Mass Transfer and Fluctuations in the Root-Mean-Square Density of Molecules around the Critical Point

The authors calculate the concentration dependence of the coefficient of mass transfer and mean-root-square fluctuations of attracting molecules under sub- and supercritical conditions for different approximations of cluster variation (CV) that allow for indirect correlations. The simplest lattice structure is considered: a uniform flat face (100) that provides an exact solution to the problem of multiple bodies. A comparison is given of characteristics calculated for a series of simplest CVM basis clusters (2 × n, n = 2–5, 3 × 3, 3 × 4) and a quasi-chemical approximation (cluster 2 × 1) that reflects only the effects of direct correlations. The coefficient of mass transfer is calculated within the theory of the absolute reactions rates of nonideal reaction systems. The effects the dependence of the motion of molecules has upon attraction from the side of neighboring molecules that block an available volume for movements on root-mean-square fluctuations are discussed. It is shown that a region near (above and below) the critical point can be selected in which there is an abrupt drop in the coefficient of diffusion because of large fluctuations of the material’s density. The increased accuracy of indirect correlations expands the area of thermodynamic parameters responsible for hindering mass transfer. The concept of transcritical first-order phase transitions and its relationship to the effects of kinetic resistivity to mass transfer are discussed.