Fluorination effect on the thermodynamic properties of long-chain hydrocarbons and alcohols

Abstract

Structural and thermodynamic insights are reported for the series of fluorinated alcohols {oFTOH; CF3(CF2)nCH2OH, with n=5–12}, with the objective of comparing the phase behaviour of perfluorinated compounds with the analogous linear alkanes and alcohols. As expected the higher vibrational contribution of the –CF2– groups contributes for the higher heat capacities of the perfluorinated compounds. Concerning the fusion/sublimation equilibria, fluorination of the alcohols decreases ΔH and ΔS of phase transitions. Compared to the regular –CH2– alkyl chain structure, the less symmetric –CF2– spiral conformation is characterized by a less efficient molecular alignment in crystal packing, thus explaining the lower ΔH. The lower internal rotational barrier associated with the –CH2– alkyl chains of n-alkanes/alcohols, when compared to –CF2– chains, contributes to the larger values of ΔS observed. Regarding the vaporization equilibria, fluorination leads to greater values of ΔS, which are explained by the larger translational entropy of the heavier perfluoroalcohols, an effect that is more important in the gas than in the liquid phase.