Effect of surfactants charge

The existence of an electric double layer can remarkably influence the dynamic interfacial properties of ionic surfactant solutions [96, 97, 98, 99, 100]. The equilibrium state of such interfacial layers has been described in much detail in Paragraph 2.5. The dynamic problems, however, are rather complex and difficulties arise in solving the respective set of non-linear equations. 

First models have been derived by Dukhin et al. [27, 28, 30, 101], and Borwankar and Wasan [102]. They used a quasi-equilibrium model by assuming that the characteristic diffusion time is much greater than the relaxation time of the electrical double layer, and thus, the complicated electro-diffusion problem is reduced to a simply transport problem. Datwani and Stebe [103] analysed this model and performed extensive numerical calculations, however, they did not include the electro-migration term into the diffusion equation so that the results are not relevant for further discussions. 

For small periodic surface perturbations as it is the case in longitudinal wave experiments Bonfillon and Langevin [104] derived a respective solution. Joos et al. [105] demonstrated that the kinetic problem becomes extremely simple for solutions of mixed anionic and cationic surfactants, and the adsorption of the resulting electroneutral combination of the two molecules is governed by the simple diffusion model [106, 107]. 

MacLeod and Radke [108] obtained numerical solutions of the electro-diffusion problem without making simplifying assumptions. Although the advantage of their rigorous approach is 

During the process of adsorption of surfactant ions at a liquid-fluid interface the surface electric potential and charge density increase with time. This leads to the formation of an electric double layer inside the solution. The charged surface repels the new-coming surfactant molecules (Fig. 4.10), which results in an apparent deceleration of the adsorption process. On the other hand, the existence of the electric double layer (DEL in agreement with the nomination given in [2]) changes the amount of adsorbed surfaetant ions needed to reach equilibrium. This decreases the rate of adsorption so that the total rate is a counterbalance of various influences and it cannot be estimated a priori if a deceleration or an acceleration of the equilibration of an adsorption layer results. The most recent analysis of the different relaxation processes inherent in the adsorption process of ionic surfactants has been performed by Danov et al. [33]. In this work the inclusion of counterions into the Stem layer was performed for the first time. 

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The effect of surfactant charge on the reaction rate was investigated for a related reaction, ring opening of 1,2-epoxyoctane with sodium hydrogen sulfite (Scheme 2 of Fig. 2). The reaction, which was performed in a Winsor III microemulsion, was fast when a nonionic surfactant was used as the sole surfactant and considerably more sluggish when a small amount of SDS was added to the formulation [9]. 

Brackman, J. C. and J. B. F. N. Engberts (1992). Effect of surfactant charge on polymer-micelle interactions -Dodecyldimethylamine oxide. Langmuir 8(2) 424-428. 

The electrochemical response of analytes at the CNT-modified electrodes is influenced by the surfactants which are used as dispersants. CNT-modified electrodes using cationic surfactant CTAB as a dispersant showed an improved catalytic effect for negatively charged small molecular analytes, such as potassium ferricyanide and ascorbic acid, whereas anionic surfactants such as SDS showed a better catalytic activity for a positively charged analyte such as dopamine. This effect, which is ascribed mainly to the electrostatic interactions, is also observed for the electrochemical response of a negatively charged macromolecule such as DNA on the CNT (surfactant)-modified electrodes (see Fig. 15.12). An oxidation peak current near +1.0 V was observed only at the CNT/CTAB-modified electrode in the DNA solution (curve (ii) in Fig. 15.12a). The differential pulse voltammetry of DNA at the CNT/CTAB-modified electrode also showed a sharp peak current, which is due to the oxidation of the adenine residue in DNA (curve (ii) in Fig. 15.12b). The different effects of surfactants for CNTs to promote the electron transfer of DNA are in agreement with the electrostatic interactions... 

Introduction to the variety of types of surfactants, effect of surfactants on aqueous solution properties. Law of mass action applied to the self-assembly of surfactant molecules in water. Spontaneous self-assembly of surfactants in aqueous media. Formation of micelles, vesicles and lamellar structures. Critical packing parameter. Detergency. Laboratory project on determining the charge of a micelle. 

The characteristic effect of surfactants is their ability to adsorb onto surfaces and to modify the surface properties. Both at gas/liquid and at liquid/liquid interfaces, this leads to a reduction of the surface tension and the interfacial tension, respectively. Generally, nonionic surfactants have a lower surface tension than ionic surfactants for the same alkyl chain length and concentration. The reason for this is the repulsive interaction of ionic surfactants within the charged adsorption layer which leads to a lower surface coverage than for the non-ionic surfactants. In detergent formulations, this repulsive interaction can be reduced by the presence of electrolytes which compress the electrical double layer and therefore increase the adsorption density of the anionic surfactants. Beyond a certain concentration, termed the critical micelle concentration (cmc), the formation of thermodynamically stable micellar aggregates can be observed in the bulk phase. These micelles are thermodynamically stable and in equilibrium with the monomers in the solution. They are characteristic of the ability of surfactants to solubilise hydrophobic substances. 

