Abstract
To assess the surface basicity constant (pKb) of aliphatic amine films, the use of a theoretical approach recently developed to evaluate the pKa 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 pKa values of carboxylic acids and pKb values of amines increase. However, unlike carboxylic acids, there is practically no dependence of the surface pKb value on the alkyl chain length of the aliphatic amine, which is caused by almost identical contributions of one CH2 fragment to the solvation Gibbs energy of neutral and ionized monomers within the calculation error. The obtained results agree with existing experimental data.
Highlights
It is known that many factors affect the structure and stability of mono- and multimolecular surfactant films
Since in our recent work we developed an adequate scheme for estimation of the surface exponent of the dissociation constant[20] for carboxylic acids during monolayer formation, it gives an impetus to testing it for assessment of the protonation constant exponent by the example of aliphatic amines
As in our earlier study,[21] we consider the structures of aliphatic amines in which the hydrocarbon chain is in the most elongated “linear” conformation
Summary
It is known that many factors affect the structure and stability of mono- and multimolecular surfactant films. Along with the experimental methods, calculation methods are widely used for estimation.[7−9] In modern studies,[10−12] it was found that a significant part of the organic functional group composition of atmospheric marine aerosol particles falls on hydroxyl, alkane, amine, and carboxylic acid functional groups. This stipulates interest to investigate the acid−base properties of surfactants with such head groups during their aggregation at air/water interface that can affect the properties of the surface microlayer and further aerosol particles formed from it
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