Formation and elimination of surfactant foam

When the air enters the liquid, because it is insoluble in water, it will be separated into many bubbles by the liquid when there is an external force, forming an uneven system. When the air enters the liquid to form a foam, the contact area between the gas and the liquid increases, and the free energy of the system also increases.

When the surfactant concentration increases, the surface tension of the system will decrease rapidly at the beginning stage, but with the further increase of the surfactant concentration, the surface tension of the system will no longer change and reach a state of equilibrium. Milles and Shedlovsky concluded through experiments that the lowest point anomaly may be caused by the impurities contained in the system, or by the interaction between multiple surfactants. The lowest point is what we call the critical micelle concentration. Therefore, when the concentration of surfactants reaches the critical micellar concentration, there will be a lot of surfactants in the system, and these surfactants are enough to closely arrange on the liquid surface to form a single molecular film layer without gaps, thereby minimizing the surface tension of the system. When the surface tension is small, the free energy required for the system to produce foam will be reduced, which will lead to the formation of foam easily.

In the actual production process, in order to make the emulsion can be stably stored, we often adjust the concentration of surfactant above the critical micelle concentration. Although this is beneficial to the stability of the emulsion, there are certain negative effects. Because too much surfactant not only reduces the surface tension of the system to a minimum, but also the excess surfactant will surround the air entering the emulsion, forming a relatively solid liquid film, and the bimolecular film layer is formed on the liquid surface, which is not conducive to the elimination of foam.

The foam is a collection of many bubbles, and the bubble is formed by the gas dispersed in the liquid, the gas acts as the dispersed phase, the liquid acts as the continuous phase. The gas in the bubble may run from one bubble to another, or it may be transferred from the system to the neighboring atmosphere, which is the merger and disappearance of bubbles.

For a single water or surfactant, because its composition is more uniform, because the resulting foam film wall is not elastic, so the foam is unstable and easy to eliminate itself. Thermodynamic theory holds that the foam produced by pure liquid is only temporary, and the foam will be eliminated by the drainage of the membrane.

As mentioned earlier, in water-based coatings, there are not only dispersing medium water, but also emulsifiers used to emulsify polymers and surfactant coating additives such as dispersants, wetting agents, thickeners, etc. Because these substances exist in a system at the same time, it is easy to produce foam in the system. And these surface-active substances will also play a stabilizing role in the resulting foam.

After the foam is produced, under the action of intermolecular force, the surfactant will be adsorbed to the membrane wall of the bubble, and the hydrophilic group will reach into the emulsion outside the start, and the hydrophobic group will reach into the air inside the bubble, so that an elastic film layer is regularly formed on the gas-liquid interface of the bubble and the emulsion, thus inhibiting the rupture of the foam membrane wall. After the foam is formed, the liquid on the bubble membrane wall will begin to flow toward the bottom of the bubble under the action of gravity, which will cause the local membrane wall of the bubble to thin. When the liquid film becomes thinner, the bubble has two mechanisms that enable the liquid film to heal itself. One repair mechanism is that when the liquid film becomes thinner, the surface tension increases due to the increase in the area of the liquid film, so the surface tension gradient causes the liquid film to shrink, which is Gibbs elastic contraction. Another repair mechanism is the Marangoni effect: after the appearance of surface tension gradient, the surfactant molecules adsorbed on the surface of the bubble film wall move from the region with low surface tension to the region with high surface tension to keep the surface tension balanced. Under the combined action of these two effects, the foam is stable.

When the emulsifier used is ionic, the membrane wall of the bubble will be charged, due to the existence of a large repulsion between the charges, the repulsion between the bubbles will not gather because of the existence of a large charge, which inhibits the bubble to become large and then in the process of elimination. Therefore, it is unfavorable to the elimination of bubbles and stabilizes bubbles.

When the emulsifier used has a long molecular chain, the gravitational effect between the molecular chains is relatively strong, the membrane wall of the bubble is therefore relatively elastic, the mechanical strength is relatively large, and it is not easy to break, thus inhibiting the elimination of the foam and stabilizing the foam.

In order to improve the viscosity of water-based coatings, thickening agents are usually added to the system, and the flow of liquid between bubbles will be hindered after the viscosity increases, and the bubble film wall will be difficult to thin and the rupture of the bubble film wall will be hindered. It can be seen that the increase in viscosity will also play a role in stabilizing the foam, which is very unfavorable to defoaming.