Studies on the hydrophobic hydration by the measurements of supercooling temperature and enthalpies of mixing

Abstract

In order to observe, more directly, the structural organization of water molecules around a nonpolar molecule in an aqueous solution, supercooling temperatures and enthalpies of mixing with some organic solvents were measured using two kinds of aqueous solutions (solution I and II) of tetrabutylammonium chloride. Solution I was prepared by melting a hydrate solid formed from the ammonium salt and water, and by keeping its temperature only 1° higher than its dissolution temperature; solution II was prepared by adding urea into solution I at the same temperature by keeping the molar ratio of urea to water as 1 : 49. Supercooling temperatures of solution I were found to be about 20° higher than those of solution II. The differences in enthalpies of mixing between solution I and solution II, ΔH I– ΔH II, were found to be about 0.17 kJ/mol of water regardless of the kind of solvents. It has been concluded that these directly reflect the difference in the stability of hydrogen-bonded water networks around a butyl chain of the ammonium cation in aqueous solutions.