Static and dynamic cross correlation in thermodynamically stable and unstable mixtures

A reactive force field molecular dynamics study of molecular nitrogen and water mixtures under high temperature and high pressure

Establishment of an equation of state (EOS) for detonation products is challenging through molecular modeling approaches. Molecular dynamics (MD) simulations with the reactive force fields (ReaxFF) were carried out in this work for molecular N₂ and H₂O and their mixtures, which are key components of detonation products, with high density and under high temperature, and compared with the experiment-based Becker-Kistiakowsky-Wilson (BKW) and the Lennard-Jones potential-based Tsien-Zhao (TZ) equations. Phase separations were noted for the mixtures, enhanced for denser systems under low temperature, and detracted for sparse systems under high temperature. ReaxFF-based MD simulations reproduce the BKW predictions for the systems with a wide range of densities, while the TZ predictions deviate at relatively low density. The deviations among the MD, BKW, and TZ results reduce with increasing temperature and decreasing density. Our calculations revealed the significant influence of phase separation on the predicted pressures, and verified that the MD/ReaxFF approach is potentially a feasible way to establish the EOS for detonation products.

Mutual diffusion in a binary isotopic mixture

The mass dependence of the mutual diffusion coefficient, in a binary equimolar mixture of Lennard-Jones fluids, is studied within Mori's memory function formalism. A phenomenological form of the memory function is used to study the time evolution of the self- and relative velocity correlation functions. The diffusion coefficients are calculated from the relevant velocity correlation functions using the Green-Kubo integral formula. Like the self-diffusion coefficient, the mutual diffusion coefficient is also found to be weakly dependent on the mass ratio. The present study shows that the minimum value that the mutual diffusion coefficient in an equimolar mixture of isotopic fluids can have is √(1/2) times the self-diffusion coefficient of any of the species when in isolation. Further, the contribution of the dynamic/distinct cross correlations to the mutual diffusion coefficient is found to be small and positive for the whole range of the mass ratio which is consistent with earlier molecular dynamics results.

Computational study of structural and dynamical properties of formamide-water mixtures

A molecular dynamics simulation study of structural and dynamical properties in liquid mixtures of formamide and water is presented. Site-site radial pair distribution functions, local mole fractions, pair energy distributions, and tetrahedral orientational order are the quantities analyzed to investigate the local structure in the simulated mixtures, along with a review of the intermolecular structure in terms of the distribution of hydrogen bonds. Our results indicate that there is a substitution of formamide molecules by water in the hydrogen bonds and a formation of a common hydrogen bond network. By analyzing the extent of tetrahedral order in the liquid as a function of composition, it is observed that whereas the tetrahedral network of liquid water is progressively lost by increasing the formamide concentration, the water structure within the first coordination shell is preserved and somewhat enhanced. The hydrogen-bond mean lifetimes were estimated by performing a time integration of the autocorrelation functions of bond occupation numbers. The lifetimes associated with hydrogen bonds between water, formamide, and interspecies pairs are found to increase with increasing formamide concentration. The lifetimes of the water hydrogen bonds show the largest variations, supporting the picture of an enhancement of the water structure among the nearest neighbors within the first coordination shell. We have used two different force field models for water, SPC/E [J. C. Berendsen et al., J. Phys. Chem. 91, 6269 (1987)] and TIP4P/2005 [J. L. F. Abascal and C. Vega, J. Chem. Phys. 123, 234505 (2005)]. Our results for structural and dynamical properties yield very small differences between those models, the TIP4P/2005 predicting a slightly more structured liquid and, consequently, exhibiting a slightly slower translational and librational dynamics.

Phase separation in H2O:N2 mixture: Molecular dynamics simulations using atomistic force fields

A class II atomistic force field with Lennard-Jones 6-9 nonbond interactions is used to investigate equations of state (EOS) for important high explosive detonation products N(2) and H(2)O in the temperature range of 700-2500 K and pressure range of 0.1-10 GPa. A standard sixth order parameter-mixing scheme is then employed to study a 2:1 (molar) H(2)O:N(2) mixture, to investigate, in particular, the possibility of phase separation under detonation conditions. The simulations demonstrate several important results, including (i) the accuracy of computed EOS for both N(2) and H(2)O over the entire range of temperature and pressure considered, (ii) accurate mixing-demixing phase boundary as compared to experimental data, and (iii) the departure of mixing free energy from that predicted by ideal mixing law. The results provide comparison and guidance to state-of-the-art chemical kinetic models.

An apparent critical point in binary mixtures: Experimental and simulation study

We report the experimental and simulation studies for the system of nitrobenzene-cyclododecane, showing an apparent critical point, which lies in their metastable, experimentally inaccessible state, below their melting point, affecting physical and chemical properties of this system in the stable liquid phase. The nonlinear dielectric effect (NDE) was measured in the mixture of nitrobenzene with cyclododecane. The mixture has been found to show an apparent critical point which lies below the melting point, manifested as anomalous NDE behavior in the vicinity of the critical concentrations in the stable liquid phase. The melting temperature of this system was estimated using the differential scanning calorimetry method. For such a system, we also performed Monte Carlo (MC) simulations that aimed to analyze the kinds of phase transitions observed and the conditions of their occurrence in Lennard-Jones mixture. The enthalpy, configurational energy, and radial distribution function have been estimated by the MC simulation method in the N-P-T system. Immiscibility conditions according to the approach by Schoen and Hoheisel [Mol. Phys. 57, 65 (1986)] are also discussed.