Try to use Lecitin as co-emulsifier, and increase the total quantity of emulsifiers

you are using above 10% p/p

You should build a ternary diagram (water, surfactant and oil) to find the microemulsion region. See Handbook of Microemulsio Science and Technology (P. Kumar and KL Mittal, Editors) Marcel Dekker 1999. ISBN: 0-8247-1979-4. Good luck!

First, re analyse the selection of the surfactant, amont and possible co-surfactant, as mentioned by Federico Svarc.

It is also quite important the amount of surfactant to be added, which in case of micro emulsions could be relative high, but dependent on the system (some to study as a screening experiment as the suggestion of Francisco Lopez-Serrano ).

More than a kinetic problem, it could be a thermodynamic problem; take into account that there is a relationship between solubilisation and system temperature.

In principle you are right about a potential problem about kinetics and viscosity. 'Micro''emulsions' are essentially not emulsions but solutions. You have a problem of viscosity, e.g., for polymers like CMC, which are highly in water but form a highly viscous layer in contact with water. However, you should observe dissolution at least at the interface towards the oil phase to be solubilized if it is really the issue in your case.

Titus, some disperse systems are thermodinamically stable. Noteworthy micro and nanoemulsions for certain

formulations. In these cases viscosity and kinetics are not the main factores.

Federico

dispersions are never thermodynamically stable, neither nanoemulsions, which, e.g., suffer from Ostwald ripening. Not thermodynamically stable can mean very very metastable, even also over centuries, e.g. so called 'thermodynamically stable' Pickering emulsions. 'Micro''emulsions' are thermodynamically stable solutions. https://www.researchgate.net/project/nanoparticle-solutions-search-for-a-perpetuum-mobile-there-are-no-particle-solutions-only-particle-dispersions

https://pubs.rsc.org/en/content/articlelanding/2012/sm/c2sm06903b/unauth

So there is at least agreement in that nanoemulsions are thermodynamically unstable. The problem starts when using a different terminology. If you consider dispersion meaning dispersed in the sense of distributed, solutions are also (molecular) dispersions. In the classical sense of colloid chemistry dispersions are related to a phase dispersed in another as phase having molecules in bulk and at the interface. The latter state is energetically less favorable leading to thermodynamic instability.

Term 'micro''emulsions' is highly misleading. Whereas most people are aware that micro is nonsense there is a widespread believe that these are still emulsions - a liquid phase dispersed in another one. Unfortunately, the theoretical description usually starts with assumed very small 'droplets' and ultralow interfacial tension. Considering 'micro''emulsions' as micellar solutions this is not necessary and in my view less contradictory.

Titus, I agree that the term "mictoemulsion" is missleading.

What I tried to point out is that under certain experimental circumstances "chemical work" at the interfaces makes spontaneous and thermodynamically favourable the formation of very small droplets in a stable phase, without any need of mechanical work.

In this case all the phenomena you pointed out (valid in normal emulsions) is not the driving force.

Thank you all for your help. I tried different surfactant mix and managed to obtain microemulsions (though I still have some issues characterizing it by DLS size measurement), using a combination of Span 80/Tween 80 in higher mass ratio and by increasing homogenization time before adding the aqueous phase.

Of note, it seems that after waiting a long time (>1 day), the very viscous emulsion I started from (with PGPR as a surfactant) lost its turbidity (without showing phase separation), suggesting that there might be a delay to microemulsion formation when the oil phase is very viscous. I did not however investigate it.

Federico Svarc, thanks for the advice. Since soy lecithin products appear to have a lot of different applications and formulations, do you have a specific type/supplier for robust and repeatable microemulsions?

I used to purify commercial soy lecithin extracting with organic solvent (acetone) its soybean oil contents (about 30 % p/p).

See: J.Dermatol Stud Treat 2018)1:01-09

I can recommend Lipoid as a supplier of standardized phospholipid products