Equilibrium state model for surfactants in oils: Colloidal assembly and adsorption

An equilibrium state model addressing the aggregation and adsorption of colloidal assemblies in apolar solvents (oils) via monomer exchange is presented. The model is based on the previously reported step-wise aggregation response of fatty acids and monoglycerides in bio-oils, and captures surface crowding via scaled particle theory (SPT). The sensitivity of key observables - mean aggregation number, adsorbed surfactant amount, and free monomer concentration - to model parameters is demonstrated. Fits to molecular modelling based aggregation and adsorption data of oleic acid and monoolein reveal that the model accurately reproduces chemically specific aggregate exponential distributions in both bulk and surface phases, even outside of its parameterization conditions. A biased state model, where the initial bulk aggregation step (dimer formation) differs from other steps results in a notable improvement in accuracy. Fits to various phospholipid adsorption isotherms demonstrate the applicability of the model to isotherm type experimental data. The fits reveal either monolayer or aggregate like adsorption structures, depending on surfactant head group charge. The presented model provides an easily accessible, computationally feasible means to estimate colloidal assembly and adsorption in oil environments, and enables assessment of surfactant aggregation propensity and adsorption energetics.