Exploring molecular insights into aggregation of hydrotrope sodium cumene sulfonate in aqueous solution: a molecular dynamics simulation study.

Abstract

Hydrotropes are an important class of molecules that enhance the solubility of an otherwise insoluble or sparingly soluble solute in water. Besides this, hydrotropes are also known to self-assemble in aqueous solution and form aggregates. It is the hydrotrope aggregate that helps in solubilizing a solute molecule in water. In view of this, we try to understand the underlying mechanism of self-aggregation of hydrotrope sodium cumene sulfonate (SCS) in water. We have carried out classical molecular dynamics simulations of aqueous SCS solutions with a regime of concentrations. Moreover, to examine the effect of temperature change on SCS aggregation, if any, we consider four different temperatures ranging from 298 to 358 K. From the estimation of densities of different solutions we calculate apparent and partial molal volumes of the hydrotrope. The changes in these quantities increase sharply at a characteristic minimum hydrotrope concentration. The determination of molal expansibility at infinite dilution for different temperatures indicates the water structure breaking by SCS molecules, which is further confirmed by the calculations of water-water pair correlation functions. In comparison with typical surfactants in micelles, a slightly lower value of volumetric change upon aggregation per carbon atom suggests the formation of a more closely packed structure of hydrotrope aggregates. A close examination of different structural properties of hydrotrope solutions reveals that the hydrophobic interactions through their hydrophobic tails significantly contribute in hydrotrope aggregation,and the dehydration of hydrophobic tail at elevated temperatures is also visible. Remarkably, the aggregates have little or no impact on the average number of water-SCS hydrogen bonds.