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

Abstract

Reasonable values of the thermodynamic characteristics for the cluster formation of odd alcohols at the air/water interface are obtained by the quantum chemical PM3 approximation. The calculated values of enthalpy Δ , entropy Δ and Gibbs energy Δ for the formation of clusters of a given structure depend linearly on the number of CH2 groups in the odd alcohol molecule. The additive approach, proposed in a recent paper, was further developed to extend the results of the calculations of the thermodynamic properties of small associates (2 to 7 alcohol molecules) to infinite clusters of odd and even homologues. Hydrogen O···H bonds, O···O interactions between the oxygen lone pairs, and four types of H···H interaction are taken into account, and the thermodynamic characteristics of the formation of linear, rhombic, rectangular, etc. clusters are calculated. Depending on the cluster type, the dependence of the Gibbs free energy on the alkyl chain length can be either the same for all homologous alcohols or can be different for the even and odd homologues. The most stable associates are clusters possessing rhombic or dodecahedral structures and linear clusters possessing one H···H bond per each methylene group. The former type exhibits a monotonic dependence of the thermodynamic parameters on the number of methylene groups, whereas for the latter type this dependence is stepwise. The results of the quantum chemical calculations agree well with the results obtained on the basis of a thermodynamic model that assumes equilibrium between oligomers and clusters within the monolayer. The experimental Π−A isotherms, indicating the existence of a first-order phase transition, and the microscopic morphology of the condensed-phase domains of tridecanol monolayers support the results of the quantum chemical and thermodynamic calculations.

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).…”

“…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.…”

Aggregation and re-organization of normal fatty alcohols at the air/water interface

“…For the cluster formation of n-alcohols at the air/water interface the quantum chemical studies of the entropy and enthalpy of cluster formation in the minimum potential energy points were performed earlier [11,12]. In the present paper the growth and re-organization of two-dimensional clusters of n-alcohols are theoretically investigated.…”