Quantum Chemical Analysis of the Thermodynamics of 2D Cluster Formation of Aliphatic Amides at the Air/Water Interface

Abstract

The semiempirical quantum chemical PM3 method is used to calculate the thermodynamic and structural parameters of formation and clusterization of aliphatic amides with the general composition CnH2n+1CONH2 (n = 6–16) at 293 and 298 K. Enthalpy, absolute entropy, and Gibbs’ energy of the formation for two stable conformations of monomers are calculated. The correlation dependencies of the calculated parameters on the alkyl chain length are found to be linear. The structures found for the monomers are used to build clusters (dimers, trimers, tetramers). The thermodynamic parameters of formation and clusterization for all cluster series are calculated. The obtained clusterzation parameters show stepwise dependencies on the alkyl chain length. It is demonstrated that the formation of two structurally different 2D films is possible. In the first film, the aliphatic amide molecules are oriented at the angles δ1 = 10° and φ1 = 20.5° to the normal to the sides of the unit cell, and in the second film, the values o...