Volume Fraction Of Micelles Research Articles

Structure of micelles at interfaces

In aqueous solutions and for high concentrations, tri-block copolymers are known toaggregate. As a critical volume fraction of micelles is reached, they crystallize. We havestudied the temperature dependence of the structure formed by Pluronic P85 micellessolved in deuterated water close to an attractive and a repulsive wall. We find that thecubic structure is suppressed for the repulsive surface and for both interfaces duringcooling.

Small-Angle Neutron Scattering and Theoretical Investigation of Poly(ethylene oxide)−Poly(propylene oxide)−Poly(ethylene oxide) Stabilized Oil-in-Water Microemulsions

The aim of this study is to determine the effects of oil solutes and alcohol cosolvents on the structure of oil-in-water microemulsions stabilized by poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers. The systems investigated involved the solubilization of 1,3,5-trimethylbenzene or 1,2-dichlorobenzene by P123 (EO(20)-PO(70)-EO(20)) pluronic surfactant micelles in water and water + ethanol solvents. The structures of these swollen micelles were determined by small-angle neutron scattering (SANS). A thermodynamic model was employed to interpret the characterization data. The results of the thermodynamic model for micellization agreed well with the SANS data from samples of micelles swollen by both oils. The model predicted the size of the micelles within 5% accuracy using only one fitting parameter, the micelle polydispersity. Ethanol had significantly different effects on the polymer micelles that contained solubilized oil compared to pure polymer micelles. For pure polymer micelles, the addition of ethanol increased the solubility of the polymer and, therefore, decreased the total volume fraction of micelles, while for polymer-oil aggregates, ethanol tended to have a positive effect on the volume fraction of micelles. SANS results showed that the greatest divergence from pure aqueous solvent results occurred at oil concentrations above the microemulsion stability limit.

Order−Disorder Transition and Critical Micelle Temperature in Concentrated Block Copolymer Solutions

The phase behavior of symmetric styrene−isoprene (SI) diblock copolymers in selective solvents in the vicinity of the order−disorder transition (ODT) was investigated by small-angle neutron scattering (SANS). Particular emphasis is placed on the region just above the ODT, where a disordered phase of micelles is observed. To understand morphological changes in more detail, one of the blocks is deuterated, i.e., PS−dPI and dPS−PI, and the scattering length density (SLD) of the solvents used were identical to the SLD of the corona chains. Two approaches for the analysis of SANS data were taken: the generalized indirect Fourier transformation (GIFT) and direct model fitting with relevant form and structure factors. With increasing temperature, the micellar aggregation number decreases, and the core radius is roughly maintained by the increased solvent swelling of the core. The critical micelle temperature (cmt) is experimentally described in the present study by an abrupt decrease in the size, aggregation number, and volume fraction of micelles. The micelles were found to dissociate into free chains approximately 20−30 °C higher than the TODT, and the cmt is found to be quite close to the mean-field spinodal temperature TS. A mean-field temperature, TMF, is located by the crossover temperature from the linear behavior of Imax-1 vs T-1 plot, and the structure factor above the experimentally determined TMF is also found to be in good agreement with the Leibler−Landau-type mean-field theory. We examined two different ODTs: body-centered cubic lattices to disorder and hexagonal cylinders to disorder, with similar results in both cases.

Dispersing Single-Walled Carbon Nanotubes with Surfactants: A Small Angle Neutron Scattering Study

We have investigated the dispersion of single-walled carbon nanotubes (SWNTs) in heavy water with the surfactant octyl-phenol-ethoxylate (Triton X-100) using small angle neutron scattering. The results indicate an optimal surfactant concentration for dispersion, which we suggest results from competition between maximization of surfactant adsorption onto SWNT surfaces and a depletion interaction between SWNT bundles mediated by surfactant micelles. The latter effect drives SWNT reaggregation above a critical volume fraction of micelles. These behaviors could be general in dispersing SWNTs using amphiphilic surfactant. The data also reveal significant incoherent scattering from hydrogen in SWNTs, most likely due to acid and water residues from the purification process.

Crystallization of micelles at chemically terminated interfaces.

In aqueous solutions and for high concentrations triblock copolymers are known to aggregate. As a critical volume fraction of micelles is reached they crystallize. We report on grazing incident small angle neutron scattering as an experimental tool to investigate the crystallization of spherical polymer micelles in the immediate vicinity of a flat solid interface. We find for an attractive surface potential a face centered close packed structure with a random orientation perpendicular to the normal of the interface. For a repulsive potential crystallization is suppressed.

