Surface tension minimum in ionic surfactant systems

Abstract

A theory is developed to explain the minimum in the surface tension of solutions of sparingly soluble ionic surfactants and its dependence on surfactant and its dependence on surfactant and electrolyte concentrations. The fundamental assumption is that at low pH, an acidic surfactant will be mostly undissolved and that this precipitate acts as a reservoir of surfactant molecules which enter the solution in the dissociated form as the pH is increased. This increase in solution concentration results in increased adsorption at the interface with a consequent lowering lowering of the surface tension. At the so-called solubility edge, the surfactant becomes completely soluble and the solution concentration of surfactant becomes constant. Thus there is no further tendency for γ to decrease as pH is increased. Indeed the conversion of neutral to charged surfactant species causes the monolayer to charge up. Thus, for pH values greater than the solubility edge the increasing electrostatic repulsion of negative surfactant from the interface causes γ to increase. A minimum in γ exists, therefore, at the solubility edge.

In this model there is no necessity to postulate complexes of the surfactant species with peculiar surface activity to explain the observed γ(pH) behaviour as has been suggested by other authors. Quantitative comparison of the theory and experiment for the oleic acid system is presented.