(291g) Monte Carlo Simulation of Chemical Reaction Equilibria at Modified Vapor-Liquid Interfaces

Chemical reactions are known to behave differently, depending upon their local environment. While the interactions with neighboring molecules may alter both the kinetics of chemical reactions and the overall equilibrium conversion, we have performed simulations of the latter. The particular environment that we address is the vapor-liquid interface, since only a few, limited studies have explored the influence of an interface on equilibrium reaction behavior. Previous investigations of reaction equilibrium at interfaces have only dealt with simple association behavior and isomerization reactions. In this work, simple dimerization reactions are modeled, as well as more complex multi-component reactions, using the reactive Monte Carlo simulation technique. We find that the conversion of a reaction can be markedly different at an interface as compared to the bulk vapor and liquid phases, and these trends are analyzed with respect to specific intermolecular interactions. In conjunction, we calculate the surface tension of the reacting fluids at the interface, which is valuable for understanding a broad range of important chemical and biological systems. We have simulated a variety of reactions, beginning with a simple Lennard-Jones model, which is used to understand the effect of the interaction parameters on both conversion and surface tension across a vapor-liquid interface. We have also tested a more realistic dimerization reaction, with parameters chosen to model nitric oxide dimerization: NO + NO = (NO)2. Finally, we have modeled more complex reaction behavior by simulating the equilibrium of Br2 + Cl2 = 2BrCl. In all three studies, interfacial composition profiles, surface tension measurements, and reaction conversions have been analyzed. We have found that the A/B equilibrium, NO dimerization, and BrCl reaction equilibrium all seem to follow similar trends at their vapor-liquid interfaces, when analyzed with respect to their intermolecular potentials. As a consequence, the reaction equilibrium tends to shift in the direction that minimizes the surface tension, depending on the intermolecular parameters of a particular system. In tandem, as the surface tension increases, the magnitudes of the equilibrium shifts are more dramatic. Our most recent efforts have focused on interfacial modifications that may potentially be used to tune the reaction conversion at the interface. In particular, we have focused on the identification of surfactant molecules, capable of significantly altering the interfacial reaction equilibrium. Beginning with the LJ dimerization as our model reaction, we have identified effective surfactant species that, even at very low concentrations, are capable of altering the reaction conversion at the interface. The accumulation of the surfactants at the vapor-liquid interface has a two-fold effect on the reaction ? a modification of the interfacial tension and an excluded volume effect. Both phenomena contribute to the overall shift in the reaction equilibrium. We believe our work will improve our understanding of reacting systems, where the interface comprises a significant fraction of the total system, such as in atmospheric aerosol chemistry.