Quantum chemical analysis of the thermodynamics of 2D cluster formation of 2-hydroxycarboxylic acids at the air/water interface

Abstract

In the framework of the quantum chemical semiempirical PM3 method, the thermodynamic and structural parameters of formation and clusterization were calculated for 2-hydroxycarboxylic acids with the general formula C n H 2n+1 CHOHCOOH (n ¼ 5-15). Dimers and tetramers were formed on the basis of optimized monomer structures. It was shown that the alkyl chains in the considered associates orientate themselves with the following values of the tilt angles with respect to the pand q-directions of the interface: d ¼ 7.2-21.2 and 4 ¼ 9.8-21.1 depending on the monomer structure of the cluster.Comparing the Gibbs energy values of clusterization for small aggregates, a possible scheme of 2D clusterization is proposed based on the formation of linear aggregates constructed by dimers with 'parallel' orientation of the functional groups for 2-hydroxycarboxylic acid monomers with n # 16 carbon atoms in the alkyl chain. The following parameters of the unit cell for the most energetically preferred structure of 2D film 3 were found: a ¼ 4.85 Å, b ¼ 5.41 Å, and the angle between them q ¼ 95 . The molecular tilt angle of 2-hydroxycarboxylic acids with respect to the air/water interface was calculated to be t ¼ 21.7 . The spontaneous clusterization threshold is 14 carbon atoms in the 2-hydroxycarboxylic acid chain at the air/water interface for a monolayer built on the basis of monomer 3 in good agreement with the existing experimental data.

Quantum-chemical analysis of hexagonal crystalline monolayers of ethoxylated nonionic surfactants at the air/water interface

“…As in our previous works the calculation of the structural and thermodynamic parameters of clusterization of monoethoxylated alcohols at the air/water interface has been carried out using the quantum chemical program package MOPAC2000 64 in the framework of the semiempirical PM3 method. This method has been successfully used for the assessment of the clusterization thermodynamic parameters of ten homologous series of surfactants [51][52][53][54][55][56][57][58][59][60][61] and of the geometric parameters of monolayers for aliphatic alcohols, 63 saturated, 61 unsaturated, 55,59 and a-substituted 57,58,62 carboxylic acids.…”

“…As indicated by previous studies, 31,37,38,49 the form and structure of C n E m domains varies depending on the temperature and alkyl chain length of molecules. Therefore it can be assumed that monoethoxylated alcohols likely form monolayers with structures similar to the structure of aliphatic alcohols, which are adequately described using the quantum chemical model [50][51][52][53][54][55][56][57][58][59][60][61][62] in a recent paper. 63 Therefore, the objective of the present work is to investigate the structural parameters of the hexagonal monolayers of monoethoxylated alcohols with normal structure at the air/water interface and to calculate thermodynamic parameters of clusterization for such monolayers using the quantum chemical semiempirical PM3 method.…”

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

“…17,18 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, 19,20 carboxylic acids, 21,22 amides, 23 α-substituted carboxylic acids, 24,25 and so on) at the air/water interface. The values obtained for the calculated alkyl chain length of the surfactants enabling monolayer formation at a given temperature are in agreement with the existing experiments and values of the structural parameters of the corresponding monolayers.…”

Quantum chemical assessment of the molecular area corresponding to the onset of the LE–LC phase transition for amphiphilic 2D monolayers at the air/water interface

“…While during the formation of an oblique monolayer LE-LC phase transition occurs on the basis of surfactant dimers. 21 This is schematically illustrated in Figs. 1 and 2 by the example of aliphatic alcohols.…”

“…Analysis of the available experimental data on the considered surfactant classes 19,21,31,34,36,42,[44][45][46] have shown that for monolayers with a hexagonal unit cell m≈3, and for films with oblique one -m≈2, with the exception for disubstituted melamine, for which this phase transition realizes on the basis of monomers. This is quite understandable, since the dialkyl-substituted melamine molecule itself has two hydrocarbon chains, participating in the formation of intramolecular CH•••HC interactions.…”