Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Institute of Theoretical and Computational Chemistry, Nanjing University, Nanjing, China.
J Chem Phys. 2011 Aug 7;135(5):054504. doi: 10.1063/1.3620404.
Molecular dynamics simulation have been performed with a wide range of densities along a near critical isotherm of supercritical water (SCW) in order to study the density dependence of the structure order and hydrogen bonding (HB). It is revealed that the translational structure order is nearly invariant while the orientational tetrahedral structure order is very sensitive to the bulk density under supercritical conditions. Meanwhile, some energetically unfavorable intermediate water dimer structures are found to appear under supercritical conditions due to the reduced energy difference and the enhanced energy fluctuation. As a consequence, a general geometrical criterion or the inclusion of a energy-based criterion instead of currently widely adopted pure r(OH)-based geometric criterion is suggested to be used in the HB statistics under supercritical conditions. It is found that the average HB number per H(2)O molecule (n(HB)) reduces with the decreasing SCW bulk density although a given pair of H(2)O molecules are shown to have a stronger ability to form a hydrogen bond under lower SCW bulk densities. Accordingly, the orientational tetrahedral structure order q decreases with the reducing bulk density under supercritical conditions. However, when the fluid is dilute with ρ ≤ 0.19ρ(c) (ρ(c) = 0.322 g/cm(3)), the energy fluctuation increases sharply and the short-range order is destroyed, signifying the supercritical fluid (SCF)-gas state transition. Accordingly, the orientational tetrahedral structure order q gets reversal around ρ = 0.19ρ(c) and approaches zero under very dilute conditions. The sensitivity of the orientational order to the density implies the microscopic origin of the significant dependence of SCF's physicochemical properties on the pressure.
已针对超临界水（SCW）的近临界等温线进行了广泛密度范围内的分子动力学模拟，以研究结构有序性和氢键（HB）对密度的依赖性。结果表明，在超临界条件下，平移结构有序性几乎不变，而取向四面体结构有序性对体密度非常敏感。同时，由于能量差减小和能量波动增强，发现一些能量不利的中间水二聚体结构在超临界条件下出现。因此，建议在超临界条件下的 HB 统计中使用通用的几何标准或包含能量的标准，而不是目前广泛采用的基于纯 r(OH)的几何标准。结果发现，尽管给定的 H(2)O 分子对在较低的 SCW 体密度下具有更强的形成氢键的能力，但每个 H(2)O 分子的平均氢键数（n(HB)）随 SCW 体密度的降低而减少。因此，在超临界条件下，取向四面体结构有序性 q 随体密度的降低而降低。然而，当流体稀ρ ≤ 0.19ρ(c)（ρ(c) = 0.322 g/cm(3)）时，能量波动急剧增加，短程有序被破坏，标志着超临界流体（SCF）-气体状态转变。因此，在非常稀的条件下，取向有序性 q 在 ρ = 0.19ρ(c) 处发生反转并趋近于零。取向有序对密度的敏感性意味着 SCF 物理化学性质对压力的显著依赖性的微观起源。
