Microemulsion characterization

Microemulsions are thermodynamically stable dispersions of oil and water stabilized by a surfactant and, in many cases, also a cosurfactant.1-4 The microemulsions can be of the droplet type, either with spherical oil droplets dispersed in a continuous medium of water (oil-in-water microemulsions, O/W) or with spherical water droplets dispersed in a continuous medium of oil (water-in-oil microemulsions, W/O). The droplet-type microemulsions can be either a single-phase system or part of a two-phase system wherein the microemulsion phase coexists with an excess dispersed phase (an upper phase of excess oil in the case of O/W and a lower phase of excess water in the case of W/O microemulsions). There are also nondroplet-type microemulsions, referred to as middle-phase microemulsions. In this case, the microemulsion phase is part of a three-phase system with the microemulsion phase in the middle coexisting with an upper phase of excess oil and a lower phase of excess water. One possible structure of this middle-phase microemulsion, characterized by randomly distributed oil and water microdomains and bicontinuity in both oil and water domains, is known as thebiccntinuous microemulsion. Numerous experimental studies have shown1 2 4 that one can achieve a transition... 

Microemulsion Characterization. Fluorescence spectra were measured on a Shimadzu RF-5000 spectrofluorometer. Samples with R values in the range of 0.50 to 4.80 were prepared in a manner similar to that used for the samples in the synthesis experiments, that is, by using a fixed amount of aqueous phase and different surfactant concentrations. The probe Ru(Bpy)32+ concentration was 9 X 10-6 M. The excitation wavelength was 460 nm. 

Microemulsion Characterization. The effects of R on the aggregation number N (i.e., the number of surfactant molecules per micelle) and on the nature of the solubilized water (i.e., bound or free) for this microemulsion system were reported (14). As R increases, dipole-dipole... 

Self-diffusion provided the first proof of bicontinuity in microemulsions and has become a routine technique of microemulsion characterization used by many groups. 

D. P. Acharya and P. G. Hardey, Progress in Microemulsion Characterization, Curr. Opin. Colloid Interface Sci., 2012, 17, 274. 

Zheng, Y. Eli, W. (2009). Study on the polarity of bmimPFe/TweenSO/toluene microemulsion characterized by UV-visible spectroscopy. Journal of Dispersion Science and Technology 30(5), 698-703. 

A beautiful and elegant example of the intricacies of surface science is the formation of transparent, thermodynamically stable microemulsions. Discovered about 50 years ago by Winsor [76] and characterized by Schulman [77, 78], microemulsions display a variety of useful and interesting properties that have generated much interest in the past decade. Early formulations, still under study today, involve the use of a long-chain alcohol as a cosurfactant to stabilize oil droplets 10-50 nm in diameter. Although transparent to the naked eye, microemulsions are readily characterized by a variety of scattering, microscopic, and spectroscopic techniques, described below. 

Dunn A S 1989 Polymerization in micelles and microemulsions Comprehensive Polymer Science—the Synthesis, Characterization, Reactions and Applications of Polymers vo 4, ed G C Eastmond, A Ledwith, S Russo and P Sigwalt (New York Pergamon) pp 219-24... 

In the real space the correlation function (6) exhibits exponentially damped oscillations, and the structure is characterized by two lengths the period of the oscillations A, related to the size of oil and water domains, and the correlation length In the microemulsion > A and the water-rich and oil-rich domains are correlated, hence the water-water structure factor assumes a maximum for k = k 7 0. When the concentration of surfac-... 

In this section we characterize the minima of the functional (1) which are triply periodic structures. The essential features of these minima are described by the surface (r) = 0 and its properties. In 1976 Scriven [37] hypothesized that triply periodic minimal surfaces (Table 1) could be used for the description of physical interfaces appearing in ternary mixtures of water, oil, and surfactants. Twenty years later it has been discovered, on the basis of the simple model of microemulsion, that the interface formed by surfactants in the symmetric system (oil-water symmetry) is preferably the minimal surface [14,38,39]. 

In sodium bis(2-ethylhexyl) phosphate microemulsions, which are composed of cylindrical micelles in the dilute region, it has been observed that the formation of micellar clusters is characterized by a branched structure as the volume fraction (<1>) of the aggregates increases. At d> > 0.2, these clusters mutually overlap, forming a network expanded overall [283]. 

