Stability microemulsions

A microemulsion (p.E) is a thermodynamically stable, transparent (in the visible) droplet type dispersion of water (W) and oil (O a saturated or unsaturated hydrocarbon) stabilized by a surfactant (S) and a cosurfactant (CoS a short amphiphile compound such as an alcohol or an amine) [67]. Sometimes the oil is a water-insoluble organic compound which is also a reactant and the water may contain mineral acids or salts. Because of the small dispersion size, a large amount of surfactant is required to stabilize microemulsions. The droplets are very small (about 100-1000 A [68]), about 100 times smaller than those of a typical emulsion. The existence of giant microemulsions (dispersion size about 6000 A) has been demonstrated [58]. 

Typical approaches to this biphasic system have involved the immobilization of catalysts in the aqueous phase as colloids [53] or using water-soluble catalysts based on ligands such as the trisulfonated TPPTS [55, 56]. Particularly high reaction rates have been obtained with surfactant-stabilized microemulsions and emulsions that allow for intimate contact of all reagents with the catalyst during the reaction [57]. The emulsions separate readily into two phases by small pressure changes and the C02-phase is then vented to isolate the products. The catalyst RhCl(tppds)3 (tppds =... 

Several generic kinds of results are pertinent to the properties of dispersions. The surfactant solutions formulated to stabilize microemulsions, and some kinds of macroemulsions, can exhibit marked dynamic interfacial tension behaviour. Figure 3.14 shows an example in which a series of commerical surfactant additions are made to a system containing crude oil and a base. Under alkaline conditions the interfacial tension is already dynamic due to the saponification of natural surfac-... 

A 9-methoxyellipticine (2)-low density lipoprotein (LDL) complex was formulated by Soula and co-workers (742) and found to be 10 times more active than 2 against L1210 and P388 leukemia in vitro. This activity seems to depend on the LDL high-affinity receptor since LDL reduces the antitumor activity. The complex was prepared by adding 2 to a dimyristoyl phosphatidylcholine, cho-lesteryl oleate-stabilized microemulsion and then fusing with human LDL. 

We report here on the use of alkyl sulfones as novel unconventional cosurfactants in CTAB-stabilized microemulsions. Sulfones, being fully oxidized at sulfur, have good stability to oxidants such as hypohalite. 

Perfluorocarbon-based ultrasound contrast agents, for example CjFg, C4F10, or C5F12, are injected typically as stabilized microemulsions [113] which have to be activated before use. The amount of perfluorocarbon which must be administered for one examination of a 70-kg person is approximately 50 pL. 

Another polar solvent that has been used in SDS-stabilized microemulsions is glycerol. Hexanol or decanol have been used as cosurfactants and systems both with and without oil have been studied. The ternary system with hexanol as cosurfactant was examined with SANS and NMR self-diffusion measurements by two different groups and both found the microemulsions to be structureless solutions [130,131], Similar behavior was found from a self-diffusion study of the quaternary systems with p-xylcnc or decane as the oil component [131,132],... 

Nonaqueous microemulsions with nonionic surfactants have been studied. The C12E4 surfactant was found to stabilize microemulsions of formamide and dodecane [138], The ternary phase diagrams were studied at different temperatures and the solubilization of hydrocarbon was shown to be very temperature dependent (Figure 6.7). It was also observed that the temperature intervals of the three-phase regions are dependent on the hydrocarbon used larger aliphatic hydrocarbons... 

Microemulsions. Unlike emulsions, microemulsions are transparent and thermodynamically stable colloidal systems, formed under certain concentrations of surfactant, water, and oil (Fig. 18.8). The transparency is because the droplet size of the microemulsions is small enough (<100 nm) that they do not reflect light. Because of its thermodynamic stability, microemulsions may have long shelf lives and spontaneously form with gentle agitation. However, microemulsions are not infinitely stable upon dilution because dilution... 

Based on the success of these fluoro-sulfosuccinates described above di-fluorocarbon phosphates have also been investigated. In terms of synthesis and raw materials costs these surfactants have significant advantages over the sulfosuccinates. Surfactants of this kind have also been studied by DeSimone et al (27 c), and the synthesis and purification are described elsewhere (27b, c). Detailed SANS experiments are described in these papers (27b, c), and it is clear that surfactants of diis kind stabilize aqueous nano-droplets. Hence, anionics other than sulfosuccinates may be employed in water-in-C(>2 microemulsions. Significantly, one of these conq>ounds (di-HCF6-P, see ref 27 b) stabilizes microemulsions in liquid CO2 at vapor pressure a potentially useftil result that may be of importance in facilitating applications. 

Bioconversion can also be conducted in water-in-C02 microemulsions. In one case, fluorinated two-chain sulfosuccinate surfactants have been used to stabilize microemulsions in which the enzyme was located within the dispersed water droplets [39]. Two reactions, a lipaseotalyzed hydrolysis ofp-nitrophenol butyrate and a lipoxygenase-catalyzed peroxidation of linoleic acid, were investigated [Eqs. (11) and (12)]. The pH was controlled by the buffer MES [2-(N-morpho-Hno)ethanesulfonic acid] in the range of 5-6. Both enzymes showed good activity in the water-in-C02 microemulsion, largely identical to that observed in water-inheptane microemulsions. 

Nasseria, A.A. et al.. Lecithin-stabilized microemulsion-based organogels for topical application of ketorolac tromethamine. II. In vitro release study. Iranian Journal of Pharmaceutical Research, 2003. 117 123. 

Because of thermodynamic stability, microemulsions are easy to prepare and require no significant energy contribution during preparation. Microemulsions have low viscosity compared to other emulsions. 

Hydrodynamic and solvation factors, emulsion stability, microemulsions, multiple emulsions, coagulation and flocculation theory, foam stability, demulsiflcation and defoaming, the effects of adsorbed polymers on stability and flocculation... 

Alkamuls SML Ethox Stabilizer, invert emulsions production fluids Alkamuls SML Ethox Stabilizer, microemulsions Capryol 90 Capryol PGMC Stabilizer, o/w emulsions Canocol S-20 Canocol S-60 Canocol S-80 Canocol S-85 Ceteareth-2 phosphate Ceteareth-5 phosphate Cete-areth-10 phosphate Crodafos CS2A ... 

With regard to the behavior of polar polymers in microemulsions, it has been reported that polymerization in methyl methacrylate microemulsions with non-polymerizable surfactants and cosurfactants [28] leads to phase separation of the system in spite of the fact that the poly(methyl methacrylate) is soluble in its monomer. On the other hand, polar polymers such as amphiphilic block copolymers, e.g., poly(2-vinyl p5nidine-fc-ethylene oxide), have been used as cosurfactants to stabilize microemulsions [65, 66] and a specific mechanism has been proposed for polar polymers as stabilizers for emulsions and microemulsions [67]. 

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