H2O-D2O Solvent Isotope Effect on Excess Molar Volumes of 3-Methylpyridine Solutions

Densities of 3-methylpyridine (3-MP) + water and 3-methylpyridine + heavy water were measured in the 3-MP mole fraction range 0.002–0.04 from 298 to 318 K. The excess molar volumes of 3-MP + D2O mixtures were found to be more negative than those of 3-MP + H2O mixtures. The partial molar volume of 3-MP at infinite dilution is smaller in D2O than in H2O which suggests that 3-MP causes a structure-breaking effect in water which is more pronounced in D2O. It was found that the volume change with concentration in dilute solutions of 3-MP in water and heavy water can be adequately described by the pair-wise interaction of the solute molecules. The molal volume second-virial coefficient, Vxx, is positive indicating that the water molecules are less structured in the cospheres of the solute pairs than in the bulk solvent. The temperature dependence of Vxx displays a maximum at around 308 K in the case of D2O solutions, whereas Vxx increases almost linearly with temperature in H2O solutions.