Improved Estimates of the Critical Point Constants for Large n-Alkanes Using Gibbs Ensemble Monte Carlo Simulations

In this work, we present improved estimates of the critical temperature (T_c), critical density (ρ_c), critical pressure (P_c), and critical compressibility factor (Z_c) for n-alkanes with chain lengths as large as C₄₈. These are obtained for several different force field models with Gibbs ensemble Monte Carlo simulations. We implement a recently proposed data analysis method designed to reduce the uncertainty in T_c, ρ_c, P_c, and Z_c when predicted with molecular simulation. The results show a large reduction in the uncertainties compared to the simulation literature with the greatest reduction found for ρ_c, P_c, and Z_c. Previously, even the most computationally intensive molecular simulation studies have not been able to elucidate the n-alkane P_c trend with respect to larger carbon numbers. The results of this study are significant because the uncertainty in P_c is small enough to discern between conflicting experimental data sets and prediction models for large n-alkanes. Furthermore, the results for T_c resolve a discrepancy in the simulation literature with respect to the correct T_c trend for large n-alkanes. In addition, the Z_c results are reliable enough to determine the most accurate prediction trend for Z_c. Finally, finite-size effects are shown to not be significant even for the relatively small system sizes required for efficient simulation of longer chain lengths.