Multiparticle effects on the interactions of complex colloidal dispersions

A model polymer colloid with simple "recognition" ability was synthesized by dispersion polymerization using comb copolymer stabilizers that incorporate both 23-unit poly(ethylene oxide) and C18H37 side chains. Force interactions between beads of the resulting latex were examined using colloid-probe atomic force microscopy (CAFM) and Langmuir compression (LC). CAFM measurements showed an initial repulsion between beads on close approach that gives way to a strong bridging attraction and a minimum in the force-distance profile at bead surface separations comparable to twice the hydrocarbon side chain dimensions. By contrast, latex beads stabilized with combs that incorporate only the (EO)(23) side chains exhibited purely repulsive behavior. LC experiments on the same systems produced pressure-area isotherms qualitatively similar to the CAFM data; however, significant forces were registered at monolayer bead densities corresponding to surface separation distances 1-2 orders of magnitude larger than the side chain dimensions and comparable to the bead size (similar to1 mum). A simple model was developed to connect the data from these two experiments into a unified picture of how particle surface chemistry is expressed in the multiparticle behavior of complex colloidal dispersions.