SOLUBILIZATION OF AROMATIC SOLUTES IN BLOCK-COPOLYMERS

Several factors affecting solubilization of aromatic solutes in triblock copolymers (PEO(n)- PPOm-PEO(n) or PPOm-PEO(n)-PPOm) known to form micelles have been investigated. Solubilization isotherms of toluene, benzene, chlorobenzene, and p-xylene in various aqueous polymeric solutions were determined using head-space gas chromatography. The solute partition coefficient in the polymer, K-s, was shown to have a strong dependence on the solute concentration for all polymers. The amount solubilized was at its highest for the high molecular weight, hydrophobic polymers. For the less hydrophobic or lower molecular weight polymers, it was determined that solubilization capacity was a function of the polymer concentration. Comparisons between the different solute solubilization isotherms indicate that PPO-PEO, polymer-water, and polymer-solute interactions have a strong influence on aggregation and solubilization. Finally, the solubilization studies show distinct differences between conventional surfactant micelles and polymeric surfactant micelles, thereby indicating two different solubilization processes. To best understand the mechanism for solubilization in the polymeric surfactant solutions, it was postulated that (1) the addition of apolar solutes promotes aggregation of the polymeric surfactant molecules, (2) the central core of the polymeric micelles contains some water molecules, and (3) solubilization is initially a replacement process in which water molecules are displaced from the micellar core by the solubilizate.