The apparently anomalous effects of surfactants on interfacial tension in the IBA/water system near its upper critical solution temperature

We studied the effect of anionic, cationic, and nonionic surfactants on the interfacial tension between the two phases of the isobutyric acid/water system using spinning drop tensiometry. It has been found that interfacial tension decreases with increasing concentration of the surfactant in the case of sodium dodecyl sulfate (SDS) and of dodecyltrimethylammonium chloride (DTAC). However, in the case of Triton X-100 an increase of surfactant concentration leads to an increase of the interfacial tension. Such results are consistent with the upper critical solution temperature (UCST) in the presence of these surfactants: the UCST decreases with increasing SDS and DTAC concentrations while it increases in the case of Triton X-100. This apparently anomalous effect has been explained by considering the molecular structure of Triton X-100, which possesses a short polymeric chain that promotes its partitioning toward one of the two phases making them less compatible and thus raises the UCST and the interfacial tension.