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The thermodynamic parameters of the formation of monomers, dimers, trimers, tetramers, and one hexamer of alkylamines with the alkyl chain length in the range of 6 to 16 carbon atoms are calculated using the semiempirical PM3 method. The dependencies of potential energy surfaces of monomers and dimers on corresponding torsion angles are analyzed to determine the most stable conformations (local and global minima) of these entities. The thermodynamic parameters of cluster formation (enthalpies, entropies, and Gibbs' energies) are calculated for the dimers, trimers, tetramers, and the hexamer. The additive approach was further developed to extend the results of the calculations of the thermodynamic properties of small associates (2-6 amine molecules) to infinite clusters. It is shown that these parameter values are stepwise-dependent on the alkyl chain length, and spontaneous cluster formation takes place for this class of compounds when the alkyl chain length becomes 18-19 carbon atoms and higher.

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The semiempirical quantum-chemical PM3 method is used to calculate the thermodynamic parameters of clusterization of the S-form of alpha-amino acids with the general composition C(n)H(2n+1)CHNH(2)COOH (n = 5-15) at 278 and 298 K. It is shown that six stable conformations of monomers exist, for which the thermodynamic parameters (enthalpy and Gibbs' energy) of the formation and absolute entropy are calculated. The correlation dependencies of the calculated parameters on the alkyl chain length are found to be linear. The structures of the monomers are used to build larger clusters (dimers, tetramers, hexamers). For all small clusters (comprised of two to six molecules), the thermodynamic parameters of formation and clusterization are calculated. It is shown that for tetramers and hexamers the enthalpy, entropy, and Gibbs' energy of clusterization are linearly dependent on the alkyl chain length, whereas for the dimers these dependencies are stepwise. The thermodynamic characteristics of clusterization of associates tilted by angles of 9 and 30 degrees with respect to the normal to the interface are calculated. It is shown that the 30 degrees angle orientation is more energetically advantageous for this class of compounds. The geometric parameters of the unit cell characteristic for the infinite 2D film which corresponds to the most advantageous conformation of the monomer were calculated using the PM3 parametrization to be a = 4.57-4.71 A and b = 5.67-5.75 A, with the angle between the axes theta = 100-103 degrees . These values agree well with the available experimental data. Spontaneous clusterization of alpha-amino acids at the air/water interface at 278 K takes place if the alkyl chain length exceeds 11-12 carbon atoms, whereas for 298 K this clusterization threshold corresponds to 13-14 carbon atoms in the alkyl chain, also in agreement with the experimental data.