Hydrophilicity of Nonanoic Acid and Its Conjugate Base at the Air/Water Interface

[Image: see text] 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.