Determination of Equilibrium Adsorbed Morphologies of Surfactants at Metal-Water Interfaces Using a Modified Umbrella Sampling-Based Methodology

H Singh and S Sharma, JOURNAL OF CHEMICAL THEORY AND COMPUTATION, 18, 2513-2520 (2022).

DOI: 10.1021/acs.jctc.2c00078

Surfactants adsorb to metal-water interfaces in variousmorphologies, including self-assembled monolayers (SAMs), cylindricaland spherical micelles, or hemimicelles. Current molecular simulationmethods are unable to efficiently sample the formation of thesemorphologies because of the large diffusive/energetic barriers. Weintroduce a modified umbrella sampling-based methodology that allowssampling of these morphologies from any initial configuration and providesfree energy differences between them. Using this methodology, we havestudied adsorption behavior of cationic quaternary ammonium (quat) of 4and 12 carbon long alkyl tails, uncharged decanethiol, and anionicphosphate monoester surfactants and their mixtures at a gold-waterinterface. Wefind that while Coulombic repulsion between the chargedhead groups of quat-4 limits their adsorption to a sparse layer, strongerhydrophobic interactions between the alkyl tails of quat-12 promote adsorption resulting in a morphology with adsorbedhemispherical micelles sitting atop a monolayer. Decanethiol molecules adsorb in a densely packed bilayer with the moleculesstanding-up on the surface in thefirst layer and lying parallel to the surface in the second layer. Cationic and anionic surfactantmixtures display a synergistic adsorption behavior. These results elucidate the role of molecular characteristics in dictating the natureof adsorbed morphologies of surfactants at metal- water interfaces.

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