Surface tension and xenon NMR study of the behavior of anionic hydrotropes in solutions containing sodium octyl sulfate micelles

Surfactants are widely applicable and appear in everything from detergents to drug delivery systems. As a result, the ability to tune both the surface and solution properties of these systems is highly desirable. One way to achieve this is with mixed systems of surfactants and hydrotropes. The focus of the present study is to determine the properties of mixed systems of the anionic surfactant sodium octyl sulfate (SOS) with either of the anionic hydrotropes sodium cumene sulfonate (SCS) or sodium benzenesulfonate (BZS). Analysis of surface tension and ¹²⁹Xe NMR experiments were used to explore these systems. The concentration of the hydrotrope was varied while the concentration of the SOS is held constant. The hydrotropes BZS and SCS behave differently at the surface and both in solutions of just the hydrotrope and in solutions including the surfactant SOS at a constant concentration. SCS lowers the surface tension with an observable critical concentration similar to a surfactant while the surface behavior of BZS is much less pronounced and shows no critical behavior. The surface behavior of the SCS/SOS and BZS/SOS solutions was very different which is suggestive of different surface compositions: a mixture of SCS and SOS at the surface of the SCS/SOS solutions and a surface of primarily SOS for the BZS/SOS solutions. These differences are not as evident in the ¹²⁹Xe data where the ionic effect of the hydrotrope appears to dominate the chemical shift resulting in a linear dependence of chemical shift on hydrotrope concentration. There is limited to no evidence of hydrotropes forming micelles or of an increase in surfactant micellization as a result of an increase in hydrotrope concentration. The SCS causes a more dramatic increase in the chemical shift than the BZS, but the change in behavior is the similar upon addition of SOS micelles. By applying a simple model, the SCS and BZS are found to incorporate to approximately the same extent -- likely not at all -- into the SOS micelles.