Associative interactions and surface tension in ionic surfactant solutions at concentrations much lower than the cmc

Abstract The surface pressure isotherms Δσ( C S ) of the ionic surfactants sodium dodecyl sulfate (NaDOS) and sodium octyl sulfate (NaOS) were measured using at different electrolyte concentration an accurate and sensitive sphere tensiometer. At 10 −1 mole/liter NaCl, two particular points on the isotherms were noticed for surfactant concentrations below 10 −4 mole/liter. The first, at Δσ ≈ 0.5 dyn/cm, corresponds to the formation of flat premicelles in the solution, whereas the second corresponds to a phase transition in the adsorbed layer and this is probably associated with the disintegration of the premicelles on the surface but not in the bulk of the solution. The supplementary measurements of the surface potential and the time dependence of the surface tension show evidence of this hypothesis.