Have a look on zeta potential. Stability of émulsion is correlated with this measure.

[EDIT] sorry reading too quickly the question

Dear Frederic, 

Thank you for your response. Nevertheless,the system does not contain water. It is a lipid mixture, not an emulsion.

obviously for a mixture of lipids and surfactants zeta potential is of no use. Please be aware that you will always have instability as long as you have not a solution but a dispersion. If you select the surfactants that they are fully soluble in lipid your system should be stable indefinitely if there is no change in chemical composition.

Dear Titus,

Thank you for your pertinent remark. Certainly, as you said,  the nature of the mixture is a dispersion and not a solution. In fact : Surfactant A is Hydrophilic and insoluble in oil, unlike the surfactant B which is soluble in oil.

Nevertheless, I think that stability could be related to the successful miscibility of the 3 excipients, even if we don't have an oily solution.

So the real question is how could we explain the good miscibility (stability) for some combinations in contrast to the appearance of demixion for the rest of the batch.

Cheers

B.Badr