A circular disk cut from a silicon wafer is rotated about its axis after immersion in a solution containing a surfactant and dodecane. Charges formed at the silicon-solution surface attract counterions which accumulate in a thin layer of solution next to the rotating disk.
Convection of these mobile charges generates a streaming potential profile in the solution. The maximum streaming potential occurs on the axis of rotation next to the disk. This potential is monitored by placing an electrode at this location and a counter-electrode in solution far away where the potential becomes uniform (see figure at right).
After about 30 sec of rotation, the streaming potential converges to some steady value (see figure at left), which is proportional to the 3/2 power of rotation rate. The proportionality constant can be used to infer the zeta potential, which is a direct measure of the charge density at the silicon-fluid interface.
The zeta potential is frequently used to determine what concentration of surfactant is needed to stabilize suspensions of solid particles in liquids, such as paints, printing inks, foodstuffs and pharmaceuticals. More than half of all products sold by the chemical industry are two-phase mixtures requiring some sort of stability to ensure a useful shelf-life.
Measurement of zeta potential and its interpretation is a mature subject for aqueous solutions. More common techniques for its determination include electrophoresis of particles in applied electric fields or measurement of streaming potential arising from pressure-driven flow through packed beds of particles. The rotating-disk experiment offers a number of advantages, the most obvious of which is the planar shape of the interface compared to the highly curved surface of small particles. This greatly simplifies the theory for relating zeta and streaming potentials.
This project will employ the rotating-disk apparatus to measure zeta potentials in nonpolar fluids (like alkanes) doped with surfactants. Different surfactants can give rise to different signs for the zeta potential. At the present level of understanding, it is not possible to anticipate even the sign of the zeta potential for a given solution and a given solid. This project is part of a larger effort supported by a grant from the Dow Chemical Co. whose agricultural and automotive divisions produce and market a number of suspensions which are nonaqueous.