Surface-active agents are a class of substances that can significantly reduce the surface tension of water at very low concentrations. They possess a characteristic asymmetric amphiphilic structure, which allows them to exhibit two important properties. One is the oriented adsorption of their molecules at the interface between two phases, and the other is the formation of micelles within the solution once the concentration reaches a certain value. These two properties form the basis for the wide-ranging applications of surface-active agents. The concentration at which surface-active agents start to form a large number of micelles in a solution is called the critical micelle concentration (CMC). Here are some methods for determining the CMC of surfactants:
Surface Tension Method: By plotting the logarithm of surface tension and concentration, a turning point appears on the curve when the surface adsorption reaches saturation, and the concentration at this point is the critical micelle concentration. The surface tension of an aqueous surfactant solution initially decreases sharply with the increase of solution concentration, and then changes slowly or no longer after reaching a certain concentration. Therefore, the logarithmic plot of surface tension concentration is commonly used to determine cmc.
Conductivity Method (Classical Method): Using the logarithmic plot of conductivity and concentration, when the surface adsorption reaches saturation, a turning point appears on the curve, and the concentration at this point is the critical micelle concentration.
Dye Method: The color difference between certain dyes in water and micelles is significant. The titration method is used to determine cmc. First, a small amount of dye is added to a higher concentration (greater than cmc) of surfactant solution, and this dye is dissolved in the micelles to present a certain color. Using the titration method, dilute the solution with water until there is a significant change in color, at which point the concentration of the solution is cmc.
Turbidity Method: Non-polar organic compounds such as hydrocarbons generally don’t dissolve in dilute surfactant solutions (less than cmc), and the system is turbid. When the concentration of surfactants exceeds cmc, the solubility increases sharply and the system becomes clear. This is the result of the solubilization of hydrocarbons by the formation of micelles.Observe the variation of turbidity with surfactant concentration in a surfactant solution containing an appropriate amount of hydrocarbons, and the concentration at the point of turbidity mutation is the cmc of the surfactant.
Light Scattering Method: Micelles, which are aggregates of tens or more surfactant molecules or ions, have a size within the range of the wavelength of light and exhibit strong light scattering. By observing the intensity of scattered light as a function of solution concentration, a turning point in the curve can be identified, corresponding to the CMC.
