Conformation and solubility of poly(ethylene oxide) in polymer-surfactant complex at high-temperature and high-salinity conditions

The solubility of poly(ethylene oxide) (PEO) with molecular weight (MW) ranging from 300,000 to 6,000,000 g/mol was studied in aqueous solution in the presence of salt (sodium chloride, [NaCl] = 1 M) at various temperatures by cloud point measurement. All PEO solutions exhibited phase separation at temperature above 78 +/- 1 degrees C, the cloud point was independent of PEO chain length within the selected range of MW. An anionic surfactant sodium dodecyl sulfate (SDS), and a cationic surfactant dodecyl tri-methyl ammonium chloride (DTAC) was added into the PEO solutions, respectively, to successfully eliminate the precipitation of the polymer at higher temperature (up to 90 degrees C). However, the effects of SDS and DTAC on the cloud point of PEO at various surfactant concentrations, as well as the rheological behaviors of PEO/SDS and PEO/DTAC solutions were completely different. This observation leads to a study of the molecular structures formed by PEO and the surfactants. The interactions between a monodisperse and short-chain PEO (MW = 20,000 g/mol) and the surfactants at molecular level were investigated via two-dimensional NMR techniques. Based on these results, two different conformations of PEO in PEO/SDS and PEO/DTAC complex were proposed and a saturation concentration of SDS binding to PEO backbone was determined. The findings in this study provide evidences that molecular interaction between a polymer and a surfactant in solution strongly affect the solubility of the polymer and the rheological property of the solution.