Micellar kinetics process

In order to describe and optimize the reverse micellar extraction process, Dekker et al. [ 170] have proposed a mathematical model, which satisfactorily describes the time dependency of the concentration of active enzyme in all the phases, based on the flow, mass transfer, and first-order inactivation kinetics. For each phase, a differential equation is derived. For forward extraction ... 

The results obtained with the reported extraction model showed that the separation of charged species is possible, provided a suitable ligand hydrophobicity. Further analytical developments of these multiphase extraction systems will require an accurate investigation of the equilibria and kinetic processes occurring at the interfaces, as well as the study of the micellar structure and properties of the host aggregates. 

The coupling of the diffusion of monomers and micelles is given by the micellar kinetics, which consists of different physical processes a fast process in the range of microseconds (exchange of monomers between the micellar and the aqueous solution phase), and a second in the range of milliseconds (total disintegration of micelles into monomers). The entire variety of micellar kinetics was discussed by Aniansson et al. [91,92,93]. 

These equations can serve to estimate the influence of micellar kinetics on the adsorption process. Much more details will be given in Chapter 5 where the various micelle kinetics models and their practical relevance for interfacial studies are discussed. 

Fig. 1 Illustration of two kinetic processes in micellar systems, (a) Micelle formation, i.e., the kinetics associated with aggregation of single amphiphiles (unimers) into micelles and (i>) the equilibrium kinetics charactiaizing a dynamic equilibrium of unrmcas exchanging between micelles...

Time resolved SAXS/SANS allow a structural observation of kinetic processes on the nanoscale (1-100 nm) on a time scale ranging from milliseconds to hours. This allows micellar kinetics to be followed in real time, giving direct structural information of the process and its evolution. Synchrotron SAXS can reach smaller time scales and exhibits better resolution compared to neutron-based methods. However, SANS offers the possibility for contrast variation via simple H/D exchange chemistry, which opens up a world of possibilities for the investigation of kinetics in soft matter systems, in particular transport and exchange processes that otherwise would be invisible in scattering experiments. As most of these techniques have become available over recent years with advancements in both instrumentation and sample environments, there is a need for an overview of the development and the possibilities that are now available in the field of soft matter in general and micellar systems in particular. 

Fig. 2 Illustration of two important mechanisms involved in various kinetic processes in micellar systems, (a) Unimer exchange, single surfactant/block copolymer chains are interchanged one by one via the solvent medium, (b) Fusion/fission, where two micelles fuse or are fragmented to...

Non-equilibrium kinetic processes typically involve monitoring a change in micellar structure or morphology over time, or following the formation of micelles from a molecular solution (unimers), i.e., micellization kinetics. Thus, in contrast to equilibrium processes a perturbation is required. Typically this is achieved by abruptly altering the thermodynamic conditions, which can be achieved either via extensive parameters like temperature and pressure, or by changing intensive parameters such as salt concentration or pH. 

In this review, we have provided a selective overview of theoretical and experimental studies on kinetic processes in block copolymer micellar systems. We have demonstrated the strengths of time-resolved small-angle scattering techniques by highlighting recent examples from the literature. Most of the available literamre in this field is either related to equihbrium exchange kinetics or micellization kinetics. 

Despite its rather short history, TR-SAS techniques have helped to resolve many aspects of kinetic processes in micellar systems, in particularly the equilibrimn kinetics. However, many challenges remain for the future. For block copolymer micelles, these include studies of morphological transitions, drug encapsulation and... 

The first models of micellar kinetics in spatially uniform solutions have been developed by Kresheck et al. [140] and Aniansson and Wall [141]. The existence of fast and slow processes of the micellar dynamics has been established. The fast process represents exchange of separate monomers between micelles and the surrounding solution. If the micelle releases monomers, its aggregation number could decrease to a critical value, after which a complete decomposition of the micelle to monomers takes place. This decomposition is known as the slow demicellization process [141]. 

The role of micellar dimensions and what has been called the spatial extent of species in intramicellar kinetic processes has been considered [63]. Three qualitatively different types of reaction were studied (i) the diffusion of a confined excited species to a reactive surface (ii) energy transfer between two separated reactants and (iii) chemical reaction between species which are restricted in their diffusion to the surface of the micelle. Table 11.3 summarizes the main findings when r and D are fixed for these three cases. 

Effective rate constants and lifetimes for reactions in which diffusion to a reactive surface must occur are shown in Table 11.4 for a range of values of r and D, The latter are a quantification of expected trends which show to increase with increasing diffusion coefficient and to decrease with increasing micellar radius. In spite of good correspondence between experiment and theory there is some caution expressed by the authors in their paper in view of the uncertainty that macroscopic equations for normal chemical kinetics apply in the reactions explored by them. The problem, they say, is that micellar kinetics is a nonequilibrium phenomenon which can only be treated by taking the geometry of the system explicitly into account in any formulation of the process. 

In the second chapter (Preparation of polymer-based nanomaterials), we summarize and discuss the literature data concerning of polymer and polymer particle preparations. This includes the description of mechanism of the radical polymerization of unsaturated monomers by which polymer (latexes) dispersions are generated. The mechanism of polymer particles (latexes) formation is both a science and an art. A science is expressed by the kinetic processes of the free radical-initiated polymerization of unsaturated monomers in the multiphase systems. It is an art in that way that the recipes containing monomer, water, emulsifier, initiator and additives give rise to the polymer particles with the different shapes, sizes and composition. The spherical shape of polymer particles and the uniformity of their size distribution are reviewed. The reaction mechanisms of polymer particle preparation in the micellar systems such as emulsion, miniemulsion and microemulsion polymerizations are described. The short section on radical polymerization mechanism is included. Furthermore, the formation of larger sized monodisperse polymer particles by the dispersion polymerization is reviewed as well as the assembling phenomena of polymer nanoparticles. 

Most chemical processes including micellar kinetics involve several steps and are characterized by several relaxation times (relaxation spectrum). The maximum number of observable relaxation times is equal to the number of independent rate equations that can be written for the system investigated. This number is equal to that of chemical species minus the number of mass-balance equations. > ... 

The chapter is organized as follows. Section II briefly recalls the theoretical aspects of micellar dynamics and the expressions of the relaxation times characterizing the main relaxation processes (surfactant exchange, micelle formation/breakdown). Section III reviews studies of micellar kinetics of various types of surfactants conventional surfactants with a hydrocarbon chain, surfactants with a fluorinated chain, and gemini (dimeric) surfactants. Section IV deals with mixed micellar solutions. Section V considers the d5mamics of solubilized systems. Section VI reviews the dynamics of sur-... 

Last, the importance of micellar kinetics in various technological processes that all use surfactants has been clearly demonstrated. 

To reconcile the apparent mismatch between theory and experiment it is important to consider the kinetics of the micellar solubilization process. Kabalnov thought it likely that the micelles are not in equilibrium with the droplets. He proposed that the rate of oil monomer exchange between oil droplets and micelles is slow, and rate-determining. Thus, at low micelle concentrations, only a small proportion of the micelles are able to rapidly solubilize the oil. This leads to a small but measurable increase in the ripening rate with micellar concentration. 

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