a charge sign opposite to that at the interface) accumulate near the interface,
whereas coions (same charge sign as at the interface) are excluded. This is
shown in Figure 12.2a.
In this way, anelectric double layeris formed: the charge at the surface
is compensated for by the excess of counterions and a depletion of coions in
the double layer, and electroneutrality is maintained. The distribution of the
FIGURE12.2 The electric double layer. (a) The concentrationcof counterions and
coions near a negatively charged surface;c?is the bulk electrolyte concentration. (b)
The decay of the electrostatic potential cnear that surface. The (‘‘nominal’’)
thickness of the electric double layer 1/kis also indicated.