Wormlike Micellar Systems Research Articles

Worm-Like Soft Nanostructures in Nonionic Systems: Principles, Properties and Application as Templates

The principles, occurrence, structure and properties of worm-like micellar solutions in nonionic surfactant systems is reviewed, with focus in certain experimental methods used to characterize such soft nanostructured systems. Formulation plays a critical role in the design of worm-like micellar systems and derived viscoelastic networks. Micellar growth in one dimension, and hence formation of worm-like aggregates, is favoured by an increase in the average surfactant molecular packing parameter. Such an increase can be induced by addition of cosurfactant or amphiphilic oil that tends to penetrate in the surfactant palisade layer and reduce the specific surface area. On the other hand, long and bulky oils prone to be solubilized in the micellar core, cause a rod-sphere transition and therefore a decrease in viscosity. Salts have a small effect on the behaviour of nonionic worm-like micelles, contrary to what is found for ionic surfactant systems. The effect of raising temperature on worm-like micellar solutions is the result of a balance between the dehydration of the surfactant head groups, which favors elongation, the kinetics of micellar disruption and the formation of structures with nearly zero curvature. Therefore, a viscosity maximum as a function of temperature is found in many systems. Reverse worm-like micelles with a hydrophilic core can also be formed in organic solvents, even in the absence of ionic components or water. Worm-like micelles are useful as templates for the formation of ordered mesoporous oxides. The interaction of micelles with silica species results in the formation of silica-surfactant complexes that later precipitate as hexagonal phase via a cooperative mechanism.

Single-component wormlike micellar system formed by a carboxylbetaine surfactant with C22 saturated tail

C22-tailed surfactants have aroused considerable attention due to their strong tendency to self-assemble into wormlike micelles, but most of these amphiphilies were prepared from erucic acid whose mono-unsaturated hydrophobic tail is readily degraded at high temperature. Here we report for the first time the wormlike micelles formed from a sole carboxylbetaine surfactant with a saturated C22-tail, docosyl dimethyl carboxylbetaine (DDCB), and examined its aqueous solution properties through UV–vis spectroscopy, surface tension, rheology and cryo-TEM observation. Above its Krafft temperature (59.5°C) and overlapping concentration (∼0.68mM), DDCB aqueous solution displays unusually strong viscoelasticity without needing any specialized hydrotropes that are normally necessary in conventional wormlike micellar systems. Cryo-TEM observation confirmed the coexistence of wormlike micelles and viscles, in good agreement with the prediction from “packing parameter” theory. Compared with its unsaturated counterparts, DDCB worms show lower overlapping concentration, longer breaking time, better stability at high temperature and ease of preparation due to the “single-component” character.

Characterization of wormlike micellar systems using DLS, rheometry and tensiometry

Alkyl polyethoxylates are surfactants widely used in vastly different fields, from oil exploitation to pharmaceutical applications. One of the most interesting characteristics of these surfactants is their ability to form micellar systems with specific geometry, the so-called wormlike micelle. In this study, dynamic light scattering (DLS), capillary viscometry, torque rheometry and surface tensiometry experiments were carried out in systems based on xylene, water, butanol (cosurfactant), and nona-ethyleneglycol-monododecyl ether (surfactant), with fixed surfactant:cosurfactant:oil composition (with and without oil phase) and varying overall concentration. The results showed that a transition from wormlike micelles to nanodrops was characterized by maximum relative viscosity (depending on how relative viscosity was defined), which was connected to maximum effective diameter, determined by DLS. Surface tension suggested that adsorption at the air–water interface had a Langmuir character and that the limiting value of the surfactant surface excess was independent of the presence of cosurfactant and xylene.

