The adsorption of homologous alkyldimethylbenzylammonium bromides, [C6H5CH2N(CH3)(2)R]Br, on sodium montmorillonite from aqueous NaCl solutions at room temperature has been studied. R stands for the methyl-, butyl-, hexyl-, octyl-, decyl-, and dodecyl-group, and the corresponding ammonium cations will be denoted as C1(+), C4(+), C6(+), C8(+), C10(+), and C12(+), respectively. C1(+), the reference cation, attains the plateau region of adsorption at a level close to the cation exchange capacity (CEC) of the clay. The chain-length dependence on adsorptivity of the homologous cations exhibits an unexpected peculiarity. In the case of short-chain homologues of C1(+) their adsorption onto sodium montmorillonite decreases in the order C1(+) > C4(+) > C6(+). This behavior is due, presumably, to the growing steric hindrances at the surface of clay, which occur because of the limited area available for the bulky organic cations at the exchange sites. These limitations appear to be out-balanced in the case of higher homologues for which the increasingly growing hydrophobic effects lead to the expected sequence of adsorptivity of the cations, i.e., C1(+) < C8(+) < C10(+) < C12(+). The extent of adsorption of the long-chain homologous cations at the plateau region exceeds the CEC value and indicates the commencement of formation of a bilayer or admicelle. As expected, the adsorption data for the short-chain homologues fit fairly well to the Langmuir isotherm, whereas the data for the C10(+) and C12(+) cations show an increasing departure from linearity of the corresponding plots. Results of X-ray analysis of organo-clays fully loaded with C1(+), W, and C12(+) cations suggest that in the case of short-chain homologues, the ammonium cations lay flatly and interact relatively strongly with the montmorillonite packet surface, whereas the long-chain homologue forms an interdigitated system of coiled hydrocarbon chains. (C) 2003 Elsevier Inc. All rights reserved.