Macroemulsions demulsification

Macroemulsions tend to separate into two phases. This is called demulsification. Demul-sification can be very slow so that even a macroemulsion might appear stable. Often macroemulsions are just called emulsions because all classical dispersions of oil and water were macroemulsions. In this section we use the term emulsion if it concerns a property of macro- and microemulsions. If we discuss the properties primarily of macroemulsions we use the full term. From the practical point of view micro- and macroemulsions are very different. That we discuss some properties together should not hide this fact. 

Freshly prepared macroemulsions change their properties with time. The time scale can vary from seconds (then it might not even be appropriate to talk about an emulsion) to many years. To understand the evolution of emulsions we have to take different effects into account. First, any reduction of the surface tension reduces the driving force of coalescence and stabilizes emulsions. Second, repulsive interfacial film and interdroplet forces can prevent droplet coalescence and delay demulsification. Here, all those forces discussed in Section 6.5.3 are relevant. Third, dynamic effects such as the diffusion of surfactants into and out of the interface can have a drastic effect. 

Recovery of the concentrated mercury required demulsification of the liquid membranes. The coarse emulsions could be easily demulsified by electrostatic techniques whereas, the microemulsions required the addition of a cosolvent (18). The use of a cosolvent is very undesirable because it requires an additional separation step to remove the cosolvent (usually distillation). This particular aspect of the overall process would favor the use of macroemulsions over microemulsions. 

Summary of Significant Shortcomings. The emulsion liquid membranes studied each possess advantages when compared against one another. In specific, the microemulsion systems displayed separation kinetics which were typically an order of magnitude faster than the macroemulsion counterparts. However, tlm increased rate of separation was at the expense of product recovery. The concentrated mercury was easily recovered by electrostatic demulsification for the macroemulsion system, but required the addition of butanol to the microemulsion before mercury recovery was achieved. The requirement of chemical demulsification is a major disadvantage of the microemulsion-based liquid membrane system. 

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