Quantifying Double-Layer Repulsion between a Colloidal Sphere and a Glass Plate Using Total Internal Reflection Microscopy

Abstract

Total internal reflection microscopy (TIRM) has recently been developed as a technique to measure the mean potential energy of interaction between a single colloidal particle and a flat plate. Based on the total internal reflection of light at an interface separating two media of different refractive indices, TIRM provides an instantaneous measurement of the separation distance between the particle and the plate. The distance measurements are derived from measurements of scattering intensity as the particle interacts with the evanescent wave formed upon total internal reflection. We have used TIRM to quantify double-layer repulsion acting between a glass plate and polystyrene latex spheres of diameters 7, 10, and 15 μm dispersed in aqueous solutions of ionic strengths from 0.2 to 3.0 mM. This work extends our earlier measurements of double-layer potential energies in that we are now able to measure the absolute separation distance between the sphere and the plate. The potential energy profiles agree very well with those predicted by a model of double-layer and gravity forces that involves no adjustable parameters. The measured double-layer potential energy is accurately described by a simple exponential model based on the linear superposition of potential profiles and Derjaguin's approximation, with the Debye length as the decay length.