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

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.

On the inclusion of alkanes into the monolayer of aliphatic alcohols at the water/alkane vapor interface: a quantum chemical approach.

In the framework of the quantum chemical semiempirical PM3 method thermodynamic and structural parameters of the formation and clusterization of aliphatic alcohols C(n)H(2n+1)OH (n(OH) = 8-16) at 298 K at the water/alkane vapor C(n)H(2n+2), (n(CH(3)) = 6-16) interface were calculated. The dependencies of enthalpy, entropy and Gibbs' energy of clusterization per one monomer molecule of 2D films on the alkyl chain length of corresponding alcohols and alkanes, the molar fraction of alkanes in the monolayers and the immersion degree of alcohol molecules into the water phase were shown to be linear or stepwise. The threshold of spontaneous clusterization of aliphatic alcohols at the water/alkane vapor interface was 10-11 carbon atoms at 298 K which is in line with experimental data at the air/water interface. It is shown that the presence of alkane vapor does not influence the process of alcohol monolayer formation. The structure of these monolayers is analogous to those obtained at the air/water interface in agreement with experimental data. The inclusion of alkane molecules into the amphiphilic monolayer at the water/alkane vapor interface is possible for amphiphiles with the spontaneous clusterization threshold at the air/water interface (n(s)(0)) of at least 16 methylene units in the alkyl chain, and it does not depend on the molar fraction of alkanes in the corresponding monolayer. The inclusion of alkanes from the vapor phase into the amphiphilic monolayer also requires that the difference between the alkyl chain lengths of alcohols and alkanes is not larger than n(s)(0) - 15 and n(s)(0) - 14 for the 2D film 1 and 2D film 2, respectively.

Relationship between the Bulk and Surface Basicity of Aliphatic Amines: A Quantum Chemical Approach.

To assess the surface basicity constant (pK b) of aliphatic amine films, the use of a theoretical approach recently developed to evaluate the pK a of carboxylic acid monolayers on the water surface is tested. The present paper gives a new full picture of the change of acid-base properties of surfactants during their aggregation at the air/water interface. The exploited approach is simple because it does not involve the construction of thermodynamic cycles but uses the Gibbs energies of the formation and dimerization of surfactant monomers in neutral and ionized forms in the aqueous and gaseous phases. The quantum chemical semiempirical PM3 method is applied to perform calculations using a conductor-like screening model, which takes into account the aqueous phase. The calculation shows that aliphatic amines, as well as carboxylic acids, are characterized by a change of the value of the basicity/acidity constant during the film formation. The film formation of surfactants leads to a decrease in their acid-base properties, i.e., the surface pK a values of carboxylic acids and pK b values of amines increase. However, unlike carboxylic acids, there is practically no dependence of the surface pK b value on the alkyl chain length of the aliphatic amine, which is caused by almost identical contributions of one CH2 fragment to the solvation Gibbs energy of neutral and ionized monomers within the calculation error. The obtained results agree with existing experimental data.

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

The thermodynamic parameters of formation and clusterization of aliphatic alcohols C n H2n+1OH and carboxylic acids C n H2n+1COOH (n = 6-16) are calculated using the quantum-chemical semiempirical PM3 method. Four types of dimers are constructed in two directions of the spread monolayer comprising the most energetically advantageous monomer structures. The hydrophobic chains of alcohol and carboxylic acid molecules in the regarded dimers are found to be tilted within 12° to the normal of the spread monolayer. The structures of the mixed and pure surfactant dimers are the basis for the mixed alcohol-carboxylic acid monolayers of the following types: two dimensional (2D) film 1 with single distribution of the individual component in the other one, when the molecules of the first component do not interact with each other but are completely surrounded by the molecules of the second component; 2D film 2 with domain structure, when the film consists of "islands" of the individual components. The dependences of the clusterization Gibbs' energy per one monolayer molecule on the molar fraction of the components for the mixed 2D films 1 formed by surfactants with equal alkyl chain length are found to be limited from top to bottom by the corresponding dependences for pure components. This indicates the absence of synergetic interaction between the hydrophilic head groups of carboxylic acids and alcohols and conforms to the available experimental data. The formation of the described types of mixed films is competitive. The preferential formation of 2D films 1 with single distribution of the first component among the molecules of the second one is possible when the length of the carboxylic acid hydrocarbon chain is longer by Δn = 1-2 methylene units than that of the corresponding alcohol alkyl chain. According to the fractionally linear law, the highest possible content of the carboxylic acids in such 2D films 1 depends on the Δn value and does not exceed 33.3%.

Quantum chemical approach in the description of the amphiphile clusterization at the air/liquid and liquid/liquid interfaces with phase nature accounting. I. Aliphatic normal alcohols at the air/water interface.

A new model based on the quantum chemical approach is proposed to describe structural and thermodynamic parameters of clusterization for substituted alkanes at the air/liquid and liquid/liquid interfaces. The new model by the authors, unlike the previous one, proposes an explicit account of the liquid phase (phases) influence on the parameters of monomers, clusters and monolayers of substituted alkanes at the regarded interface. The calculations were carried out in the frameworks of the quantum chemical semiempirical PM3 method (Mopac 2012), using the COSMO procedure. The new model was tested in the calculations of the clusterization parameters of fatty alcohols under the standard conditions at the air/water interface. The enthalpy, Gibbs' energy and absolute entropy of formation for alcohol monomers alongside with clusterization parameters for the cluster series including the monolayer at air/water interface were calculated. In our calculations the sinkage of monomers, molecules in clusters and monolayers was varied from 1 up to 5 methylene groups. Thermodynamic parameters calculated using the proposed model for the alcohol monolayers are in a good agreement with the corresponding experimental data. However, the proposed model cannot define the most energetically preferable immersion of the monolayer molecules in the water phase.