CMC is an important parameter for the development of products and processes containing surfactants. In this experiment surfactant was added stepwise to water and concentration increment of ultrasonic velocity (relative difference between ultrasonic velocity in solution and water divided by concentration of surfactant) was measured. As different surfactants have a different concentration increment of ultrasonic velocity, for comparison only the change of the increment is plotted on the above figure. As can be seen from the graph, at small concentrations the concentration increment of ultrasonic velocity stays constant, thus demonstrating the absence of interactions between molecules of surfactants. Beginning from some concentration of surfactant, the increment decreases with some saturation at high concentrations. The concentration at which the deviation accrues, CMC, is different for different surfactants. The total change in the concentration increment of ultrasonic velocity is determined by the elasticity of the hydrophobic core of the micelle. As can be evaluated from the curve the elasticity (compressibility) of the core is proportional to the length of the hydrophobic tale of the surfactant.
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