Yes, an interesting topic.

First, one should distinguish between microemulsions, which are thermodynamically stable, and nanoemulsions, which also have very small particles but are kinetically (not thermodynamically) stable. Though this can be regarded as the standard terminology, there has often been confusion about the term “microemulsion”; you can find many examples where it has been used for systems that are not in fact thermodynamically stable.

The key to the thermodynamic stability is a very low interfacial tension at the oil-water interface. This requires optimisation of the surfactant/cosurfactant.

Consider emulsification of oil in water. New oil-water surface is created, which has a positive free energy change (thermodynamically unfavorable). However, if the surface tension is very low this unfavorable contribution is small. Dispersing the oil in water increases entropy and is thermodynamically favorable. If the entropic effect is outweighs the creation of new oil-water surface, it will be thermodynamically favourable for the oil to disperse as small droplets. The same basic argument applies to water-in-oil and bicontuous microemulsions.

For further details see: McClements, Soft Matter, 8, 1719 (2012) (DOI: 10.1039/c2sm06903b)

The thermodynamics of microemulsions are usually finely balanced. To understand the system in detail, additional effects need to be taken into account, e.g. solubility of surfactant and co-surfactant in oil and water, interaction between the droplets. Because of the fine balancing, the stability is sometimes very sensitive to changes in temperature, salinity or surfactant structure.

An alternative approach is based on phase diagrams. It is useful especially for micromulsions near the “optimum conditions”, i.e. with fairly similar amounts of oil and water. This approach starts from a consideration of the behaviour of the surfactant-water, surfactant-oil and oil-water systems at equilibrium. Then these systems are combined. To understand how this works you will need to look at the diagrams. This can be found (for example) in Klemmer’s book on microemulsions in cleaning (https://cuvillier.de/de/shop/publications/6877 chapter 2 [sample chapter]).

Hello Azka Gull :

I would like to give my personal opinion to this question :) Hope it can help to build up in a nice discussion.

I wouldn't say Microemulsions are Thermodynamically Stable in general. My picture of a Microemulsion is a dissolution (in general) with colloids (or nanocolloids) embedded in it. I hope this picture could be approximately correct enough.

So, a colloid, or colloidal particles act somehow stochastically. And I believe they don't find thermodynamic equilibrium between conventional periods of time, rather when you consider extremely brief periods of time.

But when we observe these systems in our day-to-day experiments in the laboratory, I would say they they do not show measurable thermodynamic equilibrium states.

Rather, I believe you would have to get involved with Non-equilibrium Thermodynamics, if you want to attempt to characterize one state of these kind of particles. I guess you can arrive to a definition of a state of the system considering its thermodynamic variables, approaching to the system as non-equilibrium. But being limitated by a description of its thermodynamic variables just as an extrapolation, just as if the system were in equilibrium, if that makes any sense.

In addition, there is another branch of Thermodynamics named Stochastic Thermodynamics, which is the approach you would have to take if you want to describe thermodyncamic magnitudes of an individual particle (like a colloid), which behaves stochastically in a medium (Brownian Motion). It's possible to study thermodynamic magnitudes as for Heat or the Entropy of this particle, but defenitely Classical Thermodynamics would be helpful anymore in this scenario.

For example, a Colloidal particle coupled to an Active Bath, transferring Heat and Entropy to the bath.

Best Regards ! :)

Hi.

This is an important question.

Not all microemulsions are thermodynamically stable, but self-microemulsifying emulsions (I will call them SMEs), are thermodynamically stable. (In the interest of semantics, I am not trying to separate micro- vs. nanoemulsions, etc., so please take the terminology as general.)

Microemulsions typically require energy to form.. it may be a relatively small amount of energy or it may be a lot of energy (say from microfluidizations or ultrasonics) depending on the globule sizes (smaller creates more surface area and thus requires more energy). Of course, it also depends on the chemistry (for example, the oil vs. surfactant vs. external phase for an o/w emulsion will affect the surface/interfacial energies). That being said, itt is certainly possible to have thermodynamically unstable emulsions that change toward an equilibrium configuration quite slowly (so-called kinetic stability). This can be slowed even more if polymers are added to the syste-- for instance, to provide a steric barrier to globule aggregation.

As a general rule for these systems (but far from a hard law of nature), the interfacial energy contributes more to the Gibbs energy than the increase in entropy due to forming smaller globules. More globules increase the entropy of mixing (globules in water, for instance if you are considering an o/w emulsion), but the interfacial energy associated with that is even higher than the T x Delta S contribution.

