Applying zeta potential measurements to characterize the adsorption on montmorillonite of organic cations as monomers, micelles, or polymers

A systematic study was carried out to characterize the adsorption of organic cations as monomers, micelles, or polymers on montmorillonite by monitoring zeta potential (ξ) as a function of cation loading on the clay. In general, the clay’s ξ became less negative as cation loading increased. A fairly good linear correlation between adsorption of organic cations on the clay, up to the cation exchange capacity (CEC) of the clay, and ξ potential of the composites was fitted. However, when the adsorption of the larger cation exceeded the CEC, a nonlinear increase in ξ was measured. The degree of this increase corresponds to the cation size and affinity to the clay (in the order surfactant < dye dimer < micelle). In contrast to the organic cations, ξ reached zero at polycation loadings that were significantly lower than the CEC. The zeta-adsorption plot of the polycations reached a well-defined plateau which correlates to the zeta potential of the polycations. The effect of electrolytes on ξ of the crude clay was monitored, and as expected, the extent of the effect increased with valency (Na+ < Ca2+ < Al3+) and with intrinsic cation radius (Na+ < Cs+); however, an unexpected anion effect was observed.

Graphical abstractA systematic study was carried out to characterize the adsorption of organic cations as monomers, micelles or polymers on montmorillonite by monitoring zeta potential as a function of cation loading on the clay.Figure optionsDownload full-size imageDownload high-quality image (106 K)Download as PowerPoint slideResearch highlights► The clay’s ξ increased with electrolyte valency and with intrinsic cation radius. ► A local decrease in ξ was explained by an effect of counteranions. ► A linear correlation between organic cation adsorption (up to the CEC) and ξ of the composites was fitted. ► ξ Reached zero at polycation loadings significantly lower than the clay’s CEC. ► Different types of adsorbates have different but distinct effects on ξ of montmorillonite.