Charge nonuniformity light scattering

Classical models for colloidal forces assume that the particles are uniformly charged. However, charge nonuniformity can decrease suspension stability and reduce the accuracy of colloidal assemblies. Our lab group has previously seen that polystyrene latex particles are often nonuniformly charged, through the experimental technique of “rotational electrophoresis” [J.D. Feick, N. Chukwumah, A.E. Noel, D. Velegol, Langmuir 20 (2004) 3090]. Even though rotational electrophoresis is unique in finding zeta potential nonuniformity on individual particles, the technique is currently time-consuming and for practical reasons is limited to particles larger than 1 μm in size. We now introduce a modification of the rotational electrophoresis technique: “charge nonuniformity light scattering” (CNLS). CNLS takes advantage of two effects: (1) nonuniformly charged particles will rotate in an applied electric field and (2) light scattering from anisotropic particles changes when the particles align, an effect already used in the electric light scattering technique. By “visualizing” rotational electrophoresis with light scattering and interpreting the results with electrokinetic modeling, CNLS enables a measurement of zeta potential distribution for a colloidal suspension of particles with anisotropic geometry. In order to test the method, we synthesized model colloidal doublets composed of spheres with zeta potentials ζ1 and ζ2. Using translational electrophoretic mobility experiments, we found the difference ζ2 − ζ1 for our doublets. We then conducted experiments with CNLS, which gave the same value of ζ2 − ζ1, verifying the accuracy of the technique. Thus, CNLS allows for much more rapid measurements of charge nonuniformity and will soon be applied to more challenging systems.