Basic theory of surfactants (2)
Foam plays an important role in the washing process. Foam is a dispersion system of gas dispersed in liquid or solid, with gas as the dispersed phase and liquid or solid as the dispersing medium, the former is called liquid foam, while the latter is called solid foam, such as foam, foam glass, foam cement, etc.
I. The formation of foam. The foam discussed here refers to the aggregates of bubbles separated by a liquid film. This foam is due to the large difference in density between the dispersed phase (gas) and the dispersed medium (liquid), coupled with the low viscosity of the liquid, so the bubbles can always rise to the liquid surface quickly. The process of forming foam is to bring a large amount of gas into the liquid, the bubbles in the liquid and quickly return to the liquid surface, the formation of bubble aggregates separated by a small amount of liquid gas
Foam in the form of two significant features: First, as the dispersed phase of the bubble is often polyhedral shape, this is because the intersection of the bubble, there is a tendency to thin the liquid film so that the bubble becomes polyhedral, when the liquid film becomes thin to a certain extent, it leads to the rupture of the bubble; Second, the pure liquid can not form a stable foam, the liquid can form a foam, at least two or more components. The aqueous solution of surfactant is a typical system that is easy to generate foam, and its ability to generate foam is also related to other properties.
Surfactants with good foaming power are called foaming agents. Although the foaming agent has good foaming ability, the foam formed may not necessarily be maintained for a longer period of time, i.e., its stability is not necessarily good. In order to maintain the stability of the foam, often added to the foam-forming agent can increase the stability of the foam substances, such substances are called stabilizers, commonly used stabilizers are lauroyl diethanolamine and dodecyl dimethylamine oxide.
Second, the stability of the foam. Foam is a thermodynamically unstable system, the final trend is to break the bubble after the total surface area of the liquid in the system is reduced, the free energy is reduced. The defoaming process is the liquid film separating the gas from thick to thin, until the process of rupture. Therefore, the degree of stability of the foam is mainly determined by the speed of liquid discharge and the strength of the liquid film. Its influence factors are also the following.
1、Surface tension. From the energy point of view, low surface tension is more favorable for the formation of foam, but does not guarantee foam stability. The surface tension is low, the pressure difference is small, the discharge speed becomes slower, the liquid film becomes thinner slower, which is conducive to the stability of the foam.
2、Surface viscosity. The key factor to determine the stability of the foam is the liquid film strength, and the liquid film strength is mainly determined by the solidity of the surface adsorption film, measured by the surface viscosity. The foam generated by the solution with larger surface viscosity has a longer life. This is because the interaction between the surface adsorption molecules leads to an increase in film strength, thereby increasing the life of the foam.
3、Solution viscosity. When the viscosity of the liquid itself increases, the liquid in the liquid film is not easy to discharge, the liquid film thickness thins slower, slowing down the time of film rupture, increasing the stability of the foam.
4、The “repair” effect of surface tension. Surfactant adsorbed on the surface of the liquid film, the ability to resist the expansion or contraction of the surface of the liquid film, this ability is called the repair effect. Because there is a surfactant adsorbed on the surface of the liquid film, the expansion of its surface area will reduce the concentration of adsorbed molecules on the surface and increase the surface tension. Further expansion of the surface will require greater work to be done. Conversely surface area contraction will increase the concentration of surface adsorption molecules, that is, reduce surface tension, which is not conducive to further contraction.
5, the diffusion of gas through the liquid film. Due to the existence of capillary pressure, the pressure of the small bubble in the foam is higher than the pressure of the large bubble, which will cause the gas in the small bubble to diffuse through the liquid film to the large bubble of low pressure, resulting in the phenomenon of the small bubble becoming smaller and the large bubble becoming larger, and finally the foam rupture. If surfactant is added, the foam will be uniform and fine and not easy to defoam. As the surfactant is closely arranged on the liquid film, it is difficult to breathe, and the foam is more stable.
6、The effect of surface charge. If the foam liquid film has the same symbolic charge, the two surfaces of the liquid film will repel each other, preventing the liquid film from thinning or even destruction. Ionic surfactants can play this stabilizing role.
Third, the destruction of foam. The basic principle of the destruction of foam is to change the conditions of foam generation or eliminate the stabilizing factors of foam, there are two kinds of physical and chemical defoaming methods. Physical defoaming is to maintain the chemical composition of the foam solution under the circumstances of changing the conditions of foam generation, such as external disturbance, temperature or pressure changes and ultrasonic treatment are effective physical methods to eliminate the foam. Chemical defoaming method is to add certain substances and foam-forming agent to reduce the strength of the liquid film in the foam and thus reduce the stability of the foam to achieve the purpose of defoaming, such substances are called defoamers. Most of the defoamers are surfactants. Therefore, according to the mechanism of action of defoaming, defoamer should have a strong ability to reduce surface tension, easy to adsorb on the surface, and the surface adsorption molecular interaction is weak, adsorption molecules arranged in a more relaxed structure.
There are various types of defoamers, but basically they are all nonionic surfactants. Non-ionic surfactants have anti-foaming properties near or above the cloud point and are often used as defoamers. Alcohols, especially alcohols with branching structure, fatty acids and fatty acid esters, polyamides, phosphate esters, silicone oil, etc. are also commonly used as excellent defoamers.
Fourth, the foam and washing. There is no direct connection between foam and washing effect, and the amount of foam does not indicate the good or bad washing effect. For example, the foaming performance of non-ionic surfactants is far inferior to that of soap, but their detergency is much better than that of soap. In some cases, foam can be helpful in removing dirt. For example, when washing dishes at home, the foam of the detergent can take away the oil droplets; when scrubbing carpets, the foam helps to take away dust, powder and other solid dirt. In addition, foam can sometimes be used as a sign of the effectiveness of the detergent, because fatty oils have an inhibiting effect on the foam of the detergent, and when there is too much oil and too little detergent, there will be no foam generation, or the original foam will disappear. Foam can sometimes be used as an indicator of whether rinsing is clean, because the amount of foam in the rinsing solution tends to decrease with the amount of detergent, so the amount of foam can be used to evaluate the degree of rinsing.
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