Equilibrium structure of dense gases

Abstract

The computer aided analysis of precise thermophysical data for pure real gases and denser fluids is developed here to provide a vision of their equilibrium molecular structure [1, 2]. The structure of fluids means the structural features of disordered systems. It differs from the regular structures of molecules or solids [3] and reflects the structural features of the temporarily existing local molecular complexes, such as clusters and pores of different dimensions. Pure fluids are actively used in modern chemical technologies as initial and intermediate substances and final products. For further improvement of technological processes with pure fluids it is important to know better their molecular interaction mechanisms and parameters. The purity of gases and denser fluids by itself presents a great scientific value: it opens possibility to discover the details of the molecular interactions and the cluster structure by the computer aided processing of precise thermophysical data for pure substances. The distribution of intermolecular distances in fluids is not uniform. The diminishing of the fluid density from its maximal value does not mean the uniform expansion of all intermolecular distances. Some of distances stay the same as in a dense liquid, but others become noticeably enlarged. In real gases some part of molecules forms the short distance complexes, named clusters. The clusters in real gases now attract a large attention of researches [48]. The Water vapor clusters are responsible for clouds formation in upper layers of atmosphere [9]. The factors complicating determination of the molecular interaction mechanisms [1]: Densely spaced and disarranged by thermal movement bound states in clusters; The plurality of the cluster isomer configurations, growing with a number of particles in a cluster; The difference between the thermodynamically averaged energetic level of the bound state and the minimal potential energy of a cluster; Long relaxation times for tightly bound cluster isomers preventing from a fast reaching the equilibrium and thus distorting the experimental data at low temperatures. There are various approaches to molecular interaction mechanisms. The earlier models of fluids described in [10, 11], such as the famous van der Waals equation of state or the corresponding states law, have been based on the experimental data taken with not very high precision. They provided only a general picture of fluids sacrificing very important details of their molecular interactions. Now, the individual characteristics of molecular interactions in substances are being actively studied asing in the precise experimental data [12]. The constantly growing precision of thermophysical data opens new ways to penetrate deeper into details of the fluids’ structure. But this task is not very easy. It may be referred to the class of reverse mathematical problems, the results of which widely diverge at slight changes of initial data. Therefore, we should use the methods of data processing capable to limit the level of this divergence by both the regularization of the initial data and selecting the appropriate set of the molecular interaction parameters. This set should be adjusted to real features of the molecular interactions in gases instead of the purely mathematical parameterization of experimental data. An alternative approach to the molecular interactions in fluids is based on the simulations of the many particle systems behavior with different model potentials [4-9, 13]. The problem of this approach lies in rather arbitrary assumptions about the model potentials describing interactions between molecules in fluids. I.G. Kaplan [13] used more than 50 model potentials in his investigations. He came to very important conclusions for the dispersion forces temperature dependence. The optimal way to the physically correct picture of fluids may be in the mutually correlated utilization of both approaches: from the experimental data and from the model potentials. These approaches should converge in the molecular interaction parameters to be found. That may serve as the criterion of the most appropriate model potential selection. This work deals with the main thermodynamics principles extension to real gases and denser fluids and develops the methods of the molecular interaction parameters extraction from the experimental data. It is aimed at the cluster fractions’ characteristics near the saturation Dsat and even critical Dcr densities of pure gases basing on precise data from [15].