Associating equilibria micelles

The solubilization of drugs can be treated in terms of an association equilibrium between the solutes and the micelles in a micellar solution. Thus,... 

Let us consider a simple association equilibrium between surfactant monomers S and the micelles M of aggregation number n... 

The micellization process can be described by the stepwise association equilibrium... 

The coupling between the ionization of the coronal block A and the association equilibrium of the copolymers gives rise to unique features for the self-assembly of amphiphilic block copolymers with a weak (pH-sensitive) PE block. In other words, the levels of ionization for unimers in solution and for copolymers incorporated in micelles, can be noticeably different due to different values of the pH inside micellar corona and in the bulk solution. Furthermore, the strength of the electrostatic repulsion in the corona can be affected not only by variations in the ionic strength (as it is for micelles with quenched PE corona), but also by variations in the pH, which affect the ionization of the coronal chains. 

Then there must be a minimum of energy, in other words, a limited Grosz-mizelle of a definite size is produced which is in equilibrium with the Hartley micelles. Stauff (1941) gives the following picture of this Groszmizelle (Fig. 20). These Groszmizellen accordit to Staxjff are produced from associated Hartley micelles. A definite critical concentration cannot be detected there is an equilibrium... 

The micellization enthalpy was much greater than the CP enthalpy, demonstrating the dominance of the poly(propylene oxide)-water interactions. The strongly endothermic enthalpy indicates that the micellization is an entropy-driven process, since micellization is a spontaneous phenomenon. Although cooperative, the micellization process was rather long and spanned a temperature range of 10-20°C due to the temperature dependence of the amount of unimers in equilibrium with the micelles (as temperature increases, more copolymer molecules associate into micelles) [159,160] as well as to the size polydispersity inherent in copolymers. The CMT shifted to lower temperatures as the polymer concentration increased, but its width was not affected much. The CP increased with polymer concentration. 

This model assumes a dissociation-association equilibrium between surfactant monomers and micelles - thus an equilibrium constant can be calculated. For a nonionic surfactant, where charge effects are absent, this equilibrium is simply represented by Eq. (2.1), which assumes a single equilibrium. In this case, the equilibrium constant Km is given by Eq. (2.15). 

At very low surtectant concentrations the surtectant exists as monomers dispersed in water, widi an incomplete coverage at the surfiice. At the critical micelle concentration (CMC) the surfiice monolayer is saturated, the surfiu e tension has reached a minimum, and the surfiwtant molecules associate into micelles in equilibrium with a few monomers. The so-called L phase is... 

When the mass-action model is applied to micellization, on the other hand, the following association equilibrium between surfactant monomers (S) and micelles (M) can be considered ... 

Block copolymers of polystyrene (PS) and poly(methacrylic acid) (PMA) form spherical micelles in water, with a glassy core of PS blocks and extended PMA block chains forming a corona [16-18]. However, the PS-PMA block copolymers cannot be dissolved directly into water. In a dioxane-water (80 20, v/v) mixture, the PS-PMA block copolymers undergo closed association to form equilibrium micelles. A stepwise dialysis of the dioxane-water solution of the PS-PMA block copolymers, with a gradual increase in water content, eventually allows one to obtain an optically clear aqueous solution of micelles, which are referred to as kinetically frozen micelles [18]. 

Guillaume et al. [69] presented a high performance liquid chromatographic method for an association study of miconazole and other imidazole derivatives in surfactant micellar using a hydrophilic reagent, Montanox DF 80. The thermodynamic results obtained showed that imidazole association in the surfactant micelles was effective over a concentration of surfactant equal to 0.4 pM. In addition, an enthalpy-entropy compensation study revealed that the type of interaction between the solute and the RP-18 stationary phase was independent of the molecular structure. The thermodynamic variations observed were considered the result of equilibrium displacement between the solute and free ethanol (respectively free surfactant) and its clusters (respective to micelles) created in the mobile phase. 

Association-dissociation equilibrium, in micellization, 24 128, 129-131 Association for the Advancement of Medical Instrumentation (AAMI) Standards, 26 818-819 Association of American Feed Control Officials (AAFCO), 10 856 Nutrient Profiles, 10 857, 858-859t... 

The binding constants for the association of the solute with the micelle can be evaluated with the help of model developed by Sepulveda and coworkers. According to the model, solute binds to micelles with following equilibrium ... 

At their critical micelle concentrations, surface active agents (such as sodium dodecyl sulfate, Triton X-100, lysolecithin, and bile salts) self-associate into spherical or rod-shaped structures. Because dilution to below the c.m.c. results in rapid disassembly or dissolution of these detergent micelles, micelles are in dynamic equilibrium with other dissolved detergent molecules in the bulk solution. 

The pKa for the substituted hemins in aqueous pyridine solutions is 10.5 [18] while in the micelles it ranges from 7.1 to 10.3 depending on the micelle and the porphyrin substitution (Table 1). The changes in the pKa between the aqueous pyridine and micellar solutions have been shown not to be associated with any dimer-monomer equilibrium of the heme complexes, since it has been shown that in the concentration range of the reported study the aqua (pyridinato) complexes exit as monomers even in the absence of micelles in the pH range 6.5-12.5. The changes in the pKa have also been shown to be... 

The adsorption of (+)-camphor-10-sulphonate ions on a mercury electrode is a two-dimensional associative adsorption of ions as micelles in equilibrium with free adsorbed ions (cf. camphor and borneol, Vol. 5, p. 4). ... 

This is simply the result we would expect from the law of mass action. Thus, for the reaction N monomers 1 micelle, we can immediately write that there is an equilibrium (association) constant given by... 

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