The principle of foaming and defoaming agents

Jan 26, 2018

Whenever we agitate the liquid or pass the gas, a lot of bubbles are generated inside the surface area of ​​the liquid, and some bubbles generated by the liquid quickly crumble and disappear. Some liquid not only does not disappear, but the more the more gathered to form a bubble. Foam is a dispersed system with a large number of bubbles dispersed in a liquid. The dispersed phase is a gas and the continuous phase is a liquid. The volume fraction of bubbling liquid is very small, foam occupies a large volume. The gas is separated by a continuous liquid film, forming bubbles of varying sizes, stacked to form a foam.
The foaming medium acts like a surfactant, adsorbing a layer of surfactant molecules aligned on the surface of the bubble. When it reaches a certain concentration, the bubble wall forms a solid film. Surfactant adsorbed on the gas-liquid interface, causing the liquid surface tension decreased, thereby increasing the gas-liquid contact surface, so bubbles are not easy to merge. The relative density of the bubble is much smaller than that of the liquid itself, and as the rising bubbles pass through the surface of the liquid, they again adsorb a layer of surfactant molecules on the surface of the liquid. Thus, the surfactant-exposed bubble film exposed in the air is different from the bubble film in solution in that it contains two layers of surfactant molecules that form a bimolecular film and the adsorbed surfactant has a protective effect on the liquid film effect. Defoamers is to destroy and inhibit the formation of this film, defoamers into the foam bimolecular orientation film, destroy the mechanical equilibrium directional film to achieve the foam breaking effect.
Defoamers must be substances that are insoluble in the foaming medium and can be dispersed into the foaming medium in the form of droplets, or droplets that enclose solid particles, or solid particles. Defoamers have a lower surface tension than the foaming medium and spontaneously enter the liquid film to rupture the bubbles.
Antifoam is easy to spread on the surface of the solution, automatically spread on the surface of the foam, will take away a layer of solution near the surface, so that the local thinning of the liquid film, to the critical thickness, liquid film rupture, foam destruction. Defoamers spread faster on the surface of the solution, the thinner the liquid film becomes, the speed of foam destruction is accelerated, and the defoaming effect is strengthened. Therefore, defoaming is due to the one hand, easy to spread, adsorbed defoamer molecules instead of blowing agent molecules, the formation of a less strong film. At the same time in the spreading process to take away part of the solution near the surface layer, so that the foam film thinning, reducing the stability of the foam, making it easy to damage.
An excellent defoamer must take into account both consumer and anti-foam effect, that is, not only should the foam be rapidly destroyed, but also foam can be prevented for quite a long time. The reason for this may be related to whether the critical micelle concentration of the foaming agent (surfactant) in the solution is exceeded or not. In excess of the solution, defoamers (usually organic liquids) may be solubilized, so that the role of the spread of the surface loss, defoaming efficiency greatly reduced. After a period of time, as defoamers are gradually solubilized, the defoaming effect is correspondingly weakened.