Controlling interparticle interactions, aggregation and cluster formation is of central importance in a number of areas, ranging from cluster formation in various disease processes to protein crystallography and the production of photonic crystals. Recent developments in the description of the interaction of colloidal particles with short-range attractive potentials have led to interesting findings including metastable liquid–liquid phase separation and the formation of dynamically arrested states (such as the existence of attractive and repulsive glasses, and transient gels)1,2,3,4,5,6,7. The emerging glass paradigm has been successfully applied to complex soft-matter systems, such as colloid–polymer systems8 and concentrated protein solutions9. However, intriguing problems like the frequent occurrence of cluster phases remain10,11,12,13. Here we report small-angle scattering and confocal microscopy investigations of two model systems: protein solutions and colloid–polymer mixtures. We demonstrate that in both systems, a combination of short-range attraction and long-range repulsion results in the formation of small equilibrium clusters. We discuss the relevance of this finding for nucleation processes during protein crystallization, protein or DNA self-assembly and the previously observed formation of cluster and gel phases in colloidal suspensions12,13,14,15,16,17.
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We thank the Swiss spallation source at the Paul Scherrer Institut (PSI) in Villigen, Switzerland, for the neutron beam time and we acknowledge the help of our local contacts J. Kohlbrecher and S. van Petegem. We thank J. Groenewold, W. Kegel, F. Sciortino, K. Kroy and M. Cates for discussions. We thank A. Schofield for preparing the fluorescent PMMA particles. This work was supported by the Swiss National Science Foundation, the UK Engineering and Physical Sciences Research Council, the Scottish Higher Education Funding Council, and the Marie Curie Network on Dynamical Arrest of Soft Matter and Colloids. A.S. and P.S. conceived and performed the protein experiments; F.C. prepared the pH stabilized protein samples for the control experiments; H.S., W.C.K.P. and S.U.E. carried out and analysed the experiments with the colloid–polymer samples.
The authors declare that they have no competing financial interests.
This figure shows cluster aggregation numbers Nc obtained from samples where the pH is constant at all concentrations compared with those from samples where the pH slightly increases at high concentrations. It demonstrates that there is no measurable influence on Nc upon a small shift in pH. (DOC 38 kb)
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Stradner, A., Sedgwick, H., Cardinaux, F. et al. Equilibrium cluster formation in concentrated protein solutions and colloids. Nature 432, 492–495 (2004) doi:10.1038/nature03109
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