A systematic study was carried out to characterize the adsorption of organic cations as monomers, micelles, or polymers on montmorillonite by monitoring zeta potential (xi) as a function of cation loading on the clay. In general, the clay's xi 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 xi potential of the composites was fitted. However, when the adsorption of the larger cation exceeded the CEC, a nonlinear increase in xi 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, xi 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 xi 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. (C) 2010 Elsevier Inc. All rights reserved.