Quantum Chemical Analysis of the Thermodynamics of 2D Cluster Formation of n-Carboxylic Acids at the Air/Water Interface

Abstract

Within the framework of PM3 molecular orbital approximation the thermodynamic function characteristics for the formation and geometrical structure of monomers, dimers, trimers, and tetramers of nondissociated n-carboxylic acids C(n)H(2n+1)COOH with n = 5-15 are calculated. It is shown that spontaneous aggregation of homologous fatty acids for the homologues with carbon atoms numbers n > or = 13 at the air/water interface can take place, leading to the formation of infinite plane rectangular clusters, whereas for the homologues with n < 11 spontaneous decomposition of large aggregates is energetically preferable. At the same time, the formation of trimers is more probable for the lower homologues (8 < n < 13). These results agree well both with the experimental data reported by various authors and with thermodynamic models developed earlier for soluble and insoluble monolayers. The slopes of the regressions calculated for the dependencies of the thermodynamic parameters on the alkyl chain length for all the clusters considered are all equal to each other. This fact indicates that the contributions of the CH2 groups to the thermodynamic characteristics of alcohols and acids are the same, and the differences in the formation of clusters by these substances should be attributed only to the differences in the structure and interactions of relevant functional groups. Therefore, it enables one to describe both acids and alcohols within the framework of the developed method, and it makes it possible to extend the proposed approach onto other classes of amphiphilic compounds.

Simplified method of the quantum chemical analysis for determination of thermodynamic parameters of 2D cluster formation of amphiphilic compounds at the air/water interface

“…In our previous studies, the thermodynamic parameters of the clusterization process were calculated for saturated alcohols [6,7], carboxylic acids [8], thioalcohols [9], and amines [10]. In the present work, we introduce a simplified approach for the estimation of thermodynamic parameters of clusterization of amphiphilic compounds at the air/water interface, being in general agreement with the thermodynamical parameters of clusterization calculated earlier for certain classes of amphiphiles.…”

“…Using the additive schemes on the basis of the calculated thermodynamic parameters of clusterization for these classes of compounds (see Refs. [6][7][8][9][10]) enabled us to calculate the clusterization parameters for the clusters of the studied structures and any size, including the infinite clusters. For all classes of compounds mentioned above, the thermodynamic parameters of the formation and clusterization of dimers, trimers and tetramers were calculated (for the total of 317 compounds, including the corresponding monomers).…”

“…In particular, the contributions from one intermolecular H-H interaction to the variation of standard enthalpy and entropy of cluster formation are: −9.20 ± 0.08 kJ/mol and −18.40 ± 0.60 J/(mol K) for alcohols [6], −10.20 ± 0.15 kJ/mol and −23.16 ± 0.75 J/(mol K) for carboxylic acids (for the alkyl residue the parameters used in Ref. [8] were those relevant for the alcohol residue), −10.31 ± 0.11 kJ/mol and −21.29 ± 1.17 J/(mol K) for thioalcohols [9], and −10.35 ± 0.04 kJ/mol and −27.34 ± 1.09 J/(mol K) for amines [10].…”

“…frequencies are disregarded. Therefore, similar to our previous papers, these contributions were manually calculated [6][7][8][9][10].…”

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

“…Assuming that adjacent atoms contribute to the maximal increment in the energy of the intermolecular interaction, we demonstrated in refs. [32,104,149,196,[198][199][200][201] that the interaction between two next-neighbor surfactant molecules can be expanded into the sum of increments contributed by the pairwise interactions between the nextneighbor methylene units and between the next-neighbor functional groups. The structure of the hydrocarbon substituents is like as the terminal interacting atoms of the methylene units are hydrogen atoms.…”