Entropically Driven Partitioning of Ethylene Oxide Oligomers and Polymers in Aqueous/Organic Biphasic Systems

Abstract

Partitioning of ethylene oxide oligomers and polymers (PEO) in biphasic systems of water and chloroform or dichloromethane favors transfer to the organic phase as the molecular weight increases. For systems containing chlorobenzene, partitioning into the aqueous phase is always predominant. Calorimetric determination of the enthalpies of transfer for PEO from aqueous to organic phases reveals an endothermic process for all of the systems investigated, which was ascribed to the replacement of a more energetically favored PEO solvation in water for that in the organic phase. These results indicate that spontaneous PEO transfer from water to an organic phase is driven by an entropy increase. The number of water molecules transferred to the organic phase with PEO was determined to be ca. 0.08 water molecules per EO unit, smaller than hydration numbers reported in aqueous solutions. All of these findings lead to a picture where PEO may be extracted from water to an organic phase as long as the solvation by the organic solvent is relatively strong as compared to water. The displacement of water causes an entropy increase, which drives the transfer process. Chloroform and dichloromethane are suitable solvents for PEO extraction probably because of their hydrogen bond-donating capability.