Surface p Kaof Saturated Carboxylic Acids at the Air/Water Interface: A Quantum Chemical Approach

Abstract

In the present study a theoretical approach is proposed for the pKa estimation of monolayers at the air/water interface on the basis of saturated carboxylic acids. This model involves calculating only the Gibbs energies of formation and dimerization of carboxylic acid associates in the neutral and dissociated forms, as well as the corresponding monomers in the water and gas phases. The model does not require the construction of any thermodynamic cycles. The calculations are performed using semiempirical quantum chemical methods PM3 and PM6 within the framework of the conductor-like screening model for monomers and dimers of carboxylic acids CnH2n+1COOH (n = 6-16). It is shown that the minimum clusterization Gibbs energy corresponds to associates with the degree of dissociation α = 0.5. A relationship is derived between the surface and bulk pKa values. In particular, it follows from this that, unlike the bulk pKa, the surface pKa depends on the alkyl chain length of the surfactant. It is due to the difference between the solvation energies of the alkyl chains of the corresponding neutral and dissociated monomers. Thus, the calculated data show that the lengthening of the carboxylic acid chain by one CH2 fragment leads to an increase in the surface pKa by 0.43 units. The obtained results are in good agreement with the available experimental data.