The Sponge Phase

Unlike the lamellar phase, no orientational or large scale positional order is present. The phase is optically isotropic and its diffraction pattern has no Bragg peak. 

The infinite membrane without edge has complex topology. It is everywhere curved into a saddle shape in order to multiconnect with itself throughout the sample in the three dimensions of space. 

Locally then, the sponge phase is made up of the same bilayers as the lamellar phase, but built up into a quite different large scale structure. We can sum up as follows  

We may now analyse the respective stabilities of the Lq and L3 structures in a qualitative way [5.4, 5.22]. Consider the horizontal and vertical axes of Fig. 5.22. K is relevant here because the and L3 structures have such contrasting topologies. We can estimate its contribution to the difference in elastic energies between the two phases by applying the Gauss-Bonnet theorem  

The role of the mean curvature modulus K is easy to understand. Its contribution to the ground state energy levels is zero, the mean curvature being everywhere zero in these states. It only comes in if we leave these states and consider thermal fluctuations in the mean curvature, such as we would expect to occur spontaneously at any non-zero temperature. However, these fluctuations do not act symmetrically on the two structures. In the lamellar phase Lq., smectic order is maintained by steric interaction, whereas in the sponge phase L3, cubic periodicity in the ground state is lost. Since the sponge phase exhibits liquid type disorder, whilst maintains smectic order, we may presume that stability of L3 is favoured at low K and that of at high K. 

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For a recent review on the sponge phase, see G. Porte. Curr Opin Coll Interf Sci L345-349, 1996. 

Alfons, K. and Engstrom, S. (1998) Drug compatibility with the sponge phases formed in monoolein, water, and propylene glycol or polyethylene glycoll).Pharm. Sci., 87 1527-1607. 

Figure 2.4 (a) Schematic of the sponge phase and model of the silicification process... 

The three-phase coexistence of a balanced microemulsion with oil and water The narrowness (limited swelling) of both the microemulsion and the sponge phase... 

The structures of the phases L4, La, and L3 were characterized by means of several techniques and confirmed by freeze-fracture electron microscopy [47,79,86-88]. Typical electron micrographs showing the organization of the bilayers in phases L3 and L4 are shown in Fig. 5. These clearly show the continuity of the bi layer in the sponge phases and the existence... 

Figure 11 Effect of the water/surfactant ratio X on the location of the sponge phase in the water-dodecane-pentanol-SDS system.

The sponge phase, like the lamellar phase, is usually found to be stable throughout the moderate concentration range [219]. At high dilution of the system, however, the phase behavior depends on the constituents of the studied systems. In some cases, the L3 phase simply phase-separates at high dilution and expels a very dilute dispersion of micelles... 

The effect of adding various lipids to monoolein was discussed above. These lipids are soluble in either water (bile salt) or oil (triglyceride) or hardly soluble at all (lecithin). If a substance that is soluble in both water and oil, e.g., propylene glycol, is added to the monoolein-water system, the cubic liquid crystal undergoes a transition to a sponge or L3 phase [13], as shown in Fig. 5. The structure of the sponge phase has been described as a melted bicontinuous cubic phase [14]. 

The sponge phase has the same visual appearance as the microemulsion phase, i.e., an isotropic liquid, but it differs from the latter in the way the emulsifier molecules are organized in the system. In a microemulsion the emulsifier forms a monolayer at the oil/water interface, which makes it possible to create systems with high amounts of oil and water if the emulsifier system is well balanced. In a sponge phase, the emulsifier forms a bilayer (normal or reversed), which will limit the incorporation of both oil and water. Although propylene glycol is not a food additive (however, it is used in oral pharmaceutical formulations), we find it relevant to mention it in this context, since it aids in the formation of many different types of phases [14]. 

The water dilution line starting at 6 4 GMO/ water axis and crossing the bicontinuous cubic phase runs along the Ql phase and the sponge phase (L3) marked by a dashed line [28]. 

Other nonionic surfactants, that is, GMO, formed unique structures upon addition of ethanol or Transcutol. The unique isotropic fluid Ql phase formed in the GMO/ethanol/water mixture and is surmised to be a transition phase between the cubic bicontinuous phase and the sponge phase. 

Myelins have also been observed to collapse when an intermediate sponge phase is present between the lamellar phase and the micellar phase (47). After the formation of the myelins sponge phase is observed to from at the surface (Figure 3). The delayed formation of the sponge phase in this case is consistent... 

As we will see below, bicontinuous structures are very significant in many contexts of amphiphile self-assembly. Another type of bicontinuous structure in simple surfactant-water solutions is the sponge phase , formed also in quite dilute surfactant solutions (Figure 19.26). This structure forms for all classes of surfactants but in particular for nonionics. We will also mention that the structure of the sponge phase is related to that of many microemulsions. 

Figure 19.26. Representation of the sponge phase. For many surfactants, there is an isotropic solution phase where the surfactant forms a connected three-dimensional network. Since both water and the hydrophobic regions are connected over macroscopic distances, such structures are termed bicontinuous. (Redrawn from P. Pieruschka and S. Marcelja, Langmuir, 2 (1994) 345)...

Other examples of bilayer structures already mentioned are the sponge phase and bicontinuous cubic phases. The sponge phase has been most studied for nonionic surfactants and is related to common microemulsions. Bilayers may also easily close on themselves to form discrete entities including unilamellar vesicles and multilamellar liposomes. Vesicles are of interest because of the division into inner and outer aqueous domains separated by the bilayer. Vesicles and liposomes are normally not thermodynamically stable (although there are exceptions) and tend to phase separate into a lamellar phase and a dilute aqueous solution. Lipid bilayers are important constituents of living organisms and form membranes, which act as barriers between different compartments. Certain surfactants and lipids may form reversed vesicles, i. e. vesicles with inner and outer oleic domains separated by a (reversed) amphiphile bilayer the bilayer may or may not contain some water. 

A great deal of experimental work has been carried out to examine these conjectures. An example is shown in Fig. 5.27. The osmotic compressibility of the sponge phase, measured by light scattering, has been plotted as a function of (p in axes chosen to reveal the logarithmic correction. Agreement with theory... 

Lopez-Barron et al. [55] studied a mixture of didodecyldimethylammonium bromide (DDAB) in a protic IL EAN, which shows an L -La transition as determined by SANS. From the SANS data (Fig. 2.17), there is a slight peak shift and narrowing between 80 and 82.5 wt.%. This shift corresponds to the transitions from the spongelike phase to the more ordered lamellar (La) phase at 25°C. In the case of the DDAB/EAN mixture, there is a stable spongelike phase over a much wider range of compositions compared to that of DDAB/water presumably due to the lower solvophobic effect of the IL. The DDAB/EAN mixture is therefore a more robust system for applications of the sponge phase due to the relatively narrow composition region that exists in the aqueous system. 

Surfactant micelles and bilayers are the building blocks of most self-assembly structures. One can divide the phase structures into two main groups [1) (1) those that are built of limited or discrete self-assemblies, which may be characterized roughly as spherical, prolate or cylindrical. (2) Infinite or unlimited self-assemblies whereby the aggregates are connected over macroscopic distances in one, two or three dimensions. The hexagonal phase (see below) is an example of onedimensional continuity, the lamellar phase of two-dimensional continuity, whereas the bicontinuous cubic phase and the sponge phase (see later) are examples of three-dimensional continuity. Figure 3.8 illustrates these two types schematically. 

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