Conformational Equilibria of Organic Adsorbates on Nanostructures in Aqueous Solution: MD Simulations

We have performed atomistic molecular dynamics (MD) simulations of gold nanopartides (GNPs) in aqueous NaCl solution. Alkanethiol chain-covered GNPs at grafting densities between approximately one-third and full coverage were studied with nonpolar CH3 and charged COO- and NH3+ terminations. Special attention was given to the penetration depth of water and ions into the diffuse shell of the functionalized alkanethiol chains and its dependence on grafting density and functionalization. Solutions with polar terminations were neutralized by an excess of Na+ and Cl- ions. The penetration of water and ions into the hydration shell increases with decreasing grafting density irrespective of termination. High grafting densities lead to more extended hydrocarbon chains. Charged functionalized GNPs produce nonmonotonous counter charge distributions with reduced ion mobility. Partial replacement of first shell solvation water by the charged groups leads to a drastic increase in torsional relaxation times of the chain termini. Due to the large curvature of the GNPs with a diameter of 2 nm, gold cores remain accessible to both ions and water even at the highest studied grafting densities of about 5 chains/nm(2).