Pressure-driven flow of wormlike micellar solutions in rectilinear microchannels

AbstractIn this paper the inhomogeneous response of the (two species) VCM model (Vasquez et al., A network scission model for wormlike micellar solutions. I. Model formulation and homogeneous flow predictions, J. Non-Newtonian Fluid Mech. 144 (2007) 122–139) is examined in steady rectilinear pressure-driven flow through a planar channel. This microstructural network model incorporates elastically active network connections that break and reform mimicking the behavior of concentrated wormlike micellar solutions. The constitutive model, which includes non-local effects arising from Brownian motion and from the coupling between the stress and the microstructure (finite length worms), consists of a set of coupled nonlinear partial differential equations describing the two micellar species (a long species ‘A’ and a shorter species ‘B’) which relax due to reptative and Rouse-like mechanisms as well as rupture of the long micellar chains. In pressure-driven flow, the velocity profile predicted by the VCM model deviates from the regular parabolic profile expected for a Newtonian fluid and exhibits a complex spatial structure. An apparent slip layer develops near the wall as a consequence of the microstructural boundary conditions and the shear-induced diffusion and rupture of the micellar species. Above a critical pressure drop, the flow exhibits shear banding with a high shear rate band located near the channel walls. This pressure-driven shear banding transition or ‘spurt’ has been observed experimentally in macroscopic and microscopic channel flow experiments. The detailed structure of the shear banding profiles and the resulting flow curves predicted by the model depend on the magnitude of the dimensionless diffusion parameter. For small channel dimensions, the solutions exhibit ‘non-local’ effects that are consistent with very recent experiments in microfluidic geometries (Masselon et al., Influence of boundary conditions and confinement on non local effects in flows of wormlike micellar systems, Phys. Rev. E 81 (2010) 021502).

Numerical investigation of transient contraction flows for worm-like micellar systems using Bautista–Manero models

This study is concerned with the numerical modelling of the Modified Bautista–Manero (MBM) model, for both steady-state and transient solutions in planar 4:1 contraction flow. This model was proposed to represent the structured composition and behaviour of worm-like micellar systems which have importance in industrial oil-reservoir recovery applications. A parameter sensitivity analysis for the rheology of this model is presented in both transient and steady response, covering pure shear and uniaxial extension. In addition, some features in evolutionary flow-structure are demonstrated in contraction flows due to the influence and imposition of start-up transient boundary conditions. The different effects of various model parameter choices are described through transient field response, stress and viscosity fields in the contraction flow setting. Distinction may be drawn between fluid response in the strong/moderate extension hardening regimes by matching both steady-state and transient shear and extensional viscosity peaks, contrasting between micellar (MBM) models against network-based counterparts Phan-Thien/Tanner (PTT). Simulations are performed with a hybrid finite volume/element algorithm. The momentum and continuity equations are solved by a Taylor–Galerkin/pressure-correction finite element method, whilst the constitutive equation is dealt with by a cell-vertex finite volume algorithm.

Lauric Acid-Induced Formation of a Lyotropic Nematic Phase of Disk-Shaped Micelles

Addition of small amounts of lauric acid (LA) to a micellar solution of sodium dodecyl sulfate (SDS, 11.5 wt %) and cocamidopropyl betaine (CAPB, 3 wt %) has a dramatic effect on the rheological properties and phase behavior of the system. The viscosity increases by more than 1 order of magnitude up to a weight ratio LA/SDS = 0.17 and decreases for further LA loading. The decrease in viscosity is associated with the formation of a birefringent liquid crystalline phase. The evolution of the system from isotropic micelles in the absence of LA to lyotropic liquid crystals up to a weight ratio LA/SDS = 0.30 was probed by a combination of (23)Na NMR quadrupolar splitting, measurements of water and surfactant self-diffusion coefficients via (1)H-PGSE-NMR, and rheology. The evolution of the water self-diffusion coefficients indicates that LA induced a dramatic increase in the anisotropy of disk-shaped micelles. Birefringent samples always showed a well developed (23)Na quadrupolar splitting with a line shape typical of monodomain samples. This suggests that the whole sample is easily oriented within the spectrometer electromagnet, as usually observed for nematic liquid crystals. Sample spinning first destroys the alignment (only a single peak is discernible in the (23)Na NMR spectrum). Then, upon prolonged spinning, the alignment develops again. This indicates that the system is composed by disklike micelles aligning themselves with their normal perpendicular to the magnetic field. On the other hand, the linear viscoelastic response close to the nematic transition shows features usually observed in wormlike micellar systems (e.g., nearly Maxwellian behavior). To reconciliate the rheological data and the NMR evidence of disklike micelles, the formation of columnar stacks of disklike micelles is proposed. The rheology of the isotropic phase can therefore be interpreted in terms of entanglements of "living columnar stacks" of disklike micelles, and the nematic phase observed at high LA content could be attributed to a nematic columnar phase N(Col) formed by the alignment of such stacks.

