Stem layer, ionic micelle

Solvation dynamics (SD) studies of micellar solutions have reported a timescale which does not match with the dynamics either when the probe is inside the bulk water or when it is inside the bulk hydrocarbon (core). This indicates that the probe used resided neither in the bulk water nor in the dry core region, but was located in the Stem layer. Bhattacharyya and co-workers have studied SD in several micelles and found that the SD in the Stem layer of the micelles is three orders of magnitude slower than that in bulk water (in bulk the relaxation time is on the sub-picoseond timescale) [6]. The components that could cause solvation in the Stem layer of micelles are the polar or ionic headgroups of the surficants, the counterions, and the water molecules. In such an environment water motion could be severely restricted, giving rise to the slow component of SD. 

The protonation of the triplet jtjt state of 3-bromonitrobenzene is shown to be responsible for the acid-catalysed promotion of halogen exchange which follows a S y23Ar mechanism26 (equation 23). Cationic micellar effects on the nucleophilic aromatic substitution of nitroaryl ethers by bromide and hydroxide ions have also been studied27. The quantum efficiency is dependent on the chain length of the micelle. The involvement of counter ion exchanges at the surface of ionic micelles is proposed to influence the composition of the Stem-layer. 

Figure 2 Conventional representation of micelles formed by an ionic surfactant, such as sodium dodecyl sulfate. The inner core region consists of the methylene tails of the surfactants. The Stem layer consists of surfactant headgroups and bound counterion species. The diffuse double layer consists of unbound counterions and coions which preserve the electrical neutrality of the overall solution. Also pictured are the transition moment vectors for the S-O stretching modes of sodium dodecyl sulfate.

For the solution without NaCl the occupancy of the Stem layer, r2/Ti rises from 0.15 to 0.73 and then exhibits a tendency to level off. The latter value is consonant with data of other authors, who have obtained values of r2/Fi up to 0.70 to 0.90 for various ionic surfactants pronounced evidences for counterion binding have been obtained also in experiments with solutions containing surfactant micelles." ° As could be expected, both Fj and F2 are higher for the solution with NaCl. These results imply that the counterion adsorption (binding) should be always taken into account. 

The size of the micellar aggregates is usually between 1 and 10 nm, and the aggregation number, i.e., the number of surfactant molecules per micelle, ranges from 20 to 200. Like proteins, the core of a micelle is essentially dry and consists of the hydrocarbon chains. The polar charged head groups project outward into the bulk water. Surrounding the core there is a layer composed of the ionic or polar headgroups, bound counterions, and water molecules. This layer is called the Stem layer. 

A key property of normal ionic micelles in water is their charge, which attracts counterions to the micellar surface. For a given ionic micelle one can assume that a fraction, )8, of the head groups will be neutralized by counterions in the Stem layer [22,25,26]. Alternatively one can write a as the fraction of counterions which will be lost from a (hypothetical) neutral micelle so that ... 

Figure 2.4 Representation ofan ionic micelle according to Hartley [10]. TheGouy-Chapman layer corresponds to the diffuse layer under the electric field due to the micelle. The Stem layer corresponds to the ionic groups.

Small amounts of nonpolar compounds can dissolve in die nonpolar core of the micelle, ionic compounds are located in bulk water, and polar compounds can partition between the polar layer (Stem layer in ionic micelles. Figure 2.4) of the micelle and bulk water. The polar layer of the micelles formed by nonionic surfactants is larger than the Stem layer of the ionic micelles (Figure 2.12), In nonpolar solvents, water and polar solutes are located in the core of reverse micelles, and nonpolar solutes are dissolved in the nonpolar solvent (Figure 2.8). 

Figure 2.12 Localization of solutes in the micelles. The apolar solutes are located in the micelle core. The polar solutes are located in the ionic palisade (Stem) layer. Alcohols may form mixed micelles.

Micelles of ionic surfactants are aggregates composed of a compressive core surrounded by a less compressive surface structure/ and with a rather fluid environment (of viscosity 8-17 cP for solubilized nitrobenzene in SDS and cetyltrimethylammonium bromide micelles). Copper ions attached to micelles have essentially the same hydration shell near the micellar surface as in the bulk phase, and do not penetrate into the nonpolar part of the micelle. In addition, it is known that the volume change caused by binding of divalent metal ions to micelles is very small. The rate of rotation of the hydrated Na ion at the micellar surface is unlikely to change by more than 35% upon adsorption from the bulk to the Stem layer of SDS micelles. ... 

Nearly 19- and 26-fold lower values of k than k for pH-independent hydrolysis of 2 in CTABr and SDS micelles, respectively, are explained in terms of high concentration of ionic head groups in Stem layer and electrostatic effect on partially anionic transition state. However, such an electrostatic effect cannot explain nearly 190- and 65-fold lower values of k, compared to k for pH-independent hydrolysis of 3. It has been suggested that the influence of hydro-phobic chains is more pronounced for 3 than for 2. But the nearly 3-fold larger value of k i for 3 in SDS micelles than in CTABr micelles remained unexplained. The deaease in kw for 2 from 4.8 x lO- to 2.4 x lO- seer with the increase in [NaCl] from 0.0 to 0.5 M in SDS micelles has been attributed to increased counterion binding (i.e., P value in pseudophase ion-exchange [PIE] model for-... 

The rate of spontaneous hydrolysis of phenyl chloroformate decreases with increase in the concentration of micelles of various anionic, nonionic, and cationic surfactants. A comparison of the kinetic data in nonionic micellar solutions to those in anionic and zwitterionic micellar solutions makes it clear that charge effects of micelles is not the only factor responsible for the variations in the reaction rates. Depletion of water in the interfacial region and its different characteristics as compared to bulk water, the presence of high ionic concentration in the Stem layer of ionic micelles, and differences in the stabilization of the reactant state and the transition state by hydrophobic interactions with surfactant tails can also influence reactivity. 

Micelles are in dynamic equilibrium with their monomer surfactants. Two relaxation processes are related to this equilibrium, a fast one in the microsecond time domain associated with the exchange of individual monomers between the micelles and the bulk aqueous phase and a slower one on millisecond time-scale associated with the complete dissolution of the micelles into monomers [8], For example, the exit rate for the SDS anion from its micelle is about lO s, which is considered to be a diffusion-controlled process [8a]. Nonpolar molecules are usually attracted to the relatively hydrophobic inner core of micelles, whereas ionic reactants and products are either associated with the Stem and Gouy-Chapman layers or repelled from the micelles, depending on the sign of electrostatic interaction. For example, NMR studies show that nonpolar molecules such as benzene and naphthalene are... 

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