SUPERFAST ELECTROPHORESIS OF CONDUCTING DISPERSED PARTICLES

Conducting particles can display electrophoretic velocities hundreds o f times larger than those expected for nonconducting particles. For io n-exchange particles in which colons are excluded from the interior, s uperfast electrophoresis occurs when the externally applied electric f ield exceeds that required for producing the overlimit current through the particle. Then a secondary diffuse cloud of counterions is induce d outside the primary diffuse cloud (the latter is associated with the electric double layer). This extra induced charge, which increases wi th the electric field strength, causes the much larger electrophoretic velocities observed. Using multiple-exposed videoimaging and a new in clined flowcell to separate the effects of sedimentation and electroph oresis, we measure the electrophoretic velocity of electronically cond ucting particles (Al/Mg alloy, graphite, or activated carbon; 250-500 mu m diameter) which are then compared to earlier measurements with io nically conducting particles. For ionic strengths less than 1 mM, the electrophoretic mobility (velocity/electric field) of electronically c onducting particles increases significantly with the electric field an d the particle size, but is almost independent of the ionic strength. These trends are inconsistent with Smoluchowski's equation for the mob ility of a dielectric particle, but instead are consistent with the th eory (and earlier measurements on ion-exchange particles) for superfas t electrophoresis. Although the electronically conducting particles mo ve much faster than expected for dielectric particles, the velocity is not quite as high as that for ionically conducting particles. Smaller superfast electrophoresis for electronic conductors could be caused b y the overpotentials which drive the redox reactions necessary to exch ange electrons for ions at the particle surfaces; also both positive a nd negative secondary charge clouds are induced on opposite sides of a n electronic conductor particle, which partially neutralizes the ''sup erfast'' effect. (C) 1998 Academic Press.