Facet Selectivity of Cetyltrimethyl Ammonium Bromide Surfactants on Gold Nanoparticles Studied Using Molecular Simulations.

We have studied facet selectivity of cetyltrimethyl ammonium bromide (CTAB) surfactants of varying alkyl tail lengths (CTAB and CTAB) during their adsorption on a spherical gold metal nanoparticle (MNP) using umbrella sampling and well-tempered metadynamics techniques in molecular simulations. We show that the surfactants strongly adsorb with their alkyl tails wrapped around the MNP. The adsorption morphologies are dictated by the strong preference of the polar head group of the surfactants to adsorb on to the atoms that lie between the facets of the MNP, that is, in the vicinity of low-coordinated gold atoms. The alkyl tails do not display any strong facet preference. Owing to the longer alkyl tails, CTAB molecules pack together better than the CTAB molecules in the adsorbed state on the MNP. These findings suggest that the regions near the edges of the facets and low-coordinated atoms are expected to be preferentially covered with the adsorbed surfactants.