Molecular Model for the Interaction of Charged Nano-Diamonds with Metal Surface

Prior experiments (Z. Liu, et al., RSC Advances 5, 78933 – 78940 (2015)) suggest that nano-diamond(ND) charge, as measured by the Zeta potential, can have a large influence on tribological performance. Using molecular dynamics simulations, we have characterized the interaction between charged NDs and gold surface in water. Two atomic models of octahedral NDs in water were created that mimic the negative and positive experimental NDs, one with chemisorbed carboxyl groups (COO-) and Na+ counter ions, and one with chemisorbed amino groups (NH3+) and Cl- counter ions. To explore the influence of particle/surface electrostatic interactions, the simulations were carried out with and without induced charges on gold surface. For both types of ND, the induced electrostatic interactions enhance surface adhesion and increase the force needed to slide the NDs along the gold surface. However, the magnitude and mechanism of these effects were found to depend on the ND surface groups. The simulations predict that the positive NDs are both more strongly adhered to the gold (by a factor of almost three), and require a larger force for sliding (by a factor just greater than two). These results are consistent with prior experiments from Liu et al.