Micellar aggregation numbers additives effect

In the case of nonionic surfactants, the effects of added electrolytes seem to parallel their effects on the miceUization process. When such addition produces an increase in micellar aggregation number, an increase in solubilizing capacity for hydrocarbon additives is also found. The results for the solubilization of polar materials is, again, less clearcut. 

Fluorescence quenching studies in micellar systems provide quantitative information not only on the aggregation number but also on counterion binding and on the effect of additives on the micellization process. The solubilizing process (partition coefficients between the aqueous phase and the micellar pseudo-phase, entry and exit rates of solutes) can also be characterized by fluorescence quenching. 

The preceding discussion has been confined to two-component systems, amphiphile-water. In a large number of cases of practical importance one adds one (or more) additional component(s). Depending on the nature of the additive one can recognize different effects. If it is an amphiphile it is usually found that the micelles which form in the solution are of mixed composition. Under the assumption that the amphiphiles mix ideally in the micellar aggregate, Shinoda177 has derived expressions for the effective CMC of the amphiphile mixture. For nonionics... 

Systematic studies on micellar size and structure have been published for poly(styrene-h-acrylic acid) (PS-PAAc) [7, 8], poly(styrene-fr-sodium acrylate) (PS-PAAcNa) [9], or quaternized poly(styrene-h-4-vinyl-pyridine) (PS-P4VPMeI) [10, 11]. It was concluded that the polyelectrolyte chains in the micellar corona are almost fully stretched [8]. The effect of salt concentration was investigated by Guenoun et al. on poly(f-butylstyrene-fr-sodium styrene sulfonate) (PtBS-PSSNa) who observed a weak decrease of micellar size and aggregation number when the salt concentration was increased beyond 0.01 mol/1 [12]. Using small-angle neutron scattering (SANS), the authors could provide additional support for the rod-like conformation of the polyelectrolyte chains in the micellar corona [13]. 

For ionic micelles, the effect of addition of electrolyte is to decrease the cmc and increase the aggregation number. Such changes are predictable in micellar... 

In aqueous solution, the effectiveness of micellar systems as catalysts is quite often found to increase with the length of the alkyl chain. For example, the rate of acid hydrolysis of methyl o-benzoate in the presence of sodium alkyl sulfates increases in the order Cs < Cio < Cn < Cu < Cie. Such a result may be attributed to either electrostatic or solubilizing effects, or both. It might be expected that any effects due to electrostatic interactions would also increase. Alternatively (or additionally), the increasing aggregation number found in the series may result in a significant increase in the solubilizing power of the system. The importance of each mechanism will depend upon the specifics of the reaction. 

In aqueous solution micelles are generally thought to be spherical as long as the surfactant concentration remains close to the critical micelle concentration. Rod-like micelles may form at higher surfactant concentrations [1, 2]. Addition of a third component such as neutral salt or non-electrolytes may favour longer micellar structures, for instance rod-like micelles [3-6]. An increase in temperature, on the other hand, seems to favour spherical micelles [7, 8]. The effect of pressure on the shape transition point is not known, though it appears that the aggregation number of micelles decrease with pressure at least up to about 160 MPa [9-12]. 

Earlier chapters introduced some of the wide array of chemical species that exhibit surfactant properties and are potentially useful in solubilization processes. Just as molecular structure is important to such surfactant characteristics as the cmc, aggregation number, and micellar shape, it also controls the ability of a surfactant to solubilize a third component. Conversely, the presence of a third component in a surfactant solution can often affect its aggregation characteristics. It is documented in a number of reports that the presence of a solubilized additive, even though the additive has no inherent surface activity, can change the cmc of a surfactant substantially from that of the pure system. As noted in Chapter 4, the existence of such an effect means that great care must be exercised in the interpretation of experimental data on micellization derived from solubilization results. 

The effects of added electrolytes on a micellar system were discussed in Chapter 4. For the case of ionic micelles, the effect of such addition is to decrease the cmc and increase the aggregation number. Such changes are predictable in micellar systems and might be expected to produce parallel effects on solubilization. In fact, however, the results are not always so easily analyzed. At surfactant concentrations near the cmc, it is usually found that the solubilizing power of a system will increase with the addition of electrolyte, as a result of the greater number of micelles available in the system. At surfactant concentrations well above the... 

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