Entangled Wormlike Micelles Research Articles

Drug-induced transitions from micelles to vesicles in ionic surfactant solutions

Surfactant solutions play key roles in scientific research and technological applications. Here we investigated solutions that combined an ionic surfactant, Cetylpyridinium Chloride (CPyCl), with a non-steroidal, anti-inflammatory drug, Sodium Diclofenac (Diclo). The study covered a wide range of compositions and utilized both rheology and cryogenic-Electron Microscopy (cryo-EM). The solutions were prepared at three different concentrations of CPyCl: 5.0 mM, 16.7 mM and 33.0 mM, and we maintained the ratio of surfactant to salt, R=CPyCl/Diclo, within the range of 0.24<R<1.2. Linear viscoelasticity was measured at 25°C. The use of dilute solutions allowed for detailed cryo-EM imaging. By tracking the evolution of the zero-frequency viscosity as a function of Diclo concentration, we identified distinct regions of different rheological responses. The correlation between frequency sweep data and cryo-EM images enabled us to link these regions to specific morphological transitions ranging from spherical micelles to long and entangled wormlike micelles, to branched networks and, eventually, to perforated vesicles. For some systems, we were able to measure the complete spectrum of relaxation times, including the shortest characteristic Rouse time at high frequencies, previously only achieved by Diffusive Wave Spectroscopy. The use of existing models to evaluate the relevant important microstructural parameters such as entanglement, contour, and persistence lengths allowed us to establish a clear correlation between the microstructural parameters derived through rheology and the insights from the cryo-EM analysis.

Rheology of saturated micellar networks: Wormlike micellar solutions vs. bicontinuous micellar phases

The subject of this work is to investigate the rheological behavior of mixed micellar solutions (sodium laurylethersulfate and cocamidopropyl betaine) in the presence of Mg2+ divalent counterions. With the rise of salt concentration, the viscosity of micellar solutions increases to a high maximum followed by a steep decrease because of the initial growth and entanglement of wormlike micelles and a subsequent transition to branched micelles forming of a saturated micellar network. The proposed systematic rheological measurements show considerable variations in the rheological responses of the solutions when increasing the salt concentration. Rheological behavior and data are used to distinguish the micellar phases and to study the relation to micellar structures. The wormlike micellar solutions have a typical shear thinning behavior with a well-defined zero-shear viscosity, η0, described by the Cates reptation-reaction model or the augmented Maxwell model. Our data show that the power law dependence of η0 on the surfactant concentration is stronger than that reported in the literature and it is influenced of the added electrolytes. The branched micellar structures are characterized by the lower viscosities and larger elasticities, which follow the Maxwell model up to the intermediate values of the frequency of oscillations, however peculiar deviations from the Cole-Cole plot at large frequencies are detected. The isolated bicontinuous micellar phases are Newtonian fluids with viscosity 0.4–0.7 Pa.s independent on the salt concentration up to high shear rates. The threshold salt concentration ensuring the onset of the bicontinuous micellar phase is described by a simple empirical rule. These phases are characterized by large elasticities and not negligible yield stresses. The property of the bicontinuous micellar phases to form spontaneously oil-in-water nanoemulsions could find applications in drug delivery, extraction and separation processes, pharmaceutics production, etc.

The effect and enhancement mechanism of hydrophobic interaction and electrostatic interaction on zwitterionic wormlike micelles

The effects of hydrophobic interaction and electrostatic interaction on the structure of zwitterionic wormlike micelles (WLMs) were studied. The change of the hydrocarbon chain length and addition of cetyl trimethyl ammonium chloride (CTAC) were employed to explore the effect of hydrophobic interaction and electrostatic interaction on the wormlike micelles constructed by zwitterionic viscoelastic surfactants and sodium p-toluene sulfonate (NaPts). The experimental results show that increasing hydrocarbon chain length of zwitterionic surfactant and the addition of CTAC can increase the viscoelasticity and zero-shear viscosity of the system. The calculated length of wormlike micelles also increases. This shows that the enhancement of hydrophobic interaction and electrostatic interaction can promote the growth and entanglement of wormlike micelles. In addition, scanning electron microscope (SEM) images confirmed that CTAC makes the connection between the zwitterionic micellar layer closer. This is undoubtedly beneficial to the growth of micellar structure. Finally, this work explores the enhancement mechanism of the hydrophobic interaction and electrostatic interaction on the micelle structure.

