The Effect of the pH on the Adsorption of an Ampholytic Surfactant, N-Dodecyl-β-alanine, at the Aqueous Surface

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Abstract
The effect of the pH on the adsorption of an ampholytic surfactant, N-dodecyl-β-alanine (NDA), at the nitrogen/solution interface was studied by the radiotracer method, using a tritiated compound. The saturated amount of NDA adsorption was found to depend on the pH of the bulk solution; it shows a flat maximum in the isoelectric region from pH 4 to 10, and decreases distinctly on the both acid and alkaline sides. The Gibbs adsorption isotherm for an aqueous solution of the ampholytic surfactant was derived. Using this relation, the amount of adsorption in the pH range from 2 to 6 was calculated from the surface tension vs. concentration and the surface tension vs. pH relations observed for the NDA solution. The calculated values were in good agreement with those obtained by the radiotracer method. The degree of dissociation of the adsorbed monolayer was estimated from the pH dependence of NDA and the chloride ion adsorption.

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