Charged interfaces 197The particularly high resolving power of moderately concentrated gel
media is to a large extent a consequence of molecular sieving acting
as an additional separative factor. For example, blood serum can be
separated into about 25 components in polyacrylamide gel, but only
into 5 components on filter paper or by moving boundary electro-
phoresis.Techniques for concentrated dispersionsThe electrophoretic mobilities of particles in concentrated dispersion
have been measured using (a) a relatively simple moving boundary
technique^185 and (b) a mass transport method^186. The interpretation
of such measurements may be complicated by electric double layer
overlap^186.Streaming current and streaming potentialThe development of a streaming potential when an electrolyte is
forced through a capillary or porous plug is, in fact, a complex
process, charge and mass transfer occurring simultaneously by a
number of mechanisms. The liquid in the capillary or plug carries a
net charge (that of the mobile part of the electric double layer) and its
flow gives rise to a streaming current and, consequently, a potential
difference. This potential opposes the mechanical transfer of charge
by causing backconduction by ion diffusion and, to a much lesser
extent, by electro-osmosis. The transfer of charge due to these two
effects is called the leak current, and the measured streaming potential
relates to an equilibrium condition when streaming current and leak
current cancel each other.
Figure 7.10 illustrates a suitable apparatus for studying streaming
potentials. To minimise current drain, a high-resistance voltmeter
(most digital voltmeters will suffice), must be used. Most of the
difficulties associated with streaming potential measurement originate at
the electrodes. A superimposed asymmetry potential often develops;
however, by reversing the direction of liquid flow this asymmetry
potential can be made to reinforce and oppose the streaming
potential and can, therefore, be allowed for.
The streaming current can be measured if the high:resistance
voltmeter is replaced with a microammeter of low resistance
compared with that of the plug. An alternating streaming current can