Problem: Emulsions are a common working material in laboratories and industry. An emulsion consists of two or more immiscible liquids, e.g. oil and water. To produce an emulsion, one liquid – the disperse phase – has to be dispersed in the other – the continuous phase. Due to the hydrogen bonds of aqueous systems and the Vander- Waals forces of fat molecules, liquids can be hydrophil + lipophob (=easily miscable with water) or lipophil + hydrophob (= easily miscible with oil). That means that an emulsion is an unstable mixture and does not form spontaneously. To produce a stable emulsion, the droplets have to be dispersed very finely and evenly to overcome the bonding forces. Conventional stirring equipment, colloid mills and homogenizers, do not allow the preparation of emulsions with uniformly dispersed droplets as there is no direct control of the droplet size.
|Hielscher’s ultrasonic processors, such as the UIP1000hd (in the picture with glass flow cell), are reliable devices for the production of stable emulsions. The UIP1000hd is a versatile ultrasonic processor for emulsifying in the lab as well as in R&D and the industrial production line.|
Solution: High power ultrasound is known to be a reliable tool for dispersing and emulsifying smaller volumes. Ultrasonic batch processing has been used in laboratories for a long time to produce emulsions. The emulsification effect is caused by ultrasonically generated cavitation. The sound waves propagated in the liquid media result in alternating high-pressure (compression) and low-pressure (rarefaction) cycles, with rates depending on the frequency. During the low-pressure cycle, high-intensity ultrasonic waves create small vacuum bubbles or voids in the liquid. When the bubbles attain a volume at which they can no longer absorb energy, they collapse violently during a high-pressure cycle. This phenomenon is known as cavitation. During the implosion, very high temperatures (approx. 5,000K) and pressures (approx. 2,000atm) are reached locally. The implosion of the cavitation bubble also results in liquid jets of up to 280m/s velocity.
Highly intensive ultrasound thereby supplies the power to disperse the dispersed phase in small droplets in the continuous phase. Depending on the basic materials, emulsions generated by ultrasound can be self-stable or can tend to separate. In order to stabilize the newly formed droplets of the disperse phase against coalescence, emulsifiers (surface active substances, surfactants) and stabilizers are added to the emulsion. As coalescence of the droplets after disruption influences the final droplet size distribution, efficiently stabilizing emulsifiers are used to maintain the final droplet size distribution at a level that is equal to the distribution immediately after the droplet disruption in the ultrasonic dispersing zone. Stabilizers actually lead to improved droplet disruption at constant energy density. Studies in the oil-in-water (water phase) and water-in-oil (oil phase) emulsions have shown the correlation between the energy density and droplet size (e.g. Sauter diameter). There is a clear tendency for smaller droplet size at increasing energy density. At appropriate energy density levels, ultrasound can easily achieve mean droplet sizes below 1 micron (microemulsion) or can even create nano-sized emulsions with droplet size below 100nm.
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