Analytical methods to comparison of Aqueous, Micellar and Microemulsion, you can use mocroscopic methods, light scattering methods, electrochemical methods, or spectroscopic methods...

Dear Rajesh:

The name microemulsion is sometimes restricted to droplets of size sufficiently large that the physical properties if the dispersed oil  (internal phase)  are indistiguishable from those of the corresponding water bulk . When the droplet core is too small, the medium is called “swollen micellar”. When the micellar are progressively swollen, properties of the internal phase evolve smoothly towards those of a bulk phase, without any well-defined transition. Analytical tests to differentiate between micelles and a microemulsion see in

 Chimia, 1982, 36, N6. P.241.

Best regards.

In my opinion, there is no analytical method to clearly distinguish "swollen micelles" from microemulsions, since there is no agreed definiton for these two thermodynamically stable systems. Given this lack of agreement, I am confident that the same system could be classified as "swollen micelles" by one person and microemulsion by another.

Prof. Mirgorod suggests that the properties of the oil in a microemulsion are the same as in the bulk. I wonder which properties he has in mind. For instance, I doubt that the oil freezing point in a microemulsion, measured by DSC, would be the same as in the bulk .

Dear Alan:

Any ensemble of particles 1- 100 nm has other properties than the Gibbs phase. This transition is smooth like a swollen o/w micelles. At the inflection point of this function you can define a microemulsion. The chemical shift of the system H20/Aerosol OT/i-octane demonstrate an obvious transition region between the so-called micellar solution [H2O]/[AOT]->0 and the microemulsion. Eicke H.F. Microemulsions. 1982.Plenum Press. N.-Y. New understanding of the micelles surfactant into the water can be read in our article. Yuriy A. Mirgorod, Tatiana A Dolenko. Liquid Polyamorphous Transition and Self-Organization in Aqueous Solutions of Ionic Surfactants. Langmuir. 2015. V. 31, N31. P. 8535-8547.  DOI: 10.1021/acs.langmuir.5b00479

Best regards.

It is more a problem from a historic perspective. Starting from one end term solubilization when micro'emulsion' term was unknown. So, it is known for more then a century that compounds (fluids and solids) with low aqueous solubility can be solubilized into water by micelles and in similar way polar compounds into oils by reverse micelles.

Later one came up with micro'emulsions' believing that these contain droplets so small that they cannot be seen. Droplet (and particle) means that there is an interface between phases and therefore system is thermodynamically unstable. Micro'emulsions', however, are thermodynamically stable because there are no droplets and particles, however, all molecules are at the interface, which then is different from how it is defined for a dispersed system. Therefore all micro'emulsions' are either micellar or bicontinuous.

Nanoemulsions (droplets o nano size) on the other hand are already thermodynamically unstable.

Distinctions between micellar solutions and miсroemulsion make more terminological sense. And I don't recommend to focus attention on this question strongly. And the more so to look for any analytical procedures. The term "microemulsion" was entered in the forties the last century by Shulman [Hoar T.P., Schulman J.H.//Nature (London), 1943. V. 152, P. 102.] . The main difference of the reverse microemulsions from micellar solutions consists available in microemulsions of molecules of water with properties of usual "volume" water. In the reverse micelles of water or in general not (so named "dry" micelles), or all molecules of water are connected with polar groups of the surface-active substance; the water   with volume properties isn't present. However by many researches it is shown that water with volume properties can be and in the reverse micelles even at its very low general water contents. A similar situation and for direct microemulsions, only instead of water oil molecules act. Therefore personally I use these concepts for the reverse systems as synonyms.