Our solutions for material characterization
jul 19 2018
Colloidal stability refers to the long-term integrity of a dispersion and its ability to resist phenomena such as sedimentation or particle aggregation. This is typically defined by the time that dispersed phase particles can remain suspended. Metastable colloids can still present instabilities with time, which can cause small molecules to coagulate and form larger particles.
Liquid dispersions such as emulsions, suspensions, and foams must all be assessed for colloidal stability to determine the aging properties and resistance to shear stress. Assessing a product’s shelf life is a critical measurement that often relates on monitoring the appearance of migration phenomena (creaming, sedimentation) and particle size evolution (aggregation, coalescence properties).
This article will explore and define these phenomena responsible for colloidal stability:
Particle migration: Colloid creaming and sedimentation
Particle migration is the buoyant or non-buoyant migration of dispersed phase media in a solution, essentially determining whether solid particles float to the surface of the dispersive media or settle at the bottom. This is typically dependent upon particle size and density, and the viscosity of the liquid phase of the solution. It is also affected by thixotropic thinning due to agitation or time-dependent viscosity and is critical for determining the colloidal stability of numerous food and drink products. Colloidal stability typically succumbs to this form of separation at a defined rate proportional to the thermodynamic conditions of the solution, the particle size and density, and the colloid’s surface energy.
Whereas the previous phenomena refer to separative phenomena, coalescence describes the tendency of suspended media to reassemble and form larger masses. The phenomenon occurs between media of the same composition and radically, often irreversibly, changes the macroscopic condition of the colloid. As a result, formed homogenous mixtures are often no longer suitable for purpose.
Particle aggregation refers to the aggregating potential of dispersed phase media in a colloidal system. Inter molecular interactions (Van der Waals; electrostatic, depletion...) can cause solid particles to pull together and form assemblies or clusters that significantly impact colloidal stability and reduce product efficacy. Unlike in the case of coalescence, aggregated particles are still distinct and do not merge together, however the pulling forces hinter breaking the clusters. Often, if ever, a strong mechanical stress is required in order to recover to the initial dispersion state.
Colloidal Stability Measurements with Formulaction
Colloidal stability monitoring and assessment is complex, as multiple conditions must be analyzed to determine long-term integrity under a myriad of influencing agents. Thermal fluctuations and mechanical stress (dilution or agitation) can cause instabilities to appear while the same phenomena are not observed in the end-product colloids. It is therefore important to monitor colloidal stability using multiple light scattering techniques to characterize specific destabilizing phenomena. Most importantly, physical stability needs to be monitored with minimum sample handling or user intervention. Thus, direct measurement at rest of the end product is the key to reliable results.
At Formulaction, we offer a patented technology for colloidal stability analysis that requires no sample preparation for providing quantitative stability analysis up to 200 times faster than conventional tests. If you would like any more information, please do not hesitate to contact us.