Influence of ionic strength on surface tension of cetyltrimethylammonium bromide

The effect of KCl addition on the surface tension of aqueous solutions of cetyltrimethyl ammonium bromide (CTAB) was measured by the drop-weight method. A significant reduction in the solution surface tension was observed with increasing electrolyte concentration in accordance with previous literature data obtained for the CTAB/NaBr system. However, in contrast to these results, no indication of cloud point appearance or CTAB dimerization was observed for KCI concentration reaching 1 M. The relative surface excess of CTAB at cmc was determined from the Gibbs equation. This quantity was found to decrease from 8.9 x 10(-10) to 5.3 x 10(-10) mol/cm(2) when the KCl bulk concentration was varied between 0 and 1 M. The experimental surface tension data were quantitatively interpreted in terms of the theoretical model based on the assumption that the surfactant and counterions undergo nonequivalent adsorption within the Stern layer which consequently acquires a net charge. The distribution of electric potential in this layer was calculated according to the modified Stern model, whereas the diffuse part of the double layer was described using the Gouy-Chapman model.