Thermodynamic properties of binary alcohol-hydrocarbon systems

Abstract

Head-space gas chromatography has been used to determine the partial vapour pressures of the components of seven binary alcohol-hydrocarbon systems at 298.15 K. The chain lengths of the alcohols were chosen to record the differences between a long-chain alcohol (decan-1-ol) and intermediate homologues (pentan-1-ol and butan-1-ol). The hydrocarbon solvents chosen (n-octane, cyclohexane and benzene) offer the possibility to extract the contribution of ring closure and aromaticity. The accuracy of the activities were controlled applying a commonly used thermodynamical consistency test. The excess Gibbs free energies were combined with previously determined excess enthalpies to obtain the molar excess entropy of solution and the partial molar entropies of the components. The results suggest that the mixtures can be grouped into three concentration regions with significantly different properties. First, in the dilute alcohol solutions the hydrogenbonding equilibria determine the properties of the system. The influence of the apolar interaction also contributes significantly to the energetic state of the system. Secondly, in the mid-concentration range the hydrogen bonding seems to be of only secondary importance, while the apolar interaction is gaining enhanced importance. In this range the benzene-hydroxy group interaction seems to deviate drastically from the normal behaviour of alcohol-hydrocarbon systems. Thirdly, in dilute hydrocarbon systems the interaction seems to be purely apolar in nature, erasing the previous difference between the aromatic and saturated hydrocarbon systems.