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The adsorption of surfactants on the interface
Jun 11, 2018

Surfactant molecules are lipophilic and hydrophilic, and are two affinity molecules. Water is a strong polar liquid. When the surfactant is dissolved in water, the water parent and water phase are dissolved in water according to the principle of polar similarity and polarity dissimilarity, and its parent oil base and water repel and leave water. As a result, the surfactant molecules (or ions) adsorb on the two phase interface to reduce the interfacial tension between the two phases. The more adsorption of surfactant molecules (or ions) on the interface, the greater the decrease of interfacial tension.

First, the surface pressure of the adsorbed membrane: the surface active agent is adsorbed on the gas-liquid interface to form an adsorption film, such as a frictionless movable floating floated on the interface, and a floating sheet to promote the adsorption of the plasma membrane along the surface of the solution. The membrane produces a pressure on the floating sheet, which is called the surface pressure.

Two. Surface viscosity: like surface pressure, surface viscosity is a property of insoluble molecular membrane. A white gold ring is suspended with a fine metal wire. The surface of the water is touched by the surface of the water tank, the white gold ring is rotated, the white gold ring is hindered by the viscosity of the water, and the amplitude decreases gradually. Accordingly, the surface viscosity can be measured. The method is to test the amplitude attenuation first on the pure water surface and then determine the attenuation after the formation of the surface film from both. The viscosity of the surface film was calculated by the difference. The surface viscosity is closely related to the firmness of the surface film. Because the membrane has surface pressure and viscosity, it must be elastic. The greater the surface pressure and the higher the viscosity, the greater the elastic modulus of the adsorption membrane. The elastic modulus of the surface adsorbed film is of great importance in stabilizing foam.

Three. The dilute solution of surfactant obeys the law of ideal solution. The adsorption of surfactants on the surface of the solution increases with the concentration of the solution. When the concentration reaches or exceeds a certain value, the amount of adsorption no longer increases. These surfactants are disorderly in the solution, or exist in a regular way. Both practice and theory indicate that they form a complex in solution, which is called micelle. Critical micelle concentration: the lowest concentration of surfactant forming micelles in solution is called critical micelle concentration.