Kinetic Study of the Adsorption-Desorption of Alkali Metal Ions on Carboxymethyl Sephadex C-25 by the Pressure-Jump Technique

Abstract

The pressure-jump technique with the electric conductivity detection was applied following the adsorption-desorption of Li + , Na + , and K + ions in the suspension of a cation exchanger, carboxymethyl (CM) Sephadex C-25, and a single relaxation of 0.1 s order was observed. The relaxation times became fast with increasing concentrations of the added LiOH, NaOH, and KOH. Under the same conditions as those in the relaxation measurements, alkali metal ions were strongly adsorbed on the Sephadex C-25. The selectivity of alkali metal ions on the Sephadex surface increased in the sequence Li + < K + < Na + . The ζ potentials of the Sephadex particles were negative in the presence of the bases, and the negative values increased in the sequence as Na + < K + < Li + . This indicates that the stronger adsorption counteracts the more negative charge on the particle surface. On the basis of these data, the relaxation was attributed to the conformational change of the Sephadex particles accompanying the ion-pair formation between alkali metal ions and the dissociated CM groups. Since the rate constants of the conformational change increased with a decrease in the atomic weights of alkali metal ions, the conformational change was interpreted as movement, such as the vibration of the dextran chains of the Sephadex C-25.