An approach to estimate the position of the shear plane for colloidal particles in an electrophoresis experiment

The existence of the shear layer has great influence on the stability of colloidal systems and it is an important physicochemical parameter in the field of colloid and interface chemistry. However, to date, people do not know the actual position of the shear plane, and there is no theory to calculate the shear layer thickness. Conventionally, it is thought that the shear plane is very close to the Stern plane. In this study, a new theory for calculating the thickness of the shear layer for a planar charged colloidal particle in an aqueous solution was established; the shear layer thickness can be obtained based on the surface potential (the potential at the original plane of diffusion layer) and zeta potential of colloidal particles. The results showed that the surface potential of montmorillonite in various electrolyte solutions was approximately 3 to 6 times larger than the corresponding zeta potential. The thickness of the shear plane was dependent on the surface potential in the aqueous solution, and the shear plane was located far from the Stern plane but close to the Gouy plane, contradicting the widely accepted, erroneous placing of the shear plane close to the Stern plane. This study also showed that, ionic polarization in the electric field of the diffuse layer will significantly decrease the surface potential and the position of the shear plane.