Molecular Dynamics Simulation of Polarizable Gold Nanoparticles Interacting with Sodium Citrate

To study the structure of a citrate-capped gold nanoparticle and forces involved in citrate-gold interactions, we performed a molecular dynamics simulation of a truncated-octahedron nanoparticle containing Au(111) and Au(100) surfaces with sodium citrate. In this paper, we employed an approach to the modeling of interactions of a gold nanoparticle with citrate molecules taking into account the image charge effect in the metal. First, we built models of 6 and 14 nm nanoparticles, which can reproduce the polarization effects, based on a rigid-rod gold model and the GolP-CHARMM force field. To verify the simulation results, we analyzed density plots, radial distributions, distributions perpendicular to Au(111) and Au(100) surfaces, the electric potential of the system, and the dynamics of citrate crown formation. We observed formation of a stable citrate crown around the nanoparticle and detected nonuniform surface distribution of citrate ions with the preference for Au(111) facets over Au(100) ones. Testing of the model of the citrate-capped gold nanoparticle in a simulation at high concentrations of Na⁺ and Cl^- ions (0.8 M) showed incorporation of chloride anions into the citrate crown. We compared the results of citrate crown formation between polarizable and nonpolarizable gold models and noticed a difference in the citrate distribution on the surface of the gold nanoparticle. We found that polarization effects in the metal are involved in the mechanism of interaction of the gold nanoparticle and citrate ions. The obtained results are in good agreement with experimental data and computer simulations from a number of other studies, which prove the validity of the proposed model.