Thermodynamic considerations micellar solutions

By treating the Krafft point as the melting point of the hydrated solid surfactant, the partition coefficient of the solute can be calculated from its effect on the Krafft point. Simple thermodynamic considerations lead to the following relationship at low mole fractions of solute in the micellar phase ... 

Microemulsions were initially considered to be a special case of emulsions, i.e., kinetically stable dispersions of two mutually immiscible solvents, although of an unusually high stability They are clear, seemingly one-phase systems and can be prepared without considerable input of mechanical energy. As they are thermodynamically stable, however, it is more plausible to conceive of microemulsions as being aggregated (micellar or swollen micellar) solutions [5],... 

Microemulsions and surfactant-stabilized (macro) emulsions are distinctively different with respect to thermodynamic stability and, therefore, while most significant for both types of systems, the role of studies of phase behavior is different in the two cases. For emulsions we are con-eemed with two- or multi-phase regions in the phase diagrams, and for microemulsions with one-phase regions. Beeause of that micro emulsion studies are closely related to studies of other thermo-dynamically stable phases, notably liquid crystalline phases and micellar solutions. Structural models of microemulsions have to a considerable extent been advanced on the basis of our understanding of other stable phases the formation and stability of a micro-emulsion phase for a certain surfactant results from the comope-tition with alternative phases. The principal differences between micro emulsions and emulsions, together with the related nomenclature, is bound to lead to considerable confusion for example, the persistence in literature of emulsion-based structural pictures of microemulsions can be traced to the related names. However, the term microemulsions is kept for historical reasons. 

In the acidic route (with pH < 2), both kinetic and thermodynamic controlling factors need to be considered. First, the acid catalysis speeds up the hydrolysis of silicon alkoxides. Second, the silica species in solution are positively charged as =SiOH2 (denoted as I+). Finally, the siloxane bond condensation rate is kinetically promoted near the micelle surface. The surfactant (S+)-silica interaction in S+X 11 is mediated by the counterion X-. The micelle-counterion interaction is in thermodynamic equilibrium. Thus the factors involved in determining the total rate of nanostructure formation are the counterion adsorption equilibrium of X on the micellar surface, surface-enhanced concentration of I+, and proton-catalysed silica condensation near the micellar surface. From consideration of the surfactant, the surfactants first form micelles as a combination of the S+X assemblies, which then form a liquid crystal with molecular silicate species, and finally the mesoporous material is formed through inorganic polymerization and condensation of the silicate species. In the S+X I+ model, the surfactant-to-counteranion... 

In conventional reversed phase HPLC, differences in the physicochemical interactions of the eluate with the mobile phase and the stationary phase determine their partition coefficients and, hence, their capacity factor, k. In reversed-phase systems containing cyclodextrins in the mobile phase, eluates may form complexes based not only on hydrophobicity but on size as well, making these systems more complex. If 1 1 stoichiometry is involved, the primary association equilibrium, generally recognized to be of considerable importance in micellar chromatography, can be applied (11-13). The formation constant, Kf, of the inclusion complex is defined as the ratio of the entrance and exit rate constants between the solute and the cyclodextrin. Addition of organic modifiers, such as methanol, into the cyclodextrin aqueous mobile phase should alter the kinetic and thermodynamic characteristics of the system. This would alter the Kf values by modifying the entrance and exit rate constants which determine the quality of the separation. 

The mechanisms of micelle-catalyzed reactions have been studied not only by analogy with the Michaelis-Menten equation for enzymatic reactions but also from the perspective of volume fractions of the two-part reaction system consisting of the micelles and the intermicellar bulk solutions.The kinetics of this reaction has been successfully used only when the micellar concentrations are much higher than the reactant concentrations. However, micellar concentrations near the CMC are often less than the reactant concentrations. In such cases, the distribution of reactants among micelles must be taken into consideration, which is essentially a thermodynamic problem (Chapter 9). Reactant is a better technical term than substrate for micelle-catalyzed reactions. 

The aggregation of surfactants into clusters or micelles in dilute solutions, as we will see, is a direct consequence of the thermodynamic requirements of the particular surfactant-solvent system under consideration. It has been suggested that phases occurring between the simplest micelles and true crystals are natural consequences of the removal of water from the micellar system, but do not constitute thermodynamically distinct states. In other words, the factors determining the structures of the mesophases are identical to those that control the formation of micelles in the first place. The same would be true of aggregates other than micelles, which do not fall under the classification of mesophases. 

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