Defoaming method for producing a large amount of foam when the concentration of surfactant is increased
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 decreases rapidly at the beginning, but with the further increase of the surfactant concentration, the surface tension of the system no longer changes and reaches an equilibrium state. Milles and Shedlovsky concluded through experiments that the anomaly at the nadir may be caused by impurities contained in the system, or by interactions between various surfactants. The lowest point is what we usually call the critical micelle concentration. Therefore, when the concentration of surfactant reaches the critical micelle concentration, there will be a lot of surfactant in the system, and these surfactants are enough to be closely arranged on the liquid surface to form a monomolecular film layer without voids, This minimizes the surface tension of the system. When the surface tension becomes smaller, the free energy required by the system to generate foam will decrease, which will lead to the formation of foam easily.
In the actual production and use process, in order to make the prepared emulsion can be stored stably, we often adjust the concentration of surfactant above the critical micelle concentration. Although this is beneficial to the stability of the emulsion, it also has 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 firm liquid film, and forming a bimolecular film layer on the liquid surface , which is very unfavorable for the elimination of foam.
A foam is an aggregate of many bubbles, and bubbles are formed when a gas is dispersed in a liquid, the gas acts as the dispersed phase, and the liquid acts as the continuous phase. The gas in the bubbles may run from one bubble to another, or may be transferred from the system to the adjacent atmosphere, which is the merger and disappearance of the bubbles.
For a single water or surfactant, because its composition is relatively uniform, because the generated foam film wall has no elasticity, the foam is unstable and can be easily eliminated by itself. Thermodynamic theory holds that the foam produced by pure liquid is only temporary, and the foam will be eliminated by the drainage effect of the membrane.
As mentioned earlier, in water-based coatings, there are not only water as a dispersion medium, but also emulsifiers used to emulsify polymers, as well as surface-active coating additives such as dispersants, wetting agents, and thickeners. Because of these substances At the same time, it exists in a system, so it is easy to generate foam in the system, and these surface-active substances will also stabilize the generated foam.
(1) After the foam is produced, under the action of intermolecular force, the surfactant will be adsorbed on the membrane wall of the bubble, the hydrophilic group will extend into the emulsion outside the starting point, and the hydrophobic group will extend into the air inside the bubble. , so that an elastic film layer is regularly formed on the gas-liquid interface between the bubbles and the emulsion, thereby inhibiting the rupture of the foam film wall. After the foam is formed, the liquid on the walls of the bubble film will begin to flow towards the bottom of the bubble under the action of gravity, which results in a local thinning of the bubble's film wall. When the liquid film is thinned, there are two mechanisms for the bubble to make the liquid film self-healing. 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, and the resulting surface tension gradient causes the liquid film to shrink, which is Gibbs elastic contraction. Another repair mechanism is: after the surface tension gradient appears, the surfactant molecules adsorbed on the surface of the bubble film wall move from the area with lower surface tension to the area with higher surface tension to keep the surface tension balanced, which is Marangoni effect. Under the combined action of these two effects, the foam exists stably.
(2) When the emulsifier used is ionic, the film walls of the bubbles will be charged. Due to the large repulsion between the charges, the bubbles will not aggregate due to the large charge repulsion between them, inhibiting the Small bubbles become larger bubbles and then in the process of elimination. Therefore, it is not good for the elimination of foam, which stabilizes the foam.
(3) When the molecular chain of the emulsifier used is longer, the attraction between the molecular chains is relatively strong, and the film wall of the bubble is therefore more elastic, and the mechanical strength is relatively large, which is not easy to break, thus inhibiting the elimination of foam. , stabilized the foam.
(4) In order to increase the viscosity of water-based coatings, thickeners are usually added to the system. After the viscosity increases, the flow of liquid between the bubbles will be blocked, the bubble film wall will be difficult to thin, and the rupture of the bubble film wall will be blocked. It can be seen that the increase in viscosity will also play a role in stabilizing the foam, which is very unfavorable for defoaming.