MOLECULAR-DYNAMICS STUDY ON SOLVENT CLUSTERING IN SUPERCRITICAL-FLUID SOLUTIONS BASED ON PARTICLE RADIAL KINETIC-ENERGY

The objective of this work was to study the dynamic structures of solute-solvent clusters in supercritical fluids with molecular dynamics (MD) simulation. A new definition for cluster formation, based on the relative radial kinetic energy, is proposed. The method defines a cluster member as a solvent molecule which has a relative solute-solvent radial kinetic energy less than or equal to their pair potential energy. MD Simulations of Lennard-Jones (LJ) mixtures are shown for CO2 + pyrene and CO2 + model substances using one solute molecule and 863 solvent (CO2) molecules at T*=1.40, rho*=0.35 via a Nose-Hoover thermostat. The average number of member CO2 molecules in a sphere with diameter of 13.5 sigma(CO2) surrounding the pyrene solute was found to be 58. By tagging cluster and non-cluster members, we observed that approximately half of the particles in the first cluster shell exchanged in about 4 ps. Our simulations show that the number of remaining initial cluster members decay very rapidly in the beginning and approach a constant exponential decay after around 1 ps in all systems. The behavior of the time dependent radial distribution function of the remaining initial members indicated that only those members which were initially located in the solvation shell remained as members after about 1 ps. We considered the number of cluster members on the constant decay curve as the number of solvation members. By using the exponential decay rate, the cluster half-life and the solvation number of the CO2 + pyrene system was found to be around 1 ps and 16 solvent molecules, respectively. Other LJ mixtures were also studied and it was found that the LJ energy parameter strongly influences the decay rate while the LJ size parameter directly affects the solvation number. Molecular weight only had a small effect on solvation number.