Thermosensitive sol–gel transition behaviors of poly(ethylene oxide)/aliphatic polyester/poly(ethylene oxide) aqueous solutions

AbstractThe thermoreversible gelation of Pluronic [poly(ethylene oxide) (PEO)–polypropylene oxide (PPO)–PEO] aqueous solutions originates from micelle formation and micelle volume changes due to PEO–water and PPO–water lower critical solution temperature behavior. The micelle volume fraction is known to dominate the sol–gel transition behavior of Pluronic aqueous solutions. Triblock copolymers of PEO and aliphatic polyesters, instead of PPO, were prepared by hexamethylene diisocyanate coupling and dicyclohexyl carbodiimide coupling. Through changes in the molecular weight and hydrophobicity of the polyester middle block, the hydrophobic–hydrophilic balance of each block was systematically controlled. The following aliphatic polyesters were used: poly(hexamethylene adipate) (PHA), poly(ethylene adipate) (PEA), and poly(ethylene succinate) (PESc). With the hydrophobicity and molecular weight of the middle block increasing, the critical micelle concentration at the same critical micelle temperature decreased, and the absolute value of the micellization free energy increased. The micelle size was rather insensitive to temperature but slightly decreased with increasing temperature. PEO–PHA–PEO and PEO–PEA–PEO triblock copolymers needed high polymer concentrations to form gels. This was ascribed to the tight aggregation of PHA and PEA chains in the micelle core due to strong hydrophobic interactions, which induced the contraction of the micelle core. However, because of the relatively hydrophilic core, a PEO–PESc–PEO aqueous solution showed gelation at a low polymer concentration. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 772–784, 2004

The Order−Disorder Transition and the Disordered Micelle Regime for Poly(ethylenepropylene-b-dimethylsiloxane) Spheres

Two asymmetric poly(ethylenepropylene-b-dimethylsiloxane) diblock copolymer melts have been examined by rheology, small-angle X-ray scattering (SAXS), and small-angle neutron scattering (SANS). The two copolymers have total molecular weights of 24 400 and 23 900 and PDMS volume fractions of 0.12 and 0.10, respectively. The latter copolymer was obtained by extensive fractionation of the former, the difference in composition arising primarily from the removal of traces of PDMS homopolymer. The order−disorder transition (ODT) is unambiguously identified by rheology and SAXS. Above the ODT the SANS and rheological properties are consistent with the recently proposed disordered micelle regime, whereby the copolymers are associated in micellar aggregates that are not ordered on a lattice. The viscosity of the melt in this regime is greater than that of a poly(ethylenepropylene) homopolymer of the same total molecular weight. The SANS structure factors were analyzed by a generalized inverse Fourier transform and by direct fitting to a model based on the Percus−Yevick structure factor and either a spherical or an ellipsoidal form factor. Both approaches give comparable results, and the average micelle core radius, effective hard-sphere radius, and volume fraction were extracted as a function of temperature. The fitting suggests that as temperature increases the micelles remain relatively unchanged, but their number density and volume fraction decrease steadily. The fractionated copolymer has a significantly smaller micellar core than its precursor and therefore a significantly smaller aggregation number. These differences are attributable to the partitioning of PDMS homopolymer into the micellar cores. For these samples the critical micelle temperature (cmt), where the volume fraction of micelles should become negligible, was not accessed, even 100 deg above the ODT.

The Order−Disorder Transition and the Disordered Micelle Regime in Sphere-Forming Block Copolymer Melts

The order−disorder transition in a melt of asymmetric micelle-forming diblock copolymers is studied analytically in the strong segregation limit. In contrast to previous calculations by Semenov and by self-consistent mean-field theory, both the translational entropy of the micelles in a disordered micelle regime and the intermicelle free energy are taken into account. This enables us to locate the order−disorder transition, ODT, where the long-range order (or lattice) of micelles disappears. In agreement with some experimental observations, the ODT occurs between body-centered-cubic spheres and disordered micelles. A face-centered-cubic sphere phase remains thermodynamically unstable, as it is superseded by the disordered micelle regime due to the gain in the translational entropy of micelles. The ODT is accompanied by a decrease in the micelle volume fraction and, more importantly, in the number density of micelles. The aggregation number and the average distance between micelles also change at the ODT, but only slightly. At higher temperatures the number of micelles decreases, but some small fraction of micelles may persist to very high temperatures. A “critical micelle temperature” (cmt) may be estimated, which separates the disordered micelle regime (with a finite micelle concentration) from a disordered melt (with exponentially small micelle fraction), but it is not a true phase transition. Thus, the disordered micelle regime is part of the disordered phase. The composition dependence of the ODT is also analyzed. We compare the predictions of this model with experimental data for poly(styrene-b-isoprene) diblock copolymers, with encouraging agreement.

