Micellization chemical reaction model

Following this, the thermodynamic arguments needed for determining CMC are discussed (Section 8.5). Here, we describe two approaches, namely, the mass action model (based on treating micellization as a chemical reaction ) and the phase equilibrium model (which treats micellization as a phase separation phenomenon). The entropy change due to micellization and the concept of hydrophobic effect are also described, along with the definition of thermodynamic standard states. 

Micelles are important in many uses of surfactants for their capacity to solubilize water-insoluble compounds. Vesicles are model systems for biological cells and can be used for entrapping active compounds in their insides. Besides, vesicles and micelles are used as microreactors for performing chemical reactions and preparing solid nanoparticles of varied shapes. 

Further light on enzyme action is shed by micelles of amphiphilic molecules, which catalyse many chemical reactions and are often regarded as very simple models for enzymes ( amphiphilic and micelle are defined in Section 14.0). The disposition of hydrophobic (inside) and hydrophilic (outside) groups in a micelle resembles that of enzymes. Like enzymes, too, micelles are denatured by heat or urea, and they show specificity towards substrates (Jencks, 1969). Reactions that liberate anions, e.g. the hydrolysis of esters, are catalysed by cationic micelles, e.g. those of cetyltrimethylammonium bromide 14J6)... 

The theoretical model developed by Aniansson and coworkers (65-68) describes the micelles as polydisperse aggregates, whose growth or decay happens by exchange of monomers. The general theoretical description of the diffusion in such a solution of polydisperse aggregates taking part in chemical reactions (exchange of monomers) is a heavy task nevertheless, it has been addressed in several... 

Experimental data indicate that the changes in rates of variation of physical properties near the CMC actually occur over a narrow range of concentrations and not discontinuously at a single concentration. Hraice a chemical reaction or mass action model should be more realistic than the phase separation model for describing the thermodynamics of micellization. That is, we consider that micelle formation occurs as follows ... 

The alternative to analytical theories are simulations computer experiments that generate representative statistical samples of the system from which macroscopic properties can be derived. The theory used to generate simulations is much simpler than the statistical mechanical theories of liquids. Nevertheless the results can be far more accurate and less assumptions go into the derivation of the results. Computer simulations are thus often used to test the reliability of analytical theories, using model potentials. Using realistic potentials, computer simulations can be used to understand and predict properties of large systems of complex molecules. This is not restricted to simple molecular fluids but may extend to complicated macromolecules, liquid crystals, micels, surfaces and chemical reactions. 

Thus, models based on multisite binding equilibria successfully describe the dependence of D on Cx at a fixed micelle concentration when the electroactive probe is almost totally bound to the micelles. The electrode reaction of the probe must not be accompanied by adsorption or chemical reaction. Nonlinear regression analysis of the data enables an assessment of the importance of polydispersity and an accurate estimate of diffusion coefficients of the micelles. Parameters proportional to binding constants are also obtained, and these can be converted to apparent binding constants if the micelle concentrations are known. The models also quantitatively predict the observed z decrease in measured diffusion coefficient with concentration of micelles. This work shows that the micellar system we have used for electrocatalysis are probably polydisperse. 

Further light on enzyme action is shed by micelles of amphiphilic molecules, which catalyse many chemical reactions and are often regarded as very simple models for enzymes ( amphiphilic and micelle are defined in Seaion 14.0). The disposition of hydrophobic (inside) and hydrophilic... 

The following discussion of chemical reactivity and mechanism will be based on the premise that for most thermal reactions equilibrium is maintained between water and the micelles, which can be regarded as distinct reaction media, and most kinetic treatments are based on this so-called pseudophase model (Cordes and Gitler, 1973 Bunton, 1973b). Reaction... 

We have also learned that self-replication is not a prerogative only of nucleic acids, but it can be shared by different kinds of chemical families see the formose reaction, the self-replicating peptides, and the self-reproducing micelles and vesicles. The list should include the cellular automata and the corresponding devices of artificial life. Self-reproduction of vesicles and liposomes is important because it represents a model for cell reproduction. 

The Mass Action Model The mass action model represents a very different approach to the interpretation of the thermodynamic properties of a surfactant solution than does the pseudo-phase model presented in the previous section. A chemical equilibrium is assumed to exist between the monomer and the micelle. For this reaction an equilibrium constant can be written to relate the activity (concentrations) of monomer and micelle present. The most comprehensive treatment of this process is due to Burchfield and Woolley.22 We will now describe the procedure followed, although we will not attempt to fill in all the steps of the derivation. The aggregation of an anionic surfactant MA is approximated by a simple equilibrium in which the monomeric anion and cation combine to form one aggregate species (micelle) having an aggregation number n, with a fraction of bound counterions, f3. The reaction isdd... 

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. 

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