All emulsions, with the exception of micro-emulsions, are thermodynamically unstable. The most critical factors that impact emulsion or suspension stability are density differences between the oil/water phase, viscosity of the product (yield value- zero shear viscosity) and the particle size of the dispersed phase. For water thin products the particle size is probably the most important factor along with density differences. If the particle size is around 100 nm Brownian motion will normally prevent separation of the dispersed phase, especially if the density differences are small.
Simple, Useful Predictive Emulsion Stability Tests
- The viscosity of the product measured hot at the highest stability temperature is very predictive of high temperature stability. Heat stable products will retain most of their viscosity hot.
- -15°C to 45°C product cycling is a very stressful test that can be useful. Good formulations can normally handle 3 cycles.
- -15°C to 25°C- Monitor sample looking for water/oil syneresis or excessive product graininess.
- Use tall containers to stress gravitational effects.
- Use a Shaker table for testing vibrational stability. It is good for testing solid suspensions.
Instrumental Methods for Emulsion Stability Tests
You need to validate instrumental methods using real world suspension stability data with similar types of formulations.
- With the DualCon III you can record voltage potential changes of aqueous dispersions and emulsions over time at different temperatures and with multiple samples. These changes correlate well with structural changes and stability.
- Beiersdorf’s rheological swing test comparing G’ (storage modulus) and G” (loss modulus) using a cone plate rheometer. Stable products show no change after 4 cycles going from -10°C to 50°C.
- Tracking particle size over time is generally the best measurement of nano-emulsion stability. Typically, if an emulsion has less than 5% increase in particle size over the first 2 weeks of storage, it will remain stable for at least a year in 93% of the emulsions tested. Most stable nano-emulsions have a particle size of < 70 nanometers.
- Turboscan Classic MA2000 is an instrument that measures the near IR backscatter of dispersions that can be used to predict stability. Changes in back scatter across a sample from top to bottom can be indicative of coalescence, creaming, sedimentation or flocculation.
- Rheolaser Lab is a novel microrheology-based instrument that measures the visco-elastic properties of materials through the movement of particles due to thermal energy (Brownian motion). Microrheology can provide microscopic structural changes which can be used to predict stability. The unit can also measure up to six samples at a time, as well as multiple measures on a single sample over time since the measurement is non-contact/non-destructive.
- The LumiSizer is a dispersion analyzer that can simultaneously determine suspension stability and particle size distribution. It has a centrifuge to accelerate instability and it can measure separation of up to 12 samples simultaneously by near IR or blue light. It is said to be excellent for predicting products with slow separation of months to years. The LumiSizer is an improved version of the Lumifuge device that can’t do particle sizing. You should note that careful validation needs to be done for each formulation tested to predict the stability profile of a formulation.
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