Exchange between surfactant micelles

Rharbi, Y, Winnik, M.A. Solute exchange between surfactant micelles by micelle fragmentation and fusion. Adv. Colloid Interface Sci. 2001, 89-90, 25-46. 

The surfactant used in this study is decyltrimethylammonium bromide (DTAB). It was chosen because it has the longest hydrocarbon chain length for which the fast monomer exchange between the micelle and bulk phases can be monitored by ultrasonic techniques. 

Clearly, the Knox plot study points out that surfactant adsorption on the stationary phase is responsible for the bulk of efficiency loss observed using micellar phases. A slow solute exchange between the micelle apolar core and the aqueous phase is another possible explanation for MLC efficiency loss. 

Hartley [8] envisaged a dynamic equilibrium whereby surface active agent molecules are constantly leaving and entering, from solution, the micelles. The same applies to the counter ions with ionic surfactants, which can exchange between the micelle surface and bulk solution. 

A detailed analysis of the fluorescence decay of 2-ethyl-naphthalene in micelles of cationic surfactants has shown that the probe constantly exchange between the micelle core... 

The structure of these globular aggregates is characterized by a micellar core formed by the hydrophilic heads of the surfactant molecules and a surrounding hydrophobic layer constituted by their opportunely arranged alkyl chains whereas their dynamics are characterized by conformational motions of heads and alkyl chains, frequent exchange of surfactant monomers between bulk solvent and micelle, and structural collapse of the aggregate leading to its dissolution, and vice versa [2-7]. 

The situation is similar to the exchange between the monomer and the micellar states. Usually, the exchange between the monomer and the micellar states is fast. The spectra at surfactant concentrations above CMC, therefore, consist of a single set of peaks whose chemical shifts are averaged between the monomer and micellar states. Such an example is shown by spherical micelles formed by lithium perfluoro-octylsulfonate (FOS )... 

Interactions between cationic micelles and uni- and divalent anions have been treated quantitatively by solving the Poisson-Boltzmann equation in spherical symmetry and considering both Coulombic and specific attractive forces. Predicted rate-surfactant profiles are similar to those based on the ion-exchange and mass-action models (Section 3), but fit the data better for reactions in solutions containing divalent anions (Bunton, C. A. and Moffatt, J. R. (1985) J. Phys. Chem. 1985, 89, 4166 1986,90, 538). 

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... 

The environment of the surfactant molecules undergoes a marked change when they associate together to form micelles. The chemical shifts of certain nuclei of the surfactant molecules in the micellar state will consequently become different from those in the molecular dispersed state. It is well known that the exchange between the molecules of the two states in the solution is fast, so the observed chemical shift is the weighted value of the two species according to the following expression ... 

These are stable micelles that are formed with polymeric surfactants. Amphiphilic block copolymers such as the pluronics (polyoxyethylene-polyoxypropylene block copolymers) are able to self-assemble into polymeric micelles and hydrophobic drugs may be solubilized within the core of the micelle or, alternatively, conjugated to the micelle-forming polymer. Although micelles are rather dynamic systems that continuously exchange units between the micelle structure and the free units in solution, those composed of polyoxyethylene - poly(aspartic acid) have been found sufficiently... 

Surfactants. Surfactants come in two main types small amphiphilic molecules (for short called amphiphiles ) and polymers, among which are proteins. Small-molecule surfactants readily exchange between surface and solution, and a dynamic equilibrium is thus established, in accordance with the presumptions of the Gibbs equation. Most amphiphiles exhibit a critical micellization concentration (CMC), greatly... 

Vesicles differ considerably from micelles in several ways. Micelles are very dynamic species, so that both monomeric surfactant and solubilized materials are constantly exchanging between the aqueous and micellar pseudophases. However, vesicles are relatively long-lived species, and can be separated from other materials by techniques such as gel-permeation chromatography, and they can be isolated and the structures determined by electron microscopy (159, 160]. In addition the bilayer has an inside and an outside surface, so that the transport of solutes across the bilayer is of considerable interest. 

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