  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, around 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 to 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 - the transition temperature changes towards increasing the
two-phase domain, and 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.
