Microemulsion systems

Figure C2.3.8. Self-diffusion coefficients at 45°C for AOT ( ), water ( ) and decane ( ) in ternary AOT, brine (0.6% aqueous NaCl) and decane microemulsion system as a function of composition, a. This compositional parameter, a, is tire weight fraction of decane relative to decane and brine. Reproduced by pennission from figure 3 of [46].

Rosano FI L and Clausse M (eds) 1987 Microemulsion Systems (New York Dekker)... 

Fig. 2. The phase diagrams and terminology of a microemulsion system close to its two critical end points, where the middle phase and one of the binodals...

Modem scaling theory is a quite powerful theoretical tool (appHcable to Hquid crystals, magnets, etc) that has been well estabUshed for several decades and has proven to be particularly useful for multiphase microemulsion systems (46). It describes not just iuterfacial tensions, but virtually any thermodynamic or physical property of a microemulsion system that is reasonably close to a critical poiat. For example, the compositions of a microemulsion and its conjugate phase are described by equations of the foUowiug form ... 

H. L. Rosano and M. Clausse, eds.. Microemulsion Systems, Marcel Dekker, New York, 1987. 

Surfactant Solutions New Methods of Investigation, edited by Raoul Zana Nonionic Surfactants Physical Chemistry, edited by Martin J. Schick Microemulsion Systems, edited by Henri L Rosano and Marc Clausse Biosurfactants and Biotechnology, edited by Naim Kosaric, W. L. Cairns, and Neil C. C. Gray... 

FIG. 4 Apparent mole fraction (x) water in continuous phase of brine, decane, and AOT microemulsion system derived from the water self-diffusion data of Fig. 3 using the two-state model of Eq. (1). 

FIG. 5 Order parameter for disperse pseudophase water (percolating clusters versus isolated swollen micelles and nonpercolating clusters) derived from self-diffusion data for brine, decane, and AOT microemulsion system of single-phase region illustrated in Fig. 1. The a and arrow denote the onset of percolation in low-frequency conductivity and a breakpoint in water self-diffusion increase. The other arrow (b) indicates where AOT self-diffusion begins to increase. 

A somewhat different water, decane, and AOT microemulsion system has been studied by Feldman and coworkers [25] where temperature was used as the field variable in driving microstructural transitions. This system had a composition (volume percent) of 21.30% water, 61.15% decane, and 17.55% AOT. Counterions (sodium ions) were assigned as the dominant charge transport carriers below and above the percolation threshold in electrical... 

As described in the introduction, certain cosurfactants appear able to drive percolation transitions. Variations in the cosurfactant chemical potential, RT n (where is cosurfactant concentration or activity), holding other compositional features constant, provide the driving force for these percolation transitions. A water, toluene, and AOT microemulsion system using acrylamide as cosurfactant exhibited percolation type behavior for a variety of redox electron-transfer processes. The corresponding low-frequency electrical conductivity data for such a system is illustrated in Fig. 8, where the water, toluene, and AOT mole ratio (11.2 19.2 1.00) is held approximately constant, and the acrylamide concentration, is varied from 0 to 6% (w/w). At about = 1.2%, the arrow labeled in Fig. 8 indicates the onset of percolation in electrical conductivity. 

The majority of RDC studies have concentrated on the measurement of solute transfer resistances, in particular, focusing on their relevance as model systems for drug transfer across skin [14,39-41]. In these studies, isopropyl myristate is commonly used as a solvent, since it is considered to serve as a model compound for skin lipids. However, it has since been reported that the true interfacial kinetics cannot be resolved with the RDC due to the severe mass transport limitations inherent in the technique [15]. The RDC has also been used to study more complicated interfacial processes such as kinetics in a microemulsion system [42], where one of the compartments contains an emulsion. 

For a typical biomolecule-containing w/o-ME system, only a fraction of the w/o-ME population (e.g., 0.1-1%) will contain proteins. Because of their small concentration and the short (microsecond-scale) lifetime of any given w/o-ME droplet, due to the rapid collision and exchange rate for w/o-ME systems, isolation of the protein-containing, or filled w/o-ME populations is difficult to achieve. Various techniques have demonstrated that encapsulated enzymes can alter the structural properties and behavior of the w/o-MEs, and that filled, w/o-MEs may differ in properties from the empty w/o-MEs in a given microemulsion system [46-51]. However, a clear understanding of the structural and dimensional differences between filled and empty w/o-MEs has yet to be achieved. 

