Bicontinuous system

A colloidal system represents a multiphase (heterogeneous) system, in which at least one of the phases exists in the form of very small particles typically smaller than 1 pm but still much larger than the molecules. Such particles are related to phenomena like Brownian motion, diffusion, and osmosis. The terms microheterogeneous system and disperse system (dispersion) are more general because they also include bicontinuous systems (in which none of the phases is split into separate particles) and systems containing larger, non-Brownian, particles. The term dispersion is often used as a synonym of colloidal system. 

In contrast to MCM-41, the pore network of MCM-48 silica is three dimensional and highly interconnected. The strucmre of MCM-48 sdica belongs to the la 3 d space group, which also exists in the binary water/CTAB system (155). This interesting structure is viewed as a bicontinuous system which consists of two mutually intertwined three-dimensional networks of channels (Fig. 9.20 Reference 156). MCM-48 silica can be prepared in high phase purity via hydrothermal synthesis by mning the silica to CTAB ratio and other synthesis parameters, or at room temperamre... 

Depending on its composition, a single-phase microemulsified system can also exhibit different morphologies. The three main structures are 0/W, W/O, and bicontinuous. The latter is a structure in which both oil and water exist as a continuous phase, but aU three structures have a surfactant monolayer in the interface separating both phases. These stmctnres are shown in Fignre 32.18. Usually, an 0/W microemulsion is formed when oil concentration is low, and a W/O microemulsion is formed when water concentration is low. Bicontinuous systems are formed when the amounts of water and oil are similar (Lawrence and Rees, 2000). 

In the case of cosmetics in which fragrances, flavors, or emollient oils are incorporated at very low concentrations solubilization may be ensured by micellar take-up. This also applies to alcoholic preparations such as mouthwash, cologne, or skin tonic. Nevertheless, a high solubilization, i.e., a bicontinuous system, would be required if the amount of alcohol were restricted by safety and toxicity constraints. 

Among the reactions that are carried out in microemulsions, polymerization is probably the most studied, as can be found in recent reviews [155-157]. It takes place in a O/W or W/O microemulsion structure, in percolated microemulsion [158-160], and in bicontinuous systems [161,162]. 

The diameters determined by cryofracture are compared with those computed from an indirect determination method using F NMR of probe molecules (see below). It is interesting to note that the diameters determined by the two methods are of the same order of magnitude. The values determined by cryofracture were computed from the observed diameter from which the thickness of the interface (da) and in addition the length of a layer of the cosurfactant (dh) were subtracted. Note also that for bicontinuous systems such as sodium oleate-pentanol-water-benzene or sodium oleate-butanol-water, the ratio between the two types of diameters is equal to about 10 [20]. 

There are also other luminescence studies, for instance with microemulsions as reaction media, where the primary aim was to investigate how reaction rates were affected by the partitioning of the reactant molecules between the oil, water, and interfacial domains in bicontinuous systems [51], e.g., with AOT and QE, as surfactants. 

Luminescence studies of bicontinuous systems, in particular of systems undergoing a transition from discrete droplets to bicontinuous structures, have an unexploited potential to give more detailed information about the structures and mechanisms involved. New experiments can be tried, e.g., using Forster energy transfer as mentioned above. The... 

Using the deviations of specific parameters measured with different techniques from hard-sphere trends, it is possible to make preUminary predictions on when the discrete aggregates change into a bicontinuous system. Since there is no macroscopic phase separation it is not possible to acquire this information from the... 

Tingey et al. [128] reported an "antipercolation" feature of a water/dodecyldi-methylammonium bromide/supercritical propane microemulsion in the pressure range of 80-400 bar the conductance decreased by three orders of magnitude. The interconnected channels of the studied bicontinuous system were anticipated to break down into dispersed droplets. 

Microemulsions are stabilized by a monolayer of amphiphiles at the oil/water interface. Whether the oil disperses in the water phase or water in the oil phase depends primarily on the shape of the amphiphilic molecules, notably on their value for vla pl. For flnj 0 > v//, the amphiphile prefers the curvature to be convex toward the water and for < vH toward the oil. Accordingly, oil in water and water in oil emulsions are formed in which the dispersed phase occurs as spherical (or sometimes cylindrical) particles as illustrated in Figure 11.18a and b. Only for vla l 1 is the interface preferably planar, resulting in the bicontinuous distribution shown in Figure 11.18c. In such bicontinuous systems, the conduits may easily break and form during which the water and the oil may change places. 

Bicontinuous System. A two-phase system in which hoth phases are continuous phases. For example, a possible structure for middle-phase microemulsions is one in which hoth oil and water phases are continuous throughout the microemulsion phase. See also Middle-Phase Microemnl-sion. 

Herein, N represents the number of particles in the beam and Ap represents the average difference in electron density between particles and their surroundings. Some systems, such as bicontinuous systems, cannot be described by separate form and structure functions. These systems are beyond the scope of this chapter. More information can be found elsewhere [9,10]. 

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