No abstract

Using all-atom simulation of vapor deposition, we theoretically investigate how the molecular orientation depends on various factors such as the substrate temperature, molecular shape, and material composition.

Mass diffusion coefficients are critically related to the predictive capability of computational combustion modeling. To date, the most common approach used to determine the molecular transport of gases is the Boltzmann transport equation of the gas kinetic theory. The Chapman-Enskog (CE) solution of this transport equation, combined with Lennard-Jones potential parameters, suggests a simple analytical expression for computing self and mutual diffusion coefficients. This approach has been applied over a wide range of flame modeling conditions due to its minimal computational requirement, despite the fact that the theory was developed only for molecules that have a spherical structure. In this study, we computed the binary diffusion coefficients of linear alkanes using all-atom molecular dynamics simulations over the temperature range 500-1000 K. The effect of molecular configurations on diffusion coefficients was determined relating the radii of gyration of the molecules to their corresponding collision diameters. The comparison between diffusion coefficients determined with molecular dynamics and the values obtained from the CE theory shows significant discrepancies, especially for nonspherical molecules. This study reveals the inability of CE theory with spherical potentials to account for the effect of molecular shapes on diffusion coefficients.

Density functional theory calculations (B3LYP and BH&HLYP functionals) of the potential energy surface have been performed to investigate the mechanisms of decalin breakdown, and the Rice-Ramsperger-Kassel-Marcus and transition state theory methods have been used to compute the high-pressure limit thermal rate constants for the new reaction pathways. The new pathways connect decalin to five primary monoaromatic species: benzene, toluene, styrene, ethylbenzene, and xylene. The reactions used for the new routes are carbon-carbon bond cleavage reaction, dissociation reaction, and hydrogen abstraction and addition reactions. A kinetic analysis was performed for pyrolytic conditions, and benzene, toluene, and xylene were identified as major products.

The accurate knowledge of transport properties of pure and mixture fluids is essential for the design of various chemical and mechanical systems that include fluxes of mass, momentum, and energy. In this study we determine the mutual diffusion coefficients of mixtures composed of heptane isomers and nitrogen using molecular dynamics (MD) simulations with fully atomistic intermolecular potential parameters, in conjunction with the Green-Kubo formula. The computed results were compared with the values obtained using the Chapman-Enskog (C-E) equation with Lennard-Jones (LJ) potential parameters derived from the correlations of state values: MD simulations predict a maximum difference of 6% among isomers while the C-E equation presents that of 3% in the mutual diffusion coefficients in the temperature range 500-1000 K. The comparison of two approaches implies that the corresponding state principle can be applied to the models, which are only weakly affected by the anisotropy of the interaction potentials and the large uncertainty will be included in its application for complex polyatomic molecules. The MD simulations successfully address the pure effects of molecular structure among isomers on mutual diffusion coefficients by revealing that the differences of the total mutual diffusion coefficients for the six mixtures are caused mainly by heptane isomers. The cross interaction potential parameters, collision diameter σ(12), and potential energy well depth ɛ(12) of heptane isomers and nitrogen mixtures were also computed from the mutual diffusion coefficients.