MEASUREMENTS OF DOUBLE-LAYER REPULSION FOR SLIGHTLY OVERLAPPING COUNTERION CLOUDS

Total internal reflection microscopy (TIRM) has recently been developed as a new technique to measure directly the mean potential energy of interaction between a single colloidal particle and a flat plate. TIRM provides instantaneous measurements of the relative separation between the particle and the plate, which are derived from the intensity of light scattered by the particle from the evanescent wave established upon total internal reflection at the plate/liquid interface. Using Boltzmann's equation, potential energy profiles are then obtained from histograms or relative separation distance. We have used TIRM to obtain (noninvasively) intensity distributions for a single 10 μm polystyrene sphere located near a flat glass plate in a series of electrolyte solutions. Potential energy diagrams derived from the scattering histograms compare very well with those expected based on simple models of double-layer and gravity forces which involve no adjustable parameters. These potential energy profiles have confirmed the exponential decay of double-layer repulsion with the Debye length as the decay length. The experimental methods have been refined to permit smaller background ionic strengths and longer sampling times. The spatial resolution of TIRM is shown to be on the order of nanometers. © 1990.