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

Abstract

A simplified method is proposed to estimate the thermodynamic parameters of clusterization at the air/water interface for various classes of amphiphilic compounds with a single alkyl chain. The method is based on the calculation of thermodynamic characteristics only for one of the homologous series of dimers ( n = 6 – 16 ) governing the formation of infinite clusters. The method is used to calculate the thermodynamic parameters of clusterization for alcohols, thioalcohols, carboxylic acids and amines, and the dependencies of the Gibbs energy of clusterization on the alkyl chain length are evaluated. It is shown that the alkyl chain length, at which the spontaneous clusterization begins, as calculated using the proposed simplified method, is in fact the same as that calculated using the additive scheme developed earlier. The simplified method proposed was verified using alkylnitriles as example. In contrast to alcohols, thioalcohols and amines, infinite ‘rhombic’ clusters are formed rather than ‘rectangular’ clusters for this class of compounds. Spontaneous clusterization of nitriles is shown to start for alkyl chains containing 18–19 carbon atoms. This value agrees with that obtained from experimental data with 17–18 carbon atoms. The proposed simplified method introduces an exact and suitable tool for the estimation of thermodynamic parameters of the clusterization of amphiphilic compounds.