Hydrotropes are a class of compounds that, at high concentrations, enhance the solubility of a variety of hydrophobic compounds in water. The mechanism of hydrotropy is still incompletely understood. In this paper, we have studied thesolution-state properties in water of the hydrotropes sodium salicylate (NaS), sodium p-toluenesulfonate (NaPTS), sodium xylenesulfonate (NaXS), sodium cumenesulfonate (NaCS), and sodium butyl monoglycol sulfate (NaBMGS). We find that all these molecules self-aggregate in aqueous solution to form organized assemblies. It appears that a minimal hydrotropic concentration (MHC) is essential before hydrotropy can be displayed. We show that hydrotropy is different from salting-in or phase-mixing behavior. Hydrotropy is a collective molecular phenomenon, exhibited above the MHC. Hydrotropic compounds are seen to be surface active though somewhat less than classical surfactants. The microenvironmental features of hydrotrope assemblies are roughly comparable to those of surfactant micelles: low polarity and a microviscosity of the order of 60 cP. However, there are notable differences between hydrotrope assemblies and micelles. Solubilization by the former appears to be higher and somewhat more selective. The cooperativity displayed by hydrotrope molecules in the aggregation process is low. While hydrotropy appears above the MHC, it is not strictly analogous to the critical micelle concentrationdisplayed by surfactants.

Over 70 years ago, Neuberg1 described the large increase in the solubility in water of a variety of hydrophobiccompounds brought about by the addition of certain compounds. These solubility-enhancing molecules were termed hydrotropic agents or hydrotropes, and the phenomenon itself was named hydrotropy. Some examples of hydrotropes are sodium benzoate, salicylate,