Heat capacities, self-association and complex formation in alcohol–ester systems

Abstract

The apparent molar heat capacity, Φ_c, its associative part, Φ_c(assoc), and the excess heat capacity, C^E_p, have been obtained at 25 °C through the concentration range for methanol, hexan-1-ol and decan-1-ol in a series of proton acceptor solvents: methyl acetate, ethyl acetate, octyl acetate, methyl octanoate, decyl acetate and methyl tetradecanoate. The results are explained quantitatively by the Treszczanowicz–Kehiaian (TK) association model and the Flory lattice model in terms of alcohol self-association into tetramers, characterized by an OH—OH equilibrium constant and enthalpy, and by alcohol–ester complex formation characterized by an OH—COO equilibrium constant and enthalpy, each independent of alcohol and ester chain length. As predicted, Φ_c(assoc) at infinite alcohol dilution is independent of the choice of alcohol, but increases with increasing ester chain length. With increasing alcohol concentration Φ_c(assoc) passes through a maximum which is much reduced compared with the inert solvent case. For each alcohol, C^E_p increases with increasing ester chain length, while for the same ester, C^E_p decreases with increasing alcohol chain length. dC^E_p/dT is positive for mixtures of methanol and methyl acetate, as predicted by the TK model. It is negative for decanol with methyl acetate, contrary to the TK model, but consistent with a non-random distribution of alcohol tetramers in the solution.