On the selection of emulsion surfactants
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Surfactants used in emulsions generally fall into anionic, cationic, and non-ionic categories. But how do we usually determine which type of surfactant to choose for a given emulsion?
Most emulsions on the market are anionic, non-ionic, or a combination of both. Cationic types are rare.
It is related to whether the emulsion is oil-in-water or water-in-oil
Rotate one – calculated based on the HLB of the oil and surfactant. The HLB value and the selection of emulsifiers: Determining which emulsifier to use in a specific oil-water system in order to obtain an emulsion with optimal properties is key to emulsion preparation. The most reliable method is through experimental screening, and the HLB value helps in this process. Experiments have shown that emulsifiers for O/W (water-in-oil) emulsions typically have an HLB value between 8 and 18, while those for W/O (oil-in-water) emulsions usually have an HLB value between 3 and 6. When preparing emulsions, in addition to selecting an emulsifier based on the desired type of emulsion, the properties of the oil phase also influence the required HLB value of the emulsifier; moreover, the HLB value of the emulsifier should match that required by the oil phase to be emulsified. There is a simple way to determine the appropriate HLB value for the oil to be emulsified: visually observe how the oil droplets spread on the surface of aqueous solutions containing emulsifiers with different HLB values. When the HLB value of the emulsifier is high, the oil spreads completely; as the HLB value decreases, spreading becomes more difficult, until at a certain HLB value the oil no longer spreads at all. The HLB value of the emulsifier at this point is approximately the value required for emulsifying the oil. Although this method is rough, it is simple to apply, and the results it yields can provide some useful guidance.
The HLB value and the selection of the best emulsifier: Each emulsifier has a specific HLB value, and it is often difficult for a single emulsifier to meet the emulsification requirements of systems composed of multiple components. Typically, several emulsifiers with different HLB values are combined to form mixed emulsifiers, which can meet the requirements of complex systems and improve the emulsification efficiency. To select the best emulsifier for emulsifying a particular oil-water system, follow these steps. To determine the optimal HLB value for an oil-water system: ① Choose a pair of emulsifiers with significantly different HLB values, such as Span-60 (HLB=4.3) and Tween-80 (HLB=15). Prepare a series of mixed emulsifiers with varying HLB values by combining these two emulsifiers in different proportions. Use these mixed emulsifiers to prepare a series of emulsions from the specified oil-water system, and measure the emulsification efficiency of each emulsion (which can be represented by the stability time of the emulsion or other stability parameters). Plot these values against the calculated HLB values of the mixed emulsifiers; the HLB value corresponding to the peak of this curve is the value required for emulsifying the specified system. Clearly, using mixed emulsifiers can yield the most suitable HLB value, but this emulsifier may not necessarily be the one with the highest efficiency. An emulsifier with high efficiency means that the lowest concentration of the emulsifier is required to stabilize the emulsion, and it is also the cheapest. An emulsifier that is more expensive but requires a much lower concentration might still be more efficient than one that is cheaper but requires a higher concentration. ② Once the HLB value required for the emulsification system has been determined, select several pairs of emulsifiers to be combined, ensuring that the HLB values of all the mixed emulsifiers match those determined using the above method. Use these emulsifiers to emulsify the specified system, measure their stability, and compare their emulsification efficiencies until the pair with the highest efficiency is found. It should be noted that the concentration of the emulsifiers is not mentioned here, but this does not affect this selection method, as the HLB value required to create a stable emulsion has little to do with the concentration of the emulsifier. In regions where the emulsion is unstable, low concentrations of the emulsifier or high concentrations of the internal phase can affect this method. When using the HLB method to select emulsifiers, it is necessary to consider not only the optimal HLB value but also the affinity of the emulsifier for both the dispersed phase and the dispersion medium. An ideal emulsifier should have strong affinity both for the oil phase and for the water phase. By combining emulsifiers with low HLB values and those with high HLB values, a mixed membrane can be formed that has strong affinity for both phases, thus meeting these requirements simultaneously. Therefore, using mixed emulsifiers yields better results than using a single emulsifier. In summary, the method for determining the emulsifier formula required for emulsifying a specified system involves selecting any pair of emulsifiers, changing their mixing ratios within a certain range to find the HLB value with the highest efficiency, then changing the types and ratios of the mixed emulsifiers while still maintaining the required HLB value, until the mixed emulsifier with the highest efficiency is found.
The HLB value and the ratio of mixed emulsifiers: When preparing mixed emulsifiers, the appropriate amounts to use can be determined based on the HLB values of the individual emulsifiers and the HLB value required for the specified system. For example, in the O/W emulsion polymerization of vinyl acetate, 3% of the mixture consists of emulsifiers. If SDS and Span-65 are used as emulsifiers, with SDS having an HLB value of 40 and Span-65 having an HLB value of 2.1, the required average HLB value for emulsion polymerization is 16.0. Let w% represent the mass fraction of Span-65 in the mixed emulsifier. Then, 40(1-w%) + 2.1w% = 16. Solving this equation gives w% = 63.3%, meaning that SDS accounts for 36.7% of the mixed emulsifier. Thus, in the O/W emulsion polymerization of vinyl acetate, Span-65 makes up 3% * 63.3% = 1.9%, while SDS makes up 3% * (1-63.3%) = 1.1%. When preparing stable emulsions, choosing the most suitable emulsifiers to achieve the best emulsification effect is crucial. Currently, there is no complete theory for selecting emulsifiers. The HLB value of surfactants is very useful in selecting emulsifiers and determining the proportions of mixed emulsifiers. Its advantage lies in its additivity, allowing for simple calculations. However, it does not take into account other factors that can affect the HLB value, especially temperature, a problem that is particularly evident with non-ionic emulsifiers, which are widely used in recent years. Additionally, the HLB value can only roughly indicate the type of emulsion that will be formed; it cannot determine the concentration of the emulsifier required for optimal emulsification effects, nor can it predict the stability of the resulting emulsion. Therefore, while using the HLB value to select emulsifiers is an effective method, it also has certain limitations. In practical applications, it is necessary to combine this method with other approaches.
