Micelle free enthalpy

A correlation between the rate constants k and free enthalpy change AG of electron transfer was studied by Hashimoto and Thomas [127] for quenching of excited singlet states of both pyrene and N-ethylcarbazol and of the triplet state of N-methylphenothiazine by a number of metal ions and for back electron transfer reactions in micellar sodium taurocholate and sodium dodecylsulfate solutions. Quenching rate constants were determined from Stern-Volmer plots obtained for lifetimes of excited states at high concentration of micelles, where the exponential decay... 

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. 

Therefore, the physical meaning of the solubility curve of a surfactant is different from that of ordinary substances. Above the critical micelle concentration the thermodynamic functions, for example, the partial molar free energy, the activity, the enthalpy, remain more or less constant. For that reason, micelle formation can be considered as the formation of a new phase. Therefore, the Krafft Point depends on a complicated three phase equilibrium. 

Most of the studies on thermodynamics of mixed micellar systems are based on the variation of the critical micellar concentration (CMC) with the relative concentration of both components of the mixed micelles (1-4). Through this approach It Is possible to obtain the free energies of formation of mixed micelles. However, at best, the sign and magnitude of the enthalpies and entropies can be obtained from the temperature dependences of the CMC. An Investigation of the thermodynamic properties of transfer of one surfactant from water to a solution of another surfactant offers a promising alternative approach ( ), and, recently, mathematical models have been developed to Interpret such properties (6-9). 

The values of the CMC or CMT collected as a function of temperature or concentration can be used to extract the enthalpic and entropic contributions to the association process. For a closed association mechanism with relatively large aggregation number and a narrow distribution, the standard free energy and standard enthalpy of micelle formaMi nd AH°, per mole of the solute in the micelle) are related to the CMC and its temperature dependence in the form (Lindman and V fennerstrom, 1980 Zhou and Chu, 1994). 

The free energy of micelle formation has been found to be more dependent on entropy than on enthalpy factors (Kavanau, 1965 Elworthy, 1968). Micelle formation has been treated theoretically either... 

The free energy change of a system is dependent on changes in both the entropy and enthalpy that is, AG = AH-T AS. For a micellar system at normal temperatures the entropy term is by far the most important in determining the free energy changes (T AS constitutes approximately 90-95% of the AG value). Micelle formation entails the transfer of a hydrocarbon chain from an aqueous to a nonaqueous environment (the interior of the micelle). To understand the changes in enthalpy and entropy that accompany this process, we must first consider the structure of water itself. 

Until recently, the formation of micelles was regarded primarily as an interfacial energy process, analogous to the process of the coalescence of oil droplets in an aqueous medium. If this was the case, micelle formation would be a highly exothermic process as the interfacial free energy has a large enthalpy component. 

However, as mentioned above, experimental results have shown clearly that micelle formation involves only a small enthalpy change, and is often endothermic. The negative free energy of micellisation is the result of a large positive entropy, and this led to the conclusion that micelle formation must be predominantly an entropy-driven process. 

Alternatively, inverted micelles can also be formed in water-free medium. If no water is present in a two-component system, the difference between the solubility parameters of the hydrocarbon tail of the surfactant and the organic solvent contributes to inverted micelle formation. A large negative enthalpy change is the driving force to form spontaneous inverted micellization, in contrast with aqueous systems. 

JVo is a key parameter which significantly affects the physical properties of AOT reversed micelles. In the case of an AOT/oil solution, discontinuity of several physical properties of the solubilized water is observed at IVg 10 [16]. Below IV 10, the water is bound to the AOT polar head-groups and counterions, and further addition of water leads to the appearance of free water in the core of the water pools. However, the state of the water in the AOT reversed micelles, especially below Wg 2, appears unusual. We found that the solution enthalpy of the water in AOT/various organic solvents solutions indicated a great change in the state of the solubilized water [17,18]. 

The coalescence of hydrocarbon chains allows the ordered hydration layers to be expelled into the bulk phase, resulting in a considerable net gain in entropy. Indeed, micelle formation is primarily an entropy-driven process the enthalpy of hydrocarbon association is comparatively weak and can even be endothermic (opposing association). As an example, dimethyl-n-dodecylamine oxide (illustrated in Fig. 2) undergoes or free energy change of micellization of AG = —6.2 kcal/mol (a fairly typical value), of which the enthalpic contribution AH = 4-1.1 kcal/mol and the entropic contribution — T A S = -7.9 kcal/mol. 

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