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Updated: Jan 20, 2026
Relative Strengths of Conjugate Acid-Base Pairs
Hydrophilicity of Nonanoic Acid and Its Conjugate Base at the Air/Water Interface
Shiva Badban1, Anita E Hyde1, Chi M Phan1
1Department of Chemical Engineering and Curtin Institute of Functional Molecules and Interfaces, Curtin University, GPO Box U1987, Perth, WA 6845, Australia.
Abstract:
A general adsorption model based on partial dissociation was developed for carboxylic acids. The model was applied to the adsorption of nonanoic acid at the air/water interface. Two cases were selected for experimental verification: acid-only and acid with a constant Na+OH- concentration. The model was applied simultaneously at both conditions, and the hydrophilicity of the ionic states was quantified by the adsorption constants, K A and K AH. It was found that the adsorption constant for the acidic group is significantly higher than that for the carboxylate group, K AH /K A ∼ 272. The model lays important groundwork for modeling and predicting carboxylic/carboxylate adsorption.
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