Page 167 - Instant notes

The motion of larger ions 153

Electro-osmosis

Electro-osmosis is a complementary technique to electrophoresis and is a way of
inducing water flow (osmosis) by the application of an electric field. Large ions are
trapped in a gel or matrix so that they cannot move (or the gel or matrix simply has a
charged surface). Application of a large constant voltage between two electrodes either
side of the gel results in the motion of the diffuse double-layers of counterions towards
the plate electrode of opposite charge, as with electrophoresis. These counterions move
with their associated solvent and induce a net flow of water across the gel or matrix. This
is termed electroosmosis (electrically induced osmosis) and the rate of water flow is
determined by the effective charge on the large ion which, as with electrophoresis, can be
controlled by varying counterion type, concentration and electric field.