Quantum chemical analysis of the thermodynamics of 2D cluster formation of 2-hydroxycarboxylic acids at the air/water interface

In the framework of the quantum chemical semiempirical PM3 method, the thermodynamic and structural parameters of formation and clusterization were calculated for 2-hydroxycarboxylic acids with the general formula CnH2n+1CHOHCOOH (n = 5-15). Dimers and tetramers were formed on the basis of optimized monomer structures. It was shown that the alkyl chains in the considered associates orientate themselves with the following values of the tilt angles with respect to the p- and q-directions of the interface: delta = 7.2-21.2 degrees and phi = 9.8-21.1 degrees depending on the monomer structure of the cluster. Comparing the Gibbs energy values of clusterization for small aggregates, a possible scheme of 2D clusterization is proposed based on the formation of linear aggregates constructed by dimers with 'parallel' orientation of the functional groups for 2-hydroxycarboxylic acid monomers with n <= 16 carbon atoms in the alkyl chain. The following parameters of the unit cell for the most energetically preferred structure of 2D film 3 were found: a = 4.85 angstrom, b = 5.41 angstrom, and the angle between them theta = 95 degrees. The molecular tilt angle of 2-hydroxycarboxylic acids with respect to the air/water interface was calculated to be t = 21.7 degrees. The spontaneous clusterization threshold is 14 carbon atoms in the 2-hydroxycarboxylic acid chain at the air/water interface for a monolayer built on the basis of monomer 3 in good agreement with the existing experimental data.