The Rate Of Aggregation, Differential, Sedimentation, And Orthokinetic Aggregation Mechanism
1006 WordsJan 27, 20155 Pages
4. NEED FOR STABILISATION
Although the small size of nanocrystals leads to several advantages, but at the same time, it can lead to stability concerns as well. Theoretically, if we increase the drug surface area, it leads to an increase in Gibbs free energy. The system tries to reverse this increase by agglomeration/aggregation, ultimately leading to increased particle size and reduced stability.
Principally, the literature describes three mechanisms for aggregation-prekinetic aggregation, differential sedimentation and orthokinetic aggregation. In any dispersion, the particles are in continuous Brownian motion, and they may collide and stick together as a result of the attractive forces between them. Such a mechanism for aggregation is known as prekinetic aggregation. The rate of aggregation in such cases would be determined by the frequency of collisions as well as the chances of cohesion during collision. Differential sedimentation assumes importance when particles of varying sizes and densities settle in a suspension. Faster settling particles collide with the slower moving ones, thereby resulting in aggregation. Lastly, the orthokinetic aggregation mechanism describes the aggregation brought about by increased particle collisions, arising out of particle transportation as a result of fluid motion. Orthokinetic aggregation depends upon initial particle size and velocity gradient, but is independent of temperature.
Apart from aggregation, another stability issue