Bicontinuous system microemulsion

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 principle, we can distinguish (for surfactant self-assemblies in general) between a microstructure in which either oil or water forms discrete domains (droplets, micelles) and one in which both form domains that extend over macroscopic distances (Fig. 7a). It appears that there are few techniques that can distinguish between the two principal cases uni- and bicontinuous. The first technique to prove bicontinuity was self-diffusion studies in which oil and water diffusion were monitored over macroscopic distances [35]. It appears that for most surfactant systems, microemulsions can be found where both oil and water diffusion are uninhibited and are only moderately reduced compared to the neat liquids. Quantitative agreement between experimental self-diffusion behavior and Scriven s suggestion of zero mean curvature surfactant monolayers has been demonstrated [36]. Independent experimental proof of bicontinuity has been obtained by cryo-electron microscopy, and neutron diffraction by contrast variation has demonstrated a low mean curvature surfactant film under balanced conditions. The bicontinuous microemulsion structure (Fig. 7b) has attracted considerable interest and has stimulated theoretical work strongly. 

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]. 

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. 

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. 

Microemulsions are ternary systems containing oil, water, and surfactant. The terms oil and water in a microemulsion system normally refer to oil phase (oil and oil soluble components such as cyclosporine) and aqueous phase (water and water soluble components such as sodium chloride), respectively. The phase behavior of water-oil-surfactant mixtures was extensively studied by Winsor (1948). Based on his experimental observations, Winsor classified equilibrium mixtures of water-oil-surfactant into four systems (1) type I (Winsor I) system where water continuous or oil-in-water (0/W) type microemulsion coexists with the oil phase. In these systems, the aqueous phase is surfactant-rich (2) type II (Winsor II) system where oil continuous or water-in-oil (W/0) type microemulsion coexists with the aqueous phase. In these systems, the oil phase is surfactant-rich (3) type III (Winsor III) system where bicontinuous type microemulsion (also referred to as surfactant-rich middle-phase) coexists with excess oil at the top and excess water at the bottom and (4) type IV (Winsor IV) system where only a single-phase (microemulsion) exists. The surfactant concentration in type IV microemulsion is generally greater than 30 wt%. Type IV microemulsion could be water continuous, bicontinuous, or oil continuous depending on the chemical composition. The phase behavior of microemulsions is often described as a fish diagram shown in Figure lO.I (Komesvarakul et al. 2006). 

Figure 10.4 Pseudotemary phase diagram showing mono- and biphasic regions of a system comprising surfactant/cosurfactant (s), oil (o), and water (w) and elucidation of w/o-, o/w-, and bicontinuous-type microemulsion zones along with their dispersion level scheme and electron micrographs.

Silica microrods with nanosized pores were explored by Li et al. [115] templated in the microemulsion system with water/TX-100/[bmim][PFJ. Synthesis of silver (Ag) nanoparticles by the photoreduction of silver perchlorate (AgClO ) in water-in-ionic liquid microemulsions of water/Tween-20/([bmim][BFJ) and l-octyl-3-methylimid-azolium tetrafluoroborate ([omim][BFJ) was reported by Harada et al. [116]. The average diameters of the metalhc Ag particles were 8.9 and 4.9nm in the water-in-([bmim][BF ]) and water-in-([omim][BFJ) microemulsions, respectively. We have [117] synthesized CdS nanoparticles in a bicontinuous-type microemulsion in the system comprising water/lL-2-HlP (1 l-wt./wt.)/IPM as 6.8/86.6/6.6wt.%. Particle diameter of approximately 3.2 nm had marginal semiconductivity and moderate antibacterial activity against E. coli. Li et al. [118] synthesized tetragonal ZrO ... 

Block copolymer (BCP) systems possess a rich variety of microdomain structures lamellar, cylindrical, spherical and other bicontinuous structures [1, 2]. Such kinds of structures are frequently observed in other systems microemulsion, Benard convection, reaction-diffusion systems, etc. [3], Therefore, BCP is also regarded as a model system to investigate the pattern formation in nature. 

Fig, XIV-12. Freeze-fracture transmission electron micrographs of a bicontinuous microemulsion consisting of 37.2% n-octane, 55.8% water, and the surfactant pentaethy-lene glycol dodecyl ether. In both cases 1 cm 2000 A (for purposes of microscopy, a system producing relatively coarse structures has been chosen), [(a) Courtesy of P. K. Vinson, W. G. Miller, L. E. Scriven, and H. T. Davis—see Ref. 110 (b) courtesy of R. Strey—see Ref. 111.]... 

