EXCESS VOLUME OF MIXING AND EQUATION OF STATE THEORIES

Equation-of state theories of Flory and of Sanchez and Lacombe describ e both enthalpy and volume of mixing of binary systems using single co mponent properties and only one binary parameter X12. We have evaluate d this parameter from literature enthalpy data for numerous mixtures o f two aromatic hydrocarbons, of alkanes with aromatic compounds, and o f alkanes with carbonyl compounds. We have used this X12 for calculati on of excess volumes and compared the results with our previously meas ured experimental data. The agreement was fair for mixtures of two non polar components. Nevertheless, mixtures containing either cyclohexane or benzene displayed anomalies that could be traced to special packin g of molecules in these compounds when pure. For mixtures of carbonyl compounds with alkanes, the theories predicted the qualitative trends correctly, but the quantitative agreement was rather poor. These resul ts tend to support a model in which the enthalpy(cohesive energy) is i nversely proportional to volume (as in the theories considered) only f or dispersive interaction. When polar-polar interactions are involved, the dependence of excess volume on the excess enthalpy is much weaker .