Counterions, surfactants

The concentration of free surfactant, counterions, and micelles as a function of overall surfactant concentration is shown in Fig. XIII-13. Above the CMC, the concentration of free surfactant is essentially constant while the counterion concentration increases and... 

The rate constants for the reaction of a pyridinium Ion with cyanide have been measured in both a cationic and nonlonic oil in water microemulsion as a function of water content. There is no effect of added salt on the reaction rate in the cationic system, but a substantial effect of ionic strength on the rate as observed in the nonionic system. Estimates of the ionic strength in the "Stern layer" of the cationic microemulsion have been employed to correct the rate constants in the nonlonic system and calculate effective surface potentials. The ion-exchange (IE) model, which assumes that reaction occurs in the Stern layer and that the nucleophile concentration is determined by an ion-exchange equilibrium with the surfactant counterion, has been applied to the data. The results, although not definitive because of the ionic strength dependence, indicate that the IE model may not provide the best description of this reaction system. 

A pseudophase ion exchange model has been applied to reactions in micellar systems with varying success (1-7). According to this model, the distribution of nucleophile is considered to depend on the ion-exchange equilibrium between the nucleophile and the surfactant counterion at the micelle surface. This leads to a dependence on the ion-exchange constant (K g) as well as on the degree of dissociation (a) of the surfactant counterion. The ion exchange (IE) model has recently been extended to oil in water microemulsions (8). 

Finally, as an example of the effect of ion-pairing of surfactant counterions with co-moving ions, the reaction of methyl 4-nitrobenzenesulfonate with Br was accelerated when the concentration of Br in the Stern region was increased through the appropriate choice of cation. 

Compared to micellar bimolecular reactions involving reactive surfactant counterions, considerably less work has been done on micellar bimolecular reactions involving two neutral reactants. We will discuss here micellar effects on cycloaddition reactions though this is by no means the only system for which micellar catalysis has been investigated (see, e.g., Bonollo et al. °). 

For a surface active betaine ester the rate of alkaline hydrolysis shows significant concentration dependence. Due to a locally elevated concentration of hydroxyl ions at the cationic micellar surface, i.e., a locally increased pH in the micellar pseudophase, the reaction rate can be substantially higher when the substance is present at a concentration above the critical micelle concentration compared to the rate observed for a unimeric surfactant or a non-surface active betaine ester under the same conditions. This behavior, which is illustrated in Fig. 10, is an example of micellar catalysis. The decrease in reaction rate observed at higher concentrations for the C12-C18 1 compounds is a consequence of competition between the reactive hydroxyl ions and the inert surfactant counterions at the micellar surface. This effect is in line with the essential features of the pseudophase ion-exchange model of micellar catalysis [29,31]. 

The surfactant counterion is important in MECC because it affects the Kraft point, that is, the temperature above which the solubility of the surfactant increases sharply as a result of micelle formation SDS has a lower Kraft point than potassium dodecyl sulfate and will therefore reach its CMC at a lower temperature. In MECC, many separation problems can be solved with standard MECC buffers and operating conditions Table 5.4 provides a list of standard operating conditions.19... 

For the ionic surfactant solution in the presence of electrolyte containing non-surfactant counterion, the surface activity can be quantified with eqn 2.4. The more complicated Gibbs... 

It has been shown that the addition of a small amount of the anionic surfactant sodium dodecyl sulfate (SDS) to a microemulsion based on nonionic surfactant increased the rate of decyl sulfonate formation from decyl bromide and sodium sulfite (Scheme 1 of Fig. 2) [59,60]. Addition of minor amounts of the cationic surfactant tetradecyltrimethylammonium gave either a rate increase or a rate decrease depending on the surfactant counterion. A poorly polarizable counterion, such as acetate, accelerated the reaction. A large, polarizable counterion, such as bromide, on the other hand, gave a slight decrease in reaction rate. The reaction profiles for the different systems are shown in Fig. 12. More recent studies indicate that when chloride is used as surfactant counterion the reaction may at least partly proceed in two steps, first chloride substitutes bromide to give decyl chloride, which reacts with the sulfite ion to give the final product [61]. 

The first type of relaxation processes reflects characteristics inherent to the dynamics of single droplet components. The collective motions of the surfactant molecule head groups at the interface with the water phase can also contribute to relaxations of this type. This type can also be related to various components of the system containing active dipole groups, such as cosurfactant, bound, and free water. The bound water is located near the interface, while free water, located more than a few molecule diameters away from the interface, is hardly influenced by the polar or ion groups. For ionic microemulsions, the relaxation contributions of this type are expected to be related to the various processes associated with the movement of ions and/ or surfactant counterions relative to the droplets and their organized clusters and interfaces [113,146]. 

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. 

These results indicate that the surfactant counterion is capable of imparting either hydrophobic or hydrophilic properties to the polymer, depending on the substrate material. This may be due to a different orientation of the surfactant counterion within the polymer for each substrate. Because the carbon foil has a much more hydrophobic surface, the surfactant counterions could be expected to align themselves more, with the nonpolar end toward the substrate and the charged end toward the solution on the carbon-foil material. This would explain the difference in surface interactions between the polymers on these two substrates. 

Husein, M., Rodil, E. and Vera, J.H. (2004) Formation of silver bromide precipitate of nanoparticles in a single microemulsion utilizing the surfactant counterion. /. Colloid Interface Sci., 273,... 

A similar physical picture of counterion binding can be adopted for systems containing surfactant counterions, although in this case some additional effect may be expected. The main factors that influence the binding of ionic surfactants to polyelectrolytes with opposite charge are (1) the charge density of the polyion, A, (2) the hydrophobic character of the surfactant (the length of its hydrocarbon chain), (3) the additional attractive forces between the... 

Second-order rate constants in the micelles (k2m) do not depend in any obvious way upon the hydrophobicities of the reactants or, for anionic nucleophiles, upon the surfactant counterion. The relatively small inhibitory micellar medium effect on reactions of nonionic nucleophiles is readily explained by the lower polarity of the micellar surface relative to water (32, 33). The generally small effects of the medium upon the ionic reactions are also understandable because water activity and ionic hydration are similar at the micellar surface and in water (34, 35, 38). 

A numeric solution of a model for diffusive transport of ionic surfactants to an adsorbing interface was recently proposed by MacLeod Radke (1994). The model considers both the diffusion and migration of surfactant, counterions, and background electrolyte in the electric field that develops as the charged surfactant adsorbs at the interface. 

However micelles are very mobile structures. Monomeric surfactant, counterions and solutes exchange rapidly between micelles and solvent [27-29], and it appears that monomers and incorporated solutes enter the micelle at a diffusion-controlled rate. 

Table 2.2 Surfactants used for templating, conditions under which mesoporous silicas have been formed, and the interaction between surfactant (S) and inorganic species (I), from the charge-density matching model.Note, X is the surfactant counterion, H is a hydrogen ion...

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