Dirhamnolipid ester - formation of reverse wormlike micelles in a binary (primerless) system.

We report new dirhamnolipid ester forming reverse wormlike micelles in nonpolar solvents without the addition of any primer. Therefore, these compounds represent a rare case of a binary system showing this gel-like behavior. In this study, the influence of the concentration of the rhamnolipid ester and the ester alkyl chain length on the rheological properties of the reverse wormlike micelles in toluene was investigated in detail. Highly viscoelastic solutions were obtained even at a relatively low concentration of less than 1 wt %. The phase transition temperatures indicate that the formation of reverse wormlike micelles is favored for dirhamnolipid esters with shorter alkyl chain lengths. Oscillatory shear measurements for the viscoelastic samples reveal that the storage modulus (G') and the loss modulus (G'') cross each other and fit the Maxwell model very well in the low-ω region. As is typical for wormlike micelle systems, the normalized Cole–Cole plot of G''/G''max against G'/G''max was obtained as a semicircle centered at G'/G''max = 1. The formation of network structures was also verified by polarized light microscopy. The sample was birefringent at ambient temperature and anisotropic at an elevated temperature. Differential scanning calorimetry analysis yielded a transition enthalpy of about ΔHSG/GS = ±7.2 kJ/mol. This value corresponds to a strong dispersion energy and explains the formation of the highly viscous gels by the entanglement of wormlike micelles through the interaction of the alkyl chains.

Molecular structure of maltoside surfactants controls micelle formation and rheological behavior

HypothesisThe anomeric configuration (α or β) of n-hexadecyl-d-maltopyranoside (C16G2) has been shown to affect the morphology of the micelle, from elongated for α-C16G2 to worm-like micelles for β-C16G2. The entanglement of worm-like micelles often leads to strong modifications of the rheological behavior of the system and, as such, the anomeric configuration of C16G2 could also provide the possibility of controlling this. Furthermore, mixing these surfactants are hypothesized to result in mixed micelles allowing to finely tune the rheology of a system containing these sustainable surfactants. ExperimentsThe rheology of α- and β-C16G2, and mixtures of those, was determined by rotational and oscillatory rheology at different temperatures and surfactant concentrations. Micelle structure and composition for these systems were characterized using contrast variation small-angle neutron scattering and small-angle X-ray scattering. The results from these were connected in order to elaborate a molecular understanding of the rheological response of the system. FindingsThe self-assembly of these surfactants have been found to result in different rheological properties. β-C16G2 show a high viscosity with a non-Newtonian viscoelastic behavior, which was linked to the formation of worm-like micelles. In contrast, α-C16G2 self-assembled into short cylindrical micelles, resulting in a Newtonian fluid with low viscosity. Furthermore, mixtures of these two surfactants lead to systems with intermediate rheological properties as a result of the formation of micelles with intermediate morphology to those of the pure anomers. These results also show that the rheological properties of the system can be tuned to change the micelle morphology, which in turn depends on the anomeric configuration of the surfactant. Also, surfactant concentration, temperature of the system, and micelle composition for surfactant mixtures provide control over the rheological properties of the system in a wide temperature range. Therefore, these results open new possibilities in the development of sustainable excipients for formulation technology, where the characteristics of the system can be easily tailored through geometric variations in the monomer structure whilst maintaining the chemical composition of the system.

