Acid-soap formation in aqueous oleate solutions

Abstract Surfactants have been considered in the past to form premicellar multimers such as dimers and acid soap. In a previous publication, we presented a thermodynamic model for the surface tension dependence on surfactant concentration taking into account premicellar aggregation and we considered the possibility of formation of dimers in alkaline potassium oleate solutions. In this paper, the surface tension data for potassium oleate solutions at two pH values is analyzed using the above model and the observed differences from predictions of the model are attributed to the formation of acid soaps in the neutral pH region. The formation constant for acid soap is estimated from this analysis to be 1.75 × 10 5 (kmole/m 3 ) −1 .