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 hav e a Lennard-Jones (LJ) spherical core with various off-centre associat ion 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 ref erence fluid potential contains the association sites plus a soft repu lsive core. Predictions from the theory are compared with Monte Carlo computer simulation results. The liquid structure of the reference flu id 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 pseudom ixture theory, compared with perturbation theory, requires fewer compu ter simulation results for its implementation, and should be useful in developing a realistic theory for polyatomic associating fluids such as methanol and water.