Viscosity of surfactant stabilized emulsions

A new scaling parameter for the viscosity of surfactant stabilized emulsions is proposed. We suggest that the attractive force between emulsion droplets is caused by the small surfactant micelles in the continuous phase of an emulsion. The new scaling parameter will be referred to as the depletion flow number, Fld=4πηsa2am/kTϕm, and is defined as the ratio between the viscous energy needed to separate the droplets and the depletion energy that opposes this separation. Here ηs, a, am, and ϕm are the solvent viscosity, dispersed phase droplet radius, micelle radius, and micelle volume fraction, respectively. Fld is of the order of unity at the onset of shear thinning and is capable of explaining all previously observed effects of drop size, solvent viscosity, and surfactant concentration. With master curves which are obtained by using Fld as the running parameter, a relatively simple empirical model is constructed which can reproduce the viscosity curves of many previously reported in the literature.

Interaction, critical, percolation and kinetic glass transitions in pluronic L-64 micellar solutions

We briefly discuss results of analyses of an extensive set of small angle neutron scattering (SANS) intensity distributions from a class of Pluronic tri-block copolymer micelles in aqueous solutions. This class of Pluronic has a symmetric structure (PEO MPPO NPEO M), with PPO/PEO molecular weight ratio of 60/40. It is shown that the micelles are spherical, each consisting of a hydrophobic core and a diffuse hydrophilic corona region having substantial hydration. We use a previously developed ‘cap-and-gown’ model for the microstructure of the micelle, taking into account the polymer segmental distribution and water penetration profile in the core and corona regions. We treat the inter-micellar correlations using a sticky hard sphere model. With this combination, we are able to fit all SANS intensities satisfactorily for micellar solutions within the range of disordered micellar phase in absolute scale. The structure and interaction of micelle stay is essentially the same as the concentration increases. But the aggregation number and surface stickiness increases, and the micelle becomes less hydrated with increasing temperature. Micellar core is not completely dry but contains up to 20% (volume fraction) of solvent molecules at lower temperatures. We then discuss, in some details, Pluronic L64 (PEO 13PPO 30PEO 13) micellar system. This system shows an inverted bi-nodal line with a lower critical consolute point and a percolation line. We investigated the structure and interaction between these micelles as temperature approaches the bi-nodal line along iso-concentration lines. The model developed above is able to describe SANS data in critical region also satisfactorily. As one approaches the bi-nodal line at constant weight fraction of the copolymer, the aggregation number and the stickiness parameter increase. In particular, at weight fraction of 5%, the stickiness parameter approaches the critical value 10.2 at T=330.9 K. We investigated the effect of hydrophobic impurities in the commercial polymer on the critical phenomenon. We conclude that in both the pure and impure systems, the micellar solution shows a critical demixing point where micelles stay spherical but interact strongly with each other by a short range temperature dependent attraction. Furthermore, from this fitting procedure we find relationships between the stickiness parameter and temperature, the volume fraction of micelles and the polymer concentration. Using these two relations, we are able to map the phase diagram of the sticky sphere model onto that of the micellar solution. The agreement between the theory and experimental phase behavior is satisfactory. Finally, we briefly describe the recently found kinetic glass transition line in this system using a scaling plot of SANS data below and above the transition.

Rheological and small-angle neutron scattering studies of aqueous solutions of branched PEO-PPO-PEO copolymers.

