(486a) Monte Carlo Simulation of Lennard-Jones Nonionic Surfactant Adsorption at the Liquid/vapor Interface: Branched Surfactants

Monte Carlo simulations are presented of symmetric double-tailed, asymmetric double-tailed, and branch-tailed nonionic surfactants adsorbed at the liquid/vapor interface of a monatomic solvent. Surfactant molecules consist of an amphiphilic chain with a solvophilic head and two solvophobic tails. All molecules in the system, solvent and surfactant, are characterized by the Lennard-Jones (LJ) potential. Thermodynamic adsorption and surface-tension isotherms are generated and compared to those for single-tailed surfactants. Surface-tension isotherms calculated from the adsorbed amounts and the Gibbs adsorption isotherm agree with the simulated tensions, confirming equilibrium in our simulations. The classical Langmuir isotherm is obeyed for our LJ double and branch-tailed surfactants. In addition, symmetric double-tailed, asymmetric double-tailed, and branch-tailed surfactants exhibit lower critical aggregation concentrations (CAC) and more effectively reduce tension than their single-tailed counterparts. We hypothesize that this is related to the geometric configuration at the interface, i.e., the placement of two terminal solvophobic atoms closer to the head group as compared to a single-tailed surfactant enhances the ?solvophobic effect.? Specifically, the double-tailed and branch-tailed surfactants adsorb closer to the vapor phase, which allows for increased surfactant activity. When compared to each other, the asymmetric double-tailed surfactant is the more surface active, but exhibits aggregation at a lower concentration than the symmetric double-tailed surfactant. Surfactant architecture of the asymmetric double-tailed molecule has less steric hindrance as compared to the symmetric double-tailed surfactant which allows for relatively larger adsorption at the interface as well as aggregate formation at lower bulk surfactant concentration. Furthermore, as compared to both double-tailed surfactants, the branch-tailed surfactant has identical surface-tension behavior as the symmetric double-tailed surfactant, but exhibits the highest CAC and has an adsorption isotherm similar to the asymmetric double-tailed surfactant. We attribute the adsorption behavior of the branch-tailed surfactant to its reduced steric hindrance (similar to the asymmetric double-tailed surfactant) as compared to the symmetric double-tailed surfactant allowing for more adsorption at the liquid/vapor interface. Furthermore, we attribute the branch-tailed surfactant's highest CAC to the placement of additional solvophobic groups closer the end of the surfactant tail, making it difficult for surfactant tails to aggregate in the bulk liquid film. Ultimately, we establish that a coarse-grained LJ surfactant system can be used to design surfactant architecture to attain desirable adsorption and tension behavior for specific application.