Abstract:

The solubilization of 5 hydrophilic water-sol. aroma compds. in self-aggregating triblock amphiphilic copolymers of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO), with similar percentages of PEO and different mol. wts., was studied. The 5 hydrophilic compds. (diacetyl, 2-methylpyrazine, pyrrole, furfural, guaiacol) were carefully selected to represent hydrophilic mols. with a similar mol. wt. and mol. vol., but with different abilities to interact with the micellar core of PPO moieties and with the PEO palisade side chains. The solubilized solute mole fraction increased and the aggregate-water partition coeffs. of the solutes decreased with increasing free solute concn. in the aq. phase. The partition coeffs. were smaller than those obtained for hydrophobic compds. and equil. was reached at lower solubilization values. Guaiacol was the least hydrophilic mol. and had the highest partition coeff. Diacetyl was the most water-sol. compd. and exhibited the smallest partition coeff. The data reveal that the higher mol. wt. polymers solubilized more solute than the low-mol.-wt. polymers. Moreover it is supposed that at low solute concns., guaiacol (contg. a hydroxyl electron acceptor group) penetrates the core of the micelle and displaces water while at more elevated concns. it seems to be solubilized in the micelle corona. Diacetyl, the most hydrophilic solute investigated (consisting of electron donor groups), prefers mainly the corona since its affinity for the polymeric core is very weak. The solubilization occurs in the palisade layer and the partition coeff. is independent of the free solute concn. Selective site (palisade vs. core) solubilization of hydrophilic compds. in polymeric micelles can be a powerful tool to protect sensitive materials from reactants present in the continuous water phase and to conduct surface-sensitive org. reactions. Furthermore, selective release properties of reactants and products can be designed. (c) 2000 Academic Press. [on SciFinder(R)]