(260b) Adsorption and Self-Assembly of Surfactants on Metallic Surfaces Studied Using Molecular Simulations

Surfactant molecules, such as hydrocarbons with imidazoline-based polar groups, are employed in the oil and gas industry for inhibiting corrosion inside transportation pipelines. The adsorption of these molecules at the metal-water interface is understood to be important for their efficacy as corrosion inhibitors. Experimental and theoretical investigations suggest that these molecules self-assemble at the metal-water interface in organized layers. The mechanism of adsorption and self-assembly, while considered critical for their inhibition properties, are only poorly understood because of the difficulties in probing the interface at molecular length-scales. Using classical molecular simulations and a coarse-grained representation of surfactant molecules, we study the adsorption of these molecules on metallic surfaces, focusing on the effect of hydrophobic character of the alkyl tail, the tail length and the size of the polar head group. Our simulations reveal that the hydrophobic character of the alkyl tail facilitates adsorption by promoting formation of a self-assembled monolayer on the metal surface. Surfactant molecules with longer alkyl tails show a decrease in the adsorbed amount, because of larger entropic loss of these molecules upon adsorption. Upon increasing the size of the polar group, we observe that the nature of the adsorbed layer changes, with a significant fraction of molecules adsorbed with their polar groups towards the aqueous phase. This arrangement of adsorbed molecules achieves better packing efficiency and hence lower free energy because of enthalpic effects.