You add the minimum amount of surfactant to obtain a stable emulsion and for that you need to do experimental work starting with surfactant and co-surfactant in different ratios monitoring both the physical and chemical stability of your solution.

Hi there!! but if your microemulsion is already clear, why do you need to add more surfactant and cosurfactant? In any case, you can try with the titration method and add surfactant/cosurfactant until the clear and stable microemulsion is formed. I would not leave it with the minimum amount of surfactant/cosurfactant, I would add a bit more. Also, you don't always need cosurfactant, it depends on your system. Also take into account the temperature at which you will run your HPLC, you don't want your microemulsion to destabilize in your column. What is your mobile phase? will your microemulsion remain stable once in contact with your mobile phase? Probably you need to run stability tests by diluting your microemulsion with your mobile phase. Certain types of O/W microemulsions can be diluted to infinity with water and they remain stable.

Thanks margarita and Andrei Blasko