The liquid structure and thermodynamic properties of Lennard-Jones spheres with association sites

The liquid structure and thermodynamic properties are investigated for simple models of associating fluids. All of the fluids considered have a Lennard-Jones (LJ) spherical core with various off-centre association sites. The second virial coefficient and bond length distribution are calculated. Wertheim's theory of associating fluids is extended to molecules with an LJ core by way of a perturbation expansion; the reference fluid potential contains the association sites plus a soft repulsive core. Predictions from the theory are compared with Monte Carlo computer simulation results. The liquid structure of the reference fluid is in good agreement with that of the fluid with the full potential (LJ plus association sites). In addition to the perturbation theory, a pseudomixture theory is developed in which a pure associating fluid is treated as a mixture of bonded and unbonded components. The pseudomixture theory, compared with perturbation theory, requires fewer computer simulation results for its implementation, and should be useful in developing a realistic theory for polyatomic associating fluids such as methanol and water.