Excess partial molar enthalpies in the water-rich region of the isobutyric acid – water system

Abstract

The excess partial molar enthalpies of isobutyric acid (IBA), H m E (IBA), and those of H 2 O, H m E (H 2 O), 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 H 2 O 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{∂H m E (IBA)/∂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, N 2 {∂ 2 H m E (IBA)/∂n B 2 }.Key words: excess partial molar enthalpies, isobutyric acid – water, transition in mixing scheme.