STANDARD PARTIAL MOLAL HEAT CAPACITIES OF SODIUM TETRAPHENYLBORON IN AQUEOUS SOLUTION FROM 0 TO 90 DEGREES . EFFECT OF WATER STRUCTURE AND HYDROPHOBIC HYDRATION

The standard partial molal heat capacities of sodium tetraphenylboron in aqueous solution from 0 to 90° have been determined by measuring the integral heats of solution as a function of temperature over the range 5-85°. ΔCp0 and C̄p20 values at all temperatures studied are positive and larger than those reported for any other solute. The plots of ΔCp0 and Cp20 against temperature show two discontinuities which correspond to a minimum around 50° (ΔCp0 = 154 cal deg-1 mol-1) and a maximum around 70° (ΔCp0 = 245 cal deg-1 mol-1). The minimum is explained in terms of hydrophobic interactions" and the maximum is presumed to be due to a reduction in the structure-making capacity of the solute above 70°. No quantitative interpretation for the discontinuities is available at present. This is the first instance that such pronounced discontinuities are observed in the temperature dependence of partial molal heat capacities of solutes (containing a large amphiphilic ion) in aqueous solution. This work suggests the need of more experimental data on the temperature dependence of thermodynamic properties of aqueous solutions of nonpolar solutes or solutes containing nonpolar groups in order to have a better understanding of the structure of aqueous solutions."