Study of intermolecular interactions and microstructure of supercritical mixtures water +n-heptane and water +n-hexane based onPVT x measurements

The partial molar volumes of n-heptane and n-hexane in super- and nearly critical water are determined based on the PVT_x mesurements of the binary mixtures water + n-heptane and water + n-hexane near the critical point (CP) of the pure solvent (water). The partial molar volumes of n-heptane and n-hexane show nonclassical (scaling) behavior ( ) as the system asymptotically approaches (x → 0) the CP of pure water along the critical isobar-isotherm (T_c, P_c). The microstructure of the supercritical mixture is studied based on our experimental (PVT_x, ) data and Kirkwood-Buff’s fluctuation theory of dilute solutions; the dependence of the cluster size (N ₁ ^∞ ) on thermodynamic conditions (T, P) is established. With minor variations of temperature and pressure near the CP of pure water, the cluster size changes anomalously abruptly. The parameters of the equation of state of the SPHCT model are determined based on the PVT_x measurements, and direct solvent-solute correlation integrals C ₁₂ ^∞ (of water + n-heptane and water + n-hexane) are calculated. The Krichevskii parameter (∂P / ∂x) _VT ^C , adequately defining the behavior and structure of an infinitely dilute mixture near the CP of the pure solvent, is calculated. The effects of interactions between the solvent and solute molecules on the structure of a supercritical fluid are examined. Institute of Geothermal Problems, Dagestan Scientific Center, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 39, No. 1, pp. 74–85, January–February, 1998. This work was supported by RFFR grant No. 96-02-16005.