The Effect of Intermolecular Interactions on Local Density Inhomogeneities and Related Dynamics in Pure Supercritical Fluids. A Comparative Molecular Dynamics Simulation Study

Abstract

The effect of intermolecular interactions of different strength on the local density inhomogeneities in pure supercritical fluids (scfs), with different intramolecular structure, was investigated by employing molecular dynamics (MD) simulation techniques. The simulations were performed at state points along an isotherm close to the critical temperature of each system (T(r) = T/T(c) = 1.03). The molecular fluids under study have been chosen on the basis of the electrostatic character of their intermolecular interactions as follows: monatomic, dipolar and hydrogen bonding (HB), quadrupolar, and octupolar. In the case of dipolar scfs, their HB nature when present was systematically explored and related to the behavior of the created local density inhomogeneities at all densities. The results obtained reveal strong influence of the dipolar and HB interactions of the investigated systems upon the local density augmentation. We found that this effect is fairly larger in the case of the dipolar and HB fluids (H2O, CH3OH, and NH3) compared to those for the non-dipolar ones (Xe, CH4, CO2, and N2). In the case of sc CO2, the dependence of the local density augmentation on the bulk density is in agreement with available experimental data as also reported previously. The estimated average number of hydrogen bonds per molecule (nHB) in these HB fluids shows an analogue nonlinear trend compared to the behavior of the average coordination numbers Nco(rho) of a particle with bulk density. The local density dynamics of the first and second solvation shell of each fluid were further analyzed and related to our previously proposed [Skarmoutsos, I.; Samios, J. J. Chem. Phys. 2007, 126, 044503] different time-scale relaxation mechanisms. Finally, the effect of the different strength of the molecular interactions corresponding to these fluids upon the local density dynamics has also been revealed in the behavior of the predicted appropriate time correlation functions and their corresponding correlation times.