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Flexible models of the radical and water molecules including short-range interaction of hydrogen atoms have been employed in molecular dynamic simulation to understand mechanism of •OH hydration in aqueous systems of technological importance. A key role of H-bond connectivity patterns of water molecules has been identified. The behavior of •OHaq strongly depends on water density and correlates with topological changes in the hydrogen-bonded structure of water driven by thermodynamic conditions. Liquid and supercritical water above the critical density exhibit the radical localization in cavities existing in the solvent structure. A change of mechanism has been found at supercritical conditions below the critical density. Instead of cavity localization, we have identified accumulation of water molecules around •OH associated with the formation of a strong H-donor bond and diminution of non-homogeneity in the solvent structure. For all the systems investigated, the computed hydration number and the internal energy of hydration ΔhU showed approximately linear decrease with decreasing density of the solvent but a degree of radical-water hydrogen bonding exhibited non-monotonic dependence on density. The increase in the number of radical-water H-acceptor bonds is associated with diminution of extended nets of four-bonded water molecules in compressed solution at ∼473 K. Up to 473 K, the isobaric heat of hydration in compressed liquid water remains constant and equal to -40 ± 1 kJ mol-1. [ABSTRACT FROM AUTHOR]

Assembling of water molecules via hydrogen bonding has been studied by molecular dynamics simulations using flexible potential model. The relationship between the number of H-bonds per molecule, nHB, the size of H-bonded nets, k, and the size of patches of four-bonded molecules, k4, has been examined for several thermodynamic states of water ranging from ambient to supercritical conditions. Two kinds of structural inhomogeneity have been found: the patchlike associated with the mean [formula]> 2.0 and the clusterlike for [formula]< 1.9. In compressed water up to ∼473 K patches coexist with less ordered nets, both constituting the gel-like H-bonded network. The size of patches steeply decreases with the increasing temperature and the decreasing density of water. The inhomogeneity resulting from the presence of patches disappears above 473 K. This feature is associated with the rapid increase in the fraction of unbound molecules and with the breakage of the gel-like network into a variety of H-bonded clusters leading to the clusterlike structural inhomogeneity. In contrast to the patchlike inhomogeneity an increase in temperature and a decrease in density make this kind of inhomogeneity more pronounced. A degree of connectivity of H-bonds has been characterized by a parameter Pg defined as the total fraction of molecules belonging to the H-bonded clusters of size k ≥ 5. The simulation-derived values of Pg agree well with the predictions of the random bond theory giving the explicit expression for Pg as a function of the mean nHB. Going from ambient to supercritical conditions, we have found that the patchlike inhomogeneity is connected with the very slight reduction in Pg, whereas the clusterlike inhomogeneity generates a steep linear decrease of Pg with the decreasing mean nHB. The self-diffusion coefficient calculated for the thermodynamic states of water showing the clusterlike inhomogeneity has occurred to be inversely proportional to the density. We have also found that the clusterlike inhomogeneity is associated with the linear correlation between Pg and the macroscopic properties of water: the static dielectric constant, the viscosity, and the density. The provided relationships allow one to estimate the degree of connectivity of hydrogen bonds from the measured macroscopic quantities. [ABSTRACT FROM AUTHOR]

Ionic polyimides (i-PI) are a new class of polymer materials that are very promising for CO 2 capture membranes, and recent experimental studies have demonstrated their enhanced separation performance with the addition of imidazolium-based ionic liquids (ILs). However, there is very little known about the molecular-level interactions in these systems, which give rise to interesting gas adsorption and diffusion characteristics. In this study, we use a combination of Monte Carlo and molecular dynamics simulations to analyze the equilibrium and transport properties of CO 2 molecules in the i-PI and i-PI + IL composite materials. The addition of several different common ILs are modeled, which have a plasticization effect on the i-PI, lowering the glass transition temperature ( T g ). The solubility of CO 2 strongly correlates with the T g , but the diffusion demonstrates more unpredictable behavior. At low concentrations, the IL has a blocking effect, leading to reduced diffusion rates. However, as the IL surpasses a threshold value, the relationship is inverted and the IL has a facilitating effect on the gas transport. This behavior is attributed to the simultaneous contributions of the increased i-PI plasticization at higher IL concentrations (facilitating gas hopping rates from cavity-to-cavity) and the increased IL continuity throughout the system, enabling more favorable transport pathways for CO 2 diffusion.