Influence of boundary conditions and confinement on nonlocal effects in flows of wormlike micellar systems

In this paper we report on the influence of different geometric and boundary constraints on nonlocal (spatially inhomogeneous) effects in wormlike micellar systems. In a previous paper, nonlocal effects were observable by measuring the local rheological flow curves of micelles flowing in a microchannel under different pressure drops, which appeared to differ from the flow curve measured using conventional rheometry. Here we show that both the confinement and the boundary conditions can influence those nonlocal effects. The role of the nature of the surface is analyzed in detail using a simple scalar model that incorporates inhomogeneities, which captures the flow behavior in both wide and confined geometries. This leads to an estimate for the nonlocal "diffusion" coefficient (i.e., the shear curvature viscosity) which corresponds to a characteristic length from 1 to 10 microm.

Competition between Shear Banding and Wall Slip in Wormlike Micelles

The interplay between shear band (SB) formation and boundary conditions (BC) is investigated in wormlike micellar systems (CPyCl-NaSal) using ultrasonic velocimetry coupled to standard rheology in Couette geometry. Time-resolved velocity profiles are recorded during transient strain-controlled experiments in smooth and sandblasted geometries. For stick BC standard SB is observed, although depending on the degree of micellar entanglement temporal fluctuations are reported in the highly sheared band. For slip BC wall slip occurs only for shear rates larger than the start of the stress plateau. At low entanglement, SB formation is shifted by a constant Delta gamma, while for more entangled systems SB constantly "nucleate and melt." Micellar orientation gradients at the walls may account for these original features.

Review of multi-scale particulate simulation of the rheology of wormlike micellar fluids

We provide a review of our recent work on the development of a multi-scale simulation methodology to calculate the rheology and flow of wormlike micelles. There is a great need for understanding the link between the detailed chemistry of surfactants, forming wormlike micelles, and their macroscopic rheological properties. We show how this link may be explored through particle simulations. First, we calculate the mechanical properties of small units of wormlike micelles from atomistic molecular dynamics simulations. These mechanical properties are subsequently used in a coarse-grained Brownian dynamics model, where the persistence length is the smallest length scale. We show that the non-Newtonian rheology of wormlike micellar systems can be accurately determined from our simple mesoscopic simulation model. Finally, we show that this mesoscopic model can be used to study the flow of wormlike micelles in contraction–expansion geometries.

Viscoelastic wormlike micellar solutions in mixed environmentally friendly nonionic surfactant systems

A study of the phase and rheological behavior of polyoxyethylene(20)sorbitan monooleate (Tween 80)/monolaurin (ML)/water is presented at 25 °C. In water/surfactant binary system, Tween 80 forms an isotropic micellar solution over wide concentration range (∼42 wt%). With successive addition of ML to the aqueous Tween 80 solution, viscosity increases rapidly and a viscoelastic solution is formed. The viscoelastic micellar solution formed follows the Maxwell model typical of wormlike micellar systems at low-frequency region. With further addition of ML, the viscosity drops after a maximum and phase separation occurs. On increasing the concentration of Tween 80, micellar growth starts at a slightly lower concentration of ML. The micellar growth can be simply explained by reducing the effective area of Tween 80 head group upon addition of ML.

