When I understand it right interfacial tension is defined for a pair of two phases. When there is a third phase I think only an 'apparent interfacial tension' can be measured directly.
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Dear Dr Sobisch,
It is a question that theoriticians like very much. The answer is, if your measuring tool is much larger than your particles, you can use the average surface energy including the wetting energy of the particles, there is no problem. In a small system, where the size of the particles is comparable to the size of the droplet, things become more complicated. There the macroscopic free energy of the surface becomes dependant on droplet size. You can check the work of A. Marmur regarding Wenzel equation, it is a similar problem.
Regards
Dear Radomir,
I do not think that it is this way, i.e. our measuring device in ratio to the particles. I would assume that it is about the ratio of the particles relative to molecular size. With some assumptions of course one could fit anything with a theoretical approach.