Tetronic 908 is a four-branched copolymer comprised of polyethylene oxide (PEO) and polypropylene oxide (PPO) blocks fixed on an aliphatic diamine. In aqueous solution, this polymer has an amphiphilic character due to the difference in PEO and PPO solubilities with increase in temperature. This system presents interesting rheological properties depending on the polymer concentration in solution and temperature. At low temperatures and/or concentrations, the solution behaves as a Newtonian fluid. For a weight percentage p comprised between 2% and 22%, the viscosity of the solution goes through a maximum. Small-angle neutron scattering experiments show that the increase in viscosity is due to progressive aggregation of chains. For weight percentages higher than p=22%, the viscosity of the system diverges and the mixture becomes gel-like. In fact, spectromechanical measurements show that the solution behaves as an entangled polymer with a high relaxation time in the flow zone. Small-angle neutron scattering (SANS) studies of the same solution reveal the presence of several peaks indicating that the micelles are organized in a cubic structure. We have developed a model that suitably describes the SANS curves of this system in the region where micelles are formed. Our model considers that micelles are trilayered spherical entities that coexist with unimers by obeying an equilibrium reaction. Moreover, it allows the calculation of several parameters such as the volume fraction of micelles, their size, and composition. The evolution of these parameters with temperature and polymer concentration is detailed. Comparisons with literature models are presented and discussed. This compilation of results has permitted the establishment of precise relations between the structure of this polymer in solution and its rheological properties.

Dielectric Relaxation of Cationic Surfactants in Aqueous Solution. 1. Solvent Relaxation

Complex dielectric permittivity spectra, ε(ν), in the frequency range 0.09 ≤ ν/GHz ≤ 89 were recorded for aqueous solutions of octyl-, dodecyl-, and hexadecyltrimethylammonium bromide and dodecyltrimethylammonium chloride at room temperature and concentrations c ≤ 1 mol dm-3. Additionally, for selected solutions the temperature dependence of ε(ν) was examined in the range of 0 ≤ ϑ/°C ≤ 65 in order to obtain Eyring activation enthalpies and entropies of the relaxation times. It was found that a superposition of up to five Debye relaxation processes is required for a consistent fit of all spectra. The low-frequency dispersion steps with relaxation times τ1 ≈ 1.5 ns and τ2 ≈ 300 ps can be assigned to the diffusion of free and bound counterions surrounding the charged micelles. The remaining relaxation processes are due to the solvent. As in the pure solvent, a small contribution of fast mobile water molecules, τ5 ≈ 1 ps, can be observed besides the dominating relaxation of the hydrogen bond network of bulk water, τ4 ≈ 8 ps. Additionally, a slower relaxation process, τ3 ≈ 25 ps, is detected which can be attributed to water molecules within the micelles and in the vicinity of the hydrophobic tail of monomeric octyltrimethylammonium ions. From the amplitudes of the water relaxation processes hydration numbers of the surfactant ions and micelles as well as the volume fraction of micelles were calculated.

Dielectric behavior during sol-gel transition of PEO-PPO-PEO triblock copolymer aqueous solution

Thermoreversible sol-gel transition of the PEO-PPO-PEO triblock copolymer(Pluronic F127) aqueous solution was monitored by dielectric behavior measurement. Thermoreversible gelation originates from the micelle formation and micelle volume change due to PEO-water, PPO-water LCST behavior. Micelle volume fraction is the critical parameter that dominates the sol-gel transition behavior of Pluronic aqueous solution. By measuring the dielectric constant of 5-22% Pluronic F127 aqueous solution, critical micelle temperature and micelle volume fraction was obtained. The micelle volume fraction of the gel region exceeded the critical volume fractions of simple cubic packing(0.523). Gelation temperature determined by dielectric measurement agreed well with that determined by the tube tilting method.

Micellar Solutions of Associative Triblock Copolymers: The Relationship between Structure and Rheology

We report the rheology of the micellar solutions formed from narrow-molecular-weight poly(ethylene oxide) chains fully endcapped with C16 or C18 alkanes and correlate the viscoelasticity, characterized by the high-frequency modulus G∞‘ and a single relaxation time λ, with the measured characteristics of the micellar solutions. Scaling G∞‘ by p3/2kT/τRH3 (p = aggregation number, RH = hydrodynamic radius, 1/τ = attractive virial coefficient, k = Boltzmann's constant, and T = temperature) and plotting against the hydrodynamic volume fraction of micelles collapses the two sets of data. The relaxation time scales as the diffusion time, μRc3/kT, for a free hydrophobe escaping from a micellar core of radius Rc times a Boltzmann factor accounting for an association energy that increases linearly with hydrophobe length. The low shear viscosity follows as ηo = λG∞‘.

