Surfactant HLB value and choice of emulsifier

What kind of emulsifier is used in a specific oil-water system to obtain the best performance emulsion, which is the key to preparing emulsions. The most reliable method is through experimental screening, HLB value helps screening work. Through experiments It is found that the emulsifier of O / W type (oil-in-water) emulsion often has an HLB value of 8 to 18; as the emulsifier of W / O type (water-in-oil) emulsion, its HLB value is often at Between 3 and 6. When preparing an emulsion, in addition to selecting the emulsifier according to the type of emulsion desired, the different oil phase properties have different requirements for the HLB value of the emulsifier, and the HLB value of the emulsifier should be The emulsified oil phase needs to be consistent. [4] There is a simple method to determine the required HLB value of the emulsified oil: visually inspect the spread of oil droplets on the surface of the emulsifier aqueous solution with different HLB values. When the emulsifier HLB value is large, the oil Fully spreading, as the HLB value decreases, spreading becomes difficult until the oil does not spread on a certain HLB value emulsifier solution, the HLB value of this emulsifier is approximately the HLB value required for emulsified oil. Although rough, it is easy to operate, and the results obtained have certain reference value.

HLB value and 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 a system composed of multiple components. Generally, multiple emulsifiers with different HLB values ​​are mixed and used to form a mixed emulsifier. To meet the requirements of complex systems, it can greatly improve the emulsification effect. To emulsify an oil-water system, the best emulsifier can be selected according to the following steps.

Determination of the best HLB value of oil-water system:
Select a pair of emulsifiers with large differences in HLB value, for example, Span-60 (HLB = 4.3) and Tween-80 (HLB = 15), and formulate a series of mixed emulsifiers with different HLB values ​​in different proportions. The series of mixed emulsifiers respectively make the specified oil-water system into a series of emulsions, and measure the emulsification efficiency of each emulsion (which can be represented by the stability time of the emulsion or other stability properties), and the calculated mixed emulsifier HLB, drawing, you can get a bell-shaped curve, the HLB value corresponding to the highest peak of the curve is the HLB value required for the emulsification of the specified system. Obviously, the most suitable HLB value can be obtained by using a mixed emulsifier, but this emulsification The agent is not necessarily the most efficient. The so-called good efficiency of the emulsifier means that the concentration of the emulsifier required to stabilize the specified emulsion is the lowest! The price is the cheapest. The emulsifier is expensive but the required concentration is much lower than the price! The high-concentration emulsifier has high efficiency.

Determination of emulsifier:
Under the premise of maintaining the required HLB value of the selected emulsifying system, select several pairs of emulsifiers to mix, so that the HLB value of each mixed emulsifier is the value determined by the above method. Stability, compare the emulsification efficiency until you find the most efficient pair of emulsifiers. It is worth noting that the concentration of emulsifier is not mentioned here, but this does not affect this matching method, because a stable emulsion is prepared The required HLB value has little to do with the emulsifier concentration. In the unstable area of ​​the emulsion, when the emulsifier concentration is very low or the internal phase concentration is too high, it will affect this method. [6] The HLB method is used to select emulsification In addition to the best HLB value, we should also pay attention to the affinity of the emulsifier with the dispersed phase and the dispersion medium. An ideal emulsifier should not only have a strong affinity with the oil phase, but also have a comparison with the water phase. Strong affinity. Mix the emulsifier with a small HLB value and the emulsifier with a large HLB value to form a mixed film that has a strong affinity with the oil phase and the water phase, which can take into account both requirements. Therefore, use mixing Emulsifier is more effective than using a single emulsifier In summary, the method of determining the emulsifier formulation required for the emulsification of the specified system is: arbitrarily choose a pair of emulsifiers, change the mixing ratio within a certain range, and after obtaining the HLB value with the highest efficiency, change the compound emulsifier Type and ratio, but still need to maintain the required HLB value until the most efficient compound emulsifier is found.

Proportion of HLB value and mixed emulsifier:
When compounding the emulsifier, the appropriate amount can be obtained from the respective HLB value and the HLB value required by the specified system. For example, when performing O / W emulsion polymerization of vinyl acetate, the amount of emulsifier is 3% , Using SDS and Span-65 as emulsifiers, it is known that the HLB value of SDS is 40, the HLB value of Span-65 is 2.1, and the average HLB value required during emulsion polymerization is 16.0. Let Span-65 in the mixed emulsifier The mass fraction is w%, then 40 (1-w%) + 2.1w% = 16, the solution is w% = 63.3%, then the mass fraction of SDS in the mixed emulsifier is 36.7%. It can be seen that in acetic acid In the O / W emulsion polymerization system of vinyl ester, the amount of Span-65 accounts for 3% * 63.3% = 1.9%; the amount of SDS accounts for 3% * (1-63.3%) = 1.1%.

When preparing a stable emulsion, it is a key issue to choose the most suitable emulsifier to achieve the best emulsification effect. There is no perfect theory for the choice of emulsifier. The HLB value of the surfactant is in the choice of emulsifier and the determination of composite emulsification The dosage ratio has a lot of use value. Its advantages are mainly reflected in its additive nature, which can be simply calculated; the problem is that it does not consider the impact of other factors on the HLB value, especially the temperature. In recent years, non-ionic emulsifiers with large amounts have been particularly prominent. In addition, the HLB value can only roughly predict the type of emulsion formation, and cannot give the best emulsification effect when the emulsifier concentration, nor the stability of the resulting emulsion. Therefore, the use of HLB value to choose emulsifier is a more effective method, but it also has certain limitations, in practice, it needs to be combined with other methods.

