(524a) Molecular Dynamics Simulation Study of Pure and Surfactant-Laden Water Surfaces: Comparison of Flexible Water Models

Surfactants having complex molecular architectures have been used extensively in industrial applications for many years. Many of these so-called high-performance surfactants possess unusual interfacial and surface characteristics, compared to linear alkyl surfactants such as linear alkyl sulfonates and alkyl ethoxylates. To facilitate the design of industrial processes involving the use of these surfactants, it is essential to develop a fundamental understanding of their properties at interfaces, particularly at the air/water interface. Molecular dynamics simulation is a powerful technique for studying these systems at the molecular level. In applying this technique, however, it is important to have a model system that can adequately capture the characteristics of the water surface. Different water models have been shown to result in very different surface tensions, and a survey of the literature has revealed that bond flexibility may play an important role. In addition, it is generally recognized that simulated values of water surface tension are dependent on the simulation parameters, such as the Lennard-Jones cut-off distance, the system size, and the simulation time. Indeed, the differences in simulation parameters used in previous studies of water surfaces make any direct comparison of water models rather difficult.

The primary objectives of this study are to: (i) compare various flexible water models using a common set of simulation parameters, with particular emphasis on their ability to predict the water surface tension, (ii) study the use of these water models with common surfactants, such as sodium dodecyl sulfate and n-octyl glucoside, for simulating the surface pressure of surfactant-laden surfaces, and (iii) develop a better understanding of the relation between surfactant molecular architecture and surface properties. The simulated surface tensions of the flexible three-center (F3C) model and of the flexible SPC/E model at 300 K are 66.8 mN/m and 70.2 mN/m, respectively, in reasonable agreement with the experimental value of 71.7 mN/m. However, preliminary simulation results have indicated that, in the presence of adsorbed surfactants, the simulated surface pressures are considerably smaller than those observed experimentally. Molecular properties such as interfacial dipole moments, the normal and lateral pressure profiles, two-dimensional radial distribution functions, as well as the interactions between surfactant and water molecules are being examined in detail in order to elucidate the key characteristics in the simulation of water surfaces.