MOLECULAR-ORGANIZATION IN HYDROTROPE ASSEMBLIES

Hydrotropes are a class of compounds that are widely used in chemical, cosmetic, and pharmaceutical industries. Though amphiphilic in charac ter, they have short hydrophobic regions and thus differ from classica l surfactants. Yet they display substantial (and often selective) abil ity to solubilize lipophilic compounds in water. Beyond the minimal hy drotropic concentration, the molecules self-aggregate to produce the o perating assembly. We show in this paper, based on crystal structure a nalysis of several hydrotropes, that these compounds form open-layer a ssemblies, reminiscent of lamellar liquid crystals consisting of alter nating hydrophobic clustering of the nonpolar regions adjacent to ioni c or polar regions that are knitted together in a two-dimensional netw ork. Stacking of aromatic rings is not seen. Two types of assemblies a re seen, one with a more open and extended hydrophobic layer than the other. We suggest that the solubilizates enter the hydrophobic layers of microunits producing a cooperative and mutual stabilizing effect. T he observed open layer structure of hydrotropes might also account for the occasional ability of these compounds to solubilize even better t han micelles.