The HLB value and the selection of the best emulsifier: Each emulsifier has a specific HLB value, and it is often difficult for a single emulsifier to meet the emulsification requirements of systems composed of multiple components. Typically, several emulsifiers with different HLB values are combined to form mixed emulsifiers, which can meet the requirements of complex systems and improve the emulsification efficiency. To select the best emulsifier for emulsifying a particular oil-water system, follow these steps. To determine the optimal HLB value for an oil-water system: ① Choose a pair of emulsifiers with significantly different HLB values, such as Span-60 (HLB=4.3) and Tween-80 (HLB=15). Prepare a series of mixed emulsifiers with varying HLB values by combining these two emulsifiers in different proportions. Use these mixed emulsifiers to prepare a series of emulsions from the specified oil-water system, and measure the emulsification efficiency of each emulsion (which can be represented by the stability time of the emulsion or other stability parameters). Plot these values against the calculated HLB values of the mixed emulsifiers; the HLB value corresponding to the peak of this curve is the value required for emulsifying the specified system. Clearly, using mixed emulsifiers can yield the most suitable HLB value, but this emulsifier may not necessarily be the one with the highest efficiency. An emulsifier with high efficiency means that the lowest concentration of the emulsifier is required to stabilize the emulsion, and it is also the cheapest. An emulsifier that is more expensive but requires a much lower concentration might still be more efficient than one that is cheaper but requires a higher concentration. ② Once the HLB value required for the emulsification system has been determined, select several pairs of emulsifiers to be combined, ensuring that the HLB values of all the mixed emulsifiers match those determined using the above method. Use these emulsifiers to emulsify the specified system, measure their stability, and compare their emulsification efficiencies until the pair with the highest efficiency is found. It should be noted that the concentration of the emulsifiers is not mentioned here, but this does not affect this selection method, as the HLB value required to create a stable emulsion has little to do with the concentration of the emulsifier. In regions where the emulsion is unstable, low concentrations of the emulsifier or high concentrations of the internal phase can affect this method. When using the HLB method to select emulsifiers, it is necessary to consider not only the optimal HLB value but also the affinity of the emulsifier for both the dispersed phase and the dispersion medium. An ideal emulsifier should have strong affinity both for the oil phase and for the water phase. By combining emulsifiers with low HLB values and those with high HLB values, a mixed membrane can be formed that has strong affinity for both phases, thus meeting these requirements simultaneously. Therefore, using mixed emulsifiers yields better results than using a single emulsifier. In summary, the method for determining the emulsifier formula required for emulsifying a specified system involves selecting any pair of emulsifiers, changing their mixing ratios within a certain range to find the HLB value with the highest efficiency, then changing the types and ratios of the mixed emulsifiers while still maintaining the required HLB value, until the mixed emulsifier with the highest efficiency is found.
The HLB value and the ratio of mixed emulsifiers: When preparing mixed emulsifiers, the appropriate amounts to use can be determined based on the HLB values of the individual emulsifiers and the HLB value required for the specified system. For example, in the O/W emulsion polymerization of vinyl acetate, 3% of the mixture consists of emulsifiers. If SDS and Span-65 are used as emulsifiers, with SDS having an HLB value of 40 and Span-65 having an HLB value of 2.1, the required average HLB value for emulsion polymerization is 16.0. Let w% represent the mass fraction of Span-65 in the mixed emulsifier. Then, 40(1-w%) + 2.1w% = 16. Solving this equation gives w% = 63.3%, meaning that SDS accounts for 36.7% of the mixed emulsifier. Thus, in the O/W emulsion polymerization of vinyl acetate, Span-65 makes up 3% * 63.3% = 1.9%, while SDS makes up 3% * (1-63.3%) = 1.1%. When preparing stable emulsions, choosing the most suitable emulsifiers to achieve the best emulsification effect is crucial. Currently, there is no complete theory for selecting emulsifiers. The HLB value of surfactants is very useful in selecting emulsifiers and determining the proportions of mixed emulsifiers. Its advantage lies in its additivity, allowing for simple calculations. However, it does not take into account other factors that can affect the HLB value, especially temperature, a problem that is particularly evident with non-ionic emulsifiers, which are widely used in recent years. Additionally, the HLB value can only roughly indicate the type of emulsion that will be formed; it cannot determine the concentration of the emulsifier required for optimal emulsification effects, nor can it predict the stability of the resulting emulsion. Therefore, while using the HLB value to select emulsifiers is an effective method, it also has certain limitations. In practical applications, it is necessary to combine this method with other approaches.