Microemulsions principle

D. O. Shah and W. C. Hsieh, Microemulsions, Liquid Crystals and Enhanced Oil Recovery, in Theory, Practice, and Process Principles for Physical Separations, Engineering Foundation, New York, 1977. 

Since by changing the nature and/or the concentration of the components of the w/o microemulsions it is possible to change size and/or shape (spheres, needles, cubes, wires, bundles, etc.) of the hydrophilic microregions, in principle, the size, size distribution, and shape of the nanoparticles could easily be modulated [197,202, 212,213],... 

Different surfactants are usually characterised by the solubility behaviour of their hydrophilic and hydrophobic molecule fraction in polar solvents, expressed by the HLB-value (hydrophilic-lipophilic-balance) of the surfactant. The HLB-value of a specific surfactant is often listed by the producer or can be easily calculated from listed increments [67]. If the water in a microemulsion contains electrolytes, the solubility of the surfactant in the water changes. It can be increased or decreased, depending on the kind of electrolyte [68,69]. The effect of electrolytes is explained by the HSAB principle (hard-soft-acid-base). For example, salts of hard acids and hard bases reduce the solubility of the surfactant in water. The solubility is increased by salts of soft acids and hard bases or by salts of hard acids and soft bases. Correspondingly, the solubility of the surfactant in water is increased by sodium alkyl sulfonates and decreased by sodium chloride or sodium sulfate. In the meantime, the physical interactions of the surfactant molecules and other components in microemulsions is well understood and the HSAB-principle was verified. The salts in water mainly influence the curvature of the surfactant film in a microemulsion. The curvature of the surfactant film can be expressed, analogous to the HLB-value, by the packing parameter Sp. The packing parameter is the ratio between the hydrophilic and lipophilic surfactant molecule part [70] ... 

This chapter focuses on silica synthesis via the microemulsion-mediated alkoxide sol-gel process. The discussion begins with a brief introduction to the general principles underlying microemulsion-mediated silica synthesis. This is followed by a consideration of the main microemulsion characteristics believed to control particle formation. Included here is the influence of reactants and reaction products on the stability of the single-phase water-in-oil microemulsion region. This is an important issue since microemulsion-mediated synthesis relies on the availability of surfactant/ oil/water formulations that give stable microemulsions. Next is presented a survey of the available experimental results, with emphasis on synthesis protocols and particle characteristics. The kinetics of alkoxide hydrolysis in the microemulsion environment is then examined and its relationship to silica-particle formation mechanisms is discussed. Finally, some brief comments are offered concerning future directions of the microemulsion-based alkoxide sol-gel process for silica. 

In principle, silica growth kinetics may be controlled by (1) slow release of monomer via alkoxide hydrolysis in the particle-free reverse micelles, (2) slow surface reaction of monomer addition to the growing particle, and (3) slow transport processes as determined by the dynamics of intermicellar mass transfer. There is strong experimental evidence to support the view that the rate of silica growth in the microemulsion environment is controlled by the rate of hydrolysis of TEOS (23,24,29). Silica growth kinetics can be analyzed in terms of the overall hydrolysis and condensation reactions ... 

Myers, D., Surfaces, Interfaces, and Colloids Principles and Applications, VCH Publishers, New York, 1991. (Undergraduate level. A qualitative overview of micelles, microemulsions, and their applications.)... 

The same principles that lead to interest in microemulsions for displacing trapped crude oil from pores in reservoir rock also make them potentially useful for industrial cleaning [234],... 

Lif and Holmberg have demonstrated the efficiency of microemulsions as a medium for both organic and bioorganic hydrolysis of a 4-nitrophenyl ester see Scheme 3 of Fig. 3 [7]. The reactions were performed in a Winsor I type microemulsion and took place in the lower phase oil-in-water microemulsion. After the reaction was complete a Winsor I—>111 transition was induced by a rise in temperature. The products formed, 4-nitrophenol and decanoic acid, partitioned into the upper oil phase and could easily be isolated by separation of this phase and evaporation of the solvent. The principle is outlined in Fig. 4. The surfactant and the enzyme (in the case of the lipase-catalysed reaction) resided in the middle-phase microemulsion and could be reused. 

Emulsions, microemulsions, and liquid crystalline systems are suspensions of deformable particles, and many of the principles stated earlier for suspensions are valid to a similar extent. The effect of the phase volume, however, is less pronounced. ... 

A variant of the microemulsion method was first applied in the preparation of hydrogenation catalysts [3]. The principle of this and following studies consists in the preparation of solid particles in water droplets of w/o microemulsions. The resulting mbcture is used a) after separation as a catalyst [3,4] or b) as an initial suspension for further catalyst preparation [5]. 

Since the gravity force is proportional to R, then if R is reduced by a factor of 10, the gravity force is reduced by 1000. Below a certain droplet size (which also depends on the density difference between oil and water), the Brownian diffusion may exceed gravity and creaming or sedimentation is prevented. This is the principle of formulation of nanoemulsions (with size range 20-200 nm) that may show very little or no creaming or sedimentation. The same applies for microemulsions (size range 5-50 nm). 

Using the above principle, Lindman and coworkers [16-18] measured the selfdiffusion coeflBdents of all microemulsion components, with particular emphasis on the role of the cosurfactant. For microemulsions consisting of water, hydrocarbon, an anionic surfactant and a short-chain alcohol (C4 and C5), the setf-diffusion coefficient of water, hydrocarbon and cosurfactant was quite high (on the order of 10 s ), which was two orders of magnitude higher than the value expected... 

Oldfield, C., Enzymes in water-in-oil microemulsions ( reversed micelles ) principles and appbcations, in Biotechnology and Genetic Engineering Reviews, Tombs, M.P., Ed., Vol. 12, Intercept Ltd., Andover, U.K., 155-ill, 1994. 

Whereas the surfactant will always reside in the microemulsion phase, the product is likely to partition into an excess oil phase if it is an apolar substance and into an excess water phase if it is a polar compound. The principle is illustrated in Fig. 5.14 for hydrolysis of a lipophilic ester in a Winsor I system (an oil-in-water microemulsion in equilibrium with excess oil) followed by transition into a Winsor III system [60]. The ester partitions between the excess oil phase and the oil droplets and the hydroxyl ions reside in the continuous water domain of the microemulsion. The reaction takes place at the interface. After completed reaction, acid is added to protonate the alkanoate formed and the temperature is raised so that a Winsor I to Winsor III transition occurs. The lipophilic... 

Schwan, M. (2005) Uberkritische Mikroemulsionen zur Herstellung nanozellularer Schaume - Principle of Supercritical Microemulsion Expansion (POSME). Ph.D. Thesis, University of Cologne. 

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