Structure from solute-solvent interactions in supercritical polyatomic molecular mixtures

摘要

Solute-solvent and solute-solute microstructures at the sueprcritical state are important in elucidating chemical reaction and spectroscopic data. We can classify the range of pair correlations into two regions: a short range (r (le) r(sub min)) and a long range (r from r(sub min) to 20 (approximately) 50 molecular diameters). The often cited local density enhacement referes to the short range corelations and is shown not to be related to the critical behavior of the mixture. The long-range behavior of the pair correlations is responsible for the buildup of osmotic susceptibility and is related to the closeness to solvent's critical point. In the class of attractive supercritical mixtures, the solute-solvent short-range pair correlation exhibits complicated bheavior: enhanced solvation (within the first neighborhood) at low to medium densities and peak attenuation followed by peak growth at higher densities. Earlier theor. studies (integral equations) dealt with idealized model of solute molecules as spheres (molecular size effects accounted for, but not molecular shape effects). This study examines behavioral change due to molecular shapes and orientations as the solutes take the shape of polyatomic molecules. For example the case of di-tert-butylnitroxide radicals in supercritical ethylene can be modeled as linear dimeric molecules. Reference interaction site model integral equation is used to produce site-site correlation functions. We map out the density ranges where structural changes take place and elucidate their practical implications.