The effect of KCl addition on the surface tension of aqueous solutions of cetyltrimethylammonium
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 KCl 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
6.9 × 10-10 to 5.3 × 10-10 mol/cm2 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.
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