The paper is devoted to the application of a quantum chemical approach for assessment of the surface acidity of perfluorinated carboxylic acids (PFCAs) CnF2n+1COOH (n = 4–16) using the semiempirical PM3 method. This calculation scheme is based on the use of Gibbs energy of formation and association of neutral and deprotonated PFCA molecules in the gaseous and aqueous phases without involving bulky thermodynamic cycles, however, with the need to correct the obtained values of solvation Gibbs energies for PFCA anions. The findings revealed the dependence of the surface acidity value <InlineEquation ID="IEq1"> <EquationSource Format="TEX">${pK}_{a}^{\text{surf}}$</EquationSource> <EquationSource Format="MATHML"><math> <msubsup> <mrow> <mi mathvariant="italic">pK</mi> </mrow> <mrow> <mi>a</mi> </mrow> <mtext>surf</mtext> </msubsup> </math></EquationSource> </InlineEquation> on the surfactant chain length, just as in the case of fatty carboxylic acids. The elongation of the surfactant chain leads to an increase in the number of intermolecular CF∙∙∙FC interactions during the formation of supramolecular acid aggregates and, consequently, to a weakening of their ability to release a proton. Thus, the contribution of one CF2 fragment is 0.50 units for perfluorinated acids, which is close to the value <InlineEquation ID="IEq2"> <EquationSource Format="TEX">${\left(\frac{\text{d}{pK}_{a}^{\text{surf}}}{\text{d}n}\right)}_{T}=\text{0,43}$</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mfenced close=")" open="("> <mfrac> <mrow> <mtext>d</mtext> <msubsup> <mrow> <mi mathvariant="italic">pK</mi> </mrow> <mrow> <mi>a</mi> </mrow> <mtext>surf</mtext> </msubsup> </mrow> <mrow> <mtext>d</mtext> <mi>n</mi> </mrow> </mfrac> </mfenced> <mi>T</mi> </msub> <mo>=</mo> <mtext>0,43</mtext> </mrow> </math></EquationSource> </InlineEquation> for carboxylic acids with a hydrocarbon chain. The estimated values of <InlineEquation ID="IEq3"> <EquationSource Format="TEX">${pK}_{a}^{\text{surf}}$</EquationSource> <EquationSource Format="MATHML"><math> <msubsup> <mrow> <mi mathvariant="italic">pK</mi> </mrow> <mrow> <mi>a</mi> </mrow> <mtext>surf</mtext> </msubsup> </math></EquationSource> </InlineEquation> for PFCAs are in good agreement with the few available experimental records, among which there are several contradictory ones. This gives rise to use the proposed scheme for predictive purposes, as well as to assess the reliability of some controversial experimental data.

Clustering impact on the acidity of perfluorinated carboxylic acids at the air/water interface assessed within semiempirical quantum chemical approach

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2026年3月13日

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Elena S. Kartashynska

摘要

The paper is devoted to the application of a quantum chemical approach for assessment of the surface acidity of perfluorinated carboxylic acids (PFCAs) C_nF_2n+1COOH (n = 4–16) using the semiempirical PM3 method. This calculation scheme is based on the use of Gibbs energy of formation and association of neutral and deprotonated PFCA molecules in the gaseous and aqueous phases without involving bulky thermodynamic cycles, however, with the need to correct the obtained values of solvation Gibbs energies for PFCA anions. The findings revealed the dependence of the surface acidity value ${pK}_{a}^{\text{surf}}$ ${pK}_{a}^{surf}$ on the surfactant chain length, just as in the case of fatty carboxylic acids. The elongation of the surfactant chain leads to an increase in the number of intermolecular CF∙∙∙FC interactions during the formation of supramolecular acid aggregates and, consequently, to a weakening of their ability to release a proton. Thus, the contribution of one CF₂ fragment is 0.50 units for perfluorinated acids, which is close to the value ${\left(\frac{\text{d}{pK}_{a}^{\text{surf}}}{\text{d}n}\right)}_{T}=\text{0,43}$ ${(\frac{d {pK}_{a}^{surf}}{d n})}_{T} = 0,43$ for carboxylic acids with a hydrocarbon chain. The estimated values of ${pK}_{a}^{\text{surf}}$ ${pK}_{a}^{surf}$ for PFCAs are in good agreement with the few available experimental records, among which there are several contradictory ones. This gives rise to use the proposed scheme for predictive purposes, as well as to assess the reliability of some controversial experimental data.