The hydrocarbon/water interfacial behaviour of alkyl aryl sulphonates

Abstract The interfacial tension and solubilisation characteristics of the alkane/water interfaces of a series of alkyl aryl sulphonates synthesised with 1‐butanol as a co‐surfactant have been studied. It was observed that there is a narrow range of carbon numbers of these sulphonates at which maximum solubilisation of a particular alkane occurred. This range shifted progressively to a higher value, with a corresponding increase in the carbon number of the n‐alkane. The extent of solubilisation of the alkane is highly dependent on the structure of the hydrocarbon portion of the sulphonate. Unexpectedly, an ultra‐low interfacial tension seems not to be an important prerequisite for the solubilisation of hydrocarbon in the aqueous surfactant phase. The free energy decrease required for solubilisation seems to be far more dependent on the energetic interactions connected with entropy parameters. The conformational aspects of the hydrocarbon portion of sulphonate molecules, as a consequence, seem to play a decisive role in molecular interactions, leading to solubilisation of a large quantity of a particular hydrocarbon in aqueous solution of a given sulphonate. The present work aims at understanding the selection criteria for sulphonates in applications involving solubilisation and ultra‐low interfacial tension at the hydrocarbon/water interface, such as those in insecticidal/pharmaceutical formulations, fire‐resistant hydraulic fluids and enhanced oil recovery.