MB Charro, GI Vilas, TB Mendez, MAL Q, JP Marty, RH Guy. Delivery of a hydrophilic solute through the skin from novel microemulsion systems. Eur J Pharm Biopharm 43(l) 37-42, 1997. 

In an earlier study calorimetry achieved this objective for the compositional boundaries between two and three phases (2). Such boundaries are encountered both in "middle-phase microemulsion systems" of low tension flooding, and as the "gas, oil, and water" of multi-contact miscible EOR systems (LZ). The three-phase problem presents by far the most severe experimental and interpretational difficulties. Hence, the earlier results have encouraged us to continue the development of calorimetry for the measurement of phase compositions and excess enthalpies of conjugate phases in amphiphilic EOR systems. 

Olas and Wachowicz (2002) investigated the effects of tranx-resveratrol and vitamin C on oxidative stress in blood platelets. The level of 02 in control blood platelets and platelets incubated with resveratrol or vitamin C was recorded using a chemiluminescence method. On the other hand, Oh and others (2006) reported the x02 quenching activities of various freshly squeezed fruit and vegetable juices by measuring their inhibitory effects on the rubrene oxidation induced by x02 from disproportionation of hydrogen peroxide by sodium molybdate in a microemulsion system. 

Furlanetto, S., Orlandini, S., Marras, A. M., Mura, P., and Pinzauti, S. (2006). Mixture design in the optimization of a microemulsion system for the electrokinetic chromatographic determination of ketorolac and its impurities method development and validation. Electrophoresis 27, 805-818. 

Fig. 15.4 Schematic ternary-phase diagram of an oU-water-surfactant microemulsion system consisting of various associated microstructures. A, normal miceUes or O/W microemulsions B, reverse micelles or W/O microemulsions C, concentrated microemulsion domain D, liquid-crystal or gel phase. Shaded areas represent multiphase regions.

D. Langevin In S.-H. Chen, J. S. Huang, and P. Tartaglia (eds), Low interfacial tensions in microemulsion systems. Structure and Dynamics of Strongly Interacting Colloids and Supramolecular Aggregates in Solution. 325. p. Kluwer, Dordrecht (1992). 

T. Sottmann and R. Strey Shape Similarities of Ultra-Low Interfacial Tension Curves in Ternary Microemulsion Systems of the Water-AUcane-CiEj Type. Ber. Bunsenges Phys. Chem. 100, 237 (1996). 

The coexistence of three phases is typical of many microemulsions systems (11). 

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. 

Consequently, the SDS microemulsion system is the best model for indirect measurement of log Pow. However, this is valid only for neutral solutes. We reported that the relationship between MI and log Pow for ionic solutes is different from that for neutral solutes (49). This would be caused by the ionic interaction between ionic solutes and the ionic microemulsion as well as ionic surfactant monomer in the aqueous phase. Kibbey et al. used pH 10 buffer for neutral and weak basic compounds and pH 3 buffer for weak acidic compounds (53). Although their purpose was to avoid measuring electrophoretic mobility in the aqueous phase, this approach is also helpful for measuring log Pow indirectly. 

Generally, heptane or octanol has been used as the colloidal oil phase in the microemulsion systems (21-23). Octanol, heptane, and hexane show similar selectivity and migration time for the separation of a number of neutral solutes (24). A number of other oils were used in the microemulsion systems, for instance, ethyl acetate (25), hexanol (26), butyl chloride (27), and a chiral oil for chiral separation (28). 

Phosphate-borate buffer is the most frequently used solution for the preparation of microemulsion systems. An increase in the buffer concentration... 

Determination of the Enthalpy AH° and Entropy AS0 of Solubilization in Microemulsion Systems... 

The MEEKC technique is effective for the separation of complex mixtures. A typical example is the separation of proteins (33). This separation was carried out using a microemulsion system consisting of SDS-heptane-butane-... 

Table 3 Linear Relationships Between log Pm in Octanol-Water System and log k in Microemulsion System at pH 7.0...

Fig. 4 Electropherogram of cephalosorins (1—cefpim, 2—cefpirom, 3—cepha-loridin, 4—cephlexin, 5—cefaclor, 6—cefuroxim, 7—cefotaxin) in a microemulsion system. Buffer, pH 7.0, 10 mM phosphate containing 6.49% l-butanol/0.82% ra-heptane, 1.44% glycodeoxycholic acid (GDC), and 5.685% Tween. Capillary, 48.5- (40 cm to detector) X 50-/rm ID 30 kV detection, 265 nm. (From Ref. 14.)...

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