The Physics of Clusters in Real Gases

The physics of real gases is interesting both for science and practice. For scientists it is important to move forward from the ideal gas model and to include in the thermal physics of gases the characteristics of molecular interactions. The practitioners need in advanced models of real gases to predict with greater precision their practically valuable properties. The universal laws, such as the van der Waals or corresponding states laws cannot satisfy constantly growing requirements to precision of models for technological gases. The detailed individual characteristics of molecular interactions in gases should be taken into account. But the molecular interactions are shadowed by the thermal movement that complicates their determination. The paper demonstrates the power of the original computer aided analysis of precise experimental thermophysical data contained in modern databases, such as the NIST Thermophysical Properties for Fluid Systems Database. The analysis discovers the cluster structure of pure real gases and the structural transition between different isomers of clusters, provides knowledge of their bond parameters, and opens new features of molecular interactions in gases. The analysis is based on the new variable ? the monomer fraction density Dm, which develops further the fugacity concept. The series expansion coefficients of thermophysical values by powers of Dm reflect the cluster fractions’ properties and provide the knowledge of their equilibrium constants, bond energies and entropic factors of bound states. The found equilibrium constants for isomers of clusters, like equilibrium constants for chemical compounds, obey to the Boltzmann law. The developed method describes clearly the pair interactions in the dimer fraction of pure real gases and creates the basis to move step-by-step further to trimer and tetramer fractions. For larger numbers of particles in clusters there is a possibility to estimate some averaged characteristics of cluster fractions. This approach helps also to understand better the structure and properties of supercritical fluids.