pH-switchable wormlike micelles

A novel pH-switchable wormlike micellar system was prepared by mixing N-erucamidopropyl-N,N-dimethylamine and maleic acid with molar ratio of 2 : 1. The viscosity of the micellar solution is switchable via tuning the pH through the addition of minor acid or base. Such a system possesses the characteristics of a facile, rapid, cost-effective reversible process and recyclable cheaper materials.     

Polymerization of anionic wormlike micelles

Polymerizable anionic wormlike micelles are obtained upon mixing the hydrotropic salt p-toluidine hydrochloride (PTHC) with the reactive anionic surfactant sodium 4-(8-methacryloyloxyoctyl)oxybenzene sulfonate (MOBS). Polymerization captures the cross-sectional radius of the micelles (approximately 2 nm), induces micellar growth, and leads to the formation of a stable single-phase dispersion of wormlike micellar polymers. The unpolymerized and polymerized micelles were characterized using static and dynamic laser light scattering, small-angle neutron scattering, 1H NMR, and stopped-flow light scattering. Stopped-flow light scattering was also used to measure the average lifetime of the unpolymerized wormlike micelles. A comparison of the average lifetime of unpolymerized wormlike micelles with the surfactant monomer propagation rate was used to elucidate the mechanism of polymerization. There is a significant correlation between the ratio of the average lifetime to the monomer propagation rate and the average aggregation number of the polymerized wormlike micelles.     

Rheometry-PIV of shear-thickening wormlike micelles

The shear-thickening behavior of an equimolar semidilute aqueous solution of 40 mM/L cetylpyridinium chloride and sodium salicylate was studied in this work by using a combined method of rheometry and particle image velocimetry (PIV). Experiments were conducted at 27.5 degrees C with Couette, vane-bob, and capillary rheometers in order to explore a wide shear stress range as well as the effect of boundary conditions and time of flow on the creation and destruction of shear-induced structures (SIS). The use of the combined method of capillary rheometry with PIV allowed the detection of fast spatial and temporal variations in the flow kinematics, which are related to the shear-thickening behavior and the dynamics of the SIS but are not distinguished by pure rheometrical measurements. A rich-in-details flow curve was found for this solution, which includes five different regimes. Namely, at very low shear rates a Newtonian behavior was found, followed by a shear thinning one in the second regime. In the third, shear banding was observed, which served as a precursor of the SIS and shear-thickening. The fourth and fifth regimes in the flow curve were separated by a spurtlike behavior, and they clearly evidenced the existence of shear-thickening accompanied by stick-slip oscillations at the wall of the rheometer, which subsequently produced variations in the shear rate under shear stress controlled flow. Such a stick-slip phenomenon prevailed up to the highest shear stresses used in this work and was reflected in asymmetric velocity profiles with spatial and temporal variations linked to the dynamics of creation and breakage of the SIS. The presence of apparent slip at the wall of the rheometer provides an energy release mechanism which leads to breakage of the SIS, followed by their further reformation during the stick part of the cycles. In addition, PIV measurements allowed the detection of apparent slip at the wall, as well as mechanical failures in the bulk of the fluid, which suggests an extra contribution of the shear stress field to the SIS dynamics. Increasing the residence time of the fluid in the flow system enhanced the shear-thickening behavior. Finally, the flow kinematics is described in detail and the true flow curve is obtained, which only partially fits into the scheme of existing theoretical models for shear-thickening solutions.     

Temperature-induced growth of wormlike copolymer micelles

We study the temperature-induced growth of polymer micelles based on Pluronic P84 in brine (2 M NaCl) using small-angle neutron scattering, static and dynamic light scattering, and viscometry as a function of temperature and polymer concentration. Spherical micelles below 30 degrees C are shown to grow between about 30 and 40 degrees C into wormlike micelles long enough to enter the semidilute regime for polymer volume fraction larger than 0.005. The entanglements in this regime are responsible for a huge increase in the viscosity. Above about 41 degrees C, the micellar aggregates become denser as the cloud point is approached and the viscosity drops.     

Rheology of wormlike micelles in aqueous systems of a mixed amino acid-based anionic surfactant and cationic surfactant

Amino acid-based anionic surfactant, N-dodecanoylglutamic acid, after neutralizing by 2, 2′, 2″-nitrilotriethanol forms micellar solution at 25 °C. Addition of cationic cosurfactants hexadecyltrimethylammonium chloride (CTAC), hexadecylpyridinium chloride (CPC), and hexadecylpyridinium bromide (CPB) to the semi-dilute solution of anionic surfactant micellar solutions favor the micellar growth and after a certain concentration, entangled rigid network of wormlike micelles are formed. Viscosity increases enormously ～4th order of magnitude compared with water. With further addition of the cosurfactants, viscosity declines and phase separation to liquid crystal occurs. The wormlike micelles showed a viscoelastic behavior and described by Maxwell model with a single stress-relaxation mode. The position of viscosity maximum in the zero-shear viscosity curve shifts towards lower concentration upon changing cosurfactant from CPB to CTAC via CPC; however, the maximum viscosity is highest in the CPB system showing the formation of highly rigid network structure of wormlike micelles. In all the systems, viscosity decays exponentially with temperature following Arrhenius type behavior.     

