Internal Pressure in Binary Mixtures of Methylpyridine Isomers with H2O and D2O

Abstract

Internal pressures and their excesses were calculated in the strict thermodynamic way for binary mixtures of 2-,3-,4-methylpyridine, 2,6-dimethylpyridine, and 2,4,6-dimethylpyridine with water and heavy water at T = 296.15 K. The excess internal pressure approach proved to be useful in studies of relatively subtle thermodynamic effects. Positive correlation was found between the association energy of the 1:1 water–amine complexes and the maximum values of the excess internal pressure. It was shown that the differences caused by the exchange of H2O by D2O in the mixtures are evident in the excess internal pressure isotherms. The larger excesses for the D2O mixtures may indicate that the difference in energy between the O–D· · ·N and O–D· · ·O bonds is higher than that between the O–H· · ·N and O–H· · ·O ones. That probably leads to different phase properties of the studied systems.