Atomistic Study of Surface Effects on Polyelectrolyte Adsorption: Case Study of a Poly(styrenesulfonate) Monolayer

Abstract

Extensive all-atom molecular dynamics simulations have been performed to investigate the effect of surface features on the adsorption of poly(styrenesulfonate) (PSS) oligomers on top of a modified graphite substrate. In particular, we have investigated hydrophilic and hydrophobic model surfaces, accompanied by a variable surface charge density σs in the range 0−0.164 C/m2. Our results demonstrate that short-range interactions originating from the adsorbing substrate play a significant role in the layer structure of the adsorbed PSS, and they alone are already sufficient to induce a stable PSS adsorption layer. The presence of additional hydrophilic hydroxyl groups and charges on the adsorbing surface can further enhance the adsorption of PSS sulfonate groups at lower σs, whereas for the case of σs = 0.164 C/m2, the influence of the surface hydroxyl groups becomes negligible compared to that of the surface charges. The adsorbed PSS chains show mostly conformations where the PSS backbones are approximately parallel to the adsorbing surface. In some case, however, also the PSS backbones stand on top of the surface. Both the obtained surface charge overcompensation and the surface coverage are in good agreement with a previous experimental work [Ahrens et al. Macromolecules 2001, 34, 4504−4512]. Between the first PSS adsorption layer and the adsorbing substrate, we always find a water-rich region. The orientation of the water molecules in this region depends crucially on the features of the adsorbing surface. Our simulations suggest that the water-involved hydrogen bondings play a dominant role in determining the orientation of the water molecules. We also observe a decrease of the dielectric constant of water in the region close to the adsorbing surface in all of the investigated systems that is more pronounced for the hydrophilically modified surface and moreover increases with rising surface charge density. We suggest that this effect could lead to an electrostatic stabilization of the monolayer surface.