When preparing water-in-oil (W / O) microemulsion fuel, the appropriate HLB value is 4-6. In terms of the synergistic effect when different surfactants are compounded, compared with mixed surfactants, the optimal surfactant dosage when a single surfactant is used to form microemulsion fuel is larger, that is, the efficiency of a single surfactant is low The mixed anionic and cationic surfactants can greatly increase the water solubility of the microemulsion fuel due to the mutual attraction of the hydrophilic groups, and its efficiency is higher than that of the mixed positive (or negative) -nonionic surfactant, so the preparation of microemulsion fuel It is advisable to use anionic and cationic surfactants for compounding. In anionic and cationic mixed surfactants, the mixed fatty acid salt has a good compatibility effect due to the unequal hydrocarbon chain length, so its surfactant efficiency is greater than that of a single fatty acid salt.

When preparing microemulsion fuel oil with ionic surfactants, co-solvent (alcohol) is indispensable. The most widely used is C4-7 medium carbon alcohol, among which n-butanol, n-pentanol, n-heptanol and n-octanol are better. Alcohol is mainly distributed in the oil-water interface layer, its hydroxyl group is close to the polar group of the surfactant, and the hydrocarbon chain is between the hydrocarbon chain tail of the surfactant. Its function is to further reduce the interfacial tension and increase the fluidity of the interface film. Adjust the HLB value of the surfactant, which can promote the miscibility of oil and water, reduce the surfactant concentration, and increase the amount of oil and water added. By studying the thermodynamics of the formation process of oleic acid / ammonia water, fuel oil, alcohol, and water microemulsion system, the results show that the absolute value of the standard free energy change in the formation process of microemulsion fuel oil increases with the increase of the alcohol carbon chain, and the relative molecular weight of fuel decreases With the increase of fuel content, it is easier to form microemulsion fuel. In addition, carbon amines and ethers in C4-7 can also be used as co-solvents, such as n-hexylamine and glycol ether are very effective co-solvents. During the formation of microemulsion, the proper addition of electrolyte (such as NH4N03, NaCl, etc.) can increase the hardness of the surface film of micelles, reduce the content of co-solvent, thereby reducing the concentration of surfactant and increasing the efficiency of surfactant. However, salts are detrimental to the combustion of fuel and accelerate the corrosion of cylinders and other components.

Adjust HLB value of surfactant

When preparing microemulsions, surfactants with unsuitable HLB values ​​can be adjusted to a suitable range with co-surfactants.

When selecting a co-surfactant, the considerations are similar to the selection of a surfactant. Commonly used co-surfactants are medium and high carbon fatty alcohols, lanolin derivatives, cholesterol, ethylene glycol and so on. Since non-ionic surfactants are an effective solubilizer, non-ionic surfactants with low HLB values ​​are generally classified as co-surfactants. Friberg et al. Pointed out that in W / O emulsions, polyoxyethylene alkyl ethers can be used as co-surfactants for ionic surfactants. The chain length of polyoxyethylene affects the solubilization of the microemulsion on water Key factor.

The term microemulsion was first coined by Hear and Schalmer in 1943.

Another distinguishing feature of microemulsion from traditional emulsion is the large variability of microemulsion structure. Traditional microemulsions can be basically divided into two types: W / O and O / W. Microemulsion can continuously change from W / O type structure to O / W type structure. When the system is rich in water, the oil phase is dispersed in the continuous phase in the form of uniform beads, forming an O / W-type normal-phase microemulsion; when the system is rich in oil, the water phase is dispersed in the form of uniform beads In the continuous phase, a W / O reverse microemulsion is formed; for the case where the amounts of water and oil in the system are equivalent, the water phase and the oil are the continuous phase when they are the same, and the two are randomly connected, called the double continuous phase structure, At this time, the system is in the reverse area.

Reduce interfacial tension

If only surfactant is used, the interfacial tension will no longer decrease after reaching CMC. If a certain concentration of co-surfactant with different properties from the surfactant is added at this time, the interfacial tension can be further reduced, resulting in more Surfactants and co-surfactants adsorb on the interface. When the interfacial tension of the droplet is y <10 ”N / cm, it can spontaneously form a microemulsion, and when y> 10-5N / cm, a coarse emulsion is generated. Of course, there are a few ionic surfactants such as succinic acid Sodium octyl sulfonate (AOT) is characterized by a polar head with two hydrocarbon groups, so it can generate microemulsions without the need for co-surfactants. Some nonionic surfactants also have near HLB values. Similar characteristics.

Increase the fluidity of the interface film

When forming microemulsion droplets, large droplets are dispersed into small droplets, and the interface must be deformed and reformed, which requires interface bending energy. Adding a co-surfactant can reduce the rigidity of the interface, increase the fluidity of the interface, reduce the bending energy required when the microemulsion is generated, and make the microemulsion droplets easily generate themselves.


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