Assembly of Polyethyleneimine in the Hexagonal Mesophase of Nonionic Surfactant: Effect of pH and Temperature

We investigate the dispersion of a pH responsive polymer, polyethyleneimine, PEI, in a hexagonal (H₁) mesophase of a nonionic surfactant, C₁₂E₉, and water, at pH ranging from basic (pH = 12.8) to acidic (pH = 1). While the C₁₂E₉/H₂O phase behavior is independent of pH, we demonstrate that, in the PEI/C₁₂E₉/H₂O system, changing the pH influences PEI-C₁₂E₉ interactions, and thus, influences the isotropic-H₁ phase transition. With decrease in pH, there is increasing protonation of the PEI chain, and consequently, the chain extends. We show, using a combination of SAXs, optical microscopy and visual experiments, that the inclusion of PEI in a 1:1 surfactant–water mixture, lowers the hexagonal-isotropic transition temperature, T_HI. At higher pH = 12.8, T_HI shows a pronounced decrease from 50 to 13 °C on addition of PEI, and the PEI/C₁₂E₉/H₂O system forms a transparent gel. At pH = 1, we observe qualitatively different behavior and an opaque gel forms below T_HI = 25 °C. The isotropic-H₁ transition, in turn, influences the phase separation of PEI chains from the C₁₂E₉/H₂O system. 2D NMR ROESY data provides evidence that there are strong surfactant–PEI interactions at high pH that significantly reduce at lower pH. The NMR data is in accord with molecular dynamics simulations that show that surfactants strongly aggregate with unprotonated PEI chains, but not with fully protonated chains; thus, in this system, the pH controls a cascade of microstructural organization: increasing pH decreases chain protonation and increases polymer–surfactant interactions, resulting in suppression of the isotropic-H₁ transition to lower temperatures, thus, influencing the phase separation of PEI from the surfactant/water system.