Calculation of nanocolloidal liquid time scales by molecular dynamics simulations

Abstract

Molecular dynamics, MD, simulations have been used to calculate the translational and rotational relaxation dynamics of model atomistically rough spherical 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 computed from time correlation functions. For translational motion these were τ_v, the colloidal velocity relaxation time, τ_f, the hydrodynamic relaxation time and the time scale for significant particle displacement, τ_d. We show that τ_v ≃ τ_f when the relative mass density of the colloidal particle divided by the bulk density of the solvent is ca. ρ∗ = 20, in agreement with theoretical predictions. Preliminary evidence from the velocity autocorrelation functions, VACF, of the nanocolloidal particle also supports the theoretical treatments that the transition from the Liouville to Fokker—Planck description (evident by exponential decay in the VACF) is determined by both the colloidal particle mass and size. We calculated the relaxation times for angular velocity relaxation, τ_w and reorientation, τ_u and found them to scale reasonably well with the relaxation time for the free rotor, for size dependence but not so well for mass dependence. The angular velocity correlation function of 13 atom clusters departed from Langevin (exponential) relaxation also for ρ∗ < 20. The rotational self-diffusion coefficient was also non-classical in this range.