Effect of pH on the Kinetics of Crystal Growth by Oriented Aggregation

The kinetics of oriented aggregation are sensitive to solution conditions, including pH. Decreasing the difference between solution pH and the point of zero net proton charge results in an increasing second-order rate constant for nonclassical crystal growth by oriented aggregation. Results are discussed in the context of Derjaguin and Landau, Verwey and Overbeek (DLVO) theory through the use of quantitative statistical comparisons. The magnitude of the increase depends on the primary particle size; however, the magnitude and trend in size dependence deviate from predictions based on DLVO theory. Oriented aggregation occurs in a wide array of important materials often resulting in nanocrystals with unique, anisotropic, and symmetry-defying morphologies. The growth of acicular goethite (alpha- FeOOH) was used as a model system for this research. Transmission electron microscopy with time-resolved sampling was employed using semiautomated image analysis to quantify the rate constant for growth by oriented aggregation.