CO2-switchable foams stabilized by a long-chain viscoelastic surfactant

Smart foams sensitive to external stimulation have gained increasing attention recently. However, reversibly switchable CO2 foams have been less documented. In this work, a novel kind of CO2-switchable foams was developed using a long-chain cationic surfactant, N-erucamidopropyl-N,N-dimethylammonium bicarbonate (UC22AMPM⋅H+), as both the foaming agent and stabilizer. The foams can be rapidly transformed between stable and unstable states at ambient temperature with CO2/NH3·H2O as the triggers. The foaming properties and switchable performance were examined by a combination of confocal microscopy, cryogenic transmission electron microscopy, and rheological techniques. The results demonstrated that the enhanced foam stability in the presence of CO2 is attributed to the high bulk phase viscosity and gas/liquid surface viscosity, resulting from the entanglement of wormlike micelles (WLMs) formed from UC22AMPM⋅H+. When NH3·H2O is added, the network structure of WLMs is disrupted, and the bulk phase viscosity and surface viscosity subsequently drop, consequently leading to an ultimate foam destabilization. Such a CO2-sensitive viscoelastic surfactant could not only be used to fabricate smart CO2 foams but can also enable CO2 to play dual roles as both the dispersed phase, as most gases do, and an “activator” to protonate long-chain tertiary surfactants into cationic analogs to form viscoelastic WLMs to stabilize foams.

Glucose Responsive Rheological Change and Drug Release from a Novel Worm-like Micelle Gel Formed in Cetyltrimethylammonium Bromide/Phenylboronic Acid/Water System.

A novel glucose (Glc)-responsive gel formed by worm-like micelles (WLMs) has the potential to provide a self-regulating insulin delivery system. We have prepared a WLM gel system using 75 mM cetyltrimethylammonium bromide, 75 mM phenylboronic acid, and water. At pH 9.4, this gel-like system was highly viscous and supported its own weight, and dynamic viscoelasticity measurement indicated that it contained long and entangled WLMs. The visual observation of gels prepared to include >6 mM Glc revealed that these adopted a sol-like appearance, whereas those prepared to include a control compound (2-10 mM diethylene glycol) retained their gel-like appearance. The storage modulus ( G') of this system decreased as the Glc concentration increased (2-10 mM), indicating a gradual shortening of the WLMs. In vitro release was evaluated using a test compound (fluorescein isothiocyanate dextran) in a microsized flow system. By 120 min, the release of this compound from the WLM gel was around 27-fold greater in the presence of 100 mM Glc than without Glc or with 100 mM diethylene glycol. This demonstrated the successful preparation of a WLM gel that showed an altered drug release rate, depending on Glc concentration.

Effects of Calcium Ions on the Solubility and Rheological Behavior of a C22-Tailed Hydroxyl Sulfobetaine Surfactant in Aqueous Solution.

Effects of calcium ions (Ca2+) on the solubility, aggregate structure, and rheological behavior of a C22-tailed zwitterionic surfactant, erucyl dimethyl amidopropyl hydroxyl sulfobetaine (EHSB), have been investigated in aqueous solution. In comparison with sodium ions (Na+), Ca2+ ions exhibit a much higher efficiency in decreasing the Krafft temperature (TK) of EHSB. Specifically, contrary to Na+ ions which have no obvious effect on the rheological properties of the EHSB solution, Ca2+ ions increase the viscosity of the EHSB solution at lower EHSB concentration, and enhance its elasticity at higher EHSB concentration. Moreover, Ca2+ ions raise the temperature needed for the elastic-to-viscous transition of the EHSB solution at higher concentration. At lower EHSB concentration, the hydrophobic interaction between the ultralong hydrocarbon chains induces a tighter packing of the hydrophobic chains by forming a more stretched configuration, while at higher EHSB concentration, the electrostatic attraction between Ca2+ ions and the sulfonate groups of EHSB induces a tighter packing of the headgroups by forming Ca2+-mediated bridges among the EHSB headgroups. Besides, the above interactions may strengthen the hydrogen bonding of OH groups and/or of C═O amide groups, which in turn facilitates the compact packing of the surfactant molecules in aggregates and promotes the growth and entanglement of wormlike micelles. Thus, the EHSB solution shows Ca2+-dependent rheological behaviors. The solubility and rheological properties of the ultralong chain surfactant solution can be simultaneously improved with the addition of divalent Ca2+ ions.

