The principle of action of foaming and defoaming agents
Whenever we stir a liquid or introduce gas, many bubbles are generated within the surface area of the liquid, and some of the bubbles produced by the liquid quickly break and disappear. Some liquids not only do not disappear, but accumulate more and more, forming foam. Foam is a kind of dispersion system formed by a large number of bubbles dispersed in liquid. Its dispersion phase is gas and the continuous phase is liquid. The volume fraction of foaming liquid is very small, and the volume of foam is very large. The gas is separated by a continuous liquid film to form bubbles of different sizes, which accumulate to form foam.
A medium that can produce bubbles is like a surfactant, which adsorbs a layer of surfactant molecules arranged in a directed direction on the surface of the bubble. When it reaches a certain concentration, a strong thin film is formed on the bubble wall. Surfactants adsorb on the gas-liquid interface, causing a decrease in surface tension on the liquid surface, thereby increasing the gas-liquid contact surface, making it difficult for bubbles to merge. The relative density of bubbles is much smaller than that of the liquid itself. When the rising bubbles pass through the liquid surface, they also adsorb a layer of surfactant molecules on the liquid surface. Therefore, the bubble film with adsorbed surfactants exposed to the air is different from the bubble film in the solution. It contains two layers of surfactant molecules, forming a bimolecular film. The adsorbed surfactant has a protective effect on the liquid film. Defoamer is to destroy and inhibit the formation of this film. Defoamer enters the bimolecular directional film of foam to destroy the mechanical balance of the directional film and achieve the role of foam breaking.
Defoamers must be substances that are insoluble in the foaming medium and can be dispersed into the foaming medium in the form of droplets, droplets that encapsulate solid particles, or solid particles. Defoamers have lower surface tension than foaming media and can spontaneously enter the liquid film, causing bubbles to rupture.
Defoamer is easy to spread on the solution surface, and automatically spreads on the foam surface, which will take away a layer of solution on the adjacent surface, making the liquid film locally thin, reaching the critical thickness, breaking the liquid film, and destroying the foam. The faster the defoamer spreads on the solution surface, the thinner the liquid film will become, the faster the foam will be destroyed, and the defoaming effect will be strengthened. Therefore, one reason for defoaming is that it is easy to spread, and the adsorbed defoamer molecules replace the foaming agent molecules, forming a film with poor strength. At the same time, part of the solution near the surface layer is taken away during spreading, which makes the foam liquid film thinner, reduces the stability of foam, and makes it easy to be damaged.