Emulsion :
Liquid droplets dispersed in another immiscible liquid.
The dispersed phase droplet size ranges from 0.1 - 10 µ m.
Let's talk about what every one knows more : food.
Important oil-in-water food emulsions, ones in which oil or fat is the dispersed phase and water is the continuous phase, include milk, cream, ice cream, salad dressings, cake batters, flavour emulsions, meat emulsions, and cream liquers.
Examples of food water-in-oil emulsions are butter or margarine.
Actually in cosmetics, the system is the same, imagine the margarine is a facial cream. The same way you got the margarine, you get your cream by introducing a phase containing water
and other elements in an oily phase.
The unique difference between the food products and cosmetics are in raw materials used.
Even though some raw materials used in food can also be used in cosmetics, I will take an example : Argan oil you can consum but used in cosmetics, rich in vitamins E it prevents skin against aging, because anti oxydant. It is also advised for very dried skin. It also has a restruturing role. Of course, being a unique oil coming from Morocco, its price is expensive and not used in big quantity. However, it can be used as an active.
Emulsions are unstable and thus do not form spontaneously.
Energy input through shaking, stirring, homogenizers, or spray processes are needed to form an emulsion. Over time, emulsions tend to revert to the stable state of oil separated from water.
Surface active substances (surfactants) can increase the kinetic stability of emulsions greatly so that, once formed, the emulsion does not change significantly over years of storage.
Homemade oil and vinegar salad dressing is an example of an unstable emulsion that will quickly separate unless shaken continuously. This phenomenon is called coalescence, and happens when small droplets recombine to form bigger ones.
Fluid emulsions can also suffer from creaming, the migration of one of the substances to the top of the emulsion under the influence of buoyancy or centripetal force when a centrifuge is used
Emulsions are part of a more general class of two-phase systems of matter called colloids. Although the terms colloid and emulsion are sometimes used interchangeably, emulsion tends to imply that both the dispersed and the continuous phase are liquid.
There are three types of emulsion instability: flocculation, where the particles form clumps; creaming, where the particles concentrate towards the surface of the mixture while staying separated; and breaking, where the particles coalesce and form a layer of liquid
To recapitulate, by adding well chosen surfactants you can realise an emulsion by "mixing" a water phase and a oily phase all together, thing that is normally impossible :D (imagine a water drop added to your tournesol oil..they don't mix)
Actually the two phases are not mixed in reality, but from a human eye the two phases seem "mixed". They are maintened by the surfactants.
A little practical schema :
A : an oily phase B is added to a a water phase A.
Two immisicible liquids, not emulsified
B : With shaking strongly you can have an emulsion of phase B dispersed in phase A.
C : But because the emulsion is unstable, the two phases tend to separate from each other since the tension between the two interfaces is too strong.
D : By adding surfactants, the surfactants (purple outline) position themselves on the interfaces between the two phases, stabilizing the emulsion.
source : http://en.wikipedia.org/wiki/Emulsion
I will talk about those impressive surfactants in the next chapter. By their structure, you will understand their properties detailed above.
For those who discover formulation, I tried not to make it sound too complicated, anyway if you have questions, feel free to ask :-)
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