Solubilization of Hydrophilic Compounds in Copolymer Aggregates

The solubilization of five hydrophilic water-soluble aroma compounds 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 molecular weights, was studied. The five hydrophilic compounds (diacetyl, 2-methylpyrazine, pyrrole, furfural, guaiacol) were carefully selected to represent hydrophilic molecules with a similar molecular weight and molecular volume, but with different abilities to interact with the micellar core of PPO moieties and with the PEO palisade side chains. It was found that the solubilized solute mole fraction increased and the aggregate-water partition coefficients of the solutes decreased with increasing free solute concentration in the aqueous phase. The partition coefficients were smaller than those obtained for hydrophobic compounds and equilibrium was reached at lower solubilization values. Guaiacol was the least hydrophilic molecule and had the highest partition coefficient. Diacetyl was the most water-soluble compound and exhibited the smallest partition coefficient. The data reveal that the higher molecular weight polymers solubilized more solute than the low-molecular-weight polymers. Moreover it is supposed that at low solute concentrations, guaiacol (containing a hydroxyl electron acceptor group) penetrates the core of the micelle and displaces water while at more elevated concentrations 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 coefficient is independent of the free solute concentration. Selective site (palisade vs core) solubilization of hydrophilic compounds 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 organic reactions. Furthermore, selective release properties of reactants and products can be designed. Copyright 2000 Academic Press.