Reverse micelles entropy

The small and positive values of enthalpy of solution of water in AOT-reversed micelles indicate that its energetic state is only slightly changed and that water solubilization (unfavorable from an enthalpic point of view) is driven mainly by a favorable change in entropy (the destructuration of the water at the interface and its dispersion as nanodroplets could be prominent contributions) [87],... 

Other molecular thermodynamic models for protein-reverse micelle complexes have also emerged. Bratko et al. [171] presented a model for phase transfer of proteins in RMs. The shell and core model was combined with the Poisson-Boltzmann approximation for the protein-RM complex and for the protein-free RM. The increase in entropy of counterions released from RMs on solubilization of a protein was the main contribution to the decrease in free energy of com-plexation. Good agreement was found with SANS results of Sheu et al. [151] for cytochrome C solubilization and the effect of electrolytes on it. However, this model assumes that filled and empty RMs are of the same size, independent of salt strength and pH, which is not true according to experimental evidence available since then. 

Aqueous micelles are thermodynamically stable and kinetically labile spherical assemblies. Their association-dissociation process is very fast and occurs within milliseconds. The actual order is less than shown in Figure 1. Driving forces for the formation of aqueous micelles or vesicles are the solvation of the headgroup and the desolvation of the alkyl chain ( hydrophobic effect ). Because of the rapid exchange of surfactants, the core of the micelle contains a small percentage of water molecules. Aqueous assemblies are preferentially stabilized by entropy, and reverse micelles by enthalpy [4]. The actual formation of micelles begins above a certain temperature (Krafffs point) and above a characteristic concentration (critical micelle concentration, CMC). Table 1 shows a selection of typical micelle-forming surfactants and their CMCs. 

Gel electrophoresis of poly(A) in the AOT reversed micellar system revealed that the size of poly(A) in the precipitate (4.0 kb, 1.6 kb -0.6 kb) was nearly the same as that in the supernatant (2.5 kb-1.0 kb, 0.4 kb), in which the asterisk denotes the highest distribution of poly(A) [85]. A similar tendency was observed in the Ci2Eg reversed micelles. In the case of HTAC reversed micelles, the size of poly(A) (7.4 kb-5.3 kb -1.0 kb) in the supernatant was larger than that in the AOT reversed micelles. ADP molecules are strongly adsorbed at the cationic charged interface of the water pool of HTAC, resulting in enhancement of the polymerization in the water pool due to the entropy effect. 

Micelles (normal and reverse) are aggregates of surfactants, which are formed spontaneously in a liquid phase when the surfactant concentration is increased than the critical micelle concentration (CMC). Micellar solutions are formed in aqueous continuous systems, whereas, the reverse micelles are formed in oily continuous systems. In micelles (oil-in-water micelle), the polar heads of the surfactant lie outside in the aqueous phase, whereas the lipophilic hydrocarbon chains lie inside (Figure 58.3). When the surfactant concentration is increased, the free energy of the system increases due to the inauspicious interactions between the water molecules and the lipophilic portions of the surfactant. The water molecules around the oil droplets structures themselves, thereby, resulting in the decrease in the entropy. The reverse micelles (water-in-oil micelle) have opposite structure, that is, the polar heads lie at the centre, while the lipophilic tails are present outside in the oil phase (Figure 58.3). The surfactant concentration need not necessarily be higher than the CMC for the formation of reverse micelle. Many scientists reported formation of reverse micelles by lecithin in different oil phases. " ... 

The hydrolysis of alkyl 4-nitrophenyl carbonates is catalysed by dodecyl-ammonium propionate reversed micelles in benzene and hexane. The rate enhancement is attributed to an entropy effect of trapping the reactants in the water pool and to general acid-base catalysis by the ammonium-carboxylate ion-pairs. ... 

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