Quantum chemical semi-empirical approach to the thermodynamic characteristics of oligomers and large aggregates of alcohols at the water/air interface

In the framework of quantum chemical approximations, namely MINDO/3, MNDO, AM1 and PM3, the thermodynamic functions characteristic of the formation of dimers of alkanols (n=8-16) are calculated both in vacuum and in water. These values are compared with those calculated from experimental Π versus A isotherms of n-dodecanol, n-tetradecanol and n-hexadecanol using a thermodynamic model that assumes equilibrium between oligomers and highly aggregated condensed phase domains in the monolayer. The results indicate consistency between experimental data, the molecular-statistical theory developed to describe surfactant aggregation (formation of small oligomers in the fluid (gaseous) state) in the monolayer, and the values obtained by the quantum chemical method PM3. In particular, it was shown by PM3 calculations that, in perfect agreement with the experiments, the aggregation degree of an oligomer increases with the alkyl chain length. The increased chain length leads also to increasing absolute values of the free energy per monomer for the formation of highly aggregated domains; and the enthalpy and entropy constituents of the free energy.