Interfacial behaviors of betaine and binary betaine/carboxylic acid mixtures in molecular dynamics simulation

The interfacial properties of betaine surfactants and betaine/carboxylic acid mixtures at decane-water interface have been studied via molecular dynamics simulations. The effect of surfactant structure on the surfactant orientation is discussed firstly based on the pure betaine systems. Then Synergistic effect of mixed Betaine/carboxylic acid systems at the interface is explored using mass density profile, interfacial thickness and spatial distribution function. Based on the simulated results, one can find the hydrophilic tail of betaine has a strong tendency to flat on the interface, long alkyl main chain tends to stretch into oil body and dominates the orientation of hydrophobic chain while benzene ring can change and fix the order of alkyl chain to some extent. Carboxylic acid molecules locate in the crack of ASB18 hydrophobic chains and the addition of appropriate carboxylic acid molecules could induce an impressive decrease of interfacial tension. Our simulated results prove experimental conjecture and reveal the mechanism about the decrease of interfacial tension at molecule level, which is important for the enhanced oil recovery processes.