A Langmuir monolayer consists of amphiphilic surfactant molecules that adsorb at a water/air interface to form a unimolecular layer. As well as having multiple applications in templating, chemical sensing, and coatings, these monolayers can serve as a model for lipid bilayers. The precise fluid phase behavior of Langmuir monolayers and other two-dimensional systems is difficult to accurately obtain through experiments. Molecular simulations are needed to probe the vapor-liquid equilibria of these complex systems. In 1994, Siepmann, Karaborni, and Klein [J. Phys. Chem. 1994, 98, 6675] used configurational-bias Monte Carlo simulations in the Gibbs ensemble (CBMC-GE) to study the vapor-liquid coexistence curve (VLCC) of a pentadecanoic acid monolayer, but these simulations were limited to a small system comprised of only 60 surfactant chains and relatively short trajectories due to the computational resources available. In this work, we carry out CBMC-GE simulations for much larger systems and using longer trajectories utilizing the TraPPE united-atom force field for the alkyl tails to probe the VLCC of a pentadecanoic monolayer. Furthermore, we investigate modification of the headgroup-headgroup potential to improve the accuracy of the predictions and to allow for extension of the simulations to coexistence of liquid-expanded and liquid-condensed phases.

Vapor-liquid and liquid-liquid equilibria for a langmuir monolayer of pentadecanoic acid: A Monte Carlo study

253rd National Meeting (2017)

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