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 temptlating, chemical sensing, and coatings, these monolayars 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 surfactant chains and relatively short tragectories due to the computational resources available. In this work, we carry out CBMC-GE simulations for much larger systems and using longer tragectories 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.