THEORY OF HYDROGEN-BONDING IN SUPERCRITICAL FLUIDS

The degree of hydrogen bonding and macroscopic thermodynamic properties for pure and mixed fluids are predicted with the hydrogen bonding lattice fluid (LFHB) equation of state over a wide range in density encompassing the gas, liquid and supercritical states. The model is successful for molecules forming complex self-associated networks, in this case pure methanol, ethanol, and water, and the mixture 1-hexanol-SF6. In supercritical water, significant hydrogen bonding is still present despite all the thermal energy and is highly pressure- and temperature-dependent. A fundamental description of pressure and temperature effects on hydrogen bonding is presented for a well-defined case, the formation of a complex between a donor and acceptor in an inert solvent, where no self-association is present. The partial molar enthalpy and volume change on complexation both become pronounced near the critical point, where the density is highly variable with temperature and pressure.