Lets Mix Oil and Water Part 2: Thermodynamics

Emulsions are so fun we had to split it into two blog posts!
This post will focus and go more in depth on emulsions, particularly the thermodynamics of making emulsions:

Emulsions can be considered from a thermodynamic standpoint as well.
They are considered metastable - i.e. the emulsion has a thermodynamic drive to return to the state of lowest energy. The only point in time where an emulsion is completely stable is when it is separated. Thus, mixing two liquids will have a thermodynamic effect.

If two liquids are completely compatible, they do not form an interface, which is the surface between two different phases. In this case, the free energy of mixing is negative, because the two compatible liquids want to be together, and release energy when they mix.
However, if two incompatible components are mixed, they do form an interface, and the free energy of formation is positive because these two liquids do not want to be near each other, and thus require energy to be mixed together. As in the previous blog post, you saw how emulsifiers cause small micelles, or little balls of oil to form in the water.

Micelles are small droplets surrounded by an emulsifier suspended in liquid

This causes an increase in interfaces, because now each droplet has its own interface surrounding it, as opposed to before, when the separate layers only had a single interface between the two. The increase in interface causes more interactions between two substances that don't want to be together, and so clearly the energy and thermodynamics will be affected.

In order to get an idea about the thermodynamics of an emulsion, we can look at the Gibbs free energy of the system, which, as we learned in class, is the supreme equation of all of life, and is represented by the following equation:

The entropy (∆S) is a measure of the extent of disorder in the system and in the mixing of two liquids measures the size reduction of the droplets (or increase in the number of droplets). During the formation of an emulsion, we want to increase the number of droplets, and make each droplet smaller, and so ∆S is going to be positive.
∆H is the enthalpy of the system, and can basically be considered the energy input needed to achieve a certain average droplet size, and can be expressed by γ∆A, where γ is the tension of the interface, ∆A is the change in area of the interface, T is the temperature, and ΔS is the entropy of mixing.
Therefore, the thermodynamics of emulsions can be expressed as a derivation of the Gibbs free energy equation:

ΔG = (γ∆A) – (TΔS)


ΔG gives us information about the stability of the emulsion. If ∆G is positive, then energy is required, and so spontaneous emulsification is highly unlikely, similar to a ball spontaneously rolling back up a hill. If ∆G is negative, however, then spontaneous emulsification will occur. This happens with two liquids that are miscible, and will readily mix.
As stated above, if two incompatible components, like oil and water, are mixed, an interface is formed, and the ∆G is positive, so it will always be non-spontaneous, i.e. no matter how long you leave a bottle of salad dressing on the counter, the oil and water will never magically mix. However, the closer ∆G is to zero, the easier the emulsification process.

The oil and water here will never mix unless you add some energy to the system

Let's try to better explain ∆G and emulsion formation with some fun diagrams!


Free energy of emulsion formation (between components 1 and 2)


In general, γ∆A >> T∆S


and so ∆G >> 0


the formation of an emulsion requires energy








Free energy of emulsion breakdown





∆Gbreak << 0
(if ∆Gform >> 0)



Breakdown is spontaneous






Considering the thermodynamics of emulsions is important because it can help cosmetic companies figure out how their lotions and creams will react over time. Because breakdown of an emulsion is spontaneous, ultimately, all lotions and creams will separate into their separate components, but the addition of emulsifiers can slow this process down, or make it easier to evenly mix the oil and liquid by lowering ∆G.

Watch out! That lotion you're using is thermodynamically unstable, and will break down into its oil and water components eventually

jennymu