Viscoelastic wormlike micelles and their applications

Viscoelastic wormlike micelles are important microstructures that relate to rhelogical properties of fluid in different applications. Recently, studies of structure and dynamic properties of wormlike micelle have extended to different surfactant type such as anionic, zwitterionic and polymeric surfactants. Applications have been found in oil fields, drag-reducing agents for district heating and cooling and thickeners for personal and home care products.     

Polyoxyethylene cholesteryl ether-based aqueous wormlike micelles

Formation of wormlike micelles (WLMs) in an aqueous mixture of polyoxyethylene cholesteryl ether (ChEO_n; where n = 20 and 30) and polyoxyethylene dodecyl ether (C₁₂EO_m; where m = 3 and 4) has been reported; rheological and small angle X-ray scattering (SAXS) measurements have been performed in the micellar solutions of ChEO_n as a function of C₁₂EO_m for the structural elucidation. When lipophilic cosurfactant, C₁₂EO_m is added to the micellar solutions of ChEO_n, it favors the sphere-to-cylinder transition due to the penetration of C₁₂EO_m in the palisade layer of ChEO_n micelle accompanying an increase in viscosity. When the concentration of C₁₂EO_m is increased, entangled network of WLMs is formed. A strong shear thinning has been observed in highly viscous samples indicating the presence of transient networks. Such samples exhibited viscoelastic behavior and could be described by the Maxwell model with a single stress relaxation mode. A maximum is observed in zero-shear viscosity-C₁₂EO_m plot. With further addition of C₁₂EO_m, viscosity declines and ultimately a phase separation occurs with the formation of turbid solution of vesicular dispersion. This decline has been interpreted in terms of micellar branching induced by an increase in endcap energy, E _c (which is compensated by the formation of branch points, having a mean curvature opposite to that of endcaps). The C₁₂EO_m induced one-dimensional micellar growth has been confirmed by SAXS.     

Tuning rheochaos by temperature in wormlike micelles

We investigate the critical role played by the mean micellar length during the route to rheochaos for wormlike micellar gels of surfactant cetyltrimethylammonium tosylate in the presence of salt sodium chloride that show coupling of flow to concentration fluctuations. To this end, we have carried out stress/shear rate relaxation experiments at a fixed shear rate/stress but at different temperatures to take the sample through the route to rheochaos. We see the type-II intermittency route to rheochaos in stress relaxation measurements and the type-III intermittency route to rheochaos in shear rate relaxation measurements. We have also carried out linear rheology measurements at different temperatures to estimate the mean micellar length (-)L, the reptation time tau(rep), and the breaking time tau(break). It is shown that (-)L changes by approximately 58%, as the sample goes through the route to rheochaos.     

Shear-induced phase separation in solutions of wormlike micelles

Polymer solutions in the vicinity of the theta-point are known to undergo shear-induced turbidity or phase separation. The present study shows that a similar phenomenon also occurs for certain wormlike micellar solutions. Wormlike micelles are the self-assembled counterparts of polymers and are characterized by their ability to reversibly break and recombine. In the system of interest, the micelles are formed by the cationic surfactant erucyl bis(hydroxyethyl)methylammonium chloride (EHAC), in conjunction with a salt such as sodium chloride (NaCl) or sodium salicylate (NaSal). Micellar samples that become turbid under shear show evidence of critical concentration fluctuations and may contain predominantly branched micelles. The shear-induced turbidity in these samples correlates with the appearance of flow-dichroism in rheooptic experiments and with an increase in low-q scattering in small-angle light scattering under flow (flow-SALS) experiments. The characteristic "butterfly" pattern, with enhanced scattering in the flow direction and a dark streak perpendicular to the flow direction, is typically observed in flow-SALS. The results suggest that the turbidity is due to a shear-induced growth of concentration fluctuations, which in turn manifests as large anisotropic domains, typically oriented along the vorticity axis.     

