Microemulsions and micelles

Chemical methods of preparing nanoparticles offer many possibilities - the case of metal particles being illustrative. Microemulsions and micelles can be employed as the media to produce small particles of sulfides and oxides of 1.5-10 nm diameter with narrow size distribution (e.g. CdS). The sol-gel technique also gives small particulates of many oxidic materials. Recently, homogeneous nanoparticles of ZnO and of the... 

The multicomponent version of an inverse micelle is a water-in-oil microemulsion and that of a normal micelle is an oil-in-water microemulsion. Not unexpectedly, the similarities between such microemulsions and micelles are striking. Their principal advantage over conventional micelles is in their much higher capacity for enhancing solubility of desired substrates. The presence of the additional components, e.g., a co-surfactant, does decrease, however, the attainable surface charge, which in turn makes the dynamic interchange of organizates more facile. Thus, compartmentalization is less secure and sometimes normal kinetics can be observed in microemulsions. 

Only limited evidence is available on these questions. For example Mackay and coworkers have estimated an effective dielectric constant of ca. 20 for a variety of microemulsion droplets, based on pK measurements [154], i.e., they appear to be somewhat less polar than normal micelles in water. However fluorescence shifts in microemulsions are similar to those in micelles, suggesting that the polarities of o/w microemulsions and micelles are similar [155]. We do not know whether these differences stem from the different probes used to estimate polarity, or whether there are marked differences between the surfaces of the various microemulsion aggregates. 

Since the simplest oil-in-water (O/W) and water-in-oil (W/O) microemulsions are ternary systems in which the particles are swollen direct and reverse micelles, respectively, the examples given for the application of electrical birefringence will include both microemulsions and micelles. As the studies reveal, the experiments are usually carried out to find answers to specific questions instead of the complete physical characterization of the particles. Often, however, interesting additional information is derived such as the mechanism of phase separation or the elasticity constant of the monolayer in W/O microemulsions. 

When one compares microemulsions and micelles, the demarcation line can become quite blurred and, in some cases, does not exist. There is some controversy as to the true definition of clear, isotropic solutions of oil, water, and surfactant (and cosurfactant if needed) as microemulsions rather than swollen micelles. Although the differences between the two systems may appear to many to be more semantic than real, several arguments have been presented that strongly support a differentiation of the two systems. 

A third type of analyzer uses a modified form of photon correlation spectroscopy to generate particle-size distributions in the range of 0.005. 5 pm, which is useful for the measurement of microemulsions and micelles in polymers or polymer suspensions. 

D. Andelman, F. Brochard, C. Knobler, and F. Rondelez, Micelles, Membranes, Microemulsions and Monolayers, Springer-Verlag, 1994, Chapter 12. 

J. F. Rusling, in Electrochemistry in Micelles, Microemulsions and Related Microheterogeneous Fluids, Electroanalytical Chemistry, A Series of Advances, Marcel Dekker, New York, 1994. 

The issue of water in reverse micellar cores is important because water swollen reverse micelles (reverse microemulsions) provide means for carrying almost any water-soluble component into a predominantly oil-continuous solution (see discussions of microemulsions and micellar catalysis below). In tire absence of water it appears tliat premicellar aggregates (pairs, trimers etc.) are commonly found in surfactant-in-oil solutions [47]. Critical micelle concentrations do exist (witli some exceptions). 

Other solubilization and partitioning phenomena are important, both within the context of microemulsions and in the absence of added immiscible solvent. In regular micellar solutions, micelles promote the solubility of many compounds otherwise insoluble in water. The amount of chemical component solubilized in a micellar solution will, typically, be much smaller than can be accommodated in microemulsion fonnation, such as when only a few molecules per micelle are solubilized. Such limited solubilization is nevertheless quite useful. The incoriDoration of minor quantities of pyrene and related optical probes into micelles are a key to the use of fluorescence depolarization in quantifying micellar aggregation numbers and micellar microviscosities [48]. Micellar solubilization makes it possible to measure acid-base or electrochemical properties of compounds otherwise insoluble in aqueous solution. Micellar solubilization facilitates micellar catalysis (see section C2.3.10) and emulsion polymerization (see section C2.3.12). On the other hand, there are untoward effects of micellar solubilization in practical applications of surfactants. Wlren one has a multiphase... 

Cosolvents ana Surfactants Many nonvolatile polar substances cannot be dissolved at moderate temperatures in nonpolar fluids such as CO9. Cosolvents (also called entrainers, modifiers, moderators) such as alcohols and acetone have been added to fluids to raise the solvent strength. The addition of only 2 mol % of the complexing agent tri-/i-butyl phosphate (TBP) to CO9 increases the solubility ofnydro-quinone by a factor of 250 due to Lewis acid-base interactions. Veiy recently, surfac tants have been used to form reverse micelles, microemulsions, and polymeric latexes in SCFs including CO9. These organized molecular assemblies can dissolve hydrophilic solutes and ionic species such as amino acids and even proteins. Examples of surfactant tails which interact favorably with CO9 include fluoroethers, fluoroacrylates, fluoroalkanes, propylene oxides, and siloxanes. 

