Macroion clustering in solutions and suspensions: the roles of microions and solvent

Under certain conditions, macroion systems undergo a transition from a homogeneous state to a heterogeneous state in regard to spatial configurations and dynamic processes. Examined herein are three specific cases: highly ordered "crystalline-like" structures with very little kinetic activity in equilibrium with disordered spatial regions that exhibit normal diffusive motions, as first examined by Ise and co-workers (class I); an anomalously slow relaxation domain coexisting with "normal" relaxation modes but with no spatially ordered regions, as first reported by Schurr and co-workers (class II); and the formation of "vesicle-like" structures by inorganic molybdenum oxide macroions (polyoxometalates, POM), as examined in detail by Liu and coworkers (class III). A common feature of these three cases is that the heterogeneous structures do not involve macroions in physical contact, thereby ruling out the possibility of van der Waals attraction as the source of attraction. The commonalities of these three cases are that they involve electrostatic attraction as mediated by counterion and polar solvent structures, the extent of which is a function of the charge magnitude, distribution of charge on the macroion surface, and the relative macroion-solvent particle sizes. The results of juxtaposition of potential fields (JPF) profiles and Brownian dynamics (BD) simulations on these three systems suggest a family of states in the charge-size landscape of heterogeneous configurations.