Microemulsions kinetics

Microemulsions consist of either three or four components two solvents, a surfactant, and sometimes an alcohol/cosurfactant. This complexity of composition means that there are potentially many relaxation processes. Despite this, microemulsion kinetics has been relatively well researched due to sustained interest in their structure and optimization. There have been several important reviews of the area, including summaries of work on the dynamic processes in such systems [100,101]. 

Gan LM, Chew CH, Lye I (1992) Styrene polymerization in oil-in-water microemulsions— kinetics of polymerization. Makromol Chem Mactomol chem Phys 193 1249-1260... 

Tjandra et al. (1998) have proposed an interfacial reaction model for the kinetics of the reaction between 1-bromo octane and sodium phenoxide to give 1-phenoxyoctane in a nonionic microemulsion. In this model the microemulsion is assumed to consist of the aqueous phase and the interface is covered by a monolayer of surfactant molecules. It is thus possible to assess the interfacial area from the concentration of the surfactant in the microemulsion medium. 

The ITIES with an adsorbed monolayer of surfactant has been studied as a model system of the interface between microphases in a bicontinuous microemulsion [39]. This latter system has important applications in electrochemical synthesis and catalysis [88-92]. Quantitative measurements of the kinetics of electrochemical processes in microemulsions are difficult to perform directly, due to uncertainties in the area over which the organic and aqueous reactants contact. The SECM feedback mode allowed the rate of catalytic reduction of tra 5-l,2-dibromocyclohexane in benzonitrile by the Co(I) form of vitamin B12, generated electrochemically in an aqueous phase to be measured as a function of interfacial potential drop and adsorbed surfactants [39]. It was found that the reaction at the ITIES could not be interpreted as a simple second-order process. In the absence of surfactant at the ITIES the overall rate of the interfacial reaction was virtually independent of the potential drop across the interface and a similar rate constant was obtained when a cationic surfactant (didodecyldimethylammonium bromide) was adsorbed at the ITIES. In contrast a threefold decrease in the rate constant was observed when an anionic surfactant (dihexadecyl phosphate) was used. 

The majority of RDC studies have concentrated on the measurement of solute transfer resistances, in particular, focusing on their relevance as model systems for drug transfer across skin [14,39-41]. In these studies, isopropyl myristate is commonly used as a solvent, since it is considered to serve as a model compound for skin lipids. However, it has since been reported that the true interfacial kinetics cannot be resolved with the RDC due to the severe mass transport limitations inherent in the technique [15]. The RDC has also been used to study more complicated interfacial processes such as kinetics in a microemulsion system [42], where one of the compartments contains an emulsion. 

Kinetics. The reaction of N-dodecyl 3-carbamoyl pyridinium bromide (I) with cyanide ion in the microemulsions was observed by following the 340 nm absorption maximum of the 4-cyano adduct (II). See equation (1). Following the work of Bunton, Romsted and Thamavit in micelles ( ), a 5/1 mole ratio of KCN to NaOH was employed to prevent cyanide hydrolysis. The pH of each reaction mixture was measured on a Coleman 38A Extended Range pH meter to insure that the system was sufficiently basic to allow essentially complete ionization of the cyanide. The appropriate amounts of cyanide and hydroxide were added to the mlcroemulslon sample within 10 minutes of running a reaction. Cyanide concentration varied between 0.02 and 0.08 M with respect to the water content. Substrate was Injected via a Unimetrics model 1050 syringe directly into a known volume of the yE-nucleophlle mixture in a 1.0 cm UV quartz cell. Absorbance at 340 nm was followed as a function of time on a Perkln-Elmer model 320 spectrophotometer at 25.0 + 0.3 C. Since the Initial bulk concentration of substrate was 10 M, cvanide was always present in considerable excess. 

The most commercially important mechanism of all is the kinetics of solute transfer from an aqueous to a reverse micelle phase. The kinetics of extraction of metal ions have not received the same research attention as the extraction capacity of W/O microemulsions. As the mechanism of extraction of metal ions is chemical, the effect of creating a microemulsion in an organic phase that contains the reactant can be measured experimentally. Results indicate that, as in the case of extraction equilibrium, the rate of extraction may increase substantially by the presence of the microemulsion as compared with the conventional system [20,38,44] or decrease it to... 

Boso, R.L., Bellini, M.S., Miksik, I., Deyl, Z. (1995). Microemulsion electro-kinetic chromatography with different organic modifiers separation of water-and lipid-soluble vitamins. J. Chromatogr. A 709 11-20. 

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... 

Fletcher PDl, Howe AM, Robinson BH (1987) The Kinetics of Solubilisate Exchange between Water Droplets of a Water-in-Oil Microemulsion. J Chem Soc Faraday Trans 1 83 985-1006... 

Chang CL, Fogler HS (1996) Kinetics of silica particle formation in nonionic w/o microemulsions from TEOS. AIChE J 42 3153-3163... 

Towey TF, Khanlodhi A, Robinson BH (1990) Kinetics and Mechanism of Formation of Quantiun-Sized Cadmium-Sulfide Particles in Water Aerosol-Ot Oil Microemulsions. J Chem Soc Faraday Trans 86 3757-3762... 

The kinetics of an enzyme catalysed reactions in a w/o-microemulsions is dependent on several parameters. For example, the substrates and enzymes distribute within the different parts of a one-phase microemulsion with different concentrations. The enzymes are located in the water and hydrophobic substrates are mainly dissolved in the oil. Additionally, the choice of oil and surfactant, the water concentration, and the structure of the interfacial layer can influence the activity and stability of biocatalysts. The influences of the main parameters on the kinetics will be discussed in this chapter. 

The basic kinetic model for enzyme catalysed conversions in water and in w/o-microemulsions is based on the theory of MichaeHs and Menten [83]. Although the Michaelis-Menten-model is often sufficient to describe the kinetics, the bi-bi-models (e. g. random bi-bi, orderedbi-bi or ping-pongbi-bi), which describe the sequences of substrate bindings to the enzyme are the more accurate kinetic models [84]. 

Many investigations have been undertaken regarding the effect of the water concentration in the microemulsion on the catalytic behaviour of enzymes. The surfactant concentration of the microemulsion defines the size of the internal interface but it often has no measurable influence on the enzyme kinetics. On the other hand, the physical properties of the water located inside the reverse micelles differ from those of bulk water, and the difference becomes progressively smaller as the water concentration, expressed in the w -value, increases. 

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. 

Systematic investigations into the kinetic aspects of silica formation in microemulsions have been undertaken by only a few researchers (23,24,29). Various forms of useful qualitative and semi-quantitative kinetic information are available, however. For example, size exclusion chromatography (SEC) was used by Espiard et al. (22) to monitor TEOS concentration as a function of time for different R values (R = 4, 6, 10). An initial period of rapid reaction (approximately first 6 h) was observed,... 

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 ... 

In addition to development of full laboratory courses, several individual experiments that can be included in such integrated laboratories have been developed recently. They include experiments on photocatalysis (148), synthesis, kinetics and thermodynamics of an inorganic compound (149), studies on conformational analysis (150), synthesis and variable temperature proton NMR of an inorganic compound (151), and the study of microemulsions (152). As such laboratories become more common, we can expect more of these experiments to appear in the literature. These integrated laboratory courses and experiments can be found in Table XL... 

Microemulsions are stable with respect to separation into their components coarse emulsions may have a degree of kinetic stability, but ultimately separate. 

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