Micellar systems microemulsions

Meinzer, A., Mueller, E., and Vonderscher, ]., Microemulsion — A suitable galenical approach for the absorption enhancement of low soluble compounds , in Absorption of Micellar Systems Bulletin Technique Gattefosse, Barthelemy, P, Ed., Gattefosse S.A., Saint-Priest, France, 1995, p. 21. 

A pseudophase ion exchange model has been applied to reactions in micellar systems with varying success (1-7). According to this model, the distribution of nucleophile is considered to depend on the ion-exchange equilibrium between the nucleophile and the surfactant counterion at the micelle surface. This leads to a dependence on the ion-exchange constant (K g) as well as on the degree of dissociation (a) of the surfactant counterion. The ion exchange (IE) model has recently been extended to oil in water microemulsions (8). 

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... 

Romsted LS (1977) A general kinetic theory of rate enhancements for reactions between organic substrates and hydrophUic ions in micellar systems. In Mittal KL (ed) Micellization, Solubilization, Microemulsions. Plenum Press, New York... 

Tween 85 is used extensively for RME [84]. Russell and coworkers [234] used Tween 85/isopropanol microemulsions in hexane to solubilize proteins and not only showed >80% solubilization of cytochrome C at optimum conditions, but also proved that Tween 85 does not have a detrimental effect on the structure, function, and stability of subtilisin and cytochrome C. There are other reports available on the extraction and purification of proteins using Tween 85-RMs and also on the stability of protein activity in these systems [234]. It has also been shown that Tween 85-RMs can solubilize larger amounts of protein and water than AOT. Tween 85 has an HLB of 11, which indicates that it is soluble in organic solvents. In addition, it is biodegradable and can be successfully used as an additive in fertihzers [235,236]. Pfammatter et al. [35] have demonstrated that RMs made of Tween 85 and Span 80 can be successfully used for the solubilization and growth of whole cells. Recently, Hossain et al. [84] showed an enhanced enzymatic activity of Chromobacterium viscosum Hpase in AOT/Tween 85 mixed reverse micellar systems when compared to that in classical AOT-RMs. This is due to the modification of the interface in AOT-RMs caused by the co-adsorption of Tween 85, and increased availability of the oHve oil molecules (substrate) to the enzyme. 

An overview of other forms of micellar systems follows in the next three sections. Formation of reverse micelles, in nonaqueous media, is discussed briefly in Section 8.8. Sections 8.9 and 8.10 present an introduction to microemulsions (oil, or water, droplets stabilized in water or oil, respectively) and their applications. 

C. Malcolmson, M. J. Lawrence, Comparison of the incorporation of model steroids into non-ionic micellar and microemulsion systems, J. Pharm. Pharmacol. 45 141 — 143 (1993). 

The size and composition dispersions of droplets can be estimated by using the maximum-term method, without performing detailed distribution calculations. This is accomplished by adapting the approach used for micellar systems,17-18 as described in Appendix C. The aggregate corresponding to the maximum a Xgo or Xgy/ is considered to provide the number-average size and composition of the equilibrium droplets. For each component k (=S, A, O, or W) present in the microemulsion, the mean-square deviation o 2(k) from the number-average number of molecules g (k) can be shown to be... 

This chapter describes novel inkjet inks based on a variety of vehicles, and demonstrates several optical applications utilized by inkjet inks. It aims to provide a general description of inks which are based on unique components and structures, mainly micellar systems, polyelectrolyte complexes, microemulsions, miniemulsions, emulsions, liquid crystals, and interesting phase... 

A characteristic property of surfactant molecules is their tendeney to aggregate at interfaces. Examples are adsorptions onto solids and monolayer formation at an air-water interface. Surfactants sometimes ereate their own interface by forming very small aggregates like mieelles or vesieles to remove a portion of their structure from direct contact with a solvent. In ease of a mieelle formed with a surfactant such as Triton X-IOO, the hydroearbon ehains are in closer contact in the center and form a hydrophobic microenvironment. The ethylene oxide moieties are exposed to water with mueh greater frequeney. If a hydrophobic species is added into this micellar system, there will be a tendeney for the hydrophobic molecules to be concentrated inside a mieelle. At low concentration, the micelle system and the added hydrophobic additives ean reach a thermodynamic equilibrium, which is often called microemulsion system. At high concentration, the hydrophobic additives form their own separate phase and the surfactant molecules serve only as a decorative layer... 

Ti icroemulsions are transparent thermodynamically stable colloidal dispersions containing high amounts of both water and hydrocarbons. The colloidal state is stabilized by a proper balance between a hydrophobic and a hydrophilic surfactant. Initially microemulsions were considered to be different from colloidal solutions (I, 2, 3, 4, 5) an opinion that is still held by some (6) although it is accepted generally that microemulsions belong to micellar systems (7, 8, 9, 10). 

Microemulsions consist of apparently homogeneous transparent systems of low viscosity which contain a high percentage of both oil and water and high concentrations (15-25%) of emulsifier mixture. They were first described by Schulman as disperse systems with spherical or cylindrical droplets in the size range 8-80 nm. They are essentially swollen micellar systems, but obviously the distinction between a swollen micelle and small emulsion droplet is difficult to assess. 