Rheological properties of wormlike micellar gels formed by novel bio-based isosorbide surfactants

Utilization of renewable resources is gaining significance. Isosorbide, which is derived from corn, has been attracting attention as a basic component in various industrial applications. Isosorbide surfactants having an isosorbide group as a building block are called bio-based surfactants. In this study, we report wormlike micelle formation with a mixed surfactant solution of sodium 5-O-dodecyl isosorbide sulfate (SDSS) and 2-O- or 5-O-dodecyl isosorbide (2-DS or 5-DS, respectively). We studied the viscosity behavior of the 3wt%NaCl aq./SDSS system with 2-DS or 5-DS at a constant surfactant concentration (5wt%). The maximum viscosity was achieved at the same surfactant mixing composition for both systems but was higher in the 5-DS system. We also studied the dynamic rheological behavior. Oscillatory-shear (frequency sweep) measurements were performed on the viscoelastic samples that were formed around the viscosity maximum composition. Liquid-like behavior (G′<G′′) was observed in the low-frequency region, whereas solid-like behavior (G′>G′′) was observed in the high-frequency region. The Maxwell-type oscillatory rheological behavior of viscous micellar solutions can be related to the transient network formed by the entanglement of wormlike micelles. The Maxwell model was also fitted to the data, indicating wormlike micelle formation. The mesh size for the 2-DS system (58nm) was larger than that for the 5-DS system (47nm) around the maximum viscosity. This suggests that the 2-DS system forms shorter wormlike micelles with SDSS than does the 5-DS system, because of the possible larger head group area of 2-DS in the SDSS-2-DS mixed layer. Therefore a higher maximum η0 was observed in the 5-DS system.

Synergistic Behavior and Microstructure Transition in Mixture of Zwitterionic Surfactant, Anionic Surfactant, and Salts in Sorbitol/H2O Solvent: 1. Effect of Surfactant Compositions

AbstractThe phase behavior and rheological properties of a multi‐component system, made of a zwitterionic surfactant cocoamidopropyl betaine (CAPB), an anionic surfactant sodium lauryl sulfate (SLSS), and mixed salts (tetrasodium pyrophosphate, sodium acid pyrophosphate, sacharrin, and sodium fluoride) in sorbitol/H2O mixed solvent at different mass fraction of SLSS (XSLSS) were systematically investigated by steady and dynamic rheology, dynamic light scattering, and diffusion ordered spectroscopy (DOSY). When fixing the salt concentration and the mass ratio of sorbitol in mixed solvent (R), the zero‐shear viscosity increases first and then decreases showing a maximum with increasing XSLSS, resulting from the formation and entanglement of wormlike micelles. Especially when XSLSS is between 0.33 and 0.80, the mixture is dominated by entangled wormlike micelles coexisting with small micelles and separated wormlike micelles, and shows high viscoelasticity. The maximum of the zero‐shear viscosity is ca. 5 orders of magnitude larger than that of sorbitol/H2O mixed solvent or the CAPB/SLSS aqueous solution. The characteristic structural parameters for the micellar solutions at different XSLSS are also estimated from further analysis of the rheological results, and indicate the stronger network structures of the wormlike micelles are formed in our systems compared with the wormlike micelles formed by a traditional zwitterionic/anionic surfactant aqueous solutions. The great improvements of rheological properties are attributed to the strong screening effects of the mixed salts and the strong solvophobic effect of sorbitol on the electrostatic and hydrophobic interaction between the CAPB and SLSS molecules. The present work has improved our understanding about the aggregation behavior of zwitterionic/anionic mixed surfactants with salts in less polar solvent/H2O mixture, which would be of widely practical importance to optimize the formulation of products for personal care and household cleaning.

Synergistic Growth of Giant Wormlike Micelles in Ternary Mixed Surfactant Solutions: Effect of Octanoic Acid.

