EXCESS PARTIAL MOLAR ENTHALPIES IN THE WATER-RICH REGION OF THE ISOBUTYRIC ACID - WATER-SYSTEM

The excess partial molar enthalpies of isobutyric acid (IBA), H(m)E(IBA), and those of H2O, H(m)E(H2O), were measured in aqueous solutions of IBA. The temperature dependence of H(m)E(IBA) at the infinite dilution suggested that the structure enhancement of the solvent H2O by IBA is weaker than those by tert-butanol (TBA) or 2-butoxyethanol (BE). The concentration dependence of H(m)E(IBA), and that of the enthalpic IBA-IBA interaction, N{partial H(m)E(IBA)/partial n(B)}, shows that there are two distinct mixing schemes bounded at about x(B) = 0.03, before reaching the two phase separation. Namely, the IBA-IBA interaction is repulsive below this boundary, while above this boundary it becomes attractive leading eventually to phase separation at a higher concentration. The transition between the two schemes is associated with a peak(negative) anomally in the fourth derivative of the free energy, N2 {partial H-2(m)E(IBA)/partial n(B)2}.