Ionic strength controls the transition across distinct states of nanoparticle aggregation and assembly

Predicting the pathways and outcomes of nanomaterial systems requires a detailed understanding of the interplay between particle forces, interfacial structure, and emergent properties. Here we demonstrate that the ionic strength controls transitions among five distinct states of nanoparticle aggregation and assembly. Using gibbsite (γ-Al(OH)₃) nanoplatelets as building blocks, we observed the slow assembly of mesocrystals with long-range order when the suspensions were aged in pure water for several months. Upon addition of salt, the particles statistically favored oriented aggregation along the (001) basal plane to form stacks that were only locally ordered. At higher ionic strength, aggregation became more rapid and random, producing fractal aggregates with D_f = 2.4-2.6 that increased the viscosity of the suspension by 18-fold. Above 1 m, beyond the Debye-Hückel regime, the aggregation timescales dramatically increased and the steady-state aggregate size decreased, eventually stabilizing as monodisperse particles at extreme ionic strength. By establishing a unified "state diagram" that spans mesocrystal assembly, locally ordered stacking, diffusion-limited aggregation, and re-entrant stabilization, this work provides a framework for understanding nanomaterial assembly and the resultant collective response, particularly under extreme conditions relevant to environmental management and nuclear waste processing.