Controlling Nanorod Oligomer Aggregation in Solutions
HY Chen and E Ruckenstein, JOURNAL OF PHYSICAL CHEMISTRY C, 120, 16913-16918 (2016).
Controlling the size of ordered nanorod aggregates in colloidal dispersions is a challenge. Here, we employ Brownian dynamics to explore the dependence of the nanorod aggregate size and structure on the molecular weight the a polyelectrolyte attached to one end of the nanorod and on the nature of the solvent. Upon increasing the polyelectrolyte molecular weight (the length of the polyelectrolyte), the size of the aggregates decreases because of increasing electrostatic repulsion. The critical van der Waals interaction strength for transition from individual nanorods to nanorod dimers/trimers+ (trimers and larger aggregates) increases with increasing polyelectrolyte molecular weight. In a medium with a dielectric constant of 1.0, upon increasing the van der Waals interaction between nanorods, the individual nanorods aggregate as dimers and trimers+, with approximately 50% of individual nanorods forming dimers. In a solvent with a sufficiently large dielectric constant (e.g., >= 10.0), upon increasing the van der Waals interaction between nanorods, most nanorods aggregate into trimers or larger aggregates; few dimers are generated. A lower solvent dielectric constant and a higher polyelectrolyte molecular weight favor formation of more :uniform aggregates. As the charge of the segments of the polyelectrolyte increases, the fraction of nanorod dimers increases and the fraction of nanorod trimers+ decreases. In media with the dielectric constant of water, aggregation was insensitive to the temperature. In low dielectric constant media, aggregate formation was sensitive to the temperature for high molecular weight polyelectrolytes, but insensitive to the temperature for low molecular weight polyelectrolytes.
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