Compressibilities of aqueous tert-butanol in the water-rich region at 25 degrees C: Partial molar fluctuations and mixing schemes

We measured the speeds of sound, u, and the densities, d, and, hence, determined the adiabatic compressibilities, kappa(S), of aqueous tert-butanol (TBA) at small increments in mole fraction of TBA, x(TBA). Existing data for heat capacity, C-p, and thermal expansivity, alpha(p), were used to convert kappa(S) to the isothermal compressibilities, kappa(T). Using these kappa(T) data, the normalized volume fluctuations, (V)Delta, and their composition derivative, the partial molar volume fluctuations of TBA, (V)Delta(TBA), (Y. Koga and P. Westh, Bull. Chem. Soc. Jpn., 1996, 69, 1505) were calculated. By using C-p data available in the literature we also calculated the normalized entropy fluctuations, (S)Delta, and the partial molar entropy fluctuations of TBA, (S)Delta(TBA). Furthermore, the density data at 15, 25 and 35 degrees C in the literature were used to evaluate the thermal expansivities, alpha(p), from which the normalized cross (entropy-volume) fluctuations, (SV)Delta, and the partial molar cross-fluctuations of TBA, (SV)Delta(TBA), were calculated. The composition dependence of these fluctuation functions were interpreted in the Light of the mixing schemes in aqueous solutions of TBA and other non-electrolytes and the transition between them (Y. Koga, J. Phys. Chem., 1996, 100, 5172).