Measuring double layer repulsion using total internal reflection microscopy

Total internal reflection microscopy has been developed as a new 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 total internal reflection microscopy (TIRM) provides an instantaneous measurement of the separation distance between the particle and the plate with the nanometer resolution of scanning electron microscopy. Unlike conventional scanning electron microscopy, however, TIRM can be used to observe noninvasively the dynamics of particle motion in situ. We have used TIRM to quantify double layer forces between a glass plate and polystyrene latex spheres of diameters 7,10, and 15 mu m dispersed in aqueous solutions of varying ionic strength. The potential energy profiles agree very well with those predicted by a model of double layer and gravity forces which 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. The double layer potential energy is also shown to scale with the first power of the particle size, as predicted by Derjaguin's approximation.