Hydrotropes are amphiphilic molecules that are too small to spontaneouslyform equilibrium structures in aqueous solutions, but form dynamic, noncovalentassemblies, 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 mesoscalesolubilization. In this work, we focus on the unusual mesoscopic properties ofaqueous solutions of a nonionic hydrotrope, namely tertiary butyl alcohol(TBA), on addition of various hydrophobic compounds. Aqueous TBA solutions, inabout 3 to 8 mol percent TBA concentration range and about 0 to 25 deg. Ctemperature range, show the presence of short-ranged (0.5 nm), short-lived(tens of picoseconds) molecular clusters which result in anomalies of thethermodynamic properties. These clusters are transient but do not relax bydiffusion, thus distinctly different from conventional concentrationfluctuations. In this concentration and temperature range, upon the addition ofa third (more hydrophobic) component to TBA-water solutions, long-livedmesoscopic droplets of about 100 nm size are observed. In this work, we clarifythe ambiguity behind the definition of solubility and elucidate the phenomenonof mesoscale solubilization. A systematic study of the macro and meso phasebehavior of three ternary systems TBA-water-propylene oxide, TBA-water-isobutylalcohol, and TBA-water-cyclohexane has been carried out. We differentiatebetween molecular solubility, mesoscale solubilization, and macroscopic phaseseparation. We have confirmed that practically stable aqueous colloids can becreated from small molecules, without addition of surfactants or polymers. Suchkind of novel materials may find applications in the design of variousprocesses and products, ranging from pharmaceuticals to cosmetics andagrochemicals.
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