Self-microemulsifying systems spontaneously form with even mild agitation (so the energy input is negligible). Systems that emulsify spontaneously typically require a higher surfactant-to-oil ratio than emulsions that can be made with sufficient energy input. (Again, using an o/w emulsion as an example.)

A convenient picture, although not a rigorous physical model, is that for an amount of surfactant that is well above the CMC, the same tendency to form micelles leads to forming micelles with oil cores. (Unlike micellar solubilization of drug molecules, the oil cores tend to be continous and not individual molecules).

Hopefully, this will add some food for thought to the discussion!

Yes, an interesting topic.

First, one should distinguish between microemulsions, which are thermodynamically stable, and nanoemulsions, which also have very small particles but are kinetically (not thermodynamically) stable. Though this can be regarded as the standard terminology, there has often been confusion about the term “microemulsion”; you can find many examples where it has been used for systems that are not in fact thermodynamically stable.

The key to the thermodynamic stability is a very low interfacial tension at the oil-water interface. This requires optimisation of the surfactant/cosurfactant.

Consider emulsification of oil in water. New oil-water surface is created, which has a positive free energy change (thermodynamically unfavorable). However, if the surface tension is very low this unfavorable contribution is small. Dispersing the oil in water increases entropy and is thermodynamically favorable. If the entropic effect is outweighs the creation of new oil-water surface, it will be thermodynamically favourable for the oil to disperse as small droplets. The same basic argument applies to water-in-oil and bicontuous microemulsions.

For further details see: McClements, Soft Matter, 8, 1719 (2012) (DOI: 10.1039/c2sm06903b)

The thermodynamics of microemulsions are usually finely balanced. To understand the system in detail, additional effects need to be taken into account, e.g. solubility of surfactant and co-surfactant in oil and water, interaction between the droplets. Because of the fine balancing, the stability is sometimes very sensitive to changes in temperature, salinity or surfactant structure.

An alternative approach is based on phase diagrams. It is useful especially for micromulsions near the “optimum conditions”, i.e. with fairly similar amounts of oil and water. This approach starts from a consideration of the behaviour of the surfactant-water, surfactant-oil and oil-water systems at equilibrium. Then these systems are combined. To understand how this works you will need to look at the diagrams. This can be found (for example) in Klemmer’s book on microemulsions in cleaning (https://cuvillier.de/de/shop/publications/6877 chapter 2 [sample chapter]).

Microemulsions BY DEFINITION are thermodynamically stable. There is no uncertainty about that. They form spontaneously and do not coalesce. They do NOT require energy to form.

Read this: http://www.chm.bris.ac.uk/eastoe/Surf_Chem/3%20Microemulsions.pdf

(The author is a leading authority on this topic.)

The problem is that non-colloid science disciplines don't know the scientific history. It is polluting the literature with bad science.

If your "microemulsions" do not form spontaneously then you are using the wrong terminology. The introduction of the term "nanoemulsion" has led to much ignorance. It is a terrible situation.

I agree with everything Dennis John Miller says, not because of his name.

I do not want to sound harsh but other answers to your questions are just wrong.

I agree with both of the above answers (Miller and Miller)... while I tried to qualify my answer with regard to semantics, I could have been more clear.

I stand by the notion that not all emulsions on a sub-micron scale are stable, but those that self-emulsify are stable. However, in trying to make the distinction (which would be between nanoemulsions and microemulsions), I neglected to acknowledge that the term MICROEMULSION has a specific meaning, defined to form spontaneously and be thermodynamically stable (plus optically clear, which specifies the size range).

Sorry for the semantic error, but I'm glad it was caught and I don't mind at all being called on it.

Thanks!

Robert A Bellantone It seems you and I struggle with the same issue, namely having to explain the original meaning of terms used.

It used to be simple: "This is a microemulsion." Everyone knew that it meant one thing and one thing alone. Now I have to say "This is a microemulsion. By that I mean a thermodynamically stable...etc etc". There was no confusion over nano, micro, macro etc.

The advent of "nano" for nearly everything has created chaos.

I feel I have to be a bilingual translator between colloid science and the technological fields that have adopted it (e.g., nanomedicine) but confused it. The first paper I read about "protein corona" left me blank. It took a lot of additional reading to even understand what the phrase was refering to. Once I found out, I realized it was just the same as had been studied in the 1970s-1990s but with a more meaningful description and based on robust thermodynamics. I have yet to see that large body of research referenced in modern nanomedicine papers. To make matters worse, leading figures in nanomedicine who champion "protein corona" and ignore previous research are editors for flagship colloid journals.