APPLICATION OF THE ZEROTH APPROXIMATION OF THE DISQUAC MODEL TO CYCLOHEXANE PLUS N-ALKANE MIXTURES USING DIFFERENT COMBINATORIAL ENTROPY TERMS

Literature data on molar excess functions, Gibbs energy G(E), enthalpy H-E, and heat capacity C-p(E), on activity coefficients gamma(i)(infi nity), and partial molar excess enthalpies H-i(E,infinity), at infinit e dilution and on solid-liquid equilibria, SLE, of the cyclohexane + n -alkane mixtures are examined on the basis of the zeroth approximation of the DISQUAC group contribution model. The model provides a quite s atisfactory description of the thermodynamic properties for the mixtur es under study, although the symmetry of the calculated excess functio ns differs from the experimental one for systems containing long-chain n-alkanes. This may be due to the so-called Patterson effect. The inf luence of different combinatorial entropy terms (Flory-Huggins, Staver mann-Guggenheim or Kikic equations) on the prediction of thermodynamic properties such as G(E), ln gamma(i)(infinity) and SLE is also examin ed. H-E, C-p(E) or H-i(E,infinity) are represented by an interactional term only. The results calculated using the Flory-Huggins term are sl ightly better than those obtained applying the Stavermann-Guggenheim e quation. Results based on the Kikic expression are poorer than those g iven by Flory-Huggins, particularly at high concentration of cyclohexa ne in systems containing the longer n-alkanes. So, the Kikic equation leads to poorer results for ln gamma(2)(infinity) for these systems. S LE predictions are determined mainly by the physical constants of the pure compounds. So, essentially they do not depend on the combinatoria l term used. A comparison between the zeroth approximation of DISQUAC and the modified UNIFAC model (Lyngby version) is also presented. Such comparison shows that both methods lead to similar results; although the latter gives poorer predictions on the temperature dependence of t he excess functions than the former. On the other hand, the number of interaction parameters needed in modified UNIFAC is larger than when t he zeroth approximation of DISQUAC is applied and, more important, the y change with the number of carbon atoms of the n-alkane in a rather e rratic way for the first members of the series. This makes the predict ive task of UNIFAC more difficult.