Quantum Chemical Analysis of the Thermodynamics of 2-Dimensional Cluster Formation of Alkylamines at the Air/Water Interface

Abstract

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.

Simplified method of the quantum chemical analysis for determination of thermodynamic parameters of 2D cluster formation of amphiphilic compounds at the air/water interface

“…In our previous studies, the thermodynamic parameters of the clusterization process were calculated for saturated alcohols [6,7], carboxylic acids [8], thioalcohols [9], and amines [10]. In the present work, we introduce a simplified approach for the estimation of thermodynamic parameters of clusterization of amphiphilic compounds at the air/water interface, being in general agreement with the thermodynamical parameters of clusterization calculated earlier for certain classes of amphiphiles.…”

Theoretical description of 2D-cluster formation of nonionic surfactants at the air/water interface

“…Correspondingly, they are termed as hydrogen-hydrogen (СН···НС) interactions. Investigations [104,187,196] proved that it is possible to distinguish six interaction types depending on the mutual orientation of the surfactant molecules in the cluster (see Fig. 5).…”

“…Theoretical background for such scheme is the pairwise additivity of the intermolecular interactions and the fact that their force decreases much stronger than Coulombic interactions [21]. As the calculations [104,187,196] showed, the clusterization enthalpy, entropy, and Gibbs' energy depend significantly on the mutual orientation of the hydrocarbon chains of interacting molecules. This is proved by anisotropy of these interactions revealed in study [197].…”

“…Assuming that adjacent atoms contribute to the maximal increment in the energy of the intermolecular interaction, we demonstrated in refs. [32,104,149,196,[198][199][200][201] that the interaction between two next-neighbor surfactant molecules can be expanded into the sum of increments contributed by the pairwise interactions between the nextneighbor methylene units and between the next-neighbor functional groups. The structure of the hydrocarbon substituents is like as the terminal interacting atoms of the methylene units are hydrogen atoms.…”

“…It is specified by the fact that geometric parameters of the hydrophobic chains of surfactant monomers and associates are the same (as obtained using quantum chemical calculations); they are defined by the mutual orientation of the molecules in the clusters. In our preceding papers [35,196,232], the procedure for calculation of the general tilt angle t of the molecule in the monolayer using the following expressions is described as follows:…”