Structural difference between liquidlike and gaslike phases in supercritical fluid.
Structural difference between liquidlike and gaslike phases in supercritical fluid.
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DOI：
10.1103/physreve.78.051503
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发表时间：
2008-11
期刊：
影响因子：
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通讯作者：
Takashi Sato;M. Sugiyama;K. Itoh;K. Mori;T. Fukunaga;M. Misawa;T. Otomo;Shin‐ichi Takata
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文献类型：
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作者：
Takashi Sato;M. Sugiyama;K. Itoh;K. Mori;T. Fukunaga;M. Misawa;T. Otomo;Shin‐ichi Takata
用小角中子散射(SANS)观测了超临界二氧化碳沿等温线和等容线的密度涨落结构。低Q区的所有散射强度均可用Ornstein-Zernike(OZ)方程很好地描述。结果表明，在等温线上存在OZ关联长度和Q=0时散射强度极值的轨迹：该轨迹被解释为超临界状态划分为类液体和类气态的边界。为了弄清类液体和类气相的涨落结构的差异，用反蒙特卡罗方法得到了分子的实空间分布。计算了CO2分子在所有测量状态下的数密度分布，并得到了真实的分子空间分布。此外，还考察了数密度分布、标准差和偏斜度的统计参数。数密度分布的标准差与OZ分析结果基本一致。另一方面，描述数密度分布不对称性的偏斜度清楚地显示出两个相之间的不同：在所有等温线中，偏斜度在液体相中变为负值，在气态相中变为正值，并且在离脊线最近的状态处几乎为零。用简单的统计力学方程证明了偏斜度可描述为密度涨落幅度对压力的一阶微分。我们认为，SANS数据的RMC分析得到的偏斜度是区分类液体和类气体的重要结构参数。
Density fluctuation structures of supercritical carbon dioxide along the isothermal and isochoric lines were observed with small-angle neutron scattering (SANS). All the scattering intensities in the low-Q range were well described with the Ornstein-Zernike (OZ) equation. It was confirmed that there exists a locus where the OZ correlation length and scattering intensity at Q=0 exhibit extrema on the isothermal lines: this locus, named the ridge, was interpreted as the boundary by which the supercritical state is divided into liquidlike and gaslike phases. In order to clarify the difference of the fluctuation structure between the liquidlike and the gaslike phases, a real-space molecular distribution was obtained with a reverse Monte Carlo (RMC) method. Number density distributions of CO2 molecules at all measured states were calculated with the real-space molecular distributions obtained. In addition, the statistical parameters of the number density distributions, the standard deviations, and the skewnesses, were examined. The standard deviations of the number density distributions almost coincide with the results of the OZ analysis. On the other hand, the skewnesses, which describe the asymmetric nature of the number density distribution, clearly showed a difference between the two phases: the skewness became negative in the liquidlike phase, positive in the gaslike phase, and almost zero at the nearest state to the ridge in all isotherms. It was proved with simple equations of statistical mechanics that the skewness is described as the first differential of the magnitude of the density fluctuation with respect to the pressure. We conclude that the skewness, obtained with a RMC analysis for SANS data, is an important structural parameter distinguishing between the liquidlike and gaslike phases.