Local density augmentation in neat supercritical fluids: the role of electrostatic interactions

Motivated by a recent study of Saitow et al. [Chem. Phys. Lett. 368 (2003) 209–214 ], computer simulations and dielectric analyses are used to explore the effect electrical interactions have on density augmentation in supercritical fluoroform. Simulations of 2-site models of fluoroform and ethane show that electrical interactions increase local densities but not dramatically. The density augmentation simulated in both fluids (Δρmax=0.12ρc and 0.15ρc) is much smaller than the value Δρmax≅0.5ρc deduced by Saitow et al. from Raman measurements. This discrepancy can be reconciled if the observed vibrational shifts reflect mainly dipolar interactions, an interpretation justified by ab initio calculations.