Motions of Water, Decane, and Bis(2-ethylhexyl)sulfosuccinate Sodium Salt in Reverse Micelle Solutions

Reverse micellar solutions made from the surfactant bis(2-ethylhexyl)sulfosuccinate sodium salt (AOT), water, and decane have been studied using NMR inversion−recovery and pulsed-field gradient experiments. Spin-lattice relaxation times, T1, and diffusion coefficients, D, were measured for the individual components of the reverse micellar phase of the water−AOT−decane microemulsion as a function of temperature, 277 < T < 313 K, and reverse micelle volume fraction, 0 < φ < 1. Activation energies for the decane rotational motions probed by the T1 measurements and those for the translational motions probed by the D measurements are compared as a function of φ. Both types of activation energies increase by a factor of ∼2 as φ increases from zero (pure decane) to 0.8 (near the lamellar phase boundary), indicating the influence of the AOT tails on the decane motions. A T−φ phase diagram has been constructed showing an empirical boundary for the onset of percolation. This percolation characterization, together w...

Micelles and gels of oxyethylene–oxybutylene diblock copolymers in aqueous solution: The effect of oxyethylene-block length

Block copolymer E90B10 (E=oxyethylene, B=oxybutylene) was synthesised and characterised by gel permeation chromatography and 13C NMR spectroscopy. Dynamic light scattering (DLS) and static light scattering (SLS) were used to characterise the micelles in solution (both in water and in aqueous 0.2 mol dm-3 K2SO4), yielding the micellar association numbers, the hydrodynamic and thermodynamic radii, and related expansion factors. Micellar parameters were also obtained by small-angle neutron scattering (SANS) for solutions of a similar copolymer, E86B10, in water, i.e., the association number, the hard-sphere radius, the micelle volume fraction and the corresponding expansion factors. A comparison of the appropriate quantities showed good agreement between the two techniques. SANS gave additional information e.g., volume fraction profiles for the micelles and volume fraction of water in the micelle core. Moderately concentrated solutions of copolymer E90B10 were studied in the gel state by small-angle X-ray scattering (SAXS) in tandem with rheology (oscillatory shear). Values for the dynamic elastic modulus (G′) of the gels significantly exceeded 104 Pa across the range of temperature (25–80°C) and frequency (0.1–100 rad s-1) explored. The SAXS patterns for the orientated gels showed them to have a body-centred-cubic structure, as expected for packed, spherical micelles. This structure persisted over a wide concentration range, e.g., until crystallisation of the E blocks occurred at high concentration (70 wt.% copolymer). By combining the present and published results, a comparison was made of the micelle and gel properties of copolymers with the same B-block length but different E-block lengths, i.e., E90B10, E40B10 and E18B10. As would be expected, as the E-block length was increased, the micelle association numbers decreased while the micelle radii and expansion factors increased. The critical gel concentration (cgc) also decreased as the E-block length was increased. Moreover, the cgc correlated quantitatively with the thermodynamic expansion factors obtained by SLS and SANS from the exclusion properties of the micelles.

Vesicle to micelle transitions in surfactant mixtures induced by shear

Small angle neutron scattering data from aqueous solutions of mixtures of a catanionic surfactant and a detergent at rest and under shear flow are presented. These data show a shear-induced vesicle to worm-like micelle transition. A thermodynamic model of the vesicle to micelle transition in lipid-detergent mixtures is extended to include shear effects. Predictions of the extended model are compared with the shear dependence of the micelle volume fraction estimated from scattering experiments. The experimental results agree qualitatively with the predictions.

The Effects of Salts on the Lower Consolute Boundary of a Nonionic Micellar Solution

