Predicting collision efficiency and fractal floc morphology for charged nanoparticles.

摘要

There are significant discrepancies between theoretical and experimental predictions of kinetics of aggregation of colloidal particles. Collision efficiency ( a ) is under-predicted for 'stable' conditions, as determined by DLVO models. In this research, influences on collision efficiency of charged nano-particles were investigated. 'Thought experiments' were performed dealing with Brownian motion of very small particles. These 'experiments' resulted in theoretical and conceptual models for molecular-level transport and interaction phenomena, including a model for estimation Brownian jump length that was anisotropic due to inclusion of interaction forces and effect of water structure on kinetic energy. The impact of the variation of dielectric constant of structured water molecules on electrostatic energy was also included. A computer model was developed to quantify the effect of these phenomena on collision efficiency. Through this model, aggregation was examined by 1-dimensional movement of particles in the formation of doublets. A 2-dimensional model was then developed to ascertain collision efficiency between particles and pre-formed clusters. A fractal aggregation computer model was developed, using a values from the 2-dimensional particle-cluster model. Aggregates formed with this model were quantitatively described using fractal geometry and compared to published model and experimental results.;The conclusions from this work help resolve the discrepancy between theory and experiment. From the conceptual model, it was found that Brownian jump length is anisotropic in the approaching particles zone, due to water structure and other interaction forces. In addition, electrostatic forces between approaching particles are less than generally recognized in the very near-field, due to lower dielectric constant of bound water. In 1-dimensional computer modeling, short and anisotropic Brownian jump length was found to allow primary aggregation for small particles(10nm) and secondary-minimum aggregation for larger (100nm), where DLVO theory predicts stability. Two-dimensional computer modeling of small particles (10nm) showed a dependence of collision efficiency on aggregate structure due to interfacial water structure. More closed-in aggregates had higher a due lack of kinetic energy to get out of the pore space. Using collision efficiency from the 2-dimensional modeling, fractal aggregation results showed that both increased collision efficiency and variation of efficiency with aggregate geometry effected the resulting aggregate.