The synergistic growth of giant wormlike micelles in ternary mixed solutions composed of an anionic surfactant (sodium laurylethersulfate, SLES), a zwitterionic surfactant (cocamidopropyl betaine, CAPB), and octanoic acid (HC8) is studied. Rheological data and their analysis in terms of Cole-Cole plots and micellar characteristic times are presented, and the micellar structures behind the observed rheological behavior are revealed by cryo-TEM micrographs. The surfactant composition is fixed near the maximal micelle size of the binary SLES + CAPB system, whereas the concentration of HC8 is varied. At a given HC8 concentration, the viscosity of the ternary micellar solutions exhibits a very high and sharp peak. Polarized-light optical microscopy indicates that all investigated solutions are isotropic rather than liquid-crystalline. The cryo-TEM imaging shows complex phase behavior: wormlike micelles to the left of the peak, giant entangled wormlike micelles at the peak, and long wormlike micelles coexisting with multiconnected micellar aggregates to the right of the peak. The formation of multiconnected micelles leads to a drop in viscosity at the higher concentrations. The results contribute to a better understanding of the structure-rheology relations in micellar surfactant solutions and could be useful for controlling the properties of formulations in personal-care and house-hold detergency.

The Formation of pH-Sensitive Wormlike Micelles in Ionic Liquids Driven by the Binding Ability of Anthranilic Acid.

Wormlike micelles are typically formed by mixing cationic and anionic surfactants because of attractive interactions in oppositely charged head-groups. The structural transitions of wormlike micelles triggered by pH in ionic liquids composed of N-alkyl-N-methylpyrrolidinium bromide-based ILs (ionic liquids) and anthranilic acid were investigated. These structures were found responsible for the variations in flow properties identified by rheology and dynamic light scattering, and account for the structures observed with cryogenic transmission electron microscopy (Cryo-TEM). High-viscosity, shear-thinning behavior, and Maxwell-type dynamic rheology shown by the system at certain pH values suggested that spherical micelles grow into entangled wormlike micelles. Light scattering profiles also supported the notion of pH-sensitive microstructural transitions in the solution. Cryo-TEM images confirmed the presence of spherical micelles in the low-viscosity sample and entangled wormlike micelles in the peak viscosity sample. Nuclear magnetic resonance spectroscopy analysis revealed that the pH sensitivity of ionic liquid systems originated from the pH-dependent binding ability of anthranilic acid to the cationic headgroup of ionic liquids.

Thermal and pH dual stimulated wormlike micelle in aqueous N-cetyl-N-methylpyrrolidinium bromide cationic surfactant-aromatic dibasic acid system

Dual-stimulated wormlike micelles regulated by pH and temperature in a surface active ionic liquid system comprising of a cationic surfactant (N-cetyl-N-methylpyrrolidinium bromide (C16MDB)) and an aromatic dibasic acid (pyrocatechol) were investigated. Such a system can be reversibly switched between liquid-like and gel-like states on adjusting the solution pH value. The rheology, light scattering techniques and cryogenic-transmission electron microscopy (cryo-TEM) results show the growth of spherical micelles into long and entangled wormlike micelles was responsible for the changes in flow properties such as high viscosity, shear-thinning behaviour and Maxwell-type dynamic rheology. The nuclear magnetic resonance results reveal the binding capacity of pyrocatechol to C16MDB is proposed as the key factor of the unusual rheological and micellar responses of this pH-stimulated system. In addition, with the increase of temperature, molecular motion of surfactant molecules becomes more active, which endow the property of thermo-stimulated to the surface active ionic liquid system.

Quantitative correlation between counterion (X) binding affinity to cationic micelles and X – Induced micellar growth for substituted iodobenzoates (X)

A new semi-empirical kinetic (SEK) method has been used to calculate the values of KXBr or RXBr (X represents substituted iodobenzoates), with KX and KBr representing CTABr micellar binding constants of counterions X− (in the presence of either spherical or non-spherical micelles) and Br− (in the presence of only spherical micelles), respectively. Steady-shear rheological properties of mixed 0.015M CTABr/[MX] aqueous solutions reveal the presence of flexible wormlike micelles where MX represents sodium 3- and 4-iodobenzoates. The maxima of the plots of viscosity vs. [MX] at 0.015M CTABr for MX representing sodium 3- and 4-iodobenzoates support the presence of long linear and entangled wormlike micelles.

