Impact of Centrifugation on Formulation Stability

jan 24 2019

Centrifugation is a separative technique used in phase separation of dispersions, emulsions, and other formulations across many industry sectors. In some cases, it can be used for accelerated stability analysis because it enables to apply strong gravitational stress on the sample, which results in faster separation.

Separation via centrifugal force is achieved by placing a formulation in a vessel and subjecting it to rapid rotation. In principle, this is used to accelerate gravitational separation processes that act upon dispersive particles in a liquid-phase media. This is a controversial method of assessing formulation stability, as it requires simulation of mechanical stresses to assess shelf life.

Using Centrifugation to Determine Formulation Stability

Gravitational separation is a key instability phenomenon encountered in many commercially available products: food emulsions; cosmetic products; and oil blends. It is also used in the water treatment sector. The density of suspended particles in an emulsion typically differs from the surrounding dispersant, which can cause them to undergo gravitational separation due to the net force. This typically impacts formulation stability in one of two ways: lower density particles may rise to the surface – otherwise known as creaming; and higher density particles may sediment and sink to the bottom of the continuous media.

Even though some sectors may see the benefits of using centrifugation to determine formulation stability because of accelerated separation effects, there is evidence that points out the difficulty of correlating centrifugation data with real shelf life and formulation stability at rest:

It is almost impossible to predict the behavior of the complex formulation at rest using centrifugation analysis…Ageing test[s] of soft materials using centrifugation are not representative of the shelf-life stability of the product’[1].

Our latest study confirms this assertation, using both Newtonian and viscoelastic samples from a cross-section of application fields.

Predicting Formulation Stability with Centrifugation

The study was geared to observe comparative formulation stability between samples at rest and samples subject to centrifugation. Samples were graduated into glass cells and left in temperature-controlled conditions (22°C) until a 1mm layer of creaming or sediment was observed by the naked eye. A prediction of sample shelf-life was determined by extrapolating data from samples subjected to centrifugal separation. This enabled analysts to compare predictions based on centrifugal separation to the real formulation stability.

Results overwhelmingly confirmed the established academic consensus that centrifugation analysis cannot accurately simulate the gravitational separation phenomena impacting a sample while at rest. Prediction errors ranged from a factor of -7200% to 1100% across the full range of sample types. The study found that half of all cases were under-estimated due to centrifugation possibly artificially damaging the material structure and rapidly increasing the rate of instability. Similarly, formulation stability was overestimated in a fifth of cases – a factor that was explained by the back-flow effect of settling particles within the material.

Artificial mechanical agitation introduces far too many measurement inaccuracies to predict formulation stability with any certainty. Assessing formulation stability while at rest is vastly preferential, but this can be costly and time-consuming – unless conducted using TURBISCAN® stability analyzers.

Measuring Formulation Stability with Formulaction

Formulaction is the leading supplier of instruments designed to test the instability mechanics effecting complex formulations while at rest. Our innovative TURBISCAN® analyzer is the very first patented instrument for monitoring formulation stability on actual unstressed products, with extremely accurate and rapid acquisition of results.

If you would like any more information, please do not hesitate to contact us directly. 

[1] Size segregation in a Fluid-like or Gel-like suspension settling under gravity or in a centrifuge Langmuir 2008, 24, 13338-13347

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