Solvent-solute interactions and the Raman CH stretching spectrum of cyclohexane-d(11). II. Density dependence in supercritical carbon dioxide

We have measured the isotropic Raman CH stretching spectrum of cyclohexane- d(11) in supercritical CO2 at 49.7 degrees C and in liquid CO2 at room temp erature over a range of densities from 0.2 rho(c) to 2 rho(c), where the cr itical number density rho(c) for CO2 is 6.4 nm(-3). The axial and equatoria l CH stretching bands in the spectrum shift to lower frequencies and broade n with increasing density. As was the case in an earlier study of cyclohexa ne-d(11) in liquid solvents [G. J. Remar and R. A. MacPhail, J. Chem. Phys. 103, 4381 (1995)], the "perturbed hard-fluid model" of Ben-Amotz and Hersc hbach provides a satisfyingly consistent description of the observed shifts in terms of competing contributions from repulsive and attractive solute-s olvent forces along the CH bond. In particular, when the repulsive contribu tion to the shift is calculated according to the prescription developed in the liquid solution study, the attractive contribution is found to scale li nearly with the density and with the polarizability derivative of the CH bo nd, as predicted by the model. The ratio of the equatorial to axial linewid ths has a density-independent value of 1.2, nearly the same value found for the liquid solutions and numerically equivalent to the ratio of polarizabi lity derivatives for the CH bonds. This equivalence is consistent with Schw eizer and Chandler's theoretical result for the width of a band that is inh omogeneously broadened by attractive force fluctuations, but the density de pendence is not; their result would predict a nonlinear density dependence with a maximum near rho(c), whereas the observed linewidths show a nearly l inear dependence on density. Neither the frequency shifts nor the linewidth s show any clear evidence for a "local solvent density enhancement" that wo uld be predicted for this mixture near the critical point. In the accompany ing paper, Frankland and Maroncelli describe molecular-dynamics simulations of cyclohexane in supercritical CO2 that reproduce the observed linewidths nearly quantitatively. They show convincing evidence that the linewidths a re dominated by binary, collisional interactions between the hydrogen and t he solvent, and they discuss the apparent absence of a density enhancement. (C) 1999 American Institute of Physics. [S0021-9606(99)52602-0].