A theoretical description of anisotropic chemical association and its application to hydrogen-bonded fluids.

摘要

The thermodynamic and structural effects of highly anisotropic, short-ranged attraction are investigated for single- and four-site interaction models using Wertheim's multi-density graph theory of chemical association. Both models consist of associating hard spheres, where the saturable attraction sites are described by conical wells centered in the hard core and evaluated in the “sticky-spot” limit. The resulting fluids then mimic many of the directional and steric-constrained properties of hydrogen-bonded fluids.; The single-site model is used to explore the effects of dimerization upon the well-known properties of a planar liquid-vapor interface. Apart from hard sphere repulsion and sticky-spot attraction, a van der Waals-like dispersion interaction is incorporated to generate the critical point. Association is treated within Wertheim's thermodynamic perturbation theory, along with classical density functional methods to determine the interfacial density profile. The direct correlation functions—which carry all bonding information—are derived by means of the associative Ornstein-Zernike equations with a Percus-Yevick-like closure relation. The primary effects of dimerization are manifest in system thermodynamics. Critical temperatures and densities are shifted from their non-associating values and small, non-monotonic shifts in the correlation length and surface tension are also observed. While these effects are accompanied by interface compositional changes, any influence upon the density profile seems to be subsumed by use of the proper T/T_c.; The four-site, network-forming model is investigated as a prototype for the thermodynamics and structural properties of water. Bonding interactions occur between “hydrogen” and electron “lone pair” sites described in the sticky-spot limit. System properties are derived under the ideal network approximation using the same methods as for the one-site model and are found to qualitatively reproduce some thermodynamic and connectivity features characteristic of real water. Partial densities are calculated self-consistently within the theory, and most thermodynamic quantities can be written in terms of the average number of hydrogen bonds per molecule. An analytical structure factor is also derived for this model.