Development of Chemical Kinetics Models from Atomistic Reactive Molecular Dynamics Simulations: Application to Iso-octane Combustion and Rubber Ablative Degradation.

Modeling the complex chemical phenomena resulting from multiple active species and long-chain polymers is limited by uncertainties in the reaction rate parameters, which increase rapidly with the number of active species and/or reaction pathways. Reactive molecular dynamics simulations have the potential to help obtain in-depth information on the chemical reactions that occur between the polymer (e.g., ablative material) and the multiple active species in an aggressive environment. In this work, we demonstrate that molecular dynamics (MD) simulations using the ReaxFF interatomic potential can be used to obtain the reaction kinetics of complex reaction pathways at high temperatures. We report two recently developed tools, namely, MolfrACT and KinACT, designed to extract chemical kinetic pathways by postprocessing reactive MD simulation data. The pathway extraction is based on a new algorithm, Consistent Reaction Stoichiometry via an Iterative Scheme (CReSIS), for the automated extraction of reactions and kinetics from MD trajectories. As a validation of the methodology, we first report the kinetic analysis and mechanisms for the high-temperature combustion of iso-octane. The observed reaction pathways are consistent with experimental models. In addition, we compare the activation energies of select iso-octane combustion pathways with experimental data and show that nanosecond timescale molecular dynamics simulations are sufficient to obtain realistic estimates of activation energies for different fuel consumption reaction pathways at high temperatures. The framework developed here can potentially be combined with time-series forecasting and machine learning methods to further reduce the computational complexity of transient molecular dynamics simulations, yet yielding realistic chemical kinetics information. Subsequently, the CReSIS scheme applied to ethylene-propylene-diene-monomer (EPDM) rubber ablative reveals that H2O, C2H4, and HCHO are the major products during the initial stages of the polymer degradation in high-temperature oxidative environments. While prior work involving ReaxFF simulations is restricted to overall rates of formation of any species, we extract kinetic information for individual reaction pathways. In this paper, we present several reaction pathways observed during the EPDM rubber degradation into the dominant products and report the pathway-specific reaction rates. Arrhenius analysis reveals that the dominant reaction pathway activation energies for the formation of water, ethylene, and formaldehyde are 34.42, 27.26, and 6.37 kcal/mol, respectively. In contrast, the activation energies for the overall formation (across all reaction pathways) of these products are in the 40-50 kcal/mol range.

The electroneutrality constraint in nonlocal models.

We develop a nonlocal Nernst-Planck model for reaction and diffusion in multicomponent ionic systems. We apply the model to the one-dimensional liquid junction problem, in which two electrolytic solutions of different ionic concentrations are brought into contact via a permeable membrane. Transport of ions through the membrane induces an electric field which is modeled using two separate nonlocal conditions: charge conservation and Gauss' law. We investigate how well they satisfy the criterion of strict electroneutrality which stipulates that the net charge at each point in the domain is zero, by considering four different initial scenarios. Charge conservation and Gauss' law yield similar results for most practical scenarios in which the initial condition satisfies strict electroneutrality. However, Gauss' law has two important advantages over charge conservation: (i) it is numerically more stable and can be applied even when the concentration of all the charged species drops to zero and (ii) computationally, it is significantly cheaper. Further, this study provides insights on the prescription of electroneutrality conditions necessary to handle the physics of evolving charges in nonlocal peridynamic models that are aimed at modeling nonlocal reaction-diffusion or corrosion-type processes.

Modeling high-temperature diffusion of gases in micro and mesoporous amorphous carbon.

In this work, we study diffusion of gases in porous amorphous carbon at high temperatures using equilibrium molecular dynamics simulations. Microporous and mesoporous carbon structures are computationally generated using liquid quench method and reactive force fields. Motivated by the need to understand high temperature diffusivity of light weight gases like H2, O2, H2O, and CO in amorphous carbon, we investigate the diffusion behavior as function of two important parameters: (a) the pore size and (b) the concentration of diffusing gases. The effect of pore size on diffusion is studied by employing multiple realizations of the amorphous carbon structures in microporous and mesoporous regimes, corresponding to densities of 1 g/cm(3) and 0.5 g/cm(3), respectively. A detailed analysis of the effect of gas concentration on diffusion in the context of these two porosity regimes is presented. For the microporous structure, we observe that predominantly, a high diffusivity results when the structure is highly anisotropic and contains wide channels between the pores. On the other hand, when the structure is highly homogeneous, significant molecule-wall scattering leads to a nearly concentration-independent behavior of diffusion (reminiscent of Knudsen diffusion). The mesoporous regime is similar in behavior to the highly diffusive microporous carbon case in that diffusion at high concentration is governed by gas-gas collisions (reminiscent of Fickian diffusion), which transitions to a Knudsen-like diffusion at lower concentration.