionic surfactants with the same hydrophobic base and similar addition numbers of ethylene oxide are mixed, it is almost ideal. Solution, it is easy to form mixed micelles, and 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 affinity of the mixture Water is higher than the average of the two, oil-soluble varieties may be solubilized in the micelles of water-soluble surfactants.

 

In short, when the compound has an additive and synergistic effect, the various properties of the surfactant will be improved and enhanced. With the deepening of this theory, its application will become wider and wider, and it will play a greater role in various fields of the national economy.