Double Layer Interactions and Interfacial Charge. Schulman et al (42) have proposed that the phase continuity can be controlled readily by interfacial charge. If the concentration of the counterions for the ionic surfactant is higher and the diffuse electrical double layer at the interface is compressed, water-in-oil microemulsions are formed. If the concentration of the counterions is sufficiently decreased to produce a charge at the oil-water interface, the system presumably inverts to an oil-in-water type microemulsion. It was also proposed that for the droplets of spherical shape, the resulting microemulsions are isotropic and exhibit Newtonian flow behavior with one diffused band in X-ray diffraction pattern. Moreover, for droplets of cylindrical shape, the resulting microemulsions are optically anisotropic and non-Newtonian flow behavior with two di-fused bands in X-ray diffraction (9). The concept of molecular interactions at the oil-water interface for the formation of microemulsions was further extended by Prince (49). Prince (50) also discussed the differences in solubilization in micellar and microemulsion systems. 

Table I shows the effect of various systems such as micelles, swollen micelles (achieved by adding hexanol to CTAB), microemulsion systems, vesicles formed from a double-chain CTAB surfactant, and reversed micelles with water cores formed with benzyl dimethylcetylammonium bromide in benzene. Hie active chromophore exists either as pyrene, pyrene sulfonic acid or pyrene tetrasulfonlc acid. Essentially the concept here is that the polar derivatives of pyrene will always locate pyrene at the surface of the micelle as these anionic species of pyrene complex with the positively charged surface. Dimethylaniline is used as an electron donor in each case, it can be seen that for pyrene, a continual decrease in the yield of the pyrene anion (ion yield of unity in the micelle) is observed on going from micelle to swollen micelle, to microemulsion, and no yield of ions is observed in a reversed micelle system. With pyrene tetrasulfonic acid the yield of ions over the different systems is fairly constant, even across to the reverse micellar system. However, the lifetime of the ions is extremely short in the reversed micellar system. An explanation for such behavior can be given as follows as we transverse across the...

Hence, the exiplex has a sandwich structure which promotes efficient back e transfer at the water pool, and the ion yield is very small. However, a sandwich reactant pair of this sort is not formed on a micelle surface and back reaction is slower than the escape of the cation from the surface. Hie swollen micelle and microemulsion systems lead to both randomly organised ionic products and sandwich pairs, to varying extents, which are reflected in the observed yield of ions, with polar derivatives of pyrene, e.g. pyrene sulfonic acid, etc., the reactants are kept on the assembly surface where reaction occurs, giving rise to ions from a non-sandwiched type of configuration. In the reverse micellar system, these ions although they are formed, nevertheless have a short lifetime, as they cannot escape to any great distance in the small water pool. Huts, micelles are far superior to microemulsions in various aspects of... 

A significant amount of work has demonstrated the feasibility and the interest of reversed micelles for the separation of proteins and for the enhancement or inhibition of specific reactions. The number of micellar systems presently available and studied in the presence of proteins is still limited. An effort should be made to increase the number of surfactants used as well as the set of proteins assayed and to characterize the molecular mechanism of solubilization and the microstructure of the laden organic phases in various systems, since they determine the efficiency and selectivity of the separation and are essential to understand the phenomena of bio-activity loss or preservation. As the features of extraction depend on many parameters, particular attention should be paid to controlling all of them in each phase. Simplified thermodynamic models begin to be developed for the representation of partition of simple ions and proteins between aqueous and micellar phases. Relevant experiments and more complete data sets on distribution of salts, cosurfactants, should promote further developments in modelling in relation with current investigations on electrolytes, polymers and proteins. This work could be connected with distribution studies achieved in related areas as microemulsions for oil recovery or supercritical extraction (74). In addition, the contribution of physico-chemical experiments should be taken into account to evaluate the size and structure of the micelles. 

True micellar systems have low capacity for dissolving non-polar reactants, however. They are therefore of limited preparative value. Microemulsions, which contain not only surfactant and water but also an oil component, can dissolve appreciable amounts of both a polar and a non-polar reactant and are therefore much more practically useful as media for organic synthesis. There has been considerable interest in the use of microemulsions as media for organic reactions in recent years [7—11]. Not only can such a formulation be a way to overcome compatibility problems, the capability of microemulsions to compartmentalise and concentrate reactants can also lead to considerable rate enhancement compared to one-phase systems. A third aspect of interest for preparative organic synthesis is that the large oil-water interface of the system can be used as a template to induce regioselectivity. These aspects will be dealt with in this chapter. 

A first attempt at hydroformylation in a micellar system using a water-soluble rhodium catalyst (Rh-TPPTS) was made by Tinicci and Platone from Eniricerche in 1994 [ 59 ]. They converted olefins with carbon numbers up to 12 using a mixture of an anionic surfactant (SDS) and butanol (as co-surfactant). It has been shown that microemulsions made with non-ionic surfactants of the alcohol ethoxylate type are advantageous compared to ionic... 

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