Layer charge and electrophoretic mobility of smectites

The aim of this study is to determine the effect of the layer charge of smectites on their electrophoretic mobility (EM), using electrophoresis measurements. In order to cover the charge domain from 0 to 2 charges per unit structural cell, two clay series were used: thermally treated Cu-montmorillonite (0–0.7) and synthetic saponites (0.7–2). All the studied samples are negatively 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 acidic pH, the EM of smectites ranges from −3 to −0.4 10 −8 m 2 s −1 V −1 when the layer charge ranges from 0.8 to 2. High charge and low charge smectites, 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.