Fluid structure in colloid–polymer mixtures: the competition between electrostatics anddepletion

Abstract

We studied the structure of a colloid–polymer mixture, in the one-phase region, as afunction of polymer concentration. Using confocal microscopy, and Fourier transformationof the image data, we accessed the system in real and reciprocal space simultaneously, atthe single particle level. We found no change in the colloidal fluid structure as a function ofpolymer concentration. The fluid phase persisted to higher concentrations of polymer thanexpected, before an abrupt gelation transition. Furthermore, we found a dramatic change inthe local structure around each particle at the onset of gelation. We attribute thesephenomena to a screened electrostatic repulsion between the colloids, which, althoughweak, suppressed the depletion attraction due to the polymer. The repulsive and attractiveinteractions have a similar range, which results in behaviour distinct both frompure depletion and the recently observed cluster phase formed in the presence oflong-ranged electrostatic repulsions. Instead, these results are qualitatively similarto the competition between van der Waals and electrostatic interactions in theDerjaguin–Landau–Verwey–Overbeek theory. We compare our experiments with MonteCarlo simulation and find agreement with a combination of the Asakura–Oosawa–Vrij andYukawa potentials.