(22a) Contrasting the Adsorption Behavior of Quaternary Ammonium and Pyridinium Based Surfactants at Metal-Water Interfaces Via Fully Atomistic Simulations

Recent atomic force microscopy and quartz crystal microbalance results reveal that while the quaternary ammonium surfactants adsorb on the metal surface as a single layer, pyridinium based surfactants adsorb in multiple layers. Multilayer adsorption of surfactants is desired for applications like corrosion inhibition wherein a more robust corrosion mitigation is observed. We have studied adsorption behavior of these surfactants as function of concentration and alkyl tail length using well-tempered metadynamics methodology in fully atomistic system. We find that because of the large polar head, quaternary ammonium surfactants adsorb as hemispherical micelles in the surface, but pyridinium surfactants adsorb as planar multilayers. We also find that there is an optimum alkyl tail that maximizes adsorption and surface coverage. Long alkyl tails result in surfactants aggregating in the bulk aqueous phase with lower tendency to adsorb. Furthermore, we have studied the adsorption behavior of surfactants on partially corroded metal surfaces. Our simulations reveal that there is a critical size of the metal patch above which the hydrophobic interactions between the alkyl tail are sufficient to cause formation of complete adsorbed layers on the surface. These results are useful for explaining the efficacy of corrosion inhibitors on varying degrees of corroded surfaces.