Excess partial molar enthalpies of tert-butanol in water–tert-butanol mixtures

Excess partial molar enthalpies of tert-butanol (TBA) in water–tert-butanol mixtures were measured at 20.00, 26.90, 45.17, and 59.49 °C in the water-rich region, and at 30.00 °C for the entire range. The results together with those of the previous work, (Y. Koga. Can. J. Chem. 64, 206 (1986)), appear to support the following views: At XTBA ≈ 0, TBA molecules form hydrogen bonds with H2O molecules, and they enhance the structure of the hydrogen bond network of H2O. The latter effect is of very long range; the enthalpic repulsion between solutes is already operative via enhancement of hydrogen bond network when two TBA molecules are separated by 12 H2O molecules. This enthalpic repulsion becomes stronger as the concentration of TBA increases. At a certain threshold, XTBA ≈ 0.03 at 30.00 °C for example, the new scheme in which solutes tend to associate sets in, and for XTBA > 0.06 this scheme becomes predominant. The effect of higher temperature is to shift this threshold to the lower values of XTBA. Therefore, it is probable that a lower critical solution temperature may exist at a temperature higher than the boiling points of the solution. It was also shown that if species of TBA(H2O)k type persist, they exhibit a hard-sphere nature in terms of enthalpic interaction among themselves.