pKA CHANGE OF SATURATED CARBOXYLIC ACIDS AND AMINES DURING THEIR DIMERIZATION AT THE AIR/WATER INTERFACE. QUANTUM-CHEMICAL APPROACH

Abstract

A theoretical approach to the pKA assessment for dimers of saturated carboxylic acids CnH2n+1COOH( n = 6 - 16) and amines CnH2n+1NH2 (n = 6 — 16) at the air/water interface is proposed. The model is based on calculation of the formation and dimerization Gibbs energies of the surfactants in neutral and ionized form in the water and gas phases. The calculations are done using semi-empirical quantum-chemical methods РМ3 and РМ6 within the framework of the conductor-like screening model (COSMO). It is shown that the dimer formation with one ionized and one neutral surfactant molecules is the most energetically favorable. The surface pKA value for surfactant dimers is found to depend on their surfactant chain length.

Keywords

• dissociation
• aliphatic amines
• carboxylic acids
• pka
• enthalpy
• entropy
• dimerization gibbs energy
• semi-empirical methods