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

Abstract

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/ChEO15 and water/ChEO10 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/ChEO15 system, aqueous micellar (Wm), discontinuous micellar cubic (I1) with Fd3m space group, hexagonal (H1), rectangular ribbon (R1), and lamellar (Lα) phases are formed, whereas Wm, unknown, R1, defected lamellar ( ), and Lα phases are produced in the water/ChEO10 system at ambient temperatures. Compared with a conventional aqueous nonionic surfactant system, the intermediate R1 phase region is incredibly wide. As for the water/ChEO15 system, with increasing water content, the packing parameter, P, in the R1 region is gradually decreased, finally converging to 1/2 at W s ∼ 0.58, indicative of the formation of the H1 phase. The R1 phase acts as a “distorted” hexagonal phase in the system. However, in the water/ChEO10 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 Wm solution indicates that spherical micelles are present in the water/ChEO15 system in an ambient temperature range, but ChEO10 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/ChEO10 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 Wm 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 .

Wormlike micelles in Tween-80/C EO3 mixed nonionic surfactant systems in aqueous media

“…In the GIFT calculation of the SAXS data, we used a model of the averaged structure factor for a hardsphere (HS) interaction, S(q) av [37,38], which considers the Gaussian distribution of the interaction radius σ for individual monodisperse systems for polydispersity μ, and a Percus-Yevick (PY) closure relation to solve Ornstein-Zernike (OZ) equation. The detailed theoretical description on the method has been reported elsewhere [36,39,40].…”

Bending Energetics of Tablet‐Shaped Micelles: A Novel Approach to Rationalize Micellar Systems

“…[5][6][7][11][12][13][14][15] The formation of such ribbonlike micelles has been observed using a number of different techniques. [16][17][18][19][20][21][22] To account for the more complex tablet shape of surfactant micelles, we have already developed a theory in which the equilibrium thermodynamics of dispersed self-assembled aggregates is combined with the bending energetics of generally shaped, non-spherical micelles. [23,24] However, the curvature free energy of the micelles was only calculated in an approximate manner in refs.…”

Wormlike micelles in poly(oxyethylene) surfactant solution: Growth control through hydrophilic-group size variation

“…As a remarkable fact, they form a liquid crystal in a pure state above their melting temperatures [31], whereas conventional nonionic surfactants become simple liquids above the melting temperature. In dilute aqueous solutions of poly(oxyethylene) cholesteryl ether surfactants spherical micelles are formed at room temperatures [32] and increase in temperature promotes unidimensional micellar growth [33]. These surfactants, in binary systems and with cosurfactants, show interesting phase behavior, in which a rectangular phase or a defected lamellar phase in addition to conventional liquid crystals appear [32,34].…”