A new molecular model for fluids - Part I. Pure substances

This model considers free space as an essential, explicit thermodynamic component of the fluid system. The hole theory is used to formulate this principle in statistical mechanical terms. The cell free volume is dependent on hole fraction between the limits of close-packed and fully expanded states of the fluid. Here, the critical hole fraction is reproduced and the van der Waals (VW) equation is obtained as an approximation. Intermolecular attractions are regarded as weak, short-range contact bonds with chemical nature. The bond energy is defined and determined in the close-packed fluid state (CPF). A suitable analytical function approximating the Lennard-jones Devonshire cell potential is employed and the constants of this function can be determined from experimental vapor pressure data. Non-random distribution is treated by the newly developed coordination state theory (CST). The CST is based on the distribution of molecules and holes among coordination states. The bonding configuration integral and thermodynamic functions are then expressed in terms of non-random factors. The CST is general and reduces to the quasi-chemical theory in the case of pure substances. The model predicts successfully thermodynamic properties of polar and non-polar substances and extension to multi-component systems is straight forward as indicated. (C) 2008 Elsevier B.V. All rights reserved.