Examination of the hydrotropic effect of sodium p-toluenesulfonate on a nonionic surfactant (C12E6) solution

Although hydrotropy is extensively used in industry, the molecular mechanis m of hydrotropic solubilization has not been completely elucidated yet. In this paper the interaction between a nonionic surfactant (ethoxylated fatty alcohol containing between five and six oxyethylenic units) and sodium p-t oluene sulfonate is examined. Surface tension measurements confirm that the hydrotropic effect occurs at a concentration in which the hydrotropes self -associate. Photon correlation spectroscopy studies show that for this conc entration of hydrotropes a drastic reduction in the surfactant micellar rad ius occurs. Furthermore the luminescence of the hydrotrope used as a fluore scence probe indicates that at low concentrations p-toluene sulfonate disso lves in the surfactant micelles but beyond the minimum concentration for hy drotropic solubilization the hydrotrope is present in the aqueous phase. Th ese results suggest that the hydrotropic effect is related to alterations i n the water structure induced by the hydrotrope molecules and to the presen ce of hydrotrope aggregates that furnish an appropriate niche for the surfa ctant amphiphile. (C) 2000 Academic Press.