Layer charge and electrophoretic mobility of smectites

The aim of this study is to determine the effect of the layer charge of sme ctites on their electrophoretic mobility (EM), using electrophoresis measur ements. In order to cover the charge domain from 0 to 2 charges per unit st ructural cell, two clay series were used: thermally treated Cu-montmorillon ite (0-0.7) and synthetic saponites (0.7-2). All the studied samples are ne gatively charged in a pH range from 2 to 12. Neither layer charge nor ionic strength influences the EM of smectites at neutral to alkaline pH. At acid ic pH, the EM of smectites ranges from -3 to -0.4 10(-8) m(2) s(-1) V-1 whe n the layer charge ranges from 0.8 to 2. High charge and low charge smectit es, which are not expandable, tend to aggregate in aqueous suspension, and the measured EM corresponds then to the amphoteric sites of the edge faces of the layer stacks. These sites are negatively charged at pH values above 3. (C) 1999 Elsevier Science B.V. All rights reserved.