Quantum chemical analysis of the thermodynamics of 2D cluster formation of odd n-alcohols at the air/water interface

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 ΔH_m^cl, entropy ΔS_m^cl and Gibbs energy ΔG_m^cl 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.