This is something I wanted to put out there. A colleague of mine reported a situation in which his flexible, complex organic molecule was becoming more soluble in water at low temperature and he was wondering why.
The most straightforward explanation that comes to my mind is this. When the molecule is flexible it naturally exists in several conformations in solution. The lipophilic conformations are going to be higher in energy since they are exposing non-polar groups to aqueous solvent. Conversely, the conformations that are nicely solvated and expose charged or polar groups to solvent are going to be low-energy.
At low temperatures, the higher-energy lipophilic conformations become inaccessible because of less energy in the system, leading to a preponderance of the low-energy polar conformations which are more soluble. Ergo the molecule becomes more soluble.
This can be studied by a couple of different ways, most notably by changing the solvent and altering the conformer population; previous studies have indicated that changes in solvent (say from polar to non-polar) only change populations, and not the conformations themselves.
Any other explanations?
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