Polarizable force field for molecular dynamics simulations of silver nanoparticles

Abstract

Contact of silver metal surfaces with water, ions and organic ligands experiences induced charges, leading to attractive polarization. These forces play an important role at inorganic/organic interfaces and complement other non-bonded surface interactions. Despite the importance of these interactions, it, however, remains difficult to implement polarization effects to classical molecular dynamics (MD) simulations. In this contribution, we first present an overview of two popular polarizable models, such as Drude oscillator and the rigid rod model, which are utilized to mimic the polarizability of bulk metals. Second, we implemented the rigid rod model to the polarizable force field (FF) for a silver atom, which was further adapted for atomistic MD simulations of silver nanoparticles (AgNPs) composed of 1397 atoms. In our model, induced charge polarization is represented by the displacement of a charge-carrying virtual site attached rigidly to an original Ag atom. To explore the role of polarization, we compared the performance of the classical nonpolarizable FF and the new polarizable model in the MD simulations of adsorption of water and ions onto quasi-spherical AgNP and the flat crystalline silver surface. The analysis of the radial distribution function of Ag-Ag atoms demonstrated that the introduction of the polarization effect had minor effects on face-centered cubic (fcc) packing of silver atoms of bare and water-solvated AgNPs. We found that the polarizable FF causes some increase in attractive interactions between the silver surface and water molecules and Na+ ions. As a crucial test of the developed polarizable model, the structure of adsorbed interfacial water molecules was analyzed. Our data suggest that the environment-induced polarization of the silver surface contributes significantly to the structure of adsorbed interfacial water layers and it also plays an important role in the adsorption of positive ions. However, it was also found out that the polarization effect has a rather short-range effect, so that a minor contribution of silver polarization was seen for adsorption of water molecules and ions from distant solvation shells.