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

Abstract

In the framework of the quantum chemical semiempirical PM3 method the monolayers of the monoethoxylated normal alcohols CnH2n+1OCH2CH2OH with n = 6-16 (CnE1) at the air/water interface are described. The optimized structures of small clusters (dimers, trimers, tetramers, pentamers, hexamers and heptamers) comprising the hexagonal monolayer are obtained. For these aggregates thermodynamic parameters of formation and clusterization are calculated. The correlation dependencies of the clusterization enthalpy, entropy and Gibbs energy on the number of CHHC interactions and interactions between the functional groups realized in the cluster are obtained on the basis of calculated data. The calculated parameters of the hexagonal monolayer unit cell are: a = 4.02 Å; b = 7.94 Å, t = 4°, close to those for an aliphatic alcohol monolayer according to GIXD experiments: a = 5.0 Å; b = 7.5 Å, t = 0-9°. Spontaneous clusterization of monoethoxylated alcohols at the air/water interface under standard conditions is shown to be possible for molecules possessing more than 14 carbon atoms in the alkyl chain, in good agreement with the characteristics of the surface pressure-molecular area (π-A) isotherms. It is found that addition of the -O-CH2-CH2- unit to the hydrophilic part of aliphatic alcohols results in a shift of their spontaneous clusterization threshold to that of the compounds with hydrocarbon chains 3 methylene units longer. The temperature effect of CnE1 is assessed. It corresponds to the spontaneous clusterization temperature decrease of 10-20 K per two methylene units taken from the alkyl chain in agreement with experimental data. The comparison of clusterization Gibbs energy dependencies for small aggregates of CnE1 confirms the experimental fact that the crystalline monolayers are formed by preferential aggregation of trimers.

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

“…It should be noted that modeling of the structure of ethoxylated surfactants with a small number of ethoxylated units (С 18 Е 1 ) gave another results. Amphiphilic molecules in monolayers are almost vertically oriented with respect to the interface [123] in agreement with the results of quantum chemical calculations in the framework of PM3 method [124].…”

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

“…5,6 Almost all of the surfactant classes under consideration have an oblique unit cell of the condensed monolayers with the exception of surfactants with a minimum size of hydrophilic groups: ethoxylated alcohols with a hexagonal structure, as well as saturated alcohols and carboxylic acids, for which the formation of monolayers with hexagonal and oblique elementary cells is practically isoenergetic. 17,18 As shown in previous works, [18][19][20] mostly trimers are the basic units for clusterization of surfactants with a hexagonal unit cell of the monolayer. While during the formation of an oblique monolayer LE-LC phase transition occurs on the basis of surfactant dimers.…”

“…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.…”