Electrokinetics: The properties of the stagnant layer unraveled

When an aqueous solution moves tangentially to a charged surface, as in electrophoresis and all other electrokinetic phenomena, a thin water layer remains immobilized: the stagnant layer. The plane separating the stagnant layer and the mobile part of the fluid is the slip plane. The electrokinetic (or zeta-) potential, obtainable from electrokinetics is the potential at that plane. Understanding the properties of the stagnant layer is one of the most long-standing issues in colloid science. According to the most advanced physical insight we have today, this layer behaves like a two-dimensional gel in which the ions can move almost unimpeded but which nevertheless macroscopically behaves as a rigid body. Molecular Dynamics simulations confirm this picture; it also gives insight into the tangential transport mechanism of counterions and a link is made with statistical distribution functions.