Relationship between the Bulk and Surface Basicity of Aliphatic Amines: A Quantum Chemical Approach

[Image: see text] To assess the surface basicity constant (pK(b)) of aliphatic amine films, the use of a theoretical approach recently developed to evaluate the pK(a) of carboxylic acid monolayers on the water surface is tested. The present paper gives a new full picture of the change of acid–base properties of surfactants during their aggregation at the air/water interface. The exploited approach is simple because it does not involve the construction of thermodynamic cycles but uses the Gibbs energies of the formation and dimerization of surfactant monomers in neutral and ionized forms in the aqueous and gaseous phases. The quantum chemical semiempirical PM3 method is applied to perform calculations using a conductor-like screening model, which takes into account the aqueous phase. The calculation shows that aliphatic amines, as well as carboxylic acids, are characterized by a change of the value of the basicity/acidity constant during the film formation. The film formation of surfactants leads to a decrease in their acid–base properties, i.e., the surface pK(a) values of carboxylic acids and pK(b) values of amines increase. However, unlike carboxylic acids, there is practically no dependence of the surface pK(b) value on the alkyl chain length of the aliphatic amine, which is caused by almost identical contributions of one CH(2) fragment to the solvation Gibbs energy of neutral and ionized monomers within the calculation error. The obtained results agree with existing experimental data.