I constructed a pseudo ternary diagram and identified the microemulsion range. Then, I formulated microemulsion-based ibuprofen with the ratio of oil/surfactant/cosurfactant in the microemulsion range, but it turned turbid, while the formulation of its ratio without ibuprofen was clear.
And, when I increased the amount of Smix (surfactant+cosurfactant), it turned turbid (Smix 25%), then clear (Smix 30%), then turbid (Smix 35-45%), then clear (Smix >50%).
Microemulsions are systems of strictly define composition. They are very sensitive to any changes (components, temperature and others). Water-in-oil microemulsion can be made to pass to oil-in-water one by the addition of second surfactant, cosurfactant, changes of temperature etc.
I think this may be the case in your experiment.
If you prepared microemulsion with particular concetration of surfactant+cosurfactant (Smix) and system was clear. Then after addition of ibuprofen (solid) it became turbid. After addition suitable amount of surfactant ibuprofen was fully solubilized and system became clear ones again. When additional amount of surfactant was added then inversion c@ould appeared. It would be justification for was turbidity first and then becoming clear solution.
The incorporation of certain additives to microemulsions may reduce their water solubilization capacity, specially if they strongly interact with the surfactant. It seems like the system is not stable anymore for this content of aqueous ibuprofen.
You might need to check again the stabilization by reducing the concentration of ibuprofen, by increasing the amount of surfactant, or by changing the type of surfactant.
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