Light-responsive viscoelastic fluids based on anionic wormlike micelles

A new class of light-responsive viscoelastic fluids based on anionic wormlike micelles is reported. The key components are sodium oleate (NaOA) and a cationic azobenzene dye, 1-[2-(4-phenylazo-phenoxy)-ethyl]-3-methylimidazolium bromide (C0AZOC2IMB). These binary systems are gel-like fluids at certain concentration ratios of [C0AZOC2IMB]/[NaOA], e.g. 35/100, owing to the formation of long, entangled wormlike micelles. The viscosity of these fluids can be controlled reversibly by light due to photo isomerization between trans-C0AZOC2IMB and cis-C0AZOC2IMB. For example, the zero-shear viscosity (η0) of an originally gel-like sample is high up to ∼1300Pas when C0AZOC2IMB is in its trans from, whereas the mixture becomes a Newtonian fluid with η0 about 0.01Pas after UV light irradiation. For the post-irradiated cis-C0AZOC2IMB, short cylindrical micelles form, hence accounting for the lower viscosity. Evidence for the structural transition is provided by UV–vis spectra, rheology, 1H NMR and cryo-transmission electronic microscopy measurements.

Effect of Nano-SiO&lt;sub&gt;2&lt;/sub&gt; on the Rheology of Anionic Viscoelastic Solutions Formed by the Biodegradable Surfactant Fatty Acid Methyl Ester Sulfonate

The effects of nano-SiO2 on the rheology of viscoelastic solutions formed by the bio- degradable surfactant fatty acid methyl ester sulfonate (MES) were studied. The process of MES micelles transforming from sphere or rod to wormlike was investigated with the increase in NaCl concentration by rheological measurements. The increase in η0 on the introduction of SiO2 was explained by considering the increasingly entangled wormlike micelles that were formed due to the screening of the electrostatic interactions. The results of rheological measurements show that the system of MES /NaCl can transform into wormlike micelles. MES micelle solutions without SiO2 are linear viscoelastic fluids in accord with a Maxwell model having a single relaxation time. But there is a process with some distributions of the relaxation time for MES micelle solution with SiO2.

Octanol-Triggered Self-Assemblies of the CTAB/KBr System: A Microstructural Study

A micelle-vesicle transition induced by n-octanol C(8)OH was observed in an aqueous cetyltrimethylammonium bromide (CTAB)/potassium bromide (KBr) system. This transition was investigated by viscosity, rheology, dynamic light scattering (DLS), and direct imaging technique, cryo-transmission electron microscopy (cryo-TEM). Viscometry shows that the system underwent several morphological transitions with the increase in concentration of C(8)OH (regions I-IV). At low octanol concentration (region I), DLS analysis showed an increase in the apparent hydrodynamic diameter of the micelles with the addition of C(8)OH which was supported by cryo-TEM and rheology. With further addition of C(8)OH, transition of the elongated micelles occurred to a viscoelastic fluid comprising entangled wormlike micelles (region II), for which rheological data can be described by the Maxwell model. Further, the wormlike micelles transform to vesicles at [C(8)OH] ≈ 0.020 M (region III). This transition and the consequent changes in the fluid response can be explained in terms of vesicle formation caused by further addition of C(8)OH. Beyond this concentration (region IV), vesicles are the predominant microstructures in the system which shows unusual temperature response.

