Electrical work, micelle

At the end of the 1960s, Subba Rao et al. examined the influence of the interface on the CMC values [56]. They found a decrease in the CMC at the oil-water interface compared with the air-water interface. The CMC decreased by about 10% in the presence of heptane and by about 30-40% in the presence of benzene. The solubilization of the hydrocarbon in the micelle interior results in an increase in the micelle size and a slight change in the curvature of the micelle surface. The electrical potential and hence the electrical work of... 

For the ionic surfactants (1-1 type), we should take account of the electrically charged species and the possibility of doing electrical work. The micelle may be regarded as a charged pseudo-phase, and the chemical potential is replaced by the electrochemical potential (12). The effective electrical work in micelle formation is... 

There is a pronounced difference between the CMCs of ionic and non-ionic surfactants with identical hydrophobic moieties. The lower CMCs of the nonionic surfactants are a consequence of the lack of electrical work necessary in forming the micelles. 

As might be expected, the more ionized groups present in the surfactants, the higher the CMC due to the increase in electrical work required to form the micelle (Table 3.5). 

The conductivity of sodium dodecyl sulfate in aqueous solution and in sodium chloride solutions was studied by Williams et al. [98] to determine the CMC. Goddard and Benson [146] studied the electrical conductivity of aqueous solutions of sodium octyl, decyl, and dodecyl sulfates over concentration ranges about the respective CMC and at temperatures from 10°C to 55°C. Figure 14 shows the results obtained by Goddard and Benson for the specific conductivity of sodium dodecyl sulfate and Table 25 shows the coefficients a and p of the linear equation of the specific conductivity, in mho/cm, vs. the molality of the solution at 25°C. Micellization parameters have been studied in detail from conductivity data in a recent work of Shanks and Franses [147]. 

The ultrafiltration of the microemulsion is a very useful operation for separating water and oil in these mixtures [117-120]. Because of the limited availability of solvent stable membranes, most of the work pubHshed so far was performed using ceramic membranes, which show a high adsorption of surfactant at the membrane surface and comparably low rejection rates of reverse micelles. Using electro ultrafiltration, where the concentration polarisation phenomenon of the reverse micelles (using the ionic surfactant AOT) at the membrane surface is depressed by asymmetric high voltage electrical fields, the rejection rates can be increased,but not to economical values [121,122]. 

One potential application of the work on oriented nematic phases of rodlike molecules is to solutions containing cylindrical micelles. Orientation could be achieved by a shear field or perhaps by an electric field. Gotz and Heckman (9) confirmed the existence of anisotropic electrical conductivity for a concentrated surfactant solution in a shear field. They used their results to show that the solution contained cylindrical rather than platelike micelles. Of course, the magnitude of the electrical conductivity in an aqueous micellar solution should be quite different from that in the nematic phase of an organic material. So the conditions for and types of electrohydrodynamic instabilities could be different as well. 

Various photophysical techniques continue to be used in the study of polymers some particularly interesting work on electrically-conducting pol37mers has been described by Roth and Bleier, inter alia. The photophysics of thin films and colloidal systems, including micelles, continues to be an important and active field of research see e.g. Kalyansundaram Debe. Cyclodextrins have been found to increase the chemiluminescence yields from aqueous peroxyoxalates by up to 300-fold (Woolf and Grayeski). Enzymegenerated excited states of acetone have been found to induce quasi-photochemical behaviour of riboflavin in the dark (Rojas and Silva). From studies of the luminescence of Schmidt and... 

These reactions are regarded as electron tunnelling processes, and the results are interpreted in terms of the estimated distributions of occupied and unoccupied electronic levels in the redox systems aq-e, D-D+, and A-A- involved. If the overlap of the occupied levels of the donor with the unoccupied levels of the acceptor is not good, transfer is slow, irrespective of how thermodynamically favourable it may be. These levels are shifted, relative to one another, by the charge on the micellar head-group, which determines the direction of the electrical double layer at the interface. Reaction rates are therefore influenced both by the charge on the micelle and the concentration of electrolyte in the aqueous phase.47 48 The lifetime of normally unstable intermediates may be enormously enhanced by working in a micellar system.49... 

A popular representation of spherical micelles was devised by Hartley (26). As indicated in Fig. 1, the Hartley model of, e.g., an anionic micelle exhibits a spherical electric double layer composed of bulky, hydrated anionic heads of surfactant molecules and their counterions in the aqueous phase, while the hydrophobic tails, visualized as sticks, form a hydrocarbon-like micellar interior. Because of the high surface charge density of the micelle, there is only little electrolytic dissociation of counterions. The Hartley model explains the low conductivity of micellar solutions and the way surfactants work as detergents by solubilizing (i.e. incorporating) hydroi obic substrates. The model fails to explain certain NMR and fluorescence data that demonstrate some contact of... 

Finally, a third electromethod, namely electrical conductivity (k), has been employed. It is well known that micellization of an ionic surfactant like SDS leads to a slope change in its k versus concentration plot. Work by Jones (24) on the PEO/SDS system showed that intfoduction of PEO into the surfactant solution led to pre- and postmicellar breakpoints consistent with the Ti and T2 concepts mentioned above. Furthermore, interpretation of the conductivity data for PEO/SDS and PVP/SDS systems by Zana et al. provided confirmation that the degree of counterion (Na) binding for the complexes was considerably lower than that observed with simple SDS micelles (25). 

The effect of electrolyte on the CMC in aqueous media is very pronounced for ionic surfactants, and less so for zwitterionics and nonionics. The reduction in CMC is due mainly to the decrease in electrical repulsion between the ionic head-groups. A second effect is the so-called salting-out effect, which is the major reason for CMC lowering for zwitterionics and nonionic surfactants. The work needed to create the volume in water required to accommodate a non-polar compound is changed in an electrolyte solution because of the water-ion interactions. If the required work is increased by the presence of electrolyte, the surfactant monomers are salted-out and micellization is favoured. The effect of the electrolyte present in solution depends on the radius of the hydrated ion. The smaller the radius, then the greater the salting-out effect. 

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