Fourier transform IR spectroscopy is used to study the effects of solvent density on the H-bonding equilibrium between perfluoro-tert-butyl alcohol (PFTB), (CF3)3COH, and dimethyl ether (DME), (CH3)2O, in solution in SF6 (T(c), 45.5-degrees-C; P(c), 540 psi; rho(c) 5.03 mol/L). The interaction of PFTB and DME is quite strong, and thus it has been possible to use rather more dilute solutions than in previous studies of H-bonding in supercritical fluids. Both PFTB and DME are highly volatile so the equilibrium could be studied over the full range of densities of SF6 from the pure gas phase (i.e. in the absence of SF6) through the supercritical region to liquidlike densities ca. 10 mol/L (1.5 gm/L) and over the temperature range 20-65-degrees-C. Both qualitative and quantitative measurements have been made at constant temperature, constant pressure, and constant density. The experiments introduce a number of innovative features both in methodology and in data manipulation. The modified lattice-fluid hydrogen-bonding model (MLFHB) has been used to calculate the effects of density on the percent of free (uncomplexed) PFTB in the solution and on the value of the equilibrium constant K(c). Qualitative studies show explicitly and without any spectroscopic assumptions that increasing density causes an increase in the concentration of free PFTB and a concomitant decrease in the concentration of the H-bonded PFTB/DME complex. More detailed measurements have allowed these changes to be quantified and modeled; particularly interesting are (a) the variation of K(c) with temperature at constant pressure (4.4 MPa), where the rapid increase in solvent density near the critical temperature cancels almost completely the effects of lowering the temperature and (b) the isothermal dependence of K(c) with density, including the unusual behavior at 50-degrees-C in the density range ca. 3-6 mol/L of SF6, behavior which is not observed at 60-degrees-C. This unusual behavior provides good evidence of enhanced solute-solute interactions toward the solvent critical temperature, as is further demonstrated with a simplified model.