Effects of Temperature on the Rheological Behavior of Worm-like Micelles in a Mixed Nonionic Surfactant System

We examined the effects of temperature on the rheological behavior of worm-like micelles in a nonionic surfactant system consisting of polyoxyethylene (10) phytosterol (PhyEO(10))/glyceryl monocaprylate (GFA-C(8))/Water. First, the phase diagram of a PhyEO(10)/GFA-C(8)/Water system was examined when the weight ratio, R, of GFA-C(8) to the total surfactants was changed keeping the total concentration of the surfactants at 5 wt%. The formation of worm-like micelles was confirmed over a wide temperature range. Next, the effect of temperature on the rheological properties of the worm-like micelles was examined. From steady-flow viscosity measurements of the worm-like micelles, it was found that the zero-shear viscosity (eta(0)) gave a maximum value more than 1,000 times greater than the minimum value in the temperature range 20-50 degrees C. Further, the temperature at which the maximum eta(0) was observed decreased with increasing R value. These results indicate that there is an optimal temperature at which the entanglement of worm-like micelles is at its greatest. From dynamic viscoelasticity measurements, it was shown that the viscoelastic behavior observed for the worm-like micelles was consistent with the Maxwell model, which is the basic model for a viscoelastic body. In addition, the plateau modulus (G(0)), which reflects the volume fraction of entangled worm-like micelles, gradually increased with increasing temperature, while the relaxation time (tau), which reflects the disentanglement time of the worm-like micelles, rapidly decreased with increasing temperature. From these results, it was clarified that, for a nonionic worm-like micelle, tau influences the change in eta(0) more strongly than G(0).

Dynamics and rheology of wormlike micelles emerging from particulate computer simulations

We perform coarse-grained computer simulations of solutions of semidilute wormlike micelles and study their dynamic and rheological properties, both in equilibrium and under shear flow. The simulation model is tailored to the study of relatively large time and length scales (micrometers and several milliseconds), while it still retains the specific mechanical properties of the individual wormlike micelles. The majority of the mechanical properties (persistence length, diameter, and elastic modulus of a single worm) is determined from more detailed atomistic molecular dynamics simulations, providing the link with the chemistry of the surfactants. The method is applied to the case of a solution containing 8% (by weight) erucyl bis(hydroxymethyl)methylammonium chloride (EHAC). Different scission energies ranging from 15.5k(b)T to 19.1k(B)T are studied, leading to both unentangled and entangled wormlike micelles. We find a decrease in the average contour length and an increase in the average breaking rate with increasing shear rate. In equilibrium, the decay of the shear relaxation modulus of the unentangled samples agrees with predictions based on a theory of breakable Rouse chains. Under shear flow, transient over- and undershoots are measured in the stress tensor components. At high shear rates we observe a steady-state shear stress proportional to gamma(1/3), where gamma is the shear rate. This is confirmed by our high shear rate experiments of real EHAC in a parallel-plate geometry.

Structure of a New Type of Transient Network: Entangled Wormlike Micelles Bridged by Telechelic Polymers

The thermodynamics and structural behavior of a new type of transient network are reported. The network is obtained by adding in a solution of entangled surfactant wormlike micelles a telechelic triblock copolymer whose hydrophobic ends anchor into the micelles and whose hydrophilic tails are swollen in the aqueous solvent and reversibly link the entangled cylindrical micelles. For comparison, we have also studied the same surfactant system decorated with an amphiphilic diblock copolymer which corresponds exactly to a triblock telechelic copolymer cut into two identical diblock copolymers. We find that the addition of telechelic polymers induces an effective attraction of entropic origin between the surfactant that may result, in equilibrium, in the coexistence of a dilute phase and a connected network, as predicted by theory. Small-angle neutron scattering experiments show first that the locally cylindrical structure of the micelles is maintained upon copolymer addition over a wide range of copolymer-to-surfactant molar ratio. On the other hand, the addition of telechelic polymer correlates with (i) the emergence of a broad peak in the structure factor at a finite scattering vector, q, due to the short-range steric repulsion induced by the polymer between the cylinders, and (ii) the large rise of the scattered intensity at low q's (for small enough surfactant concentration), which indicates an increase of the osmotic compressibility of the solution due to the effective attractive interaction between cylinders provoked by the telechelic polymers linking them.