Surprisingly short-ranged interactions in highly charged colloidal suspensions

The interaction potential between colloidal particles in a suspension has been the topic of much research recently. Digital video microscopy techniques have come into wide use, as this method yields direct information about the structure of such systems. However, two main problems have plagued researchers working with this technique. First, what one sees through a microscope is actually a projection of a three-dimensional sample onto a two-dimensional image plane. Second, in order to achieve long-range interactions between particles, the ionicity of the surrounding medium must be as low as possible. In order to address the first problem, researchers have created quasi-two-dimensional samples by confining the system between two glass plates. However, this geometry makes it difficult to control the ionicity, and it also makes the analysis more difficult since one is dealing with an anisotropic system for which established theories of colloidal interactions formulated for the bulk do not apply. We have developed techniques to effectively address each of these two problems. Our sample cell is large enough to allow direct contact of the suspension with ion exchange resin, and allows one to make bulk measurements of the structure. In addition, we have developed techniques to handle the projection effects. We have used these methods to measure the radial distribution function of dilute suspensions of highly charged unconfined polystyrene microspheres in a density matched mixture of H2O and D2O. We found that the interaction potential between the colloidal particles was much shorter ranged than would be expected based on the Dejaguin-Landau-Verwey-Overbeek theory of colloidal interactions.