Measurement of surface charge densities on Brownian particles using total internal reflection microscopy

Due to double-layer forces a charged colloid suspended in an electrolyte is repelled from a like-charged planar wall. We demonstrate that and how a pr ecise measurement of these double-layer forces acting on a colloid near a g lass surface can be used to determine surface charge densities. The effecti ve wall-colloid potentials are measured using the total internal reflection microscopy technique, and a whole series of such potentials, taken for var ious different salt concentrations, are then analyzed in terms of a given t heoretical interaction potential, where the surface charge densities are th e only unknown parameters. We find reasonable values for the surface charge densities of silica and polystyrene spheres in water, and compare the prop osed method with other more established techniques to measure surface charg e densities on single particles. (C) 2001 American Institute of Physics.