Description of surfactant 2D monolayer formation at the air/water interface within semiempirical quantum chemistry

Abstract

The review discusses the development of a quantum chemical approach for calculating the thermodynamic clusterization parameters of surfactants at the air/water interface. It characterizes the intermolecular interactions that govern the process of surfactant association. Of particular interest are the dispersion interactions CH···HC between the hydrophobic chains of surfactants, as well as their accurate description within the exploited scheme and semiempirical methods. The scheme is based on calculating the thermodynamic parameters of formation for a certain number of surfactant monomers and small clusters with different chain lengths using the supermolecule approximation. Furthermore, it enables the construction of an additive scheme with assessed values of the increments of CH···HC interactions and interactions between the hydrophilic parts. This scheme provides equations for thermodynamic clusterization parameters per one surfactant molecule of infinite 2D films. The number of parameters adequately assessed within this approach is described in the previous review in Colloid Polym. Sci., 2015, 293, 3065–3089: the threshold chain length of spontaneous clusterization, the “temperature effect” of clusterization, and the assessment of the molecular tilt angle of the surfactant with respect to the interface. In this context, our aim is to describe additional parameters and experimental phenomena. These include the dependence of the area per surfactant molecule in a 2D monolayer at the LE-LC phase transition on temperature and chain length, the correlation of the surfactant clusterization threshold with the solubility threshold, and with the donor-acceptor properties of the substituents included in the hydrophilic part. We also explore surfactant binary mixtures, surfactant – alkane mixtures, and the role of amphiphiles in the formation of alkane adsorption layers. Additionally, we investigate the shifting of the acid or base value (pKa, pKb) of surfactants with chain elongation during monolayer formation, as well as the dendricity of surfactant domains with an increase in temperature or shortening of chain length.