Mixing scheme of aqueous butan-1-ol in the water-rich region at 25°C: Excess chemical potential, partial molar enthalpy, entropy and volume, heat capacity compressibility and thermal expansivity

Abstract

We determined excess chemical potential, partial molar enthalpy, entropy and volume, heat capacity, isothermal compressibility and thermal expansivity for aqueous butan-1-ol in the water-rich region up to the phase separation boundary at 25°C. The latter three response functions were used to calculate the mean-square fluctuation densities, which signifies the amplitude, or the intensity, of fluctuations in volume, entropy or cross (entropy–volume) fluctuations. Furthermore, we calculated the (mean-square) normalized fluctuations that are indicative of the wavelength, or the extensity, as well as the amplitude of respective fluctuations. The behaviour of these thermodynamic quantities were compared with those obtained earlier in this laboratory for aqueous methanol, ethanol, propan-1-ol, tert-butanol (tert-butyl alcohol), and 2-butoxyethanol. We conclude that in the water-rich region of aqueous butan-1-ol, mixing scheme I is operative as in other alcohols, whereby butan-1-ol molecules enhance the hydrogen bond network of H₂O in their immediate vicinities with concomitant reduction of hydrogen bond probability in the bulk H₂O away from solutes. However, before reaching the phase separation boundary there was no signature indicative of the transition of mixing scheme observed in other aqueous alcohols. Thus, in aqueous butan-1-ol phase separation occurs directly from mixing scheme I, without going through mixing scheme II, which we argued earlier to be a preparation stage for phase separation for the other alcohols.