Molecular structure of liquid alcohols

A new thermodynamic treatment of continuous association is presented, where the various equilibria between i-mers are replaced by a single equilibrium between an OH groups in the bonded and the non-bonded states, linked in both cases to an indefinite ensemble of molecules. The treatment leads to an association constant K which differs from those considered in the theories of Kretschmer and Wiebe and of Wiehe and Bagley. For pure alcohols the association constant can be estimated from the vapor pressure of the alcohol and that of the homomorphous hydrocarbon. The fraction γ of free OH groups determined in this way is markedly smaller than those calculated from the other theories. For the normal alcohols the product $\text{K}\text{V}{}_{\mathrm{<mtext>A</mtext>}}$ is approximately constant at a given temperature, $\text{V}{}_{\mathrm{<mtext>A</mtext>}}$ being the molar volume. This can be expected from the increasing of the standard entropy of the non-bonded molecules when the molecular volume increases. For secondary and tertiary alcohols the product K$\text{V}{}_{\mathrm{<mtext>A</mtext>}}$ is significantly lower due to steric hindrances. However for all the alcohols considered here the enthalpy of the hydrogen bond remains nearly constant — δH being equal to 24.8 ± 2 kJ mol^?1.