There is much interest in understanding vapor-liquid equilibrium (VLE), interfacial properties, and supercritical behavior of hydrocarbons and their mixtures because fuel is subjected to extreme temperatures and pressures in modern jet and diesel engines. Previously, we performed VLE simulations of twelve pure hydrocarbons to assess the accuracy of three MD force fields and determined that the TraPPE-UA force field provided the most accurate critical temperature values while CHARMM and OPLS-AA produced the correct qualitative VLE behavior. To mimic mixing of fuel with air, our recent work has focused on binary VLE simulations of a hydrocarbon with nitrogen. Binary simulations give insight into interfacial behavior of fuel-nitrogen systems as a function of temperature and pressure. Here, we present the results of binary VLE simulations of nitrogen with either linear or branched alkanes. The TraPPE-UA potential was used to model the hydrocarbons and the Rivera potential was used for N2. These simulations demonstrated that the Lorentz-Berthelot combining rules were insufficient to model interactions between unlike molecules. Pressure-composition phase diagrams were used to fit new hydrocarbon-N2 interaction parameters and to test the transferability of these parameters to the interaction of larger hydrocarbons with N2. The free energy of N2 relative to the vapor phase was calculated and N2 preferentially adsorbed at the liquid-vapor interface. In addition, the adsorption energy decreased as the n-alkane length increased and as temperature and pressure increased. The impact of hydrocarbon structure was investigated by comparing mixtures of N2 with either n-dodecane or isododecane. The VLE properties of these systems have different trends as a function of temperature, due to differences in hydrocarbon-hydrocabon and hydrocarbon-N2 interactions and how the relative importance of these interactions changes with temperature. We have also quantified the nature of the liquid-vapor interface and show that as temperature is increased, the interface becomes rougher.

Interfacial properties of binary mixtures of linear or branched hydrocarbons with N2: A molecular dynamics study

ACS Spring Meeting 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.