A Study of the Equilibrium Surface Tension and the Critical Micelle Concentration of Mixed Surfactant Solutions

The existence of a minimum in the surface tension (γ)−concentration (C) curve of surfactant solutions is investigated. Equilibrium surface tension and solution conductivity (κ) of anionic surfactant (SDS) solution and mixed anionic−nonionic surfactant (SDS−NP(EO)40) solutions are measured. Minima are observed in the γ−log C curves. The critical micelle concentration (CMC) inferred from the κ−C data does not match the bulk concentration at which the minimum in the γ−log C curve occurs, but corresponds to the one at which surface tension reaches the constant equilibrium value. Polyoxyethylene nonionic surfactants C m E n are mixed and solutions show no minimum in the γ−log C curves, whereas minima in γ−log C curves for solutions of C10E8 and C12E8 mixing with a trace of n-dodecanol are observed. Therefore, the existence of a minimum or a negative peak in the γ−log C profile may imply that the surfactant is a mixture, but no negative peak in the γ−log C profile does not imply that there is no impurity in the surfactant or the surfactant consists of only one component. A clear break with no negative peak in the γ−log C profile may be a better criteria for surfactant purity, but is still not a guarantee.