Frans_M_Everaerts_Isotachophoresis_378342.pdf

ADDITIVES TO THE ELECTROLYTES 173
electric field E is applied, a stationary state is reached after a short period of time. We can
divide the forces that are responsible for the electroendosmosis into two main classes: the
force exerted by the external fieldE on ihe ions in the double layer, the force being transferred
by these ions by liquid friction to the layer as a whole; and the force exerted by the
friction on the layer considered by the neighbouring layers, moving with a different
velocity. The force due to the difference in electroendosmosis in the consecutive zones
is not taken into consideration.
In order to gain an impression of the electroendosmotic velocity, vE, the following
equation can be given:
(6.16)
where E is the dielectric constant, { is the potential at the wall, E is the electric field
applied and q is the viscosity. The volume of liquid moving by electroendosmosis can be
measured for each zone, but the net result is zero, because one side is blocked. If this side
was not blocked, this volume transport would be
Q=vEO, (6.17)
where 0 is the cross-section of the narrow-bore tube. We can eliminate 0 by means of
Ohm’s law:
I
OE=-
x
where I is the current through the narrowbore tube and h is the specific conductivity of
the liquid.
For a rough estimation, the volume transport in a system in which no semipermeable
membrane is applied is
(6.18)
(6.19)
We have not considered the contribution of the surface conductance, because in the
systems applied by us they can be neglected.
Under normal operating conditions, e.g., if analyses are performed at pH 6 (Table 12.1),
an estimation can be made of the values of Q in the leading and terminating zones, the
values being about 40 and 80 pl/h, respectively. The final profde of a zone boundary is,
of course, influenced by these values.
By adding a suitable surfactant, the viscosity in the vicinity of the wall can be increased
at least 100-fold, which suppresses the electroendosmotic flow sufficiently. Finally, it
should be noted that the time of analysis is not influenced by the electroendosmosis,
again because one side is blocked by a semipcrrneable membrane.