In the earlier study of copper tetraphenylporphine (CuTPP) formation in an anionic 0/W mlcroemulslon, the rate of reaction was greatly accelerated by the addition of quinoline. We have therefore extended our electrochemical measurements to Include studies of the copper(II) - quinoline system in an anionic micro-emulsion, supplemented by some additional kinetic data. We report here the results of these studies, as well as some supplementary investigations dealing with the effect of surface charge and the nature of the surfactant counterion in Interfacial processes. 

For solutions of typical ionic surfactants with no added salts the studies of Carroll and Ward showed that solubihzation rates were much smaller than those for nonionic surfactants, presumably because the surfactant ions adsorbed at the oil-water interface repelled the micelles of like charge in the solution. Indeed, Bolsman et al. found no measurable solubilization of n-hexadecane into solutions of a pure benzene sulfonate and a commercial xylene sulfonate. They injected small oil drops into the surfactant solutions and observed whether the resulting turbidity disappeared over time due to solubilization. Similarly, Kabalnov found from Ostwald ripening experiments that the rate of solubilization of undecane into solutions of pure SDS was independent of surfactant concentration and about the same as the rate in the absence of surfactant. That is, the hydrocarbon presumably left the bulk oil phase in this system by dissolving in virtually miceUe-free water near the interface. In similar experiments TayloC and Soma and Papadopoulos observed a small increase in the solubilization rate of decane with increasing SDS concentration. De Smet et al., who used sodium dodecyl benzene sulfonate, which does not hydrolyze, found, like Kabalnov, a minimal effect of surfactant concentration. 

E. Effects of Electrostatic Charges of Surfactants and Cosurfactants on Reversed Micellar Properties... 

Nagai and coworkers reported a study of heterocoagulation driven by the hydrophobic effect of cationically charged hard poly[styrene-C( -(methacryloy-loxyphenyl-dimethylsulfonium methylsulfate)], or soft poly[styrene-co-(butyl acrylate)-co-(methacryloyloxyphenyl-dimethylsulfonium methylsulfate)] latex particles of ca. 220-240 nm in diameter onto neutral microspheres of crosslinked polystyrene (8.5[im in diameter) [37]. A separate study on the small cationic latex particles showed that their interface was hydrophobic, as the cationic surfactant cetyltrimethylammonium bromide (CTAB) adsorbed onto the surface, clearly driven by a hydrophobic effect [38]. The assembly of the cationic latex particles onto the larger microspheres was studied against increasing NaCl concentrations, which influenced the packing patterns from individually spaced to clusters (see Fig. 5). 

Figure 20.18. Effect of introducing charges in a mixture of a nonionic polymer and a nonionic surfactant, illustrated for the mixture of dextran and a polyoxyethylene surfactant (reference system in (a)). Both upon introducing a low fraction of ionic surfactant in the micelles (b) or a low amount of sulfate groups in dextran (c), the mutual miscibility is strongly enhanced. This can be eliminated either if electrolyte is added or if both the polymer molecules and the micelles are charged (d). Above the curves there is miscibility, while below there is phase separation into two solutions. The dashed curves give the miscibility limits for the reference system. (Redrawn from K. Bergfeldt and L. Piculell, J. Phys. Chem., 100 (1996) 5935)...

Figure 6 shows the effect of surfactant concentration on interfacial tension and electrophoretic mobility of oil droplets (14). It is evident that the minimum in interfacial tension corresponds to a maximum in electrophoretic mobility and hence in zeta potential at the oil/brine interface. Similar to the electrocapillary effect observed in mercury/water systems, we believe that the high surface charge density at the oil/brine interface also contributes to lowering of the interfacial tension. This correlation was also observed for the effect of caustic concentration on the interfacial tension of several crude oils (Figure 7). Here also, the minimum interfacial tension and the maximum electrophoretic mobility occurred in the same range of caustic concentration (17). Similar correlation for the effect of salt concentration on the interfacial tension and electrophoretic mobility of a crude oil was also observed (18). Thus, we believe that surface charge density at the oil/brine interface is an important component of the ultralow interfacial tension.

Taylor DJF, Thomas RK, Li PX, Penfold J (2003) Adsmption of oppositely charged poly-electrolyte/surfactant mixtures. Neutron reflectirai frran alkyl trimethylammonium bromides and sodium poly(styrenesulfonate) at the air/water interface the effect of surfactant chain length. Langmuir 19 3712... 

The spontaneous self-assembly of surfactants is an active area of research in part because weak interactions in solutions of ionic surfactants depend on both ion type and charge and these ion specific effects alter, sometimes dramatically, the chemical and physical properties of surfactant solutions. However, consensus is absent on how to model these effects. In modem science terms this is an ancient problem because specific ion effects of surfactants solutions, proteins and biomembranes have been known for... 

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