(150a) Determination of the Zeta Potential of Planar Solids in Nonpolar Liquids

An apparatus and a method for determining the zeta potential of planar solid surfaces in contact with nonpolar liquids are described. Rotation of disk-shaped mica, silica, and ITO samples in surfactant-doped dodecane generated streaming potential that was measured between a glassy carbon electrode near the sample at its axis and a second glassy carbon electrode far from the sample. After rotation began, the measured potential changed rapidly at first but asymptotically approached a steady value over a timescale from fractions of a second to seconds. The potential difference between the rest potential and the asymptote was proportional to the 3/2 power of the rotation rate, which agrees with the theory of streaming potential for the rotating disk. Also, the potential measured at the axis had the opposite polarity of potential measured by the same sensor at the disk’s periphery, which is expected from the same theory used to determine the zeta potential. This evidence indicated that the difference between the potential before rotation and its asymptotic value during rotation was streaming potential. The zeta potential of freshly cleaved mica in dodecane doped with 0.5% wt. AOT was +100 mV. The zeta potential of fused silica in dodecane doped with OLOA 11000 varied from -10 mV to -25 mV agreeing qualitatively with zeta determined from electrophoretic mobility measurements of silica particles. The magnitude of the zeta potential of ITO coated glass was smaller than 25 mV in solutions of OLOA 11000 in dodecane, which means that the diffuse layer capacitance near the ITO equals the permittivity divided by the Debye length, to a good approximation.