Phase Behavior and Self-Organized Structures in Water/Poly(oxyethylene) Cholesteryl Ether Systems

Different from conventional nonionic poly(oxyethylene) surfactants, poly(oxyethylene) cholesteryl ethers, ChEO n, possess a bulky and nonflexible hydrophobic part and form a variety of self-organized structures in water. We investigated the phase behavior and the micellar structures in the water/ChEO 15 and water/ChEO 10 systems by means of visual observation, rheometry, small-angle X-ray scattering (SAXS), dynamic light scattering (DLS), dielectric relaxation spectroscopy (DRS), and densimetry. We found that in the water/ChEO 15 system, aqueous micellar (W m ), discontinuous micellar cubic (I 1 ) with Fd 3 m space group, hexagonal (H 1 ), rectangular ribbon (R 1 ), and lamellar (L α ) phases are formed, whereas W m, unknown, R 1, defected lamellar ( ), and L α phases are produced in the water/ChEO 10 system at ambient temperatures. Compared with a conventional aqueous nonionic surfactant system, the intermediate R 1 phase region is incredibly wide. As for the water/ChEO 15 system, with increasing water content, the packing parameter, P, in the R 1 region is gradually decreased, finally converging to 1/2 at W s ∼ 0.58, indicative of the formation of the H 1 phase. The R 1 phase acts as a “distorted” hexagonal phase in the system. However, in the water/ChEO 10 system, upon reduction of W S, P shows a steplike increase and the maximum value ∼0.67 at W S ∼ 0.7, just corresponding to the threshold of discontinuous and bicontinuous structures. After that, P is decreased with decreasing W S and unknown phase that cannot be indexed to any known space group for liquid crystalline phases emerges at W S ∼ 0.5. The GIFT analysis of the SAXS data for the W m solution indicates that spherical micelles are present in the water/ChEO 15 system in an ambient temperature range, but ChEO 10 forms a short-rod micelle in water. With increasing temperature, rodlike micelles appear to be grown and a viscoelastic micellar phase is formed in water/ChEO 10 system. The hydration number for each oxyethylene unit is evaluated as ∼4 by DRS, which gives a consistent explanation for the concentration dependence of the apparent hydrodynamic radius in the W m phase obtained by DLS. Hydrated water molecules should be regarded as a constituent of the micelles. The majority of these features of novel phase behavior in the water/ChEO n systems are based on a nonflexible and bulky hydrophobic part of ChEO n .