A molecular model based in statistical thermodynamics is used to study salt effects on the lower consolute boundary of the aqueous non-ionic surfactant C8E5. The C8E5micelles are modeled as hard spheres interacting via a temperature-dependent Yukawa attraction and the salt ions are modeled as positively and negatively charged hard spheres interacting via a Coulombic potential. The excess thermodynamic properties due to the Coulombic and Yukawa potentials are evaluated using the analytical solutions to the Ornstein–Zernike equation obtained for the mean spherical approximation closure. The Yukawa parameters for the micelle–micelle attractions are determined by fitting the theoretical phase diagram for a pure Yukawa fluid to the experimental lower consolute boundary for a salt-free C8E5micelle–water solution. Ion–solvent interactions are indirectly accounted for by using previously determined adjusted values for the cation size and the dielectric constant of the medium. We evaluate theoretical coexistence curves for the C8E5micelle–salt–water mixtures in the temperature–micelle volume fraction and temperature–salt molarity planes. We calculate the changes in the lower critical solution temperature (LCST) for the C8E5micelle–salt–water mixture as a function of salt concentration for the salts NaF, NaCl, NaBr, NaI, and Na2SO4and compare the trends seen with experiments. When ion–solvent interactions are indirectly accounted for, the theory correctly predicts the salting-out trends exhibited by NaF, NaCl, and NaBr. For the 1:2 salt (Na2SO4), charge effects resulting from the higher charge on the ions play a more important role in salting-out than ion–solvent interactions do. The theory, however, cannot predict the salting-in phenomena exhibited by NaI, thus indicating that salting-in is the result of variations in the intermicellar attraction as a function of the salt type and salt concentration. The theoretical results also indicate that excluded-volume forces resulting from the different sizes of the salt ions cannot alone account for the salting-in and salting-out phenomena seen in aqueous nonionic micellar solutions.

On the Structure of Polymeric Micelles: Self-Consistent-Field Theory and Universal Properties for Volume Fraction Profiles

Polymeric micelles composed of asymmetric block copolymers of the type A n B m in a solvent S are analyzed by the self-consistent-field theory due to Scheutjens and Fleer. Micelle formation is studied in a spherical lattice geometry. An unfavorable mixing of A and S segments drives the self-assembly process, leading to micelles which can vary their aggregation number according to the conditions in solution. We study the micelle thermodynamics and the micelle structure as a function of the solvency of the B tails; the Flory-Huggins interaction parameter χ SB has been varied from 0 (good solvent) to 0.5 (Θ solvent). With decreasing solvency the critical micelle volume fraction (CMV) decreases and the micelle size increases. The micelles have a dense core and a more dilute brushlike corona structure. Our main interest is in the density profiles of the corona. We distinguish four regimes in these profiles, denoted proximal, central, parabolic, and distal. The proximal part of the B profile is near the core and is nonuniversal, the central part is a power law which is in good agreement with the scaling predictions of Daoud and Cotton, in the third regime the profile of the polymer segments is roughly parabolic, and in the distal regime an exponential decay toward the bulk solution occurs. The relative importance of these regimes Depends on the molecular architecture. For example, increasing the number of core (A) segments causes the central part first to grow and then to shrink in favor of the parabolic regime

Micellar effects on reaction kinetics: Part II. Study of the action of dodecyltrimethylammonium bromide on the reactions between Pyrogallol Red and chromium(VI), vanadium(V) and titanium(IV)

The complexation kinetics and equilibrium of the reactions of vanadium(V) and titanium(IV) with Pyrogaliol Red (PGR), and the kinetics of oxidation of chromium(VI) by PGR were studied in micellar solutions of dodecyltrimethylammonium bromide (DTAB). The 1:1 PGR-V(V) and 2:1 PGR-Ti(IV) complexes formed in an aqueous medium associate with DTAB molecules of the micellar system to yield the new complexes 1:1:2 PGR-V(WDTAB and 2:1:1 PGR-Ti(IV)-DTAB. The stability constants of the ternary complexes were calculated by considering the volume fraction of micelles and were compared with those determined from the overall solution volume. The binding constants for the associates of PGR, chromium(VI), vanadium(V), titanium(IV), PGR-V(V) and PGR-Ti(IV) complexes with DTAB micelles were spectrophotometrtcally determined. The observed rate of formation of the PGR-Ti(IV) complex was increased by the presence of DTAB micelles, but the second-order rate constant in the micellar pseudo-phase was slightly lower than in water. Therefore, the whole rate increase observed was due to the concentration of reactants in the micellar pseudo-phase. The kinetics of formation of the PGR-V(V) complex could not be investigated by using the stopped-flow technique because the reaction was too fast in both the aqueous and the micellar medium. The second-order rate constant for the oxidation reaction between PGR and Cr(VI) was found to exceed that in aqueous solutions by a factor of ca. 55. Therefore, the increased rate observed for this reaction in the DTAB micellar medium can be ascribed to intrinsic effects of the micellar pseudo-phase on reactivity.