Calculation of nanocolloidal liquid time scales by molecular dynamics simulations

Molecular dynamics, MD, simulations have been used to calculate the transla tional and rotational relaxation dynamics of model atomistically rough sphe rical nanocolloidal particles in solution at infinite dilution by immersing a single Lennard-Jones cluster in a molecularly discrete solvent. Key time scales characterizing colloidal particle dynamical relaxation were compute d from time correlation functions. For translational motion these were tau( v), the colloidal velocity relaxation time, tau(f), the hydrodynamic relaxa tion time and the time scale for significant particle displacement, tau(d). We show that tau(v) similar or equal to tau(f) when the relative mass dens ity of the colloidal particle divided by the bulk density of the solvent is ca, rho* = 20, in agreement with theoretical predictions. Preliminary evid ence from the velocity autocorrelation functions, VACF, of the nanocolloida l particle also supports the theoretical treatments that the transition fro m the Liouville to Fokker-Planck description (evident by exponential decay in the VACF) is determined by both the colloidal particle mass non size, We calculated the relaxation times for angular velocity relaxation, tau(w) an d reorientation, tau(u) and found them to scale reasonably well with the re laxation time for the free rotor, for size dependence but not so well for m ass dependence. The angular velocity correlation function of 13 atom cluste rs departed from Langevin (exponential) relaxation also for rho* < 20. The rotational self-diffusion coefficient was also non-classical in this range.