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.