Direct sponge phase

A particularly interesting case is that of systems in which the film has a zero interfacial tension and zero spontaneous curvature (i.e., it is spontaneously flat), together with a great flexibility, so that local curvature may nevertheless occur. P.G. de Gennes was able to predict the existence of bicontinuous phases which provide a good representation of the mysterious zone (III). Note that these sponge phases possess curvatures in opposite directions, unlike droplets in a dispersion. These points and the properties of these phases are discussed further in Chap. 5. 

Fig. 5.0. The structure of the La-sponge phase is built up from one single infinite surfactant bilayer, bent everywhere in a saddle like manner so to be multiconnected to itself over macroscopic distances in the three directions of space. This structure is indeed intriguing and provides a beautiful illustration of the capability of amphiphilic molecules to self assemble spontaneously into various morphologies and structures at thermodynamic equilibrium. (by courtesy of Jean-Pierre Fluxench)...

Self-diffusion data from bicontinuous structures contain, in principal, information on the average coordination number of the microstructure. The experimental results can, however, not be compared with the theoretical results directly. There is an additional reduction of D/Dq due to solvation of the surfactant film (a lateral friction felt by the solvent molecules in the solvent layers closest to the film) and an obstruction due to the finite film volume. These effects, which both increases with the surfactant-to-solvent ratio can however be included in the model, as will be discussed for the case of the sponge phase in one of the following sections. Extending the model to include solvation, however, introduces additional parameters which will affect the uncertainty. 

The reduction occurs by direct oxygen removal from the solid oxides (solid-state diffusion). The basic underlying mechanism is not known (diffusion of O, OH, H2O) and is likely to vary for different for different phase transitions. On the final reduction the metal remains pseudomorphous to the starting oxide, forming a polycrystalline metal sponge. Solid-state reactions are characteristic for low reduction temperatures (<750 °C) and the early WO3 - WO2 9 transition ( crystallographic shear transition). 

The bicontinuous structure of the L3 phase can also be seen directly from Fig. 11.30b. This phase shows a low viscosity without elasticity and is optically isotropic but shows a strong flow birefringence. According to the similarity with normal micellar or reversed micellar solutions (Li and L2 phases) these phases have been called L3 phases. The surfactant lamellae are arranged as branched tubes similar to a sponge. The low viscosity results from the very high flexibility of the lamellae which break and reform easily. 

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