Accelerated Tests and Their Limitations

With increasing temperature the viscosity of the system usually decreases, and hence sedimentation or creaming is accelerated. The assumption is usually made that if a suspension or emulsion does not show any sedimentation, creaming or separation at 50 °C for say one month, then the system will show no separation at ambient temperatures for more than one year. 

The above method is only vahd if the formulation viscosity t] follows the Arrhenius equation, which predicts a linear increase in Ini with (1/T), where T is the absolute temperature. Most practical formulations do not follow such a plot due to the possible phase changes or flocculation that may occur at high temperatures. With many surfactant systems, such phase changes may result in 

Formulation of D erse Systems Science and Technology, First Edition. Tharwat F. Tadros. 

This method, if carefiiUy studied, may offer a better accelerated method and has been particularly applied to emulsions. The assumption is also made here that, by increasing the g-force, the rate of sedimentation or creaming is significantly increased, and that this could be applied to predict the process from measurement at short time periods. 

Modern analytical ultracentrifuges allow the separation of emulsions to be followed in a quantitative manner. With typical oil-in water (O/W) emulsions, three layers are generally observed (i) a clear aqueous phase (ii) an opaque phase consisting of distorted polyhedral oil droplets and (iii) a clear separated oil phase, resulting from coalescence of the polyhedra. 

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