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

Abstract

Recent progress in modeling of the surfactant behavior from atomistic to continuous at the air/water interface across different space-time scales is reviewed. Advantages and disadvantages of modern quantum mechanical, molecular dynamical, and mesoscale methods are discussed for description of interactions between amphiphilic molecules leading to formation of 2D films. The use of nonempirical and semiempirical methods for assessment of the thermodynamic and structural parameters of large van der Waals complexes is of particular interest. An approach for calculation of the thermodynamic and structural parameters of clusterization for nonionic surfactants at the air/water interface is proposed on the basis of the quantum chemical semiempirical PM3 method. This approach implicitly takes into account the influence of the interface on the surfactant molecules via stretching and orienting effect. The calculations are carried out in the supermolecule approximation for a limited number of small amphiphilic aggregates with different alkyl chain. The correlation analysis of the calculated data array provides the increments contributing by the intermolecular CH···HC interactions and interactions between hydrophilic parts into the thermodynamic parameters of formation and clusterization. Obtained increments are further used for constructing the dependencies of the thermodynamic clusterization parameters per one monomer on the alkyl chain length for large clusters up to 2D films. In the framework of the proposed theoretical approach, the next parameters are calculated as follows: enthalpy, entropy, and Gibbs' energy of clusterization for 11 homologous series of nonionic surfactants, threshold chain length enabling the process of monolayer formation at standard conditions, the Btemperature effect^of clusterization, and the structural parameters of the monolayer unit cell (particularly the tilt angle) depending on the size of the hydrophilic headgroup of the amphiphilic compound.

Analysis of Temperature and Alkyl Chain Length Impacts on the Morphological Peculiarities of Nonionic Surfactant Clusterization. A Quantum Chemical Approach

“…The advantage of this method in the series of papers devoted to investigation of the surfactant behavior at the air/water interface is stipulated by its parametrization with respect to the formation heats. , In addition, this method includes atom–atom potentials, which enable to a certain degree the account of the dispersion interactions of molecules for calculation of van der Waals clusters. Despite the overestimation of the forces of the CH···HC interactions between the hydrophobic chains of surfactant molecules, the PM3 method reproduces adequately the thermodynamic and structural parameters of the clusterization for different nonionic surfactants …”

“…Despite the overestimation of the forces of the CH•••HC interactions between the hydrophobic chains of surfactant molecules, 31 the PM3 method reproduces adequately the thermodynamic and structural parameters of the clusterization for different nonionic surfactants. 25 The used values calculated for the thermodynamic clusterization parameters of α-substituted carboxylic acids were obtained in the framework of the quantum chemical model described in detail elsewhere. 25 Here we just note its main key points:…”

“…25 The used values calculated for the thermodynamic clusterization parameters of α-substituted carboxylic acids were obtained in the framework of the quantum chemical model described in detail elsewhere. 25 Here we just note its main key points:…”

“…The behavior of nine surfactant classes was considered in compliance with the above-described scheme (thioalcohols, amines, saturated carboxylic acids, cis - and trans -unsaturated carboxylic acids, α-amino and α-hydroxylic acids, amides of carboxylic acids, and substituted melamines) . The clusterization thermodynamic parameters for α-substituted carboxylic acids were calculated for the temperatures of 278, 298, and 318 K. It gives an opportunity to analyze the impact of this factor on the morphological features of the monolayers formed.…”

Theoretical Description of Mixed Film Formation at the Air/Water Interface: Carboxylic Acids–Alcohols

“…The large-scale development of computers and of high-performance algorithms renders possible the theoretical description of monolayers using different kinds of molecular modeling methods, particularly molecular dynamics simulation of surfactant monolayers. , However, there are few theoretical papers modeling their behavior during monolayer formation. Therefore, we developed an approach for the description of 2D clusterization of nonionic surfactants (aliphatic alcohols, , carboxylic acids, , amides, α-substituted carboxylic acids, , and so on) at the air/water interface.…”