(281d) Monte Carlo Simulation of Lennard-Jones Surfactant Adsorption at the Liquid/Vapor Interface: the Effect of Tail Attractions, Temperature, Added Hydrophiles, and Surfactant Configuration

We present Monte Carlo simulations of nonionic surfactant adsorption at the liquid/vapor interface. All molecules in the system are characterized with the Lennard-Jones (LJ) potential using differing interaction parameters. Surfactant molecules consist of an amphiphilic bead chain with a solvophilic head and a solvopohbic tail. Adjacent concerted atoms along the surfactant chain are connected by finitely extensible harmonic springs. In this work, we study intermolecular attraction amongst surfactants, temperature variation, added solvent monatomic hydrophiles (?salt?), and surfactant configuration (triblock versus diblock and double-tail versus single-tail) on the adsorption and the surface tension isotherms. We find that increasing surfactant tail attractions decreases the critical aggregation concentration of surfactants, while also allowing for more surfactants to reside at the interface. Lowering temperature has the expected effect of raising the surface tension, but also increases the ability of the surfactants to reside at the interface and increases overall tension lowering. Added hydrophiles have little to no effect on the surface tension and adsorption isotherms. We find that triblock surfactants with our model parameters have little to no surface activity. Double-tail surfactants are found to be more efficient at the interface as compared to single-tail surfactants, i.e., it requires a smaller number of surfactants at the interface to reduce the tension a given amount if they have two tails instead of one. Furthemore, double-tail surfactants are more effective, i.e., they achieve a larger overall reduction in the tension for a smaller concentration as compared to single-tail surfactants.