Effect of Solvent Quality on Reverse Micelle Formation and Water Solubilization by Poly(ethylene oxide)/Poly(propylene oxide) and Poly(ethylene oxide)/Poly(butylene oxide) Block Copolymers in Xylene

Abstract

In addition to associating into (“normal”) micelles in aqueous solutions, amphiphilic polyoxyalkylene block copolymers can form “reverse” micelles in organic solvents at sufficiently high copolymer concentrations (above the critical micellization concentration (CMC)) and in the presence of some water. The effects of solvent quality on the copolymer micellization in an organic solvent and on the solubilization of water in such systems are examined here for representative poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO–PPO–PEO) and poly(butylene oxide)-b-poly(ethylene oxide) (PBO–PEO) copolymers. The solvent quality is modulated by the addition of cosolutes and by a change in the temperature. A number of notable observations are reported: Worsening the aqueous solvent conditions by the addition of NaCl (10 wt% with respect to water) almost doubles the CMC and the water solubilization capacity (WS) of a PEO–PPO–PEO copolymer inp-xylene. An increase in temperature makes water a worse and xylene a better solvent for the copolymer. The combined result of heating is an increase of the CMC for all three copolymers studied. This indicates that the formation of reverse micelles is exothermic, which is opposite to what has been observed for normal micelles. The effects of temperature on the water uptake are nonmonotonic: WS increased with temperature for a PEO–PPO–PEO copolymer with 40% PEO but decreased for a copolymer with 20% PEO. An increase in WS with temperature, followed by a decrease, with the maximum WS efficiency occurring at the “effective” cloud point of the copolymer is proposed in order to explain this observation.