INTEGRAL-EQUATION THEORY FOR THE 4 BONDING SITES MODEL OF ASSOCIATINGFLUIDS .1. STRUCTURE FACTOR AND COMPRESSIBILITY

The Wertheim integral equation theory for associating fluids is reform ulated for the study of associating hard spheres with four bonding sit es. The association interaction is described as a square well saturabl e attraction between these sites. The associative version of the Ornst ein-Zernike integral equation is supplemented by the Percus-Yevick-lik e closure relation and solved analytically within an ideal network app roximation, in which the network is the result of the crossing of idea l polymer chains. The structure factor S(k) is obtained for both symme trical network and polymer chain cases. It is shown that S(k) exhibits a peculiarity (a so-called pre-peak) at small wavenumbers, connected with the formation of relatively large molecular aggregates due to hig hly directional saturable bonds. The magnitude and location of the pre -peak as functions of density eta and association K-s are analysed. Ba sed on the analysis of the S(k = 0) limit, the behaviour of the isothe rmal compressibility chi(T) is studied and the gas-liquid critical poi nt is predicted to exist. The result for the spinodal curve is also re ported.