(658g) Mesoscale Solubilization in Aqueous Solutions of Hydrotropes

In this work, we introduce a new concept in molecular thermodynamics, which we refer to as “mesoscale solubilization”. Aqueous solutions of small non-ionic amphiphilic molecules (also known as hydrotropes) such as ethanol, isopropanol, tert-butanol, and 3-methylpyridine, show the presence of short-lived (tens to hundreds of picoseconds) and short-ranged (order of a nanometer) molecular clusters. These clusters occur over a narrow concentration range (between 1 to 10 mol % of the hydrotrope in water) and get enhanced at temperatures below room temperature. We have shown that these clusters originate due to the transient formation and breakage of hydrogen-bonds between water and the hydrotrope molecule. The clusters also seem to be responsible for the thermodynamic anomalies observed in the aqueous solutions. We have further shown that in the presence of a hydrophobic compound, these clusters get stabilized leading to the formation of highly stable mesoscopic droplets, a phenomenon that we refer to as “mesoscale solubilization”. The mesoscopic droplets are of the size of the order of 100 nm and are extremely long-lived (over a year). To typify our findings, we have carried out static and dynamic light scattering experiments, small-angle neutron scattering experiments, and molecular dynamics simulations in aqueous solutions of tert-butanol, containing trace amounts of cyclohexane. The results obtained here are not specific to aqueous solution of tert-butanol or even just hydrotropes, but are observed in a broad set of aqueous solution.  Various works have shown the existence of mesoscale solubilization in aqueous solutions of ionic molecules, such as NaCl, as well as water-soluble polymers such as polyethylene oxide. All these studies confirm that mesoscale solubilization is a broader phenomenon, and seems to occur ubiquitously in aqueous solutions of small molecules as well as water-soluble polymers, although the origin and specifics of this phenomenon may vary for different species, such as ionic or non-ionic.