Thermodynamics of bicyclic compound + n-alkane mixtures

Abstract

Mixtures formed by a cyclic molecule A, namely cyclohexane, or benzene, or bicyclohexyl ((1,1′-bi(cyclohexane)), or cis-decalin (cis-bicyclo[4.4.0]decane), or trans-decalin (trans-bicyclo[4.4.0]decane), or cyclohexylbenzene, or tetralin (1,2,3,4-tetrahydronaphthalene) and a n-alkane have been investigated by means of excess molar functions, enthalpies (HmE) and volumes (VmE), available in the literature, and of isochoric excess properties determined in this work, internal energies (UVmE) and heat capacities (CVmE). The behaviour of the systems with respect to the GmE(excess molar Gibbs energy) vs. HmE is discussed. The mixtures have been also studied using the zeroth approximation of DISQUAC and the Flory model. Dispersive interactions are dominant in the considered systems. The HmE values increase with n, the number of C atoms in the n-alkane, along each homologous series. Mixtures containing shorter n-alkanes and a bicyclic compound are characterized by large negative VmE values, which remark the existence of large structural effects. It is shown that longer n-alkanes are poorer breakers of the A-A interactions, which leads to decreased values of UVmE when n increases in systems with a given cyclic compound. The disruption of the short orientational order of longer n-alkanes by the A molecules lead to increased values ofUVmE. This effect is predominant in systems with cyclohexane, or benzene from n = 8, and UVmE decreases up to this alkane and then increases. However, the mentioned effect seems to be only predominant in systems with a bicyclic compound for n = 16. Thus, UVmE decreases up to n = 12 and increases for n = 16. The different behaviour of the treated systems is described by the Flory model through a X12 parameter which increases with n in solutions with cyclohexane or benzene, and that changes similarly to UVmE in the remainder solutions. In terms of DISQUAC, the enthalpic interaction parameter for the aromatic/aliphatic contact is independent of the n-alkane in systems with tetralin. For solutions with bicyclohexyl or cis-decalin, the variation with n of the mentioned parameter for the aliphatic/cyclic contact is similar to that observed for cyclohexane systems.