Advancing Vapor-Liquid Equilibrium Predictions with the OPPES United Atom Forcefield for Associating Fluids

In this study, we extend the OPPES united atom force field to normal alcohols, glycols, and alkoxyethanols by optimizing new potential parameters based on the previously developed OPPES n-alkane and ether models. Bonded interaction parameters were primarily adopted from the TraPPE-UA model, except for equilibrium bond lengths and bending angles, which were obtained through density functional theory (DFT) geometry optimizations. Partial charges for ether oxygens and neighboring carbon pseudo-atoms were taken from the OPPES ether model, while those for hydroxyl oxygens and hydrogens were adopted from the AMBER model. The alpha carbon's charge was determined by enforcing charge neutrality. Lennard-Jones (LJ) parameters for hydroxyl oxygen and hydrogen were fitted to experimental liquid densities and vapor pressures of representative n-alcohols.Using the optimized parameters, we performed configurational-bias Monte Carlo simulations in the NVT ensemble for five n-alcohols (methanol to 1-octanol), two glycols (1,2-ethanediol and 1,3-propanediol), and three alkoxyethanols (2-methoxyethanol to 2-propoxyethanol). Additionally, NPT Gibbs ensemble Monte Carlo simulations were conducted for a binary n-heptane + 2-propoxyethanol system to evaluate phase behavior and local composition enhancements. Hydrogen bonding statistics were analyzed to assess the model’s performance in capturing associative interactions and fluid structure.Overall, the OPPES model yielded improved predictions of thermophysical properties compared to the TraPPE-UA model, especially near critical conditions, demonstrating its potential as a reliable and transferable force field for associating fluids.