Kirkwood-Buff integrals in aqueous solutions of 3-methylpyridine

Small-angle neutron scattering measurements were performed on 3-methylpyridine (3MP) - heavy water mixtures in the mole fraction range of 0.005-0.25 at 25 degreesC. The obtained zero-angle scattering intensities were used together with thermodynamic data to obtain Kirkwood-Buff integrals. Calculations for 3MP - water mixtures using thermodynamic data available in the literature indicated the inaccuracy of the excess Gibbs free energy data. The extraordinary high values of the Kirkwood-Buff integrals for the 3MP - heavy water system together with thermodynamic arguments show that first indications of the liquid-liquid phase separation can be seen already at 25 degreesC, which is much below the lower critical solution temperature. The analysis of the excess (or deficit) number of molecules surrounding a central one allowed us to conclude that strong clustering occurs in the mixture.