SOLVENT EFFECT ON HYDROGEN-BONDING IN SUPERCRITICAL FLUIDS

A pronounced solvent effect on the hydrogen bonding of methanol and triethylamine is observed throughout the gas, supercritical, and liquid states in the relatively inert solvent SF6, based on FTIR spectroscopy. The free energy of hydrogen bonding is stabilized by a decrease in density; i.e., the donor and acceptor are destabilized more than the complex as the solvation is reduced. Also, the hydrogen bond energy becomes stronger. A hydrogen-bonding lattice-fluid (LFHB) model is extended to treat this density dependence, and the calculations are in reasonable agreement with experiment. Near the mixture critical point, the number of hydrogen bonding encounters between the donor and acceptor is enhanced due to solute-solute clustering as expected on the basis of previous experimental and computer simulation studies.