Vapour pressure of aqueous tert.-butanol in the water-rich region: transition in the mixing scheme

Abstract

The total vapour pressure of aqueous tetrt-butanol (TBA) was measured in small increments in the range x TBA < 0.2 at 25.15 and 30.00 ° C, where x TBA is the mole fraction of TBA in the liquid phase. The partial pressure of TBA, and hence the excess partial molar free energy of TBA, G m E(TBA), were calculated by the Boissonnas method. Since the values of the excess partial molar enthalpy, H m E(TBA), had been directly measured in small increments, the values of the excess partial molar entropy, S m E(TBA), were calculated accurately. From the concentration derivative of G m E(TBA), the Kirkwood-Buff parameters were calculated for the range x TBA < 0.05. The concentration dependence of the partial molar entropy, as well as that of the enthalpy, indicated that, at about x TBA = 0.045, there is a transition from one scheme of mixing in the solution to the other. As the temperature increases this threshold value decreases; there is a boundary which is marked by the maxima in various quantities that are proportional to the third derivative of the free energy. Below this boundary, the mixing scheme is consistent with such ideas as the “iceberg formation”, the “structure enhancement of the solvent water”, and the “hydrophobic interaction (attraction)”. The second scheme above this boundary appears to be that of TBA cluster formation.