✍ Scribed by M. Minor; A.J. van der Linde; H.P. van Leeuwen; J. Lyklema
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ture much greater than the double-layer thickness k 01 ). Po-Measuring the dc electrophoretic mobility in a microelectropholarization of the double layer was not taken into account.
retic setup is traditionally performed at the stationary levels where In 1948, Henry (4) and Booth (5) independently took the electroosmotic flow due to charge on the capillary wall is abdouble-layer polarization into account in the case of spherisent. Small deviations in the measuring position can give significal particles with thin double layers (ka ӷ 1, where a is the cant errors in the measured mobility, especially if the cell wall is particle radius). It turned out that the extent to which the highly charged. An electroosmotic streaming profile does not arise electric double layer is polarized depends on the value of instantaneously upon switching on a field, but develops within a the dimensionless ratio Du (after Dukhin ( )). Du reprecharacteristic time, typically of the order of 1 s in a common sents the surface conductivity relative to the bulk conductivcapillary of 1 mm radius. From a theoretical study of the most pertinent relaxation times in electrophoresis and electroosmosis it ity (times a). When Du is of the order one or higher, the is shown that, under the usual microelectrophoretic conditions, a electric double layer is polarized significantly and, as a concolloidal particle responds much faster to an applied electric field sequence, the electrophoretic velocity is retarded.
than does the liquid inside the capillary. Therefore, it is possible Henry and Booth did not consider the polarization of neuto apply an alternating electric field with such a frequency that tral electrolyte (concentration polarization) around the partielectroosmosis is suppressed, whereas the particles are still able to cle. Deviations from the bulk densities extend from the surfollow the field according to their dc mobility. Measurements were face into the solution over distances of the order of the performed with a commercial laser-Doppler microelectrophoretic particle radius a, i.e., well beyond the double layer if ka is device in an alternating electric field as a function of frequency large. and position in order to demonstrate this principle. This new In 1970, Dukhin and Semenikhin (7) took concentration method for measuring particle mobilities overcomes the problems polarization into account. They solved the conservation associated with electroosmosis, e.g., finding the stationary levels.
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