Proton Adsorption and Electrokinetics of an Argentinean Montmorillonite

Suspensions of an argentinean Na+-exchanged montmorillonite were studied by acid–base potentiometric titrations, mass titrations and electrophoresis. Potentiometric titrations were employed to obtain proton adsorption vs pH curves at different NaCl concentrations whereas mass titrations were used to determine the dependence 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 structural negative charges and few hydroxyl surface groups: the PZNPC decreased with the ionic strength, and the different titration curves ran almost parallel to each other. No crossing point could be observed for several titration curves performed at different NaCl concentrations. The EM was also typical of particles that carry mainly negative charges: there was only a slight decrease in the negative electrophoretic mobility when the pH was decreased from 11 to 3. The kinetics of proton adsorption–desorption processes was fast in alkaline media and slower at pH lower than ca. 8.5. This behavior appears to be a consequence of edge-to-face interactions and suggests that the edges become positively charged at pH