(534c) Determination of the Zeta Potential of Planar Solids in Nonpolar Liquids
The Gouy-Chapman model for the counterion cloud is used to convert the differential capacitance into ionic strength. This requires knowledge of the equilibrium (no applied voltage) surface potential of the electrodes. In the absence of such knowledge, we have until now assumed that surface potential was negligibly small compared to the thermal voltage. Under these conditions, the capacitance is insensitive to the surface potential. However, electrophoretic measurements of zeta potentials of micron-sized particles suggest these potentials are not always small in such solutions.
Measurements of zeta potentials of planar interfaces (like our electrodes) are rarely reported in the literature compared to those of colloidal particles. The zeta potential of the latter is usually obtained by electrophoresis. Of course, we could grind our electrodes to form colloidal particles but the surfaces exposed by grinding are not necessarily the same as the original flat.
Recently, Sides & Prieve developed a new technique â called ZetaSpinÂ® â for measuring zeta potentials on flat surfaces. A circular disk having a diameter equal to a couple of centimeters is rotated around its axis and the resulting streaming potential generated on the axis near the disk is measured relative to the solution far away. All previous publications involving this technique used aqueous solutions.
In this paper, we describe modifications (to the aqueous version) which allow ZetaSpinÂ® to measure streaming potentials in nonpolar solutions. Basically, three changes are needed: 1) use of an electrometer with much higher input impedance, 2) a different choice of electrodes and 3) a much longer time (following changes in fluid flowrate) for the readings to reach steady state. Finally, we present experimental results which demonstrate that we are indeed measuring streaming potential in the nonpolar solutions. The ZetaSpinÂ® system emerges as powerful technique to quickly quantify surface charging as required for the study of electrostatic stabilization of particles dispersed in nonpolar liquids.
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