TRANSLATIONAL AND ROTATIONAL DIFFUSION OF MODEL NANOCOLLOIDAL DISPERSIONS BY MOLECULAR-DYNAMICS SIMULATIONS

Molecular dynamics, MD simulations have been applied to model nanocoll oids in solution at infinite dilution. Simulations were carried out wi th atomistically rough clusters of atoms with variable dimensions comp ared to the solvent molecule, using the Weeks-Chandler-Andersen (WCA) interaction between solute and solvent. Nanocolloidal particles contai ning between 20 and 256 atoms held together with strong Lennard-Jones interactions and with up to eight times the diameter of the solvent mo lecule were modelled. At liquid-like densities the translational and r otational self-diffusion coefficients for the nanocolloids of all size s were statistically independent of the ratio of solute to solvent par ticle densities. The clusters induced a 'layering' of solvent molecule s around them. The translational and rotational diffusion coefficients decreased with increasing cluster size and solvent density. The simul ations reveal that there is a clear separation of timescales between a ngular velocity and orientation relaxation, consistent with the classi cal small-step diffusion picture encapsulated in Hubbard's relationshi p which is obeyed well by the simulation data. Application of the Stok es-Einstein (translation) and Stokes-Einstein-Debye (rotation) equatio ns to these data indicate that the translational degrees of freedom ex perience a local viscosity in excess of the bulk value, whereas rotati onal relaxation generally experiences a smaller viscosity than the bul k, dependent on cluster size and solvent density and reasonably in acc ord with the Gierer-Wirtz model. Both of these observations are consis tent with the observed layering of the solvent molecules around the cl uster, whose effects appear to be significant for clusters on the nano scale.