Nonlocal Effects in Flows of Wormlike Micellar Solutions

The flow curve of wormlike micelles usually exhibits a stress plateau sigma* separating high and low viscosity branches, leading to shear-banded flows. We study the flow of semidilute wormlike micellar systems in a confined geometry: a straight microchannel. We characterize their local rheology thanks to particle image velocimetry. We show that flow curves cannot be described by a simple constitutive equation linking the local shear stress to the local shear rate. We demonstrate the existence of nonlocal effects in the flow of wormlike micellar systems and make use of a theoretical framework allowing the measurement of correlation lengths.

Evidence of Wormlike Micellar Behavior in Chromonic Liquid Crystals: Rheological, X-ray, and Dielectric Studies

We report rheological, X-ray, and dielectric investigations on a chromonic liquid-crystalline system formed by aqueous solutions of a food coloring agent, Sunset Yellow, in the absence and upon addition of salt. The salt-concentration dependence of the steady-state viscosity at low shear rates has a non-monotonic variation and is qualitatively similar to the behavior seen in wormlike micellar systems, a surprising result since chromonic systems are expected to be non-micellar in character. More interestingly, for a particular low concentration of the salt (20 mM), the viscosity increases by 3 orders of magnitude in comparison with that of the pure chromonic material. The dynamic (oscillatory) rheological data bring out features which can be described in terms of a microstructure formation. X-ray and dielectric studies show that certain characters of the aggregates formed by the Sunset Yellow molecules are not altered by the addition of salt.

Molecular Dynamics Simulations of Mixed Cationic/Anionic Wormlike Micelles

Simulations of mixed cationic/anionic wormlike micellar systems have been carried out for a wide range of compositions, including pure anionic and cationic systems. It was found that the wormlike micelle formed by only cationic surfactant molecules is unstable and transforms to a set of small spherical micelles. Adding anionic surfactants with a short hydrophobic chain (only eight carbon atoms) results in stable wormlike micelles. The 34/66 cationic/anionic worm is stable and symmetrical, while the 50/50 mixture yields a flattened worm, indicating a phase transition to the lamellar phase. All these observations are in excellent agreement with the experimental results of Raghavan et al. (Langmuir 2002, 18, 3797), and they provide a molecular mechanism for their observations. The addition of octyltrimethylammonium chloride increases the radius of the worm due to the bigger hydrophobic part. Meanwhile, the length of the worms decreases with the concentration of cationic surfactant and reaches a minimum for the 50/50 mixture. The latter system is of special interest due to a zero surface charge density. The worm with the electrostatically neutral surface was used to investigate intermicellar interactions. The molecular dynamics (MD) simulations show that the merging process requires a substantial activation energy even in the case of reduced electrostatic repulsion.

Dynamics of worm-like micelles: the Cox–Merz rule

The viscoelastic behaviour of worm-like micelles in small-amplitude oscillatory, steady simple shear and uniaxial extensional flows are analyzed with a model that couples the Oldroyd-B constitutive equation with a kinetic equation that accounts for the structural changes induced by the flow. In some cases, the constitutive equation predicts a viscoelastic behaviour that is consistent with the Cox–Merz rule. Departures from this rule are also predicted. Experimental data obtained for two worm-like micellar systems indicate that in these solutions, the Cox–Merz rule is not usually followed, in agreement with the predictions of our model. In uniaxial extensional flow, the model predicts a strain hardening in the extensional viscosity at low extensional rates and a strain-thinning at high extensional rates.

Cloud-Point Phenomena in Wormlike Micellar Systems Containing Cationic Surfactant and Salt

Cationic surfactants with an erucyl (C22, monounsaturated) tail display unusual phase behavior in aqueous solution as a function of temperature and added salt concentration. Low amounts of salts with binding counterions such as sodium tosylate (NaTos) lead to highly viscoelastic wormlike micellar solutions. With further addition of salt, the viscosity decreases, and the solutions cloud on heating (i.e., exhibit lower consolute phase behavior). The cloud-point temperature and the zero-shear viscosity η0 pass in parallel through minima as a function of NaTos concentration. Cloud-point behavior is seen over a wider range of salt concentrations in the case of sodium salicylate (NaSal), which has an even stronger binding counterion. In the case of a weakly binding salt (NaCl), phase transitions are observed only at much higher salt contents, and the phase behavior is predominantly of the upper consolute type. Light and neutron scattering data show evidence of critical concentration fluctuations near the cloud ...

