Effects of specific anions on the relationship between the ion-adsorption properties of sulfobetaine gel and its swelling behavior

The adsorption of cations and anions from four different halide solutions, i.e., KF, KCl, KBr, and KI solutions, onto N,N-dimethyl(acrylamidopropyl)ammonium propane sulfonate (DMAAPS) gels prepared using various cross-linker concentrations was investigated at a variety of temperatures; the swelling degrees of the gel in these solutions were also examined. The amount of K⁺ adsorbed coincided with the adsorption of the anions (i.e., I⁻, Br⁻, Cl⁻, or F⁻) because of simultaneous adsorption of the cations and anions onto DMAAPS gel. The order of anions, which gave larger amount of K⁺ adsorbed onto the gel, was opposite that of the Hofmeister series, i.e., I⁻ > Br⁻ > Cl⁻ > F⁻. Furthermore, the relationship between the degree of swelling of the gel and amount of K⁺ adsorbed onto the gel was elucidated. The data points laid on the same line for the same halide solution even at different cross-linker concentrations or temperatures. At smaller degrees of swelling, the amount of K⁺ adsorbed remained unchanged and then decreased gradually as the degree of swelling increased. In solutions containing anions with lower hydration ability, such as KI, the amount of K⁺ adsorbed decreased at larger degrees of swelling. In contrast, in solutions containing anions with higher hydration abilities, such as KCl, the adsorbed amount of K⁺ decreased at smaller degrees of swelling. Additionally, it was found that, in mixtures of these halide solutions, competitive adsorption of anions occurred. Anions with lower hydration abilities adsorbed more readily in the mixtures than in the single-anion solution. In contrast, anions with higher hydration abilities adsorbed less readily in the mixtures than in the single-anion solution.