Thermochemistry of solvation versus vaporisation: Why is the enthalpy of solvation extremely additive?

Abstract

In the present work, the enthalpies of vaporisation and solvation were compared for various homologous series. It was found that the contribution of the methylene fragment into the enthalpy of solvation in n-heptane is a constant value (4.9 ± 0.1 kJ·mol−1). Simultaneously, the contributions of the methylene group into the enthalpy of vaporisation differ considerably from each other. For example, the contributions of the methylene groups for alkyl acetates and n-alkanes are 4.39 ± 0.04 kJ·mol−1 and 4.99 ± 0.02 kJ·mol−1, respectively. To find an explanation for this phenomenon, the dissolution processes of n-alkanes in homologous series of solvents were compared with the contributions of the methylene groups into vaporisation enthalpy. A relationship between the enthalpy of solution and the contribution of the methylene group into the enthalpy of vaporisation at 298.15 K was discovered. This quantitative relationship shows that the contribution of the methylene fragment into the vaporisation enthalpy is connected with the molecular volumes of n-alkane and solvents and the solution enthalpies of the n-alkane in studied homologous series of solvents.