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 C_nH_2n+1OCH₂CH₂OH with n = 6–16 (C_nE₁) 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 CH⋯HC 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 C_nE₁ 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 C_nE₁ confirms the experimental fact that the crystalline monolayers are formed by preferential aggregation of trimers.