Associative Model of a Fluid and Its Thermodynamics

摘要

An associative model of a vapor-liquid system is created for a self-consistent description of thermodynamic functions of three-aggregate systems. The model permits consideration of the bound states of molecules and their vibrational motion (the main form of thermal motion in solid phases). It simultaneously allows for the volume excluded by neighbors that is available for the motion of a fluid molecule and its vibrational motion in harmonic and anharmonic approximations, offering a unified way of calculating thermodynamic functions in different aggregate states. Equations of the associative model of a vapor-liquid system are derived in terms of correlation functions (CFs) for a discrete-continuous version of the lattice gas model in the cluster approach and the Bogolyubov-Born-Green-Kirkwood-Yvon (BBGKY) hierarchy for vapor-liquid systems. The cluster approach is an alternative way of deriving CFs that differs from deriving the BBGKY hierarchy and allows us to consider the bound states of molecules excluded in both the Ursell-Mayer theory of non-ideal gases and the BBGKY hierarchy. In both cases, the use of CFs prevents direct consideration of the statistical sum of system Q, leading to ambiguity in calculating thermodynamic functions of a vapor-liquid system. The need to consider bound states and fluid molecules is discussed, along with the relationship between the new theory, the Ursell-Mayer theory of non-ideal gases, and the BBGKY hierarchy. Ways of calculating the thermodynamic functions of a vapor-liquid system are considered.