A new colloidal model system to study long-range interactions quantitatively in real space

We present quantitative three-dimensional real space measurements by confocal microscopy on fluorescently labelled and sterically stabilized dispersions of polymethylmethacrylate spheres dispersed in index and density-matched solvent mixtures with a relative dielectric constant 5 < epsilon(r) < 10. In this new model system Debye screening lengths (kappa(-1)) comparable to the particle size (diameter or) can be realized even for particles with sizes of several micrometres. Moreover, by addition of salt (tetrabutylammonium chloride) kappa(-1) can be varied and the surface charge of the particles can be set roughly in between the values +100 and -100 mV, as determined by electrophoresis. By a comparison of radial distribution functions and displacements from lattice positions with Monte Carlo computer simulations we found that both the structure in the liquid and the crystallization volume fraction could be described with a Yukawa potential characterized by one set of parameters, a surface potential of 36 mV and kappasigma = 5, where the particle diameter sigma = 2 mum. Anomalous ('phase') behaviour such as extreme long-range repulsions, 'coexistence' of high-density and low-density colloidal crystals and void formation, previously observed for deionized dispersions in water, was observed as well, and can now be studied in a different system without ion exchange resin. These anomalous effects are seen relatively soon after preparing the systems and are absent or short-lived in systems with grounding and at higher salt concentrations.