Solubilization of hydrophilic compounds in copolymer aggregates

The solubilization of five hydrophilic water-soluble aroma compounds in sel f-aggregating triblock amphiphilic copolymers of poly(ethylene oxide)-poly( propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO), with similar percentag es of PEO and different molecular weights, was studied. The five hydrophili c compounds (diacetyl, 2-methylpyrazine, pyrrole, furfural, guaiacol) were carefully selected to represent hydrophilic molecules with a similar molecu lar weight and molecular volume, but with different abilities to interact w ith 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 increa sing free solute concentration in the aqueous phase. The partition coeffici ents were smaller than those obtained for hydrophobic compounds and equilib rium was reached at lower solubilization values. Guaiacol was the least hyd rophilic molecule and had the highest partition coefficient. Diacetyl was t he most water-soluble compound and exhibited the smallest partition coeffic ient, The data reveal that the higher molecular weight polymers solubilized more solute than the low-molecular-weight polymers. Moreover it is suppose d that at low solute concentrations, guaiacol (containing a hydroxyl electr on acceptor group) penetrates the core of the micelle and displaces water w hile at more elevated concentrations it seems to be solubilized in the mice lle 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 conc entration. 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 cond uct surface-sensitive organic reactions. Furthermore, selective release pro perties of reactants and products can be designed. (C) 2000 Academic Press.