Sponge phases, surfactants

Figure 17. Phase diagram of C,2E05/water (L and L 2, surfactant solutions L2, liquid surfactant containing dissolved water, not fully miscible with water L3, sponge phase, isotropic solution not fully miscible with water or surfactant, otherwise as for Fig. 14) (reproduced from [102]).

Figure 8.5 Two-dimensional cut through a bicontinuous microemulsion comprising water (white) and oil (black) separated by a surfactant monolayer. The same picture can also represent the inside and outside of the so-called L3 sponge phase where a surfactant bilayer separates inner and outer regions of a single solvent. Note that the domains have a well-defined iength scaie. The detaiis of the model used to generate this representation are discussed in Ref. 28.

In order to emphasize the role of the inter facial films and to highlight the most recent viewpoints on the stability of microemulsions, sponge phases, and dilute lamellar phases, some of the experimental facts about phase behavior of microemulsion systems containing alcohol are reviewed in this chapter. The systems investigated consist of water, oil, alcohol, and sodium dodecylsulfate (SDS). In the next section, the theoretical aspects of the stability of surfactant phases are briefly discussed. Then in Secs. Ill and IV the effects of varying alcohol and oil chain lengths and the addition of a water-soluble polymer are examined. The examination of multiphase regions provides the location of lines of critical points or critical endpoints. This chapter also deals with the study of several physical properties in the vicinity of critical points. 

Figure 8 Sections at constant water/surfactant ratios X = 1.55 and X = 4.3 of the phase diagram of the water-dodecane-hexanol-SDS system (system A) at 21 °C Li and L2 are isotropic phases. L , is a lamellar phase, L.vo is an oil-rich sponge phase, tj is a three-phase region. G is a gel phase. (From Refs.

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

Over the last 10 years or so, a great deal of work has been devoted to the study of critical phenomena in binary micellar solutions and multicomponent microemulsions systems [19]. The aim of these investigations in surfactant solutions was to point out differences if they existed between these critical points and the liquid-gas critical points of a pure compound. The main questions to be considered were (1) Why did the observed critical exponents not always follow the universal behavior predicted by the renormalization group theory of critical phenomena and (2) Was the order of magnitude of the critical amplitudes comparable to that found in mixtures of small molecules The systems presented in this chapter exhibit several lines of critical points. Among them, one involves inverse microemulsions and another, sponge phases. The origin of these phase separations and their critical behavior are discussed next. 

Anderson and Wennerstrom [33] calculated the geometrical obstruction factors of the self-diffusion of surfactant and solvent molecules in ordered bicontinuous microstructures, which serve as good approximations also for the disordered bicontinuous microemulsions and L3 (sponge) phases. The geometrical obstruction factor is defined as the relative diffusion coefficient DIDq, where D is the diffusion coefficient in the structured surfactant system and Z)q is the diffusion coefficient in the pure solvent. In a bicontinuous microemulsion the geometrical obstruction factor depends on the water/oil ratio. An expansion around the balanced (equal volumes of water and oil) state gives, to leading order. 

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. 

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. 

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)...

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