Charged interfaces 193measurement; if a rectangular cell is adapted for horizontal viewing
(see Figure 7.5), sedimenting particles remain in focus and do not
deviate from the stationary levels.
The electrophoretic velocity is found by timing individual particles
over a fixed distance (c. 100 n-m) on a calibrated eyepiece scale. The
field strength is adjusted to give timings of c. 10 s - faster times
introduce timing errors, and slower times increase the unavoidable
error due to Brownian motion. Timings are made at both stationary
levels. By alternating the direction of the current, errors due to drift
(caused by leakage, convection or electrode polarisation) can largely
be eliminated. The electrophoretic velocity is usually calculated from
the average of the reciprocals of about 20 timings. In a more
sophisticated set-up, these timings can be automated using a laser-
doppler technique (see page 61).
The potential gradient E at the point of observation is usually
calculated from the current /, the cross-sectional area of the channel
A and the separately determined conductivity of the dispersion & 0 -
i.e. E = llkoA.
Particle electrophoresis studies have proved to be useful in the
investigation of model systems (e.g. silver halide sols and polystyrene
latex dispersions) and practical situations (e.g. clay suspensions,
water purification, paper-making and detergency) where colloid
stability is involved. In estimating the double-layer repulsive forces
between particles, it is usually assumed that »/rd is the operative
potential and that $& and £ (calculated from electrophoretic mobilities)
are identical.
Particle electrophoresis is also a useful technique for characterising
the surfaces of organisms such as bacteria, viruses and blood cells.
The nature of the surface charge can be investigated by studying the
dependence of electrophoretic mobility on factors such as pH, ionic
strength, addition of specifically adsorbed polyvalent counter-ions,
addition of surface-active agents and treatment with specific chemical
reagents, particularly enzymes. Figure 7.7 shows, for example, how
the rnobility-pH curve at constant ionic strength reflects the
ionogenic character of some model particle surfaces.
Moving boundary electrophoresis
An alternative electrophoretic technique is to study the movement of
a boundary formed between a sol or solution and pure dispersion