Amphiphilic properties of hydrotropes derived from isosorbide: endo/exo isomeric effects and temperature dependence.

The hydrotropic properties of short-chain monoalkyl ethers of isosorbide have been studied with emphasis on the difference between the two regioisomers (2-O- and 5-O-monoalkylisosorbide, C(i)Iso-exo and C(i)Iso-endo, respectively). On the one hand, the partitioning in a water/cyclohexane system has been measured, and on the other hand, the "optimal formulation" of C(i)Iso/oil/water systems has been determined by changing the oil polarity. In both cases, whatever the alkyl chain length (four or five carbons), C(i)Iso-exo appears to be less hydrophilic due to an intramolecular hydrogen bond that makes the free hydroxyl group less available. With one substituted position, the hydrophilicity contribution of isosorbide is slightly higher than the one of two ethylene glycols when the exo hydroxyl remains free, whereas it is closer to one ethylene glycol when this hydroxyl is substituted. With regard to the sensitivity to temperature, the hydrophilicity loss on heating seems to be similar for both isomers and close to what is obtained for poly(ethylene glycol) derivatives if we consider the evolution of the partition coefficients. In C(i)Iso-endo/oil/water systems, however, the variation of the optimal oil with temperature tends to show that the isosorbide derivatives are approximately 2 times less sensitive than their poly(ethylene glycol) counterparts to temperature changes.