isomorphic substitutions take place beneath the plane of basal Suspensions of an argentinean Na / -exchanged montmorillonite oxigens in illites, modifying their electronic population and inwere studied by acid-base potentiometric titrations, mass titracreasing their cation binding capabilities. α§ 1998 Academic Press tions and electrophoresis. Potentiometric titrations were employed Key Words: montmorillonite; illite; structural charges; permato obtain proton adsorption vs pH curves at different NaCl concennent charges; cation exchange; proton binding; electrokinetics; trations whereas mass titrations were used to determine the depenmodeling; electrical double layer. dence of the point of zero net proton charge (PZNPC) with the ionic strength. The studied Na / montmorillonite presented proton adsorption vs pH curves typical of materials carrying mainly struc-INTRODUCTION tural negative charges and few hydroxyl surface groups: the PZNPC decreased with the ionic strength, and the different titra-
The electrical and adsorptive properties of the clay-water tion curves ran almost parallel to each other. No crossing point interface are of primary importance in soil chemistry and could be observed for several titration curves performed at differenvironmental chemistry, as well as in catalysis and several ent NaCl concentrations. The EM was also typical of particles that industrial processes. The interaction of the clay surface with carry mainly negative charges: there was only a slight decrease in ions has great influence, for example, on soil fertility, soil the negative electrophoretic mobility when the pH was decreased aggregation, and chemical speciation in different environfrom 11 to 3. The kinetics of proton adsorption-desorption promental systems. Among the great variety of clays existing cesses was fast in alkaline media and slower at pH lower than ca. in nature, montmorillonites are important because they can 8.5. This behavior appears to be a consequence of edge-to-face completely delaminate in some aqueous solutions, offering interactions and suggests that the edges become positively charged a large surface area for interaction with water molecules and at pH Γ΅8.5. A model of the 2:1 type clay water interface is proposed to fit proton adsorption and electrophoretic data of the dissolved ions (1-3).
studied sample and those previously published for an illite. The