Density-dependent solvation dynamics in a simple Lennard-Jones fluid

The density dependence of time correlation functions for the solvation energy in a simple Lennard-Jones liquid has been investigated by molecular dynamics computer simulations. Considering argon dissolved in liquid argon, mechanical solvation dynamics has been studied treating interactions between excited solute states and the solvent by changes in the Lennard-Jones well-depth parameter, the Lennard-Jones size parameter, and a combination of the two. Densities have been varied from supercritical to triple point densities at a constant temperature of 151 K. In addition, a thermodynamic state close to the argon triple point has been considered. All the solvation energy time correlation functions have been broken down into their partial two- and three-body contributions giving an insight into the cancellation effect of solvation dynamics. It is found that the well-depth solvation process produces slowly decaying time correlation functions for the solvation dynamics at lower densities. In this case, the solvation dynamics becomes faster with increasing density due to long time cancellations between two-body time correlations with positive amplitudes and negative three-body contributions. In contrast, the size parameter solvation process is much faster. The corresponding solvation dynamics time correlations decay rapidly already at low liquid densities with two- and three-body contributions significantly stronger correlated than the total solvation energy time correlation function. Describing the excited solute by changes in the well-depth and the size parameter, the dynamical features resemble much of the solvation dynamics obtained from changes only in the size parameter. (C) 2003 American Institute of Physics.