ISOTOPE EFFECTS ON THE EXCESS PARTIAL MOLAR ENTHALPIES OF (XCH3(CH2)3O(CH2)2O(H OR D)+(1-X)(H2O OR D2O))

The excess partial molar enthalpies HiE were measured at the temperature 303.15 K for [x{CH3(CH2)3O(CH2)2OH (BH) or CH3(CH2)3O(CH2)2OD (BD)} + (1-x){H2O (WH) or D2O (WD)}]. Deuterium isotope effects were found to be qualitatively different in each of three composition regions, which have totally different mixing schemes, as we have pointed out earlier. In the water-rich region, the values of HEBD in {xBD + (1-x)WH} and for HEBH in {xBH + (1-x)WH} were the same within the experimental uncertainty. Those for HEBD in {xBD + (1-x)WD} and for HEBH in {xBH + (1-x)WD} were also the same, with the values about 1.3 kJ·mol-1 more negative at x→0 than those for WH rather than WD. These findings are not inconsistent with such notions as "iceberg formation", "structure enhancement of the solvent water", and "hydrophobic interaction". In the intermediate region, an isotope effect appeared mainly in B, suggesting that the B-B attraction, a precursor of phase separation, is of the direct kind. In the B-rich region, a new feature in {xBH + (1-x)WD} is evident. Our tentative interpretation is that WD molecules may cause some structural stabilization in the BH clusters. © 1992 Academic Press Limited All rights reserved.