When 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 air enters the liquid to form foam, the contact area of gas and liquid increases, and the free energy of the system also increases.
When 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 air enters the liquid to form foam, the contact area of gas and liquid increases, and the free energy of the system also increases.
When the concentration of surfactant increases, the surface tension of the system will decrease rapidly at the initial stage, but with the further increase of the concentration of surfactant, the surface tension of the system will not change any more, reaching an equilibrium state. Milles and shedlovsky concluded through experiments that the abnormality of the lowest point may be caused by the impurities contained in the system or by the interaction between various surfactants. The lowest point is what we usually call the critical micelle concentration. Therefore, when the concentration of surfactants reaches the critical micelle concentration, there will be a lot of surfactants in the system, which are enough to be closely arranged on the liquid surface to form a monolayer layer without gaps, so as to minimize the surface tension of the system. When the surface tension becomes smaller, the free energy required for the system to generate foam will decrease, which will lead to the easy generation of foam.
In the actual production and use process, in order to make the prepared lotion store stably, we often adjust the concentration of surfactant above the critical micelle concentration. Although this is beneficial to the stability of lotion, it also has some negative effects. Because too much surfactant not only reduces the surface tension of the system to the minimum, but also encloses the air entering the lotion to form a relatively solid liquid film, and forms a bimolecular film on the liquid surface, which is very unfavorable to the elimination of foam.
Foam is an aggregate of many bubbles, and bubbles are formed by gas dispersed in liquid. Gas acts as a dispersed phase and liquid acts as a 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 bubbles.
For a single water or surfactant, the composition is relatively uniform, and the resulting foam membrane wall is inelastic, so the foam is unstable and can be easily eliminated by itself. According to the thermodynamic theory, the foam produced by pure liquid is only temporary, and the foam will be eliminated due to the drainage effect of the membrane.
As mentioned above, in water-based coatings, there are not only dispersion medium water, but also emulsifier, dispersant, wetting agent, thickener and other surface active coating additives used to emulsify polymer. Because these substances exist in a system at the same time, it is easy to generate foam in the system, and these surface active substances will also stabilize the generated foam.
(1) After the generation of foam, under the action of intermolecular force, the surfactant will be adsorbed to the membrane wall of the bubble, the hydrophilic group will extend to the lotion outside the starting point, and the hydrophobic group will extend to the air inside the bubble, so that an elastic film layer is regularly formed on the gas-liquid interface between the bubble and the lotion, thereby inhibiting the rupture of the membrane wall of foam. After the formation of foam, the liquid on the bubble membrane wall will start to flow to the bottom of the bubble under the action of gravity, which will cause the local membrane wall of the bubble to become thinner. When the liquid film becomes thinner, there are two mechanisms for bubbles to self repair. One repair mechanism is that after the liquid film becomes thin, the surface tension increases due to the increase of the area of the liquid film, and the resulting surface tension gradient makes the liquid film shrink, which is Gibbs elastic contraction. Another repair mechanism is that after the surface tension gradient appears, the surfactant molecules adsorbed on the surface of the bubble membrane wall move from the area with low surface tension to the area with high surface tension, so as to keep the surface tension in balance, which is Marangoni effect. Under the joint action of these two effects, foam exist stably.
(2) When the emulsifier used is ionic, the membrane wall of the bubbles will be charged. Since there is a large repulsive force between the charges, the bubbles will not gather because there is a large repulsive force between the charges, which suppresses the process of small bubbles becoming large bubbles and then eliminating them. Therefore, it is unfavorable to eliminate foam and stabilize foam.
(3) When the emulsifier used has a long molecular chain, the gravitational effect between the molecular chains is relatively strong, so the membrane wall of the bubble has better elasticity, greater mechanical strength and is not easy to burst, thus inhibiting the elimination of foam and stabilizing the foam.
(4) In order to improve the viscosity of water-based coatings, thickeners are usually added to the system. After the viscosity increases, the flow of liquid between bubbles will be blocked, the bubble film wall will be difficult to thin, and the bubble film wall will be broken. It can be seen that the increase of viscosity will also play a role in stabilizing foam, which is very unfavorable to defoaming.
