Exothermic and endothermic chemical reactions involving very many particles modeled with molecular dynamics

The traditional continuum approach of modeling chemical reactions with specified kinetic rates suffers from numerical difficulties in reactive flows and other highly non-equilibrium situations due to the stiffness of the differential equations in both space and time. These drawbacks can be eliminated within the framework of the discrete-particle approach in which the chemical reactions are modeled by means of two-body interparticle potentials with classical molecular dynamics. Here we present a simple model for prescribing the binary endothermic and exothermic reactions of the type A+B→C, in the presence of many (greater than 10⁴) reacting particles. Two-body Lennard-Jones based potentials, V_ij, have been utilized, in which the reaction takes place via an attractive potential V_AB with a deep enough potential well. The other potentials V_AC and V_BC have deeper wells than V_AB for creating a suitable situation for inducing exothermic reactions, while a high-energy plateau for V_AC and V_BC is used for simulating endothermic reactions. We have deployed 20 000 to 100 000 particles in a computational domain with an area of around 1 million Ų. These ensembles have all been integrated out to around the order of nanoseconds. We have examined the bimolecular reactions between two layers initially consisting of A particles lying atop B particles, equally divided in the total population. Reactions take place first along this boundary. There is a nonlinear threshold phenomenon in the exothermic reactions, which are greatly accelerated by the heat liberated from the neighboring reactions. In contrast, the endothermic reactions are more subdued, as there is no positive feedback. Exothermic reactions occur in a spatially heterogeneous manner, especially at the lower temperatures, in which no single reaction rate can be assigned to the entire reaction front. More coherent spatial structures are found with decreasing temperature. These spatial structures can influence the overall flow field associated with the motions of the reacting particles. Timescales of the reactions are greatly lengthened by a reduction in the temperature in a threshold manner.