(247i) Thermodynamic Techniques to Capture Non-Ideal Surfactant Assembly at Hard Nanoscale Interfaces

Surfactant behavior and structure is well studied with industrial applications. Adsorption isotherms of surfactants are critical in determining the relationship between the interfacial properties and structures of surfactants, but providing quantitative predictions of the isotherms remains challenging. This is especially true for adsorption at hard interfaces such as on 2D layered materials or on nanoparticles where simulation techniques developed for fluid-fluid interfaces do not apply. Here we predict non-ideal adsorption at a solid-solution interface with a molecular thermodynamic theory (MTT) model. Our approach uses MD simulations for the determination of MTT parameters from two thermodynamic integrations. Furthermore, the MTT/MD model provides atomistic insights into non-ideal behavior of surfactant molecules by capturing structural details of the surfactants at different concentrations. Our approach captures structural transitions from the ideal (non-interacting) case at low concentrations, monolayer assembly, and then to the critical surface adsorption concentration (CSAC, where hemi-micelles form) and finally through the CMC where micelles are stable in the bulk. We validate our model against the original MTT model by comparing predicted adsorption isotherms of a simplified surfactant system from both approaches. We further substantiate the applicability of our model in complex systems by providing adsorption isotherms of sodium dodecyl sulfate (SDS) in a water-graphene system, in agreement with experimental observations of the CSAC for the same system.