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The Mechanism of Action of Foaming and Defoaming Agents
Release Date:
2022-08-10
When a liquid is stirred or gas is introduced into it, numerous bubbles form throughout the liquid’s volume. In some liquids, these bubbles rapidly collapse and disappear; in others, they not only persist but also accumulate, giving rise to foam. Foam is a dispersed system in which a large number of gas bubbles are evenly distributed within a liquid, with gas as the dispersed phase and liquid as the continuous phase. Although the gas occupies only a small fraction of the total volume, the resulting foam occupies a substantial space. The gas is separated by thin liquid films, forming bubbles of varying sizes that collectively constitute the foam.
When a liquid is stirred or a gas is introduced into it, numerous bubbles form throughout the liquid’s volume. In some liquids, these bubbles rapidly collapse and disappear; in others, they not only persist but also accumulate, giving rise to foam. Foam is a dispersed system in which a large number of gas bubbles are evenly distributed within a liquid, with the gas phase serving as the dispersed phase and the liquid phase as the continuous phase. Although the gas occupies only a small fraction of the total volume, the resulting foam occupies a substantial space. The gas is separated by thin liquid films, forming bubbles of varying sizes that collectively constitute the foam.
Substances capable of forming bubbles, such as surfactants, adsorb in a well‑ordered monolayer at the bubble surface. When their concentration reaches a certain threshold, a robust, plastic‑like film forms on the bubble wall. Surfactants adsorb at the gas–liquid interface, reducing interfacial tension and thereby increasing the gas–liquid contact area, which makes it harder for bubbles to coalesce. Because the relative density of a bubble is much lower than that of the liquid, as rising bubbles pass through the liquid surface, they also carry along a layer of surfactant molecules from the liquid–gas interface. Consequently, the surfactant‑stabilized bubble film exposed to air differs from that in the bulk solution: it comprises a double‑layered surfactant structure, forming a bilayer membrane, and the adsorbed surfactants confer protective effects on the liquid film. To disrupt and inhibit the formation of this plastic film, defoamers penetrate the bubble’s bilayer membrane, disturbing its structural mechanical equilibrium and achieving foam destruction.
Antifoaming agents must be substances that do not dissolve into the foaming medium; they can be dispersed into the foaming system in liquid form, as liquid droplets encapsulating solid particles, or as solid particles themselves. Compared with the foaming medium, antifoaming agents exhibit lower interfacial tension, enabling them to spontaneously penetrate the liquid film and thereby cause the bubbles to rupture.
Defoamers readily spread across the surface of an aqueous solution and spontaneously distribute themselves on the foam‑plastic surface, carrying with them a thin layer of the surrounding liquid. This weakens the liquid film to a critical thickness, causing it to rupture and leading to the destruction of the foam plastic. The faster the defoamer spreads on the aqueous surface, the thinner the liquid film becomes, accelerating the breakdown of the foam plastic and enhancing its emulsion‑breaking effect. Consequently, the emulsion‑breaking process is driven, on the one hand, by the defoamer’s ease of spreading: adsorbed defoamer molecules replace the surfactant molecules, forming a film with reduced mechanical strength. At the same time, during the spreading process, the defoamer removes a portion of the adjacent liquid layer, softening the foam’s liquid film, diminishing its stability, and making it more susceptible to rupture.
An excellent defoamer must simultaneously exhibit both defoaming and foam‑inhibiting properties: it should not only rapidly destroy existing foam but also prevent the formation of new foam over an extended period. The occurrence of such phenomena may be related to whether the critical micelle concentration of the foaming agent (surfactant) in the aqueous solution is exceeded. When this threshold is surpassed, the defoamer—typically an organic liquid—can become solubilized, thereby losing its ability to spread at the interface and significantly reducing its emulsion‑breaking efficacy. Over time, as the defoamer becomes progressively solubilized, its defoaming performance correspondingly diminishes.
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