Intermicellar interactions

Some investigations have emphasized the importance of micellar size as a control parameter of nanoparticle size [224]. It has been suggested that other factors also influence the nanoparticle size, such as the concentration of the reagents, hydration of the surfactant head group, intermicellar interactions, and the intermicellar exchange rate [198,225-228],... 

A. Intermicellar Interactions in Semidilute Solutions of Water-Containing Reversed Micelles... 

It follows that in spite of the apolar coat surrounding water-containing AOT-reversed micelles and their dispersion in an apolar medium, some microscopic processes are able to establish intermicellar attractive interactions. These intermicellar interactions between AOT-reversed micelles increase with increasing temperature or the chain length of the hydrocarbon solvent molecule, thus leading to the enhancement of the clustering process [244-246], whereas they are reduced in the presence of inorganic salts [131]. 

As expected, substitution of water for other polar substances or the presence of additives leads to changes in the intermicellar interactions [261],... 

By dynamic light scattering it was found that, in surfactant stabilized dispersions of nonaqueous polar solvents (glycerol, ethylene glycol, formamide) in iso-octane, the interactions between reversed micelles are more attractive than the ones observed in w/o microemulsions, Evidence of intermicellar clusters was obtained in all of these systems [262], Attractive intermicellar interactions become larger by increasing the urea concentration in water/AOT/ -hexane microemulsions at/ = 10 [263],... 

Two system-dependent interpretative pictures have been proposed to rationalize this percolative behavior. One attributes percolation to the formation of a bicontinuous structure [270,271], and the other it to the formation of very large, transient aggregates of reversed micelles [249,263,272], In both cases, percolation leads to the formation of a network (static or dynamic) extending over all the system and able to enhance mass, momentum, and charge transport through the system. This network could arise from an increase in the intermicellar interactions or for topological reasons. Then all the variations of external parameters, such as temperature and micellar concentration leading to an extensive intermicellar connectivity, are expected to induce percolation [273]. 

Another feature of surfactant-water systems is that they can also aggregate into lyotropic liquid crystalline phases when Intermicellar interactions are significant. Typically, non-Newtonian behavior is usually found for these liquid crystalline phases. For the 3LDA0/ISDS mixed system, all evidence suggests that they do form liquid crystalline phase. 

The intermicellar potential, deduced from Baxter model, decreases with the number of carbon atoms (25). This has been explained in term of solvent penetration bulk solvent molecules having a small number of carbon atoms penetrate easily in the surfactant alkyl chains, which are then well solvated. This induces a decrease in the intermicellar interactions. The increase in the number of carbon atoms of the bulk solvent induces a decrease in the solvation of the surfactant and then an increase in the intermicellar attractive potential. 

Influence of Intermicellar Interactions. By replacing isooctane by cyclohexane as the bulk solvent, the intermicellar potential decrease inducing a decrease in the exchange micellar rate by a factor of 10 (24,63) whereas with decane the exchange micellar rate increases by a factor of 2 (27). The size of the droplets remains the same by replacing the bulk solvent. 

The self-diffusion of the individual components is strongly affected by the formation of micelles in the solution. This applies to the surfactant, the counterion, the water, and to solubilized molecules. As illustrated in Fig. 2.11 for sodium dodecyl sulfate, surfactant and counterion diffusion are very weakly dependent on concentration below the CMC while a marked decrease in the micellar region is observed for the surfactant and a less marked one for the counterion37. Water diffusion shows a stronger concentration dependence below the CMC than above it. Self-diffusion studies using radioactive tracers have been performed to obtain information on CMC, on counterion binding, on hydration and on intermicellar interactions and shape changes. 

The data from the SANS study were fitted to a poly-disperse spherical core-plus-shell (or corona) model assuming no intermicellar interactions. The model Sharma and Bhatia used essentially fits three parameters, namely the radius of the micelle core, / i, the radius of the micelle shell, R2 and the micelle aggregation number. To fit the SANS data to the model, the authors assumed that the micellar cores were comprised only of hydrophobe (PPO) and D2O, while the shell was assumed to comprise only of hydrophile (PEO) and D2O, with no intermixing of the PPO and PEO chains. 

About 15 years ago, the first ultra (low) SANS (USANS) instrument was built. The equipment is complimentary to conventional SANS as it enables measurement to be made at lower Q than conventional SANS. USANS and Super USANS are useful for the measurement of shape and size of particles from a maximum size of 30 pm to either 0.3 pm or 0.01pm, depending upon the particular type of USANS instrument used. When combined with SANS, USANS allows the characterization of microstructure over 11 orders of magnitude and has been used to study the structure and intermicellar interactions in block polyelectrolyte assemblies. ... 

Crichton, M. Bhatia, S. Structure and intermicellar interactions in block polyelectrolyte assemblies. Journal of Apphed Crystallography 2003, 36, 652-655. 

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