A new mechanism for regulating the stability of colloidal particles has been discovered. Negligibly charged colloidal microspheres, which flocculate when suspended alone in aqueous solution, undergo a remarkable stabilizing transition upon the addition of a critical volume fraction of highly charged nanoparticle species. Zeta potential analysis revealed that these microspheres exhibited an effective charge buildup in the presence of such species. Scanning angle reflectometry measurements indicated, however, that these nanoparticle species did not adsorb on the microspheres under the experimental conditions of interest. It is therefore proposed that highly charged nanoparticles segregate to regions near negligibly charged microspheres because of their repulsive Coulombic interactions in solution. This type of nanoparticle haloing provides a previously unreported method for tailoring the behavior of complex fluids. Colloidal suspensions enjoy widespread use in applications ranging from advanced materials to drug delivery. By tailoring interactions between colloidal particles, one can design stable fluids, gels, or colloidal crystals needed for ceramics processing (1), coating (2), direct write (3), photonic (4-9), and pharmaceutical (10, 11) applications. Long range, attractive van der Waals forces are ubiquitous and must be balanced by Coulombic, steric, or other repulsive interactions to engineer the desired degree of colloidal stability.The self-organization of highly charged nanoparticles and their influence on the behavior of complex fluids in which they dwell has received scant attention. The traditional view is that small particles or other species (e.g., polyelectrolyte, polymer, or micelles) in solution can promote flocculation of stable colloidal suspensions via an entropic depletion interaction (12-15). The term ''depletion'' describes the exclusion of these smaller species from the gap region between colloidal particles that arises when their separation distance becomes less than the characteristic depletant size. The resulting concentration gradient between the gap region and bulk solution gives rise to an attractive force, whose magnitude scales with the volume fraction of smaller species, their charge, and the size ratio of large to small species (12,15,16). However, emerging theoretical work (17-19) suggests that charged species in solution may affect system stability through other self-organizing pathways. For example, charged nanoparticles have been predicted to segregate to regions surrounding large uncharged colloids, especially in systems with high size asymmetry and many more small to large spheres (18). This segregation is driven solely by a Coulombic repulsion between smaller species in solution and occurs simply because the larger particles represent a big volume without charge. The key question we wish to explore is whether this type of haloing process can provide a mechanism for stabilizing colloidal species.Here, we study the effects of highly charged nanoparticles on the behavior of ne...

Various nanoscale semiconducting superlattices have been generated by direct templating in a lyotropic organic liquid crystal. These include superlattices of CdS, CdSe, and ZnS, templated in a liquid crystal formed by oligoethylene oxide oleyl ether amphiphiles and water. The semiconductor growth process copied the symmetry and characteristic dimensions of the original mesophase by avoiding growth of mineral within regularly spaced hydrophobic regions. The final product was a superlattice structure in which a mineral continuum was featured with hexagonally arranged cylindrical pores 2−3 nm in diameter and 5 nm apart. Most importantly, the superlattice morphology of the nanostructured systems in contact with the mesophase was found to be thermodynamically stable with respect to the solid lacking nanoscale features. We also found that both the morphology of features in the nanostructured solids and their dimension can be controlled through the amphiphile's molecular structure and water content in the liquid crystal. The semiconducting solids CdS, CdSe, and ZnS were all directly templated, while Ag2S, CuS, HgS, and PbS were produced only as nonfeatured solids using identical synthetic methodologies. We propose that interactions of polar segments in template molecules with the precipitated mineral and with its precursor ions are necessary conditions for direct templating. This is based on the absence of templating in the more covalent minerals and also in the presence of salts known to bind precursor ions.

A new mechanism for regulating the stability of colloidal suspensions has been discovered, known as nanoparticle haloing. Colloidal microspheres suspended under conditions near their isoelectric point were stabilized by the addition of highly charged nanoparticles. Such species segregated to regions near negligibly charged microspheres leading to their effective charge buildup. At even higher nanoparticle volume fractions, system stability was reversed due to attractive depletion interactions. By engineering the strength of energetic and entropic interactions via nanoparticle additions, we have assembled colloidal fluid, gel, and crystalline phases from binary mixtures of colloidal silica microspheres and hydrous zirconia nanoparticles whose structure and flow behavior vary dramatically.

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