Recently an alternative approach for the description of the structure in systems with self-assembling molecules has been proposed in Ref. 68. In this approach no particular assumption about the nature of the internal interfaces or their bicontinuity is necessary. Therefore, within the same formahsm, localized, well-defined thin films and diffuse interfaces can be described both in the ordered phases and in the microemulsion. This method is based on the vector field describing the orientational ordering of surfactant, u, or rather on its curlless part s defined in Eq. (55). 

On a microscopic scale, a microemulsion is a heterogeneous system and, depending on the relative amounts of the constituents, three main types of structures can be distinguished [69] oil in water (OAV, direct micellar structure), water in oil (W/O, reverse micellar structure) and a bicontinuous structure (B) (Figure 6.1). By adding oil in water, OAV dispersion evolves smoothly to a W/O dispersion via bicontinuous phases. 

The ITIES with an adsorbed monolayer of surfactant has been studied as a model system of the interface between microphases in a bicontinuous microemulsion [39]. This latter system has important applications in electrochemical synthesis and catalysis [88-92]. Quantitative measurements of the kinetics of electrochemical processes in microemulsions are difficult to perform directly, due to uncertainties in the area over which the organic and aqueous reactants contact. The SECM feedback mode allowed the rate of catalytic reduction of tra 5-l,2-dibromocyclohexane in benzonitrile by the Co(I) form of vitamin B12, generated electrochemically in an aqueous phase to be measured as a function of interfacial potential drop and adsorbed surfactants [39]. It was found that the reaction at the ITIES could not be interpreted as a simple second-order process. In the absence of surfactant at the ITIES the overall rate of the interfacial reaction was virtually independent of the potential drop across the interface and a similar rate constant was obtained when a cationic surfactant (didodecyldimethylammonium bromide) was adsorbed at the ITIES. In contrast a threefold decrease in the rate constant was observed when an anionic surfactant (dihexadecyl phosphate) was used. 

Fatty alcohol- (or alkyl-)ethoxylates, CoE, are considered to be better candidates for LLE based on their ability to induce rapid phase separation for Winsor II and III systems. (Winsor III systems consist of excess aqueous and organic phases, and a middle phase containing bicontinuous microemulsions.) However, C,E,-type surfactants alone cannot extract biomolecules, presumably because they have no net negative charge, in contrast to sorbitan esters [24,26,30,31]. But, when combined with an additional anionic surfactant such as AOT or sodium benzene dodecyl sulfonate (SDBS), or affinity surfactant, extraction readily occurs [30,31]. The second surfactant must be present beyond a minimum threshold value so that its interfacial concentration is sufficiently large to be seen by... 

Thus, in summary, self diffusion measurements by Lindman et a (29-34) have clearly indicated that the structure of microemulsions depends to a large extent on the chain length of the oosurfactant (alcohol), the surfactant and the type of system. With short chain alcohols (hydrophilic domains and the structure is best described by a bicontinuous solution with easily deformable and flexible interfaces. This picture is consistent with the percolative behaviour observed when the conductivity is measured as a function of water volume fraction (see above). With long chain alcohols (> Cg) on the other hand, well defined "cores" may be distinguished with a more pronounced separation into hydrophobic and hydrophilic regions. 

Microemulsions are thermodynamically stable mixtures. The interfacial tension is almost zero. The size of drops is very small, and this makes the microemulsions look clear. It has been suggested that microemulsion may consists of bicontinuous structures, which sounds more plausible in these four-component microemulsion systems. It has also been suggested that microemulsion may be compared to swollen micelles (i.e., if one solubilizes oil in micelles). In such isotropic mixtures, short-range order exists between droplets. As found from extensive experiments, not all mixtures of water-oil-surfactant-cosurfactant produce a microemulsion. This has led to studies that have attempted to predict the molecular relationship. 

Many reports are available where the cationic surfactant CTAB has been used to prepare gold nanoparticles [127-129]. Giustini et al. [130] have characterized the quaternary w/o micro emulsion of CTAB/n-pentanol/ n-hexane/water. Some salient features of CTAB/co-surfactant/alkane/water system are (1) formation of nearly spherical droplets in the L2 region (a liquid isotropic phase formed by disconnected aqueous domains dispersed in a continuous organic bulk) stabilized by a surfactant/co-surfactant interfacial film. (2) With an increase in water content, L2 is followed up to the water solubilization failure, without any transition to bicontinuous structure, and (3) at low Wo, the droplet radius is smaller than R° (spontaneous radius of curvature of the interfacial film) but when the droplet radius tends to become larger than R° (i.e., increasing Wo), the microemulsion phase separates into a Winsor II system. 

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