Mesoscale solubilization and critical phenomena in binary and quasi-binary solutions of hydrotropes

Hydrotropes are substances consisting of amphiphilic molecules that are too small to self-assemble in equilibrium structures in aqueous solutions, but can form dynamic molecular clusters H-bonded with water molecules. Some hydrotropes, such as low-molecular-weight alcohols and amines, can solubilize hydrophobic compounds in aqueous solutions at a mesoscopic scale (∼100 nm) with formation of long-lived mesoscale droplets. In this work, we report on the studies of near-critical and phase behavior of binary (2,6-lutidine-H2O) and quasibinary (2,6-lutidine-H2O/D2O and tert-butanol/2-butanol-H2O) solutions in the presence of a solubilized hydrophobic impurity (cyclohexane). In additional to visual observation of fluid-phase equilibria, two experimental techniques were used: light scattering and small-angle neutron scattering. It was found that the increase of the tert-butanol/2-butanol ratio affects the liquid-liquid equilibria in the quasi-binary system at ambient pressure in the same way as the increase of pressure modifies the phase behavior of binary 2-butanol-H2O solutions. The correlation length of critical fluctuations near the liquid-liquid separation and the size of mesoscale droplets of solubilized cyclohexane were obtained by dynamic light scattering and by small-angle neutron scattering. It is shown that the effect of the presence of small amounts of cyclohexane on the near-critical phase behavior is twofold: (1) the transition temperature changes toward increasing the two-phase domain; (2) long-lived mesoscopic inhomogeneities emerge in the macroscopically homogeneous domain. These homogeneities remain unchanged upon approach to the critical point of macroscopic phase separation and do not alter the universal nature of criticality. However, a larger amount of cyclohexane generates additional liquid-liquid phase separation at lower temperatures.