Jonathan J. Booth, M. Omar, Steven Abbott
Mar 11, 2015
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Journal
Physical chemistry chemical physics : PCCP
Abstract
Nicotinamide is an effective non-micellar hydrotrope (solubilizer) for drugs with low aqueous solubility. To clarify the molecular basis of nicotinamide's hydrotropic effectiveness, we present here a rigorous statistical thermodynamic theory, based on the Kirkwood-Buff theory of solutions, and our recent application of it to hydrotropy. We have shown that (i) nicotinamide self-association reduces solubilization efficiency, contrary to the previous hypothesis which claimed that self-association drives solubilization and (ii) the minimum hydrotrope concentration (MHC), namely, the threshold concentration above which solubility suddenly increases, is caused not by the bulk-phase self-association of nicotinamides as has been postulated previously, but by the enhancement of nicotinamide-nicotinamide interaction around the drug molecules. We have thus established a new view of hydrotropy - it is nicotinamide's non-stoichiometric accumulation around the drug that is the basis of solubility increase above MHC.