A Molecular Dynamics Study of the Surfactant Surface Density of Alkanethiol Self-Assembled Monolayers on Gold Nanoparticles as a Function of the Radius

Abstract

Atomistic molecular dynamics calculations of self-assembled monolayers of alkanethiol molecules on gold nanoparticles are carried out to determine the surface per ligand molecule as a function of the size. The molecular footprint is determined by calculating the surface tension starting from its thermodynamic definition in the canonical ensemble. We use the method of Chiu et al. (Chiu, C. C. et al. J. Chem. Phys. 2010, 132, 054706) that makes use of a parametric dependence on the system size of the potential energy between the gold surface and the ligand molecules. The role of the different groups in the molecule in the surface tension is studied. An analysis of the dependence of the surface per thiol molecule on the molecular length is carried out showing that, for molecules larger than hexanethiol, this value is independent of the number of carbon groups in the molecule. The surface occupied per molecule on spherical nanoparticles as a function of the curvature is obtained, and the method is applied to flat surfaces, obtaining a very good agreement with the experimental results. A simple model for the surface density per molecule in curved and flat surfaces is developed.