Quantum chemical approach to the description of the amphiphile clusterization at the air/liquid and liquid/liquid interfaces with phase nature accounting. II. 1-( n -Alkyl)-imidazoles at the air/water interface

Abstract

Abstract In the framework of the model for surfactant clusterization description at the air/liquid and liquid/liquid interfaces proposed earlier (Vysotsky et al. J. Phys. Chem. B, 2015, 119 (7), 3281–3296) and based on the quantum chemical semi-empirical PM3 method the structural and thermodynamic parameters of the formation of 1-( N -alkyl)-imidazole monolayers with alkyl chain lengths of 6–16 carbon atoms at the air/water interface are calculated. The model allows the calculation of the thermodynamic parameters (enthalpy, entropy and the Gibbs’ energy) of monomers, clusters and monolayers at the interface between two liquids or between the air and liquid phases. To obtain such results one needs to calculate the corresponding thermodynamic parameters for both adjacent phases within the semi-empirical PM3 method (in the frameworks of the COSMО procedure), then to calculate the increments from the different types of the pairwise interactions and, with the help of the additive scheme, to calculate the required parameters. It has been shown that 1-( n -alkyl)-imidazole forms monolayers with hexagonal and oblique unit cells normal conditions. The parameters of the unit cell are calculated for each of these monolayers: 4.6 Ǻ and 7.2 Ǻ (the angle γ between the sides of the unit cell is equal to 62°) for the hexagonal unit cells and 4.6 Ǻ and 4.6 Ǻ ( γ = 80°) for the oblique unit cells. Enthalpy, entropy and Gibbs’ energy for small clusters (dimers–hexamers), representing the structural elements of the investigated monolayers, are calculated. The probable way of the clusterization process is proposed on the basis of these parameters. In the framework of the model the thermodynamic functions of the monolayer formation are calculated. These results indicate that for both types of the monolayers (with oblique and hexagonal unit cells) spontaneous clusterization is possible when the alkyl chain length is ≥14–15 carbon atoms.