Closed-Looped Micelles in Surfactant Tetramer Solutions

Formation of a dominant population of closed loops in wormlike micellar systems, theoretically predicted but never reported so far, is achieved with a new cationic surfactant tetramer. The contour length distribution N(L) of the closed-looped micelles is determined by transmission electron microscopy at cryogenic temperature. At large contour lengths, the distribution observed scales as N(L) ∝ L-5/2, as expected from ring−chain equilibrium polymerization theory. At small contour lengths, the rigidity-dependent ring closure probability provides a direct estimate of the persistence length. These results re-emphasize the new possibilities offered by surfactant dimers (gemini) and oligomers in obtaining new morphologies in self-assemblies.

Micro/mesoscopic approaches to the ring formation in linear wormlike micellar systems

AbstractWe hereby present i) a microscopic model for the self‐assembly of linear wormlike micelles for which scission/recombination processes and loop formation are allowed, and ii) a mesoscopic analytic description of such systems. Both approaches predict the extent of loop formation as function of the micellar concentration, the end‐cap energy and the flexibility of micelles. As a matter of fact, even if loop formation is unfavorable under many conditions, e.g., for stiff micelles and low end cap energies, they have to be treated correctly in any statistical approach to their behavior, since their presence can significantly affect the relaxation time spectrum, the rheological behavior and correlation function of various types.

Dynamics of living and dead polymers studied by electrooptical birefringence

We report on transient electrooptic birefringence experiments on dead (poly(styrene) in 1,4 dioxane) and living (lecithin/water/iso-octane microemulsions) polymer solutions. We find it impossible to obtain reliable information on the electrooptical behavior of dead polymers in the semi-dilute regime. Comparison with the behavior of worm-like micellar solutions forming living polymer systems is therefore also ruled out. The interesting data obtained on the latter system, however, easily compensate for this negative result. The data seem to support the idea that branching rather than entanglement plays a key role in this worm-like micellar systems.

Slow Transients and Metastability in Wormlike Micelle Rheology

The steady-state nonlinear rheology of wormlike micellar systems is thought to be subject to shear banding (the underlying shear stress vs. strain rate curve σ(γ) is nonmonotonic). Shear banding may result in a plateau (σ(γ) = σp) in the measured flow curve (at controlled mean strain rate γ). We present new rheological data for aqueous CPyCl/NaSal (100 mM /60 mM). Steady-state flow curves published previously for this system (Rehage H. and Hoffmann H., Mol. Phys. 74 (1991) 933) have since been interpreted as shear-banded flow with top-jumping, in which the steady-state shear rate γ1 in the low shear band is the largest possible (γ 1 = γ 1 max , σ p = σ max ). That would rule out the existence of a metastable branch with a stress larger than σp. We show that such a branch does, however, exist (for temperatures in the range 20 - 25 °C). Similar results are found for a 100 mM/75 mM system. The time scale for relaxation of a metastable state onto true steady state flow, τ ss , is far longer than the Maxwell time of the fluid; this is consistent with shear banding. We observe τ ss ∼ (γ - γ c ) -P in the metastable regime (γ > γ 1 ), with p an exponent that depends on composition and temperature. The critical shear rate γ c is in some cases less than γ 1 so that no actual divergence of τ ss occurs. In at least one case, though, there is evidence for a physical divergence (γ c > γ 1 ) accompanied by a small window of shear rates, γ 1 < γ < γ c , for which τ ss is effectively infinite. In some respects the observed behaviour resembles that reported previously (Berret J.-F., Roux D.C. and Porte G., J. Phys. II Prance 4 (1994) 1261) for equimolar CPyCl/NaSal in 0.5 M NaCl. Those results were interpreted in terms of nucleation and growth of a shear-induced nematic phase. However the same explanation is unlikely for the low weight fractions (Φ ≤ 5%) used in our study.