Mechanism of the Salt Effect on Micellization of an Aminosulfonate Amphoteric Surfactant

The critical micellar concentrations (cmc’s) of sodium dodecyldiaminosulfonate amphoteric surfactant (C12AS) in aqueous solutions containing seven inorganic salts with chloride anions were measured by tensiometry using the Wilhelmy plate method at 25 °C. The mathematical relationship between the cmc of C12AS and the concentration of an inorganic salt in an aqueous solution was established. The effect of an inorganic salt on micellization of C12AS was also discussed. The results show that the dependence of the cmc of C12AS on an inorganic salt within the range of the effective concentration of the inorganic salt investigated in this experiment can be well described by the linear equation. The decrease of the cmc with an inorganic salt may be mainly attributed to the salting-out effect of the hydrophobic moiety of C12AS. Except for the salting-out effect, the causes can be partly from the vital contributions of both the electrostatic interaction between ionic heads of C12AS and the hydration of inorganic ions in the process of micellization. Also, the type of inorganic ion can affect the process of micellization. The data of Gibbs energy changes show that there is a spontaneous process of micellization, and the addition of an inorganic salt in an aqueous solution is favorable thermodynamically to the process of micellization.