Studies on Characteristic Properties of Supercritical Fluids from the View Point of Fluctuation

超临界流体具有分子分布高度不均匀的特点。密度涨落是定量和直接描述非均质性的合适参数。我们利用小角X射线散射(SAXS)实验对几种超临界流体的密度涨落进行了全面的研究，发现了以下事实。(I)当密度涨落的值在相图中绘制等值线图时，密度涨落形成山脊。(Ii)脊对应于各种超临界流体物理量的极值，这些物理量与吉布斯自由能的二阶导数有关。(Iii)溶解度的增加速率在脊上变得最大。也就是说，山脊将超临界区域分成两个溶解度极不相同的区域，(4)超临界流体中各种化学反应的速率常数或平衡常数在山脊上表现出奇异的行为。结果表明，密度涨落是决定超临界流体性质的最基本因素之一，密度涨落脊是将超临界流体一分为二的边界。

Supercritical fluids are characterized by large inhomogeneity of molecular distribution. The density fluctuation is a suitable parameter for quantitative and direct description of the inhomogeneity. We have comprehensively investigated the density fluctuation of several kinds of supercritical fluids by means of small-angle X-ray scattering (SAXS) experiments and have found the following facts. .(i)The density fluctuation forms a ridge when the contour map of their values is drawn in a phase diagram. (ii)The ridge corresponds to extrema of the various physical quantities of supercritical fluids, which are related to the second derivatives of the Gibbs free energy. (iii)The rate of increase in solubility becomes maximum on the ridge. Namely, the ridge separates the supercritical region into two, regions, in which the solubility is extremely different, (iv) The rate constants or equilibrium constants of various chemical reactions in supercritical fluids show singular behaviors on the ridge. It is concluded that the density fluctuation is one of the most fundamental factors which determine properties of supercritical fluids and its ridge is the boundary which separates, the supercritical region into two.