Solvation of Metal Nanoparticles in a Subcritical — Supercritical Fluid:  A Computer Simulation Study

Molecular dynamics simulations of passivated and bare gold nanoparticles immersed in ethane have been performed in the reduced temperature (T-r = T/T-c) range 0.95-1.05 along the critical isochore of the solvent. The effects of temperature and passivation on the radial distribution of the solvent molecules about the center of mass of the metal core and on the degree of solvation (0) have been investigated. The results show that the solvation of the 38-atom bare particle is qualitatively different from that of the passivated particle: the degree of solvation of the bare particle is positive, whereas it is negative for the passivated particle. This difference in the solvation propensity of the particle in the presence and absence of the passivating layer is attributed to the different forces controlling the solvation process in the two cases. It is found that the degree of solvation of the 8-atom-core passivated particle is greater than that of the 38-atom-core passivated particle, due essentially to different configurational structures assumed by the passivating layers of the smaller and the larger particle. For the bare particle, the degree of solvation as a function of temperature passes through a maximum, a manifestation of the enhancement effect produced in the clustering of the solvent molecules around the solute as the system moves toward the critical point. The initial, minimum-energy configuration of the particle cores underwent appreciable changes as the system approached the equilibrium state in the simulations.