Page 185 - Introduction to Colloid and Surface Chemistry

7 Charged interfaces

The electric double layer

Most substances acquire a surface electric charge when brought into
contact with a polar (e.g. aqueous) medium, possible charging
mechanisms (elaborated below) being ionisation, ion adsorption and
ion dissolution. This surface charge influences the distribution of
nearby ions in the polar medium. Ions of opposite charge (counter-
ions) are attracted towards the surface and (less important) ions of
like charge (co-ions) are repelled away from the surface. This,
together with the mixing tendency of thermal motion, leads to the
formation of an electric double layer made up of the charged surface
and a neutralising excess of counter-ions over co-ions distributed in a
diffuse manner in the polar medium. The theory of the electric
double layer deals with this distribution of ions and, hence, with the
magnitude of the electric potentials which occur in the locality of the
charged surface. This is a necessary first step towards understanding
many of the experimental observations concerning the electrokinetic
properties, stability, etc., of charged colloidal systems.
Origin of the charge at surfaces

Ionisation
Proteins acquire their charge mainly through the ionisation of
carboxyl and amino grolips to give COO" and NH| ions. The
ionisation of these groups, and so the net molecular charge, depends
strongly on the pH of the solution. At low pH a protein molecule will
be positively charged and at high pH it will be negatively charged.
The pH at which the net charge (and electrophoretic mobility) is zero
is called the iso-electric point (see Table 2.3 and Figure 7.7).