Maria A. Accomando, Chemical and Biological Engineering, The University of Alabama, P. O. Box 860559, Tuscaloosa, AL 35486-0005 and Heath Turner, Chemical and Biological Engineering, University of Alabama, Box 870203, Tuscaloosa, AL 35487-0203.
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 primarily dealt with simple association behavior and isomerization reactions. We began this investigation by modeling simple dimerization reactions, as well as more complex multi-component reactions, using the reactive Monte Carlo simulation technique. We have shown 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 have been analyzed with respect to specific intermolecular interactions. In conjunction, we have calculated the surface tension of the reacting fluids at the interface, which is a valuable property 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 predicting the behavior of interfacial chemical reactions, in cases where the reactions involve significant charge separations. Examples of these types of systems are the dissociation equilibria of salts, acids, and bases at aqueous interfaces. In these simulations, the importance of the long-range interactions, as well as the issues associated with the inhomogeneity of the simulation cell, require careful treatment. Regardless of these challenges, we are able to provide a detailed molecular description of the reaction conversions, component profiles, and distribution functions across the interfaces. Our systems show unusual behavior at the interface, much different than that observed in the separate liquid and vapor regions. 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 aqueous aerosol chemistry.