Improvement of the ReaxFF Description for Functionalized Hydrocarbon/Water Weak Interactions in the Condensed Phase

The ReaxFF protein reactive force field (protein-2013) has been successfully employed to simulate the biomolecules and membrane fuel cells, but it inaccurately describes the weak interaction of functionalized hydrocarbon/water molecules in condensed phase, especially for the density. In this article, the development of a ReaxFF force field (CHON-2017_weak) on the basis of protein-2013 is presented that improves the weak interaction description for atom pairs of carbon, hydrogen, oxygen, and nitrogen. To examine the quality of the force field, we performed a series of molecular dynamics simulations with model systems. These simulations, describing density trends for pure and mixture compound systems, demonstrate that CHON-2017_weak force field predictions are in good agreement with experimental data. Furthermore, ReaxFF can also describe the phase separation in hexane-water mixture and dissolution of ethanol or tetramethylammonium (TMA) in liquid water. To validate it in the application of membrane fuel cells, we studied structural property and degradation mechanism of TMA in alkaline aqueous solution, as well as some typical chemical reactions for small compounds. On the basis of our results, an additional reaction pathway is proposed for the degradation of TMA, which seems to be more energetically favorable compared to the main mechanism predicted from quantum mechanics calculations.