Thermodynamic properties of aqueous solutions of tetraalkylammonium salts: Dependence of the hydrophobic hydration effect on the cation size

The heats of solution of tetrahexylammonium and tetraheptylammonium bromides in water were measured at 318.15 and 328.15 K. The standard enthalpies and specific heats of solution and the temperature changes in the free energy and entropy of solution were calculated. A comparison of the thermodynamic properties of solutions for the homologous series of tetraalkylammonium salts demonstrated that the enthalpic, entropic, and specific heat characteristics of solution are positive and increase almost linearly up to tetrapentylammonium bromide. On passing to larger homologues, these parameters decrease, suggesting that the hydrophobic hydration effect is substantially weaker in solutions of tetraalkylammonium salts with larger cations. Original Russian Text ? A.V. Kustov, 2009, published in Zhurnal Neorganicheskoi Khimii, 2009, Vol. 54, No. 2, pp. 368–373.