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

Hydrotropes, such as sodium alkyl benzene sulfonates and sodium butyl monoglycol sulfate, were used for the selective extraction of piperine by cell permeabilization of Piper nigrum fruits. Penetration of the hydrotrope molecules into the cellular structures and subsequent cell permeabilization were hypothesized to explain the enhanced extraction rates of aqueous hydrotrope solutions. Hydrotrope molecules, after adsorption on a cell wall, cause disorder in its structure and in the bilayered cell membrane to facilitate the rapid extraction of piperine. The hydrotrope solution showed selective and rapid extraction of piperine from black pepper. The recovered piperine was ∼90% pure and substantially free from oleoresins. The type and nature of the hydrotrope, the hydrotrope concentration, the temperature, and the particle size all had significant effects on the extraction process.

Surface-active short-chain alkyl benzene sulfonates which are known as hydrotropes interact electrostatically as well as hydrophobically with cationic surfactants in the same manner as salicylate. The interaction in mixed aggregates of the three isomers of butyl benzene sulfonate (BBS-) and cetyl trimethylammonium bromide (CTAB) has been studied by surface tensiometry and rheological techniques. These mixed aggregates form rodlike micelles, giving rise to highly viscous solutions even at low concentrations. To study the interactions in the absence of electrolytes, CTA/BBS complexes were also obtained separately and characterized by similar techniques. The steric effect due to the structure of the butyl group in butyl benzene sulfonates on the interaction with CTAB has been emphasized during these investigations. The interactions are also characterized by infrared and ultraviolet absorption and nuclear magnetic resonance spectroscopy. Molecular modeling was carried out using an MM2 force field for molecular mechanics calculations for surfactant/hydrotrope pairs in the presence and absence of water. The PM3 semiempirical molecular quantum mechanical calculations were also carried out to calculate heats of formation of complexes. The steric hindrance due to the bulky tert-butyl group was found to be the maximum in tert-butyl benzene sulfonate (TBBS-), in comparison to n-butyl benzene sulfonate (NBBS-) and isobutyl benzene sulfonate (IBBS-).

The diffusion coefficient of Li+ ions decreases with increase in LiCl concentration which depends on the size of coordination structure of ions formed in solutions.