In the present study a theoretical approach is proposed for the pK a estimation of monolayers at the air/water interface on the basis of saturated carboxylic acids. This model involves calculating only the Gibbs energies of formation and dimerization of carboxylic acid associates in the neutral and dissociated forms, as well as the corresponding monomers in the water and gas phases. The model does not require the construction of any thermodynamic cycles. The calculations are performed using semiempirical quantum chemical methods PM3 and PM6 within the framework of the conductor-like screening model for monomers and dimers of carboxylic acids C n H 2n+1 COOH (n = 6−16). It is shown that the minimum clusterization Gibbs energy corresponds to associates with the degree of dissociation α = 0.5. A relationship is derived between the surface and bulk pK a values. In particular, it follows from this that, unlike the bulk pK a , the surface pK a depends on the alkyl chain length of the surfactant. It is due to the difference between the solvation energies of the alkyl chains of the corresponding neutral and dissociated monomers. Thus, the calculated data show that the lengthening of the carboxylic acid chain by one CH 2 fragment leads to an increase in the surface pK a by 0.43 units. The obtained results are in good agreement with the available experimental data.

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The thermodynamic parameters of formation and clusterization of aliphatic alcohols C n H 2 n +1 OH and carboxylic acids C n H 2 n +1 COOH ( n = 6–16) are calculated using the quantum-chemical semiempirical PM3 method. Four types of dimers are constructed in two directions of the spread monolayer comprising the most energetically advantageous monomer structures. The hydrophobic chains of alcohol and carboxylic acid molecules in the regarded dimers are found to be tilted within 12° to the normal of the spread monolayer. The structures of the mixed and pure surfactant dimers are the basis for the mixed alcohol–carboxylic acid monolayers of the following types: two dimensional (2D) film 1 with single distribution of the individual component in the other one, when the molecules of the first component do not interact with each other but are completely surrounded by the molecules of the second component; 2D film 2 with domain structure, when the film consists of “islands” of the individual components. The dependences of the clusterization Gibbs’ energy per one monolayer molecule on the molar fraction of the components for the mixed 2D films 1 formed by surfactants with equal alkyl chain length are found to be limited from top to bottom by the corresponding dependences for pure components. This indicates the absence of synergetic interaction between the hydrophilic head groups of carboxylic acids and alcohols and conforms to the available experimental data. The formation of the described types of mixed films is competitive. The preferential formation of 2D films 1 with single distribution of the first component among the molecules of the second one is possible when the length of the carboxylic acid hydrocarbon chain is longer by Δ n = 1–2 methylene units than that of the corresponding alcohol alkyl chain. According to the fractionally linear law, the highest possible content of the carboxylic acids in such 2D films 1 depends on the Δ n value and does not exceed 33.3%.

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.

Recent progress in modeling of the surfactant behavior from atomistic to continuous at the air/water interface across different space-time scales is reviewed. Advantages and disadvantages of modern quantum mechanical, molecular dynamical, and mesoscale methods are discussed for description of interactions between amphiphilic molecules leading to formation of 2D films. The use of nonempirical and semiempirical methods for assessment of the thermodynamic and structural parameters of large van der Waals complexes is of particular interest. An approach for calculation of the thermodynamic and structural parameters of clusterization for nonionic surfactants at the air/water interface is proposed on the basis of the quantum chemical semiempirical PM3 method. This approach implicitly takes into account the influence of the interface on the surfactant molecules via stretching and orienting effect. The calculations are carried out in the supermolecule approximation for a limited number of small amphiphilic aggregates with different alkyl chain. The correlation analysis of the calculated data array provides the increments contributing by the intermolecular CH···HC interactions and interactions between hydrophilic parts into the thermodynamic parameters of formation and clusterization. Obtained increments are further used for constructing the dependencies of the thermodynamic clusterization parameters per one monomer on the alkyl chain length for large clusters up to 2D films. In the framework of the proposed theoretical approach, the next parameters are calculated as follows: enthalpy, entropy, and Gibbs' energy of clusterization for 11 homologous series of nonionic surfactants, threshold chain length enabling the process of monolayer formation at standard conditions, the Btemperature effect^of clusterization, and the structural parameters of the monolayer unit cell (particularly the tilt angle) depending on the size of the hydrophilic headgroup of the amphiphilic compound.