Page 164 - Instant notes

E8
THE MOTION OF LARGER IONS

Key Notes
Large ions are often multiply charged and are surrounded by an
electrical double-layer, consisting of a Stern layer of tightly
bound counterions and a diffuse double-layer which is an ionic
atmosphere containing a majority of counterions. As with smaller
ions, this ionic atmosphere is crucial for determining ionic
interactions. The residual charge at the outside edge of the Stern
layer is the effective charge on the ion, which can vary with
electrolyte concentration and type.
Electrophoresis is the separation of large ions by applying a field
between two plates to induce concentration polarization. Under
the influence of the field, ions that have small hydrodynamic radii
and large effective charges move further and separate from
larger, less charged ions.
Electro-osmosis is a method for inducing osmosis by an electric
field. It is the counterpart to electrophoresis and involves
applying a field across a solution with fixed larger ions. Motion
of the counterions is induced, which also produces water flow
(osmosis). As with electrophoresis, the rate of water flow is
controlled by the effective charge on the ion.
Related topics Ions in aqueous solution Macroscopic aspects of ionic motion
(E1) (E6)

The electrical double-layer

Larger ions, such as biological polymers or macromolecules, which can contain many
thousands of atoms and many ionizable groups, are often multiply charged. The resulting
electrostatic forces between these ions and their small counterions are often sufficiently
strong that an electrical double-layer is formed (Fig. 1).
Near the large ion, a layer of counterions (often partially desolvated) is held tightly to
the surface by the very strong attractive electrostatic forces. This is termed the Stern
layer, and these strongly adsorbed counterions reduce the effective charge of the ion.
Outside the Stern layer, the remaining charge imbalance is sufficiently small that an ionic
atmosphere (see Topic E1) is formed around the ion called a diffuse double-layer,
which contains an overall excess of counterions sufficient to balance the residual charge,
loosely bound to the ion by the electrostatic forces of attraction. As with smaller ions, the
interaction between the ion and its ionic atmosphere of diffuse double-layer is crucial in