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 the solution-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 concentration displayed by surfactants.Over 70 years ago, Neuberg1 described the large increase in the solubility in water of a variety of hydrophobic compounds 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, p-toluenesulfonate, and xylenesulfonate.2 A 2 M solution of the latter salt in water enhances the solubility of nitrobenzene 50-fold and that of cresol by a factor of 200 in water.3 Hydrotropy appears to operate at high concentrations of the hydrotrope in water; most hydrotropic solutions precipitate the solute (solubilizate) on dilution with water. This is convenient since it allows the ready recovery of the hydrotrope for reuse.Despite intermittent attempts over the years, there is no consensus on the mechanism behind hydrotropy. McKee2 suggested that hydrotropy may be viewed as a salting-in process; he noted

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Spin-labeled fatty acids, fluorescent amphiphiles, and similar bifunctional probe molecules are used to monitor the interior of micelles, bilayers, and biomembranes. It is usually assumed that the probe moiety of these molecules is inserted in the interior of the host assembly, an assumption that needs to be verified. We show here that these molecules are incorporated in different ways in spherical micelles, rodlike micelles, and bilayer assemblies. Looping back of the probe molecule and the consequent placing of the probe moiety in the interfacial region appear to be common in spherical micelles whereas the same molecule is incorporated in a straight-chain configuration in bilayer vesicles. The reason behind this differential incorporation appears to be related to the surface area to volume ratio of the host assemblies and their intermolecular packing differences. Micelles promote self-coiling of these molecules while membranes appear not to do so.

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