V. Degiorgio, M. Corti, eds. Physics of Amphiphiles Micelles, Vesicles and Microemulsions. Amsterdam North-Holland, 1985 W. M. Gelbart, A. Ben-Shaul, D. Roux, eds. Micelles, membranes, microemulsions and monolayers. Berlin Springer, 1994. 

Generation of nanoparticles under Langmuir monolayers and within LB films arose from earlier efforts to form nanoparticles within reverse micelles, microemulsions, and vesicles [89]. Semiconductor nanoparticles formed in surfactant media have been explored as photocatalytic systems [90]. One motivation for placing nanoparticles within the organic matrix of a LB film is to construct a superlattice of nanoparticles such that the optical properties of the nanoparticles associated with quantum confinement are preserved. If mono-layers of capped nanoparticles are transferred, a nanoparticle superlattice can be con-... 

Moreover, stable liquid systems made up of nanoparticles coated with a surfactant monolayer and dispersed in an apolar medium could be employed to catalyze reactions involving both apolar substrates (solubilized in the bulk solvent) and polar and amphiphilic substrates (preferentially encapsulated within the reversed micelles or located at the surfactant palisade layer) or could be used as antiwear additives for lubricants. For example, monodisperse nickel boride catalysts were prepared in water/CTAB/hexanol microemulsions and used directly as the catalysts of styrene hydrogenation [215]. 

A general analysis for microphase catalysis, where microphase includes micelles, swollen micelles, microemulsions and macroemulsions, can be rigorously constructed by writing the... 

The potential x as the difference of electrical potential across the interface between the phase and gas, is not measurable. But its relative changes caused by the change of solution composition can be determined using the proper voltaic cells (see Section IV). The name surface potential is unfortunately also often used for the description the ionic double layer potential (i.e., the ionic part of the Galvani potential) at the interfaces of membranes, microemulsion droplets and micelles, measured usually by the acid-base indicator technique (Section V). 

D. Sornette and N. Ostrowsky, in Micelles, Membranes, Microemulsions, and Monolayers (W. Gelbart, A. Ben-Shaul, and D. Roux, eds.) Springer-Verlag, New York, 1994, pp. 251-302. 

Other claimed matter DBT for enrichment, biocatalyst preparation contacting process Enzymes contacting process Pure compounds as feedstock Membrane fragments and extracts Cell-free extract (envelope and its fragments + associated enzyme) reversible emulsion microemulsion reverse micelles Cell-free enzyme preparation microemulsified process RR and derivatives and other biocatalyst concepts + any known microorganism active for C—S bond cleavage... 

Studies of chemical reactivity in self-assembling colloids were initially based on reactions in aqueous micelles, but recently reactivity has been examined in other colloidal systems such as microemulsions and synthetic vesicles (Mackay, 1981 Fendler, 1982 O Connor et al., 1982, 1984 Cuccovia et al. 1982b). Some hydrophobic trialkylammonium salts, which are phase-... 

It is convenient to differentiate between oil-in-water (o/w) microemulsions and water-in-oil (w/o) microemulsions in which water and oil are the respective major components. It is reasonable to regard (o/w) microemulsions as akin to swollen normal micelles and w/o microemulsions as reverse micelles (Section 1). 

As for direct emulsions, the presence of excess surfactant induces depletion interaction followed by phase separation. Such a mechanism was proposed by Binks et al. [ 12] to explain the flocculation of inverse emulsion droplets in the presence of microemulsion-swollen micelles. The microscopic origin of the interaction driven by the presence of the bad solvent is more speculative. From empirical considerations, it can be deduced that surfactant chains mix more easily with alkanes than with vegetable, silicone, and some functionalized oils. The size dependence of such a mechanism, reflected by the shifts in the phase transition thresholds, is... 

In recent years, much of the research work in the pharmaceutical sciences was focused on the development of effective vehicle systems, such as micelles, microemulsions, and liposomes, for drugs that are critical with respect to bioavailability. Knowledge of this subject is a prerequisite to developing vehicle systems for special administration routes, such as dermal, transdermal, intravenous, and nasal. 

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. 

Three different interacting phases can be distinguished in ACE the stationary, pseudostationary, and mobile phases. First, the interaction can take place at the surface of a coated capillary wall or at a stationary phase present in the capillary. This approach is analogous to CEC, as discussed previously. Second, the interaction can take place in pseudostationary phases, such as micelles, microemulsions, and liposomes. Third, the interaction can take place when both the solute and the affinity molecule are in free solution. For studying these interactions, two analysis methods have been developed. 

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