There is a well known, solubilising effect of impurities which was originally found during scale up processes in preparation for the launch of products because scale up reduces the proportion of impurities so companies were suddenly finding that their new compound was less soluble than they expected. The effect was used deliberately by Glaxo in the 1970s when they used a salt of a steroid added to the original steroid to make it more soluble. This was the intravenous steroid anaesthetic Althesin, in which the main active was alphaxalone and the deliberate impurity the acetate salt at about 30% of the total concentration. This was done to make the volume of injection to induce anaesthesia as small as possible. I think the mechanism is that when all the molecules are the same they space out but when there is an impurity the spacing is disrupted so more molecules are packed in.

Thanks very much Andrew Sutton, I don´t have any impurity in my system (only water, acetone and drug). The solubility of drug is low in acetone, increasing in water+acetone mixture until a maximum value and decreasing again in pure water.

This is way, i think this behavior is exotic.

I didn´t find other examples.

I agree it does sound very unusual. I was wondering if there were a small change in the molecule in acetone that mimics the impurity idea, possibly even something like a dimer/polymer that has the effect along the lines I suggested.

Diethanolamine may be less soluble in pure solvent but more soluble in mixture of solvent