The XRD and TEM showed that the bimetallic nanoparticles with Ag-core/Rh-shell structure spontaneously form by the physical mixture of Ag and Rh nanoparticles. Luo et al. [168] carried out structure characterization of carbon-supported Au/Pt catalysts with different bimetallic compositions by XRD and direct current plasma-atomic emission spectroscopy. The bimetallic nanoparticles were alloy. Au-core/Pd-shell structure of bimetallic nanoparticles, prepared by co-reduction of Au(III) and Pd(II) precursors in toluene, were well supported by XRD data [119]. Pt/Cu bimetallic nanoparticles can be prepared by the co-reduction of H2PtClg and CuCl2 with hydrazine in w/o microemulsions of water/CTAB/ isooctane/n-butanol [112]. XRD results showed that there is only one peak in the pattern of bimetallic nanoparticles, corresponding to the (111) plane of the PtCu3 bulk alloy. 

Hernandez J, Solla-Gullon J, Herrero E. 2004. Gold nanoparticles synthesized in a water-in-oil microemulsion Electrochemical characterization and effect of the surface structure on the oxygen reduction reaction. J Electroanal Chem 574 185-196. 

Electrical conductivity is an easily measured transport property, and percolation in electrical conductivity appears a sensitive probe for characterizing microstructural transformations. A variety of field (intensive) variables have been found to drive percolation in reverse microemulsions. Disperse phase volume fraction has been often reported as a driver of percolation in electrical conductivity in such microemulsions [17-20]. 

J Sjoblom, R Lindberg, SE Friberg. Microemulsions-phase equilibria characterization, structures, applications and chemical reactions, Adv Colloid Interf Sci 65 125-287, 1996. 

A Hafie, S Keipert. Development and characterization of microemulsions for ocular application. Eur J Pharm Biopharm 43(2) 179-183, 1997. 

A. Martinez-Arias, M. Femandez-Garcia, V. Ballesteros, L.N. Salamanca, J.C. Conesa, C. Otero and J. Soria, Characterization of high surface area Zr-Ce (1 1) mixed oxide prepared by a microemulsion method, Langmuir 15,4796-4802 (1999). 

The rate of metal complex formation is often modified (usually enhanced) by the presence of a charged interface in the aqueous phase. This may be provided by ionic micelles, e. g., SDS, microemulsions or polyelectrolytes. jjjg reactions of Ni + and Co with hydrophobic ligands pan, pap and pad 14-16 are popular ones for examining effects, since they are well characterized in the bulk water. The simple model (4.126)... 

Microemulsions are also characterized as microstructured, themodynamically stable mixtures of water, oil, surfactant, and additional components (such as cosurfactants). The study of microemulsions has shown that they are of the following types ... 

In pharmaceutics, therefore, simple and effective methods and procedures are needed to characterize the interactions of drugs with pharmaceutical excipients (polysaccharides, cyclodextrins, etc.) and vehicle systems (micelles, microemulsions, and liposomes) in order to optimize the load of vehicle systems with the drugs. 

Part II starts with the possibilities of ACE for characterizing the relevant physicochemical properties of drugs such as lipophilicity/hydrophilicity as well as thermodynamic parameters such as enthalpy of solubilization. This part also characterizes interactions between pharmaceutical excipients such as amphiphilic substances (below CMC) and cyclodextrins, which are of interest for influencing the bioavailability of drugs from pharmaceutical formulations. The same holds for interactions of drugs with pharmaceutical vehicle systems such as micelles, microemulsions, and liposomes. 

Microemulsion electrokinetic chromatography was introduced to study the affinity of various cephalosporins [cefpim, cefpirom, cefaloridin, cefaclor, cephalexin, cefuroxim, cefotaxim] in microemulsions and micellar (MC) systems. The affinity of various cephalosporins in microemulsions was characterized calculating the capacity factor. The capacity factor values of the cephalosporins in micellar systems and in microemulsions are given in Table... 

The plots of log k vs. log P w and the plots of log k (v) vs. log k (z) were studied for seven cephalosporins. A linear relationship was obtained in micellar solution and in microemulsion solution (Tables 3 and 4). The results obtained indicate that the capacity factor determined by EKC could be used both as parameter to characterize the partition behavior of drugs in ME and MC and as hydrophobic parameter instead of log Pow. k appears to be an evident parameter, and it shows a better diversification than P w. In the 1-octanol/water system, we did not found high values of the partition coefficients. In contrast, the ME systems used provide a better characterization of the drugs according to their hydrophilic/lipophilic properties. 

Mrestani, Y., El-Mokdad, N., Riittinger, H., Neubert, R. (1998). Characterization of partitioning behavior of cephalosporins using microemulsion and micellar electrokinetic chromatography. Electrophoresis 19 2895—2899. 

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