Local structural effects and related dynamics in supercritical ethanol. 1. Mechanisms of local density reorganization and residence dynamics.

The length scale effects on the relaxation processes describing the local density reorganization and residence dynamics of pure supercritical ethanol have been systematically investigated by employing molecular dynamics simulation techniques. The calculated static local density augmentation and enhancement values of ethanol have been found to be comparable to those of methanol at similar conditions but significantly lower than those of the strong associating fluid water, indicating thus the effect of hydrogen-bonding interactions on the creation of local density inhomogeneities in supercritical fluids. The bulk density dependence of local density reorganization dynamics has been studied as a function of the shell cutoff radius, revealing a significant change at length scales higher than the position of the first maximum of the radial pair distribution function. At length scales higher than this cutoff, the local density inhomogeneities affect more significantly the reorganization of the local environment and this becomes even more apparent at length scales corresponding to the second solvation shell radius. Additionally, the residence dynamics for the region extending up to the first solvation shell of ethanol was investigated, and the results obtained reveal that the density dependence of local density dynamics exhibit differences in correspondence to local density dynamics, something which becomes also apparent at length scales higher than the position of the first peak of the radial pair distribution function.