Investigations of naphthalene solubilization in aqueous solutions of ethylene oxide-b-propylene oxide-b-ethylene oxide copolymers

The enhanced apparent solubility of naphthalene in aqueous solutions of several ABA block copolymeric surfactants has been measured using HPLC. The surfactants investigated combine, within their structure, a block of propylene oxide (PO) (the hydrophobic B block) sandwiched between two blocks of ethylene oxide (EO) (the hydrophilic A blocks). This commercially produced family of surfactants encompass a variety of materials which differ from each other in terms of block sizes and thus in terms of the balance of hydrophobic and hydrophilic forces. It is this balance which controls the ability of the surfactants to effect particular solubility enhancements. Substantial increases in solubility arise from the incorporation of naphthalene into block copolymer micelles. However the experimental data also point to solubility enhancements arising from naphthalene-surfactant interactions at surfactant concentrations below the critical micelle concentration (cmc). The apparent equilibrium constant-describing the solute distribution between the aqueous phase and the surfactant-that characterizes this interaction correlates well with the surfactant cmc. It is concluded that the more hydrophobic EO-PO-EO copolymers produce micellar environments that are more favorable for naphthalene incorporation compared to the hydrophilic members of the family and that surfactant-naphthalene interactions below the cmc can substantially increase apparent aqueous solubilities. Experimental determinations of cmc values were measured by reductions in surface tension and by high-sensitivity differential scanning calorimetry and compared with the values estimated by naphthalene solubilization. The values obtained by the three methods are comparable. The data clearly demonstrate that cmc values are strongly dependent upon molecular composition and molecular size and that the cmc values are largest for small molecules with large hydrophilic blocks.