Quantization of the Molecular Tilt Angle of Amphiphile Monolayers at the Air/Water Interface

Abstract

It is shown on the basis of quantum chemical calculations that the tilt angle of the alkyl chains with respect to the interface in the amphiphilic monolayers changes discontinuously and has a sequence of discrete values depending on the structural peculiarities of the intermolecular CH···HC interactions realized between the monolayer molecules. This is caused by the fact that the geometric parameters of the alkyl chains for amphiphile monomers and their aggregates obtained using quantum chemical calculations virtually coincide for different amphiphile types and are determined by the mutual disposition of the molecules in the cluster. Two quantum numbers for the angles φ and δ of the amphiphile alkyl chains with respect to the normal to the p- and q-directions of the two-dimensional film propagation (corresponding to minimum Gibbs energy of clusterization) define the value of the polar tilt angle, t, of the monolayer molecules. Their specific values depend on the size of the hydrophilic part of the molecule. The electronic structure and geometric parameters of the dimers were examined for six amphiphile types having increasing size of the hydrophilic part in the following sequence: saturated alcohols, carboxylic acids, amides of carboxylic acids, α-hydroxycarboxylic acids, α-amino acids, and N-acyl-substituted alanine. Quantum chemical calculations of the geometric structure of the considered dimers showed that the tangent of φ and δ angles depends linearly on the number of the lost CH···HC interactions due to sterical hindrances occurring because of the orientation of voluminous hydrophilic parts of molecules in the monolayer. The values of the increment and the free term in the obtained linear dependence of tan φ or tan δ agree well with those derived using correlation analysis of the values of considered angles for optimized dimer structures of the amphiphile type in the framework of the program package Mopac2000 (PM3 method). The calculated values of the molecular tilt angle in the monolayer having oblique unit cells agree well with the existing experimental data obtained by using grazing incidence X-ray diffraction.