Heteroaggregation of oppositely charged colloids

The aggregation of oppositely charged particles (electrostatic aggregation) is a phenomenon fundamental to a variety of natural and industrial processes, where the particles may be Angstroms in size (atoms or small molecules) or tens of nanometers to microns in size (colloids). This project is aimed at developing an understanding of electrostatic heteroaggregation of colloids from both fundamental and practical points of view. The kinetics of aggregation and the resulting heterofloc structures for aqueous model colloidal systems representative of hydrophilic, hydrophobic, and polymer-coated particles are quantified by light scattering methods. The kinetics of aggregation (stability ratios) of oppositely charged, polymer coated particles are modeled successfully using an extension of DLVO theory. Depending on the polymer molecular weight, the stability of such systems is characterized by a critical salt concentration, above which the suspension becomes fully stable. For bare particles, heterofloc structures with ultra-low mass fractal dimensions (D_f = 1.2 to 1.4) are found in both the hydrophilic and hydrophobic colloidal systems, suggesting that electrostatic heteroaggregation may represent a new class of materials. Such low fractal dimensions approaching the theoretical limit of a linear object imply a chaining mechanism. Optical microscopy confirms the formation of linear chains with the charge of the particles alternating down the chains. Brownian dynamics simulations corroborate the particle chaining and alternation of charge but predict fractal dimensions higher than measured values. The results of this study have potential value in the fabrication of nanostructured composites of desired porosity and locally uniform distribution of the starting materials.