(449a) Role of Sedimentation in the Assembly of Colloidal Crystals of Oppositely Charged Particles

Ionic colloidal crystals are long-range ordered structures formed from the finely tuned interaction of charged colloids carrying opposite charges [1]. These structures have been self-assembled under near-equilibrium conditions where the solvent is refractive index and near density-matched to the particles. The allowable range of attractions is apparently limited. For example, if the attraction is too great, ionic gels form rather than crystals. Ionic colloidal crystals hold promise in applications ranging from photonic bandgap to chemical and biological sensing materials. Therefore, a full understanding of the extent to which this type of colloidal crystal structure can be formed would benefit a range of potential applications.

We investigate the role of an applied field, gravity, on ionic colloidal crystal formation through study of their sedimentation. We find that the final structure of the ionic colloidal crystal depends on the sedimentation velocity, initial volume fraction, and interparticle interactions. The strength of the gravitational field relative to the randomizing effects of Brownian motion plays a key role in this process and its effect can be quantified by the dimensionless group, Peclet number. Using poly(diphenyl-dimethyl) siloxane stabilized poly(methyl methacrylate) [DPDM-PMMA] particles (positively charged) mixed with poly-12-hydroxystearic acid (PHSA) stabilized PMMA particles (negatively charged), we evaluate the quality of ionic colloidal crystals with cesium chloride (CsCl) structure formed in a solvent system consisting of cylcohexyl bromide (CHB), decalin and tetrabutylammonium chloride (TBAC) salt added for charge screening. The three-dimensional structure of these crystals is characterized by confocal laser scanning microscopy. By varying the ratio of CHB to decalin, we manipulate the sedimentation rate, and subsequently the Peclet number, of the assembly process. We show that a range of Peclet values exists where ionic colloidal crystallization is observed.

[1] Leunissen, M.E., C.G. Christova, A.P. Hynninen, C.P. Royall, A.I. Campbell, A. Imhof, M. Dijkstra, R. van Roij, and A. van Blaaderen, Ionic colloidal crystals of oppositely charged particles. Nature, 2005. 437(7056): p. 235-240.