Microemulsions domain morphology

The most general definition of a template is as a structure-directing agent. In surfactant solutions the final templated polymers can be either discrete nanoparticles or mesostructured bulk materials as a consequence of polymerization, respectively, in the non-continuous or continuous domains of the template. Thermodynamically stable media, such as microemulsions, equilibrium vesicles, or lyotropic mesophases are especially useful as templates because of their structural definition and reproducible morphologies. The mesostructure of a thermodynamically stable template is defined by composition and temperature, but this same feature makes the structure unstable to changes in temperature, pH, or concentration. The aim of template synthesis is to transfer the self-organized template structure into a mechanically and chemically stable, durable, and processable material. 

Microemulsions comprising [bmim][PF j as lipophilic component and water stabilized by Tween-20 and by Triton X-100 surfactants were reported [40,41]. UV-Vis absorption spectra indicated that the polarity of the water domains in water-in-[bmim][PFg] reverse microemulsions was lower than that of bulk water. The experimental results indicated that the water domains were sufficiently well formed to support the solubility of conventional electrolytes in the water-in-[bmim][PFJ microemulsions, and spectroscopic data indicated that the ionic species were in the aqueous pseudophase. It was demonstrated that the hydrodynamic diameter of the [bmim][PFg]-in-water microemulsion droplets was nearly independent of the water content but increased with increasing [bmim][PFJ content due to the sweUing of the micelles by the IL. The size and morphology of IL droplet in the microemulsion depend on the water content (molar ratios of IL/TX-100), and the diameter of droplet increased with the water content as shown in Figure 18.4. The diameters of 40-47, 50-53, 55-57, and 83-90 nm were for the ratios of 0.2, 0.5, 1.0, and 1.5, respectively [29]. The surfactant aggregate sizes were somewhat higher than that of conventional microemulsions. 

Other more complex morphologies also arise for A-B mixtures. In particular, domains A and B may enclose each other, forming entangled networks, separated by a hyperbolic interface. Those cases include mesh , bicontinuous microemulsions, bicontinuous cubic phases and their disordered counterparts, sponge phases, which are discussed below. In these cases too, the sign (convex/concave) of the interfacial mean curvature sets the Type . A representation of the disordered mesostructure in a Type 2 bicontinuous microemulsion is shown in Figure 16.3. A hyperbolic interface may be equally concave and convex (a minimal surface, e.g. see Figure 16.2(c)) so that the mesophase is neither Type 1 nor Type 2. Lamellar mesophases ( smectics or neat phases) are the simplest examples. Bicontinuous balanced microemulsions, with equal polar and apolar volume fractions are further examples. 

Laradji and Here [72] have simulated the dynamics of phase separation in binary blends in the presence of nanorods the simulations predicted that rods would dramatically slow down, and possibly stop, the phase separation, leading to the stabilization of a single-phase morphology. Still, it is not clear whether this morphology is a simple homogeneous structure or, more likely, a complicated phase like a microemulsion. Similarly, there have been some experimental studies [50] suggesting that nanoplatelets could lead to microemulsion-like behavior, when platelets occupy interfaces between A-rich and B-rich domains and stabilize them. At present, to our knowledge, there is no satisfactory theory to describe this behavior, and more research is needed. 

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