Short haired wormlike micelles in mixed nonionic fluorocarbon surfactants

We have studied the rheological behavior of viscoelastic wormlike micellar solution in a mixed system of nonionic fluorinated surfactants, perfluoroalkyl sulfonamide ethoxylate, C(8)F(17)SO(2)N(C(3)H(7))(CH(2)CH(2)O)(n)H abbreviated as C(8)F(17)EO(n) (n=10 and 20). Above critical micelle concentration, the surfactant, C(8)F(17)EO(20) forms small spherical micelles in water and the viscosity of the solution remains constant regardless of the shear rate, i.e., the solutions exhibit Newtonian behavior. However, upon successive addition of the C(8)F(17)EO(10) the viscosity of the solution increases and at certain C(8)F(17)EO(10) concentration, shear-thinning behavior is observed indicating the formation wormlike micelles. Contrary to what is expected, there is a viscosity increase with the addition of the hydrophilic C(8)F(17)EO(20) to C(8)F(17)EO(10) aqueous solutions at certain temperature and concentration, which could be attributed to an increase in rigidity of the surfactant layer and to the shifting of micellar branching to higher temperatures. The oscillatory-shear rheological behavior of the viscoelastic solution can be described by Maxwell model at low frequency. Small-angle X-ray scattering (SAXS) measurements confirmed the formation of small spherical micellar aggregates in the dilute aqueous C(8)F(17)EO(20) solution. The SAXS data shows the one-dimensional growth on the micellar size with increase in the C(8)F(17)EO(10) concentration. Thus, the present SAXS data supports the rheological data.     

Rheology and structure of worm-like micelles

Although worm-like micelles have been studied for over 20 years, the diversity of macroscopic behavior and the potential analogy to polyelectrolytes has driven continued study. During the last year, development and application of more realistic scattering models has yielded a deeper understanding of micelle structure. Comprehensive studies on systematic systems are being reported that combine structural and macroscopic data. These studies provide the fundamental understanding necessary to quantify the coupling between micelle structure and rheology. Finally, with increased understanding of these systems, there is a growth in the number of novel applications of worm-like micelles.     

Anisotropic viscoelasticity and molecular diffusion in nematic wormlike micelles

Using rheo-NMR, pulsed-field-gradient NMR diffusometry and solution rheology, we observe temperature- and composition-dependent orientational order as well as director dynamics and molecular transport in a nematic wormlike micelle (WLM) system composed of cetyltrimethylammonium bromide in D₂O. We measure, for the first time over a comprehensive nematic range in WLMs, four of the five independent anisotropic viscosities by fitting nonlinear rheo-NMR director realignment data and comparing with traditional solution rheology measurements. Additionally, we extract self-diffusion coefficients in three orthogonal directions for the aligned WLM unimers as well as for the D₂O solvent molecules. Continued study and enhanced understanding of complex fluid dynamics in these anisotropic shear-modulated fluid systems can lead to advances in lubricants, coatings, oil extraction, drug delivery and tissue engineering.     

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.     

Solution behavior of rod-like polyelectrolyte-surfactant aggregates polymerized from wormlike micelles

The behavior of a rod-like, water-soluble, polyelectrolyte-surfactant aggregate system (pC16TVB) in aqueous solution is characterized to determine the partitioning of surfactant in these systems and the impact on aggregate structure. These aggregates are generated by in situ polymerization of a cationic surfactant-hydrotrope wormlike micelle system. This system differs from most other polyelectrolyte-surfactant systems in that the monomer groups and the surfactant are present in ion pairs in the absence of added salts or counterions, so the stoichiometry (with respect to charge) is 1:1 for the system. Therefore, after polymerization the surfactant acts as the counterion for the polyelectrolyte chains as other counterions (salts) are not available. Despite being present in a 1:1 molar ratio, the aggregates are surprisingly stable in water (concentrations >600 mg/mL have been achieved). The conformation of the polyelectrolyte in the aggregate is analogous to the case of a polymer chain in tight confinement in a "tube" or cylindrical pore in which the pore walls are attractive--the tube is formed by the surfactant which is free to dissociate from the aggregates. A simple model for the structure and partitioning is presented and the ability to manipulate the aggregate structure is demonstrated.     

Quasi-anomalous diffusion processes in entangled solutions of wormlike surfactant micelles

In this article, we present a detailed analysis of the dynamic properties of entangled solutions of semi-flexible, threadlike surfactant micelles. These aggregates were formed by self-association processes in aqueous solutions of cationic surfactants such as cetylpyridinium chloride (CPyCl) or cetyltrimethylammonium bromide (CTAB) after the addition of different amounts of sodium salicylate (NaSal). We performed dynamic light scattering (DLS) experiments in combination with rheological measurements in order to investigate the dynamic properties of these viscoelastic surfactant solutions. In all samples, we observed three distinct relaxation regimes: initial monoexponential decay, followed by a power-law behavior at intermediate observation times. A second monoexponential region was detected at very long times, and this terminal regime described the viscoelastic features of the samples. The fast decay mode was induced by local cooperative motions in the gellike network. The intermediate and slowest decay modes point to the existence of quasi-anomalous diffusion processes. These phenomena are characterized by linear-diffusion properties at long times, and they obeyed anomalous logarithmic slow-dynamics behavior at intermediate time zones. The anomalous diffusion properties at intermediate time scales can be induced by the bending motions of the rod-shaped micelles between two entanglement points. This regime, which was more extended at lower temperatures, was described by the power-law form of the correlation function. The power-law exponent depended on the chemical structure of the surfactants and the temperature. The power-law regime shifted toward earlier times as the gellike network evolved. The slowest mode