EXCESS AND PARTIAL MOLAR ENTHALPIES FOR MIXTURES OF METHYLPYRIDINE ISOMERS WITH LIGHT AND HEAVY-WATER AT T=298.15 K

Excess enthalpies for the mixtures of methylpyridine isomers with ligh t and heavy water were measured at T = 298.15 K within the whole conce ntration range. The excess partial enthalpies were calculated. The neg ative values of the excess enthalpies for {xC(6)H(7)N + (1 - x)H2O} ar e slightly larger than those for {xC(6)H(7)N + (1 - x)D2O}. It was sug gested that the differences between H2O and D2O, reflected in the firs t enthalpies of solution of C6H7N, could be due to a hydrophobic effec t in dilute aqueous solutions in spite of the polarity of the methylpy ridine molecules. The enthalpies of the solution of 2- and 3-methylpyr idine in D2O, significantly smaller than those in H2O, do not compensa te the large entropy effect due to the rearrangement of the solvent st ructure affected by the solute and, consequently, the mixtures of both isomers with D2O show a lower consolute point. In the case of 4-methy lpyridine the enthalpies are almost equal to one another, thus the pha se properties of the mixtures with light and heavy water are similar. At higher methylpyridine concentrations the hydrophilic effect predomi nates; because of shortage of water, the hydration spheres around the non-polar parts of the methylpyridine molecules, built of water molecu les linked tetrahedrally to each other by hydrogen bonds, decay gradua lly. Because of the equal polarity of the H2O and D2O molecules, the e nthalpies of mixing for those solutions are almost independent of the isotopic composition of water. (C) 1998 Academic Press Limited.