Mass transfer micellization kinetics

For the scale-up of reverse micelle extractions, it is important to know which factors determine the mass transfer rate to or from the reverse micelle phase. So far most work has concentrated on the kinetics of solubilization of water molecules [34,35], protons [36], metal ions [20,35,37,38 0], amino acids [41], and proteins [8,35,42,43]. There are two separate processes forward transfer, which is transfer of solute from the aqueous to the reverse micelle phase, and back transfer, which is the antithesis of the first one. 

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 ... 

The rapid development of biotechnology during the 1980s provided new opportunities for the application of reaction engineering principles. In biochemical systems, reactions are catalyzed by enzymes. These biocatalysts may be dispersed in an aqueous phase or in a reverse micelle, supported on a polymeric carrier, or contained within whole cells. The reactors used are most often stirred tanks, bubble columns, or hollow fibers. If the kinetics for the enzymatic process is known, then the effects of reaction conditions and mass transfer phenomena can be analyzed quite successfully using classical reactor models. Where living cells are present, the growth of the cell mass as well as the kinetics of the desired reaction must be modeled [16, 17]. 

Common surfactants that have been used in MEKC, are listed in Table 3.1 with the respective critical micelle concentrations the most popular are SDS, bile salts, and hydrophobic chain quaternary ammonium salts. Selectivity can also be modulated by the addition to the aqueous buffer of organic solvents (methanol, isopropanol, acetonitrile, tetrahydrofuran, up to a concentration of 50%). These agents will reduce the hydrophobic interactions between analytes and micelles in a way similar to reversed-phase chromatography. Organic modifiers also reduce the cohesion of the hydrophobic core of the micelles, increasing the mass transfer kinetics and, consequently, efficiency. Nonionic... 

On the other hand, Nomura and Harada [14] proposed a kinetic model for the emulsion polymerization of styrene (St), where they used Eq. 7 to predict the rate of radical entry into both polymer particles and monomer-swollen micelles. In their kinetic model, the ratio of the mass-transfer coefficient for radical entry into a polymer particle kep to that into a micelle kem> K lk,... 

Polymeric micelles form stable pseudostationary phases with a critical micelle concentration of virtually zero (aggregation number of 1), and are tolerant of high organic solvent concentrations in the electrolyte solution. Mass transfer kinetics are slow compared with conventional surfactant micelles, and peak distortion from mass overloading is a problem for some polymer compositions. Preliminary studies indicate that polymeric surfactants are effective pseudostationary phases in micellar electrokinetic chromatography, but only a limited number of practical applications have been demonstrated, and uptake has been slow. 

If this reaction were to occur only in the aqueous bulk, the film has no role to play, the entire system is pseudohomogeneous, and a true kinetic analysis as outlined earlier is possible. If mass transfer effects are present between the pseudophases, the analysis outlined before for such systems would apply. However, if reaction occurs in the film, two situations can arise (1) reaction occurs only in the micelles present in the film and not in the rest of the film, and (2) reaction occurs in both the micellar and aqueous phases in the film. The analysis of both the situations is very similar to that for microphase action described in Chapter 23. 

The 1983 work of the Dorsey research group established the use of 3% v/v 1-propanol in micellar phases to reduce the MLC efficiency problem [3]. It is now confirmed that the addition of alcohol to micellar phases (i) increases the rate of the solute mass-transfer between the micelles and the aqueous phase by increasing the solute micelle exit rate constant, (ii) increases the solute mass transfer kinetics between the stationary phase and the aqueous phase by decreasing the stationary phase viscosity and the amount of adsorbed surfactant. The problem of alcohol addition to micellar phases is that kinetics enhancements cannot be dissociated from thermodynamics changes. The efficiencies increase and the retention times decrease. A hybrid alcohol-micelle mobile phase has necessarily a higher solvent strength than a purely aqueous phase [34]. It was shown that alcohols were changing the micelles and the stationary phase in a comparable manner [26] as noted on the Pws and parallel variations in Table 6.4. 

In this chapter we examine some issues in mass transfer. The reader has already been introduced to some of the key aspects. In Chapter 3 (Section 7), flocculation kinetics of colloidal particles is considered. It shows the importance of diffusivity in the rate process, and in Equation 3.72, the Stokes-Einstein equation, the effect of particle size on diffusivity is observed, leading to the need to study sizes, shapes, and charges on colloidal particles, which is taken up in Chapter 3 (Section 4). Similarly some of the key studies in mass transfe in surfactant systems— dynamic surface tension, smface elasticity, contacting and solubilization kinetics—are considered in Chapter 6 (Sections 6, 7, 10, and 12 with some related issues considered in Sections 11 and 13). These emphasize the roles played by different phases, which are characterized by molecular aggregation of different kinds. In anticipation of this, the microstructures are discussed in detail in Chapter 4 (Sections 2,4, and 7). Section 2 also includes some discussion on micellization-demicellization kinetics. 

Section 1 deals with the formal kinetics of the photochemical reactions in micellar solutions and its application to determine the rate constants of photoprocesses, the critical concentration of micellization (CMC) and the aggregation number of micelles from experimental data. In Section 2 the correlations of the rate constants of the photochemical charge separation and mass transfer processes with its thermodynamics, and also the microviscosity and effective polarity of organized assemblies are considered. 

Solubilization as a bulk reaction Molecular dissolution and diffusion of oil into the aqueous phase takes place, with a subsequent uptake of oil molecules by surfactant micelles [156-161]. This mechanism is operative for oils (like benzene, hexane, etc.), which exhibit a sufficiently high solubility in pure water. Theoretical models have been developed and verified against the experiment [157,159-161]. The bulk solubilization includes the following processes. First, oil molecules are dissolved from the surface of an oil drop into water. Kinetically, this process can be characterized by a mass transfer coefficient. Next, by molecnlar diffnsion, the oil molecules penetrate in the water phase, where they react with the micelles. Thus, the concentration of free oil molecules dimmishes with the distance from the oil-water interface. In other words, solubilization takes place in a certain zone around the droplet [159,160]. 

Both the maximum solubilization capacity and the rate of solubilization depend critically on the hydrocarbon chain length. Dramatic differences in micelle facilitated emulsion growth rates were noted as a function of hydrocarbon chain length. The rather astounding increase in droplet diameter noted for the tetradecane-in-water emulsions suggests that the rate of Ostwald ripening has been dramatically increased. More careful kinetic studies are required, however, to ascertain the mechanism of the mass transfer. 

The studied triblock copolymer PS-PVP-PEO was purchased from Polymer Source (Dorval, Canada). The number-average molar masses of PS, PVP, and PEO blocks were 2.1 x 10 , 1.2 x 10 , and 3.5 x 10 g mol , respectively, and the poly-dispersity index of the sample was 1.10. The copolymer is insoluble in aqueous media, but the micelles can be prepared indirectly both in acidic and alkaline aqueous solutions by dialysis from 1,4-dioxane-methanol mixtures [88]. The micelles can be transferred from acidic to alkaline alkaline solutions and vice versa, but the addition of a base together with intense stirring promotes aggregation. Two factors contribute to the destabilization of micelles after the pH increase (a) In alkaline media, the PVP blocks become insoluble, collapse and form an upper layer of the core. Since the cores of micelles are kinetically frozen, the association number does not change. The mass of insoluble cores increases, while the length of soluble shellforming chains decreases, which results in a deteriorated thermodynamic stability of micellar solutions, (b) The PVP middle layer shrinks and PEO chains come close to each other, which worsens the solubility due to insufficient solvation of PEO blocks. 

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