Reversible aggregation on nanometer length scales (from simple salts to viruses)

Abstract

This thesis focuses on issues concerning the reversible self-assembly on nanometer length scales, starting from simple salt without and in the presence of high and low molecular weight additives, over the clusters of molybdenum oxide and finishing with the rodlike viruses. All those systems are examples of thermodynamically stable dispersions. Thermodynamically stable clusters containing up to 10 or more silver iodide pairs have been observed in aqueous electrolyte solutions. These clusters are in equilibrium with an excess phase of solid AgI. The statistical weight of a cluster in the size distribution is exponentially decreasing with its interfacial area. The clusters of AgI of this size and stability have never been seen before, not in aqueous solutions or in other solvents. If adsorbing species other than iodide (high and low molecular weight additives) are present, AgI clusters roughly twice as large as those without additives form. The larger clusters form because the interfacial tension of AgI electrolyte is reduced, and even larger clusters can be formed if the tension of the AgI-electrolyte interface is further reduced (by stronger adsorbing species). But there also are examples of thermodynamically stable dispersions of large (solid) clusters or nanosized particles in a solvent. In the presence of acids, clusters containing up to 36 metal atoms spontaneously (and reversibly) form in aqueous solutions of molybdenum oxide. The conditions under which clusters of molybdenum (VI) oxide are stable follow from two basic assumptions and we predict that clusters of increasing size appear upon increasing the proton concentrations, although there are some exceptions. It is shown that the formation of metal oxide clusters is thermodynamically equivalent to the formation of surfactant micelles. We also considered relatively small species (cyclodextrines and virus coat protein subunits) that self-assemble onto relatively large ones (PEG and RNA). In the first mentioned species the average cluster size is predicted to vary smoothly with the interaction strength between subunits and threads, as well as with the ratio between the concentrations of threads and subunits. For viruses we find a sharp transition between monomeric species and (almost) fully assembled viruses upon varying the interaction between monomers. A small excess of RNA relative to the subunit concentration dramatically increases the polydispersity of the virus. This may have important biological consequences.