Predicting the Equilibrium Adsorption Morphologies of Surfactant Molecules at Metal-Water Interfaces Via Advanced Molecular Dynamics Simulations

Self-assembly of surfactant molecules at the metal-water interfaces is understood to play a key role in many useful applications, including corrosion inhibition, improving selectivity in heterogeneous catalysis, modulating electrochemical reactions and synthesis of anisotropic metal nanoparticles. Surfactants are understood to adsorb at metal-water interfaces in high-density adsorbed morphologies. Current molecular simulation techniques are unable to effectively study the formation of these morphologies. Therefore, determining the most thermodynamically stable adsorbed morphology of surfactants has remained an unsolved problem. In this work, we report a new free energy sampling methodology that enables the study of formation of high-density adsorption morphologies. The methodology is generally applicable for studying other adsorption systems. Using our methodology, we have studied adsorption of cationic and uncharged surfactant molecules on metal-water interfaces. Our free energy calculations show that the equilibrium adsorption morphology of cationic molecules vary with their alkyl tail lengths. Cationic molecules with small tail lengths (C-4) adsorb in a monolayer of molecules lying flat onto the surface, whereas the molecules with large tail lengths (C-12) adsorb in a mixed morphology such that some molecules lie-flat on the surface and rest of the molecules are aggregated as hemi-micelles on the surface. As opposed to cationic molecules, uncharged molecules adsorb in much higher densely packed morphologies.