  Hydrotropes are amphiphilic molecules that are too small to spontaneously
form equilibrium structures in aqueous solutions, but form dynamic, noncovalent
assemblies, referred to as clusters. In the presence of a hydrophobic compound,
these clusters seem to get stabilized leading to the formation of long-lived,
highly stable mesoscopic droplets, a phenomenon that we call mesoscale
solubilization. In this work, we focus on the unusual mesoscopic properties of
aqueous solutions of a nonionic hydrotrope, namely tertiary butyl alcohol
(TBA), on addition of various hydrophobic compounds. Aqueous TBA solutions, in
about 3 to 8 mol percent TBA concentration range and about 0 to 25 deg. C
temperature range, show the presence of short-ranged (0.5 nm), short-lived
(tens of picoseconds) molecular clusters which result in anomalies of the
thermodynamic properties. These clusters are transient but do not relax by
diffusion, thus distinctly different from conventional concentration
fluctuations. In this concentration and temperature range, upon the addition of
a third (more hydrophobic) component to TBA-water solutions, long-lived
mesoscopic droplets of about 100 nm size are observed. In this work, we clarify
the ambiguity behind the definition of solubility and elucidate the phenomenon
of mesoscale solubilization. A systematic study of the macro and meso phase
behavior of three ternary systems TBA-water-propylene oxide, TBA-water-isobutyl
alcohol, and TBA-water-cyclohexane has been carried out. We differentiate
between molecular solubility, mesoscale solubilization, and macroscopic phase
separation. We have confirmed that practically stable aqueous colloids can be
created from small molecules, without addition of surfactants or polymers. Such
kind of novel materials may find applications in the design of various
processes and products, ranging from pharmaceuticals to cosmetics and
agrochemicals.
