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We report on a ternary isothermal system consisting of a poly(ethylene oxide)/poly(propylene oxide) (PEO/PPO) amphiphilic block copolymer, "water", and "oil" (where "water" and "oil" are selective solvents for the different blocks), which exhibits the richest structural polymorphism ever observed (in equilibrium) in mixtures containing amphiphiles (such as block copolymers, surfactants, or lipids). The microstructure resulting from the self-assembly of the PEO/PPO block copolymer can vary from normal (oil-in-water) micelles in solution, through all types of normal and reverse (water-in-oil) lyotropic liquid crystals (normal micellar cubic, normal hexagonal, normal bicontinuous cubic, lamellar, reverse bicontinuous cubic, reverse hexagonal, reverse micellar cubic), to reverse micelles, as the relative volume fraction of the apolar ("oil"-like) components increases over that of the polar ("water"-like) components. The structure in the liquid crystalline phases has been established with small-angle X-ray scattering; both the normal and the reverse bicontinuous cubic structures are consistent with the Ia3d crystallographic space group (and the Gyroid minimal surface), while the normal and reverse micellar cubic structures are consistent with the Im3m and Fd3m space groups, respectively. The self-assembly of amphiphilic block copolymer in selective solvents described here provides a link between the self-assembly of surfactants in water (and oil/cosurfactant) and the self-assembly of block copolymers in the absence of any solvent. Furthermore, the ability of the PEO/ PPO amphiphilic block copolymers to attain diverse microstructures is of great importance to numerous practical applications, especially since such copolymers are commercially available (as poloxamers, Pluronics, or Synperonics).

The study addresses the effects of block composition on the self-assembly (and resulting microstructure) of amphiphilic block copolymers in the presence of selective solvents (“water” and “oil”) by examining the ternary phase behavior and structure of two copolymers, E20P70E20 and E100P70E100, having the same block architecture (E x P y E x ) and P:poly(propylene oxide) middle-block size but different E:poly(ethylene oxide) end-block sizes (Pluronic P123, E20P70E20, contains 30% E and Pluronic F127, E100P70E100, 70% E). A characterization (using SAXS and deuterium NMR) of the ternary isothermal (25 °C) E20P70E20−butyl acetate−water and E20P70E20−butanol−water systems is presented first. A variety of lyotropic liquid-crystalline (LLC) phases are thermodynamically stable in the former (butyl acetate) system, both of the “normal” (oil-in-water) and of the “reverse” (water-in-oil) morphology. In the latter (butanol) system, the reverse LLC phases are not stable but are replaced by an extensive water-lean solution region under the influence of the amphiphilic character of butanol. Following the presentation of the E20P70E20−“oil”−water systems, the microstructure afforded by E20P70E20 is compared to that of E100P70E100 (Holmqvist, P.; Alexandridis, P.; Lindman, B. Macromolecules 1997, 30, 6788). In the E20P70E20 phase diagrams, the lamellar structure (of zero interfacial curvature) is the most extensive. The high-E (hydrophilic) content of E100P70E100, however, favors oil-in-water LLC structures with high interfacial curvature. The copolymer−water side of the E100P70E100 ternary phase diagrams is dominated by the micellar cubic LLC structure; for cylindrical and lamellar structures to form, significant amounts of oil are needed. No reverse LLC phases are formed by E100P70E100 in the presence of water and either butyl acetate or butanol. The normal hexagonal phase in the E100P70E100 systems occurs in approximately the same composition range as the lamellar phase in the E20P70E20 systems.

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Catanionic mixtures are aqueous mixtures of oppositely charged surfactants which display novel phase behavior and interfacial properties in comparison with those of the individual surfactants. One phase behavior property is the ability of these systems to spontaneously form stable vesicles at high dilution. The phase behavior of the mixture sodium dodecyl sulfate (SDS) -didodecyldimethylammonium bromide (DDAB) in water has been studied in detail, and two regions of isotropic vesicular phases (anionic-rich and cationic-rich) were identified. Cryo-transmission electron microscopy allowed direct visualization of relatively small and polydisperse unilamellar vesicles on the SDS-rich side. Monitoring of the microstructure evolution from mixed micelles to vesicles as the surfactant mixing ratio is varied toward equimolarity was also obtained. Further information was provided by water self-diffusion measurements by pulsed field gradient spin-echo NMR. Water molecules can be in fast or slow exchange between the inside and outside of the vesicle with respect to the experimental time scale, depending on membrane permeability and vesicle size. For the SDSrich vesicles, a slow-diffusing component of very low molar fraction observed for the echo decays was traced down to very large vesicles in solution. Light microscopy confirmed the presence of vesicles of several microns in diameter. Thus, polydispersity seems to be an inherent feature of the system.

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A reverse (water-in-oil) cubic phase was observed in a ternary system consisting of an amphiphilic diblock copolymer (EO17BO10, where EO represents ethylene oxide and BO represents butylene oxide), water, and p-xylene in the following composition range: 47-62 wt % polymer and 7-12 wt % water. This cubic phase occurs between a reverse hexagonal liquid crystalline (H2) and a reverse micellar solution (L2) region and can be considered the result of crystallization of the reverse micelles as they swell with increasing water content. Small-angle X-ray spectra from samples of this cubic phase can be indexed to the crystallographic space group Fd3m (Q227). This is one of the first times a cubic structure consisting of distinct reverse micelles has been observed in a ternary amphiphile-water-oil system; bicontinuous reverse cubic structures, such as the Gyroid (Ia3d, Q230), are more common and have been previously identified in such ternary systems between the lamellar (L R) and the H2 phases.