Stability of Emulsions
Stability of emulsions (including the composition and microstructure) is a key element for evaluation of the lifetime and temperature conditions for the storage and use of emulsion based products. Ultrasonic measurements allow very simple procedures for the evaluation of the stability of emulsions.
In this experiment, ultrasonic analysis of the thermal stability of a pharmaceutical water/oil emulsion (0.5mm) was performed during gradual heating of the sample. The changes in the attenuation and relative ultrasonic velocity were measured. The arrows in the above figure indicate the temperature corresponding to the destabilisation of the sample. (Temperature measurements were taken at various frequencies in the range 2-15 MHz). The rise in the attenuation at 44°C provides clear evidence of restructuring in the emulsion. The increase in the attenuation can be attributed to the flocculation of dispersed aqueous droplets induced by heating. As seen above, the change in ultrasonic velocity deviates from the baseline at the same temperature that the ultrasonic attenuation begins to rise. This demonstrates the sensitivity of both ultrasonic parameters (attenuation and velocity) for accurate characterisation of the emulsion. Another advantage of the ultrasonic technique is the ability to make measurements directly, in the original emulsion, without dilution, commonly required when using optical techniques.
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Analysis of Microemulsions
This example describes the application of high-resolution ultrasonic spectroscopy for analysis of the formation of a microemulsion in a system consisting of pharmaceutically accepted components, namely, isopropyl myristate, lecithin (Epikuron 200) and n-propanol as a cosurfactant in a w/w ratio 6:1:1 and water. The measurements were performed at 20ºC using HR-US 102 spectrometer equipped with standard 1ml cell in the frequency range 2-20 MHz.
The figure above shows a typical dependence of ultrasonic velocity (squares) and attenuation (circles) on concentration of water in the system (at 5 MHz). Molecular dissolving of water in the mixture at low concentrations (up to 2 wt%), which is accompanied by hydration of lipid and cosurfactant as shown by the steady increase in ultrasonic velocity. Following this a microstructural reorganisation takes place, indicated by an additional increase in ultrasonic velocity in the range between 2% and 6% of concentration of water. Formation of the microemulsion can be seen from approximately 8 wt% with ultrasonic attenuation quickly increasing, which is caused by the scattering of the ultrasonic wave on the particles. It should be noted that when compared with light scattering we found good agreement above the break point, while below the break point the possible structuring of the system is not observable with the light scattering. This shows the potential of the technique in providing information previously unattainable by method of analysis.
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