A simple method for estimation of the 2D cluster formation temperature of substituted alkanes at the air/water interface

Abstract

A simple method for estimation of the 2D spontaneous clusterization temperature of substituted alkanes at the air/water interface versus alkyl chain length is proposed using the quantum chemical semiempirical PM3 method. The method is approved by long-chain amines, alcohols, thioalcohols, saturated and unsaturated carboxylic acids, α-amino acids and amides of carboxylic acids.The method is based on the experimental fact that short chain amphiphilic molecules cannot form condensed 2D monolayers whereas above a specific alkyl chain length condensed monolayer phases can be observed under the same conditions. That is, elongation of the alkyl chain of amphiphilic molecules causes the increase of intermolecular interaction energy between them what stimulates their close packing and structuring. This leads to linear or stepwise dependencies of enthalpy and entropy of cluster formation on the alkyl chain length. It is possible to obtain an equation for the estimation of the temperature of the spontaneous clusterization threshold of the considered substituted alkane types by setting Gibbs’ energy equal to zero and by knowing the regression expressions for determination of enthalpy and entropy of cluster formation calculated under the condition of 298K using coefficients which determine the contribution of the hydrophilic head group interactions of molecules and CH⋯HC-interactions between the alkyl chains.It is shown that the temperature dependence of the spontaneous clusterization threshold on the alkyl chain length of the amphiphile is fractionally linear with a relative error of 3–8%. The effect of the alkyl chain elongation of substituted alkanes by two methylene units corresponds to subphase temperature reduction (ΔТ) by 10–20K in good agreement with the available experimental data. The difference between ΔТ values for considered types of amphiphilic compounds becomes less significant with the lengthening of the alkyl chain. This indicates the basic contribution of intermolecular CH⋯HC-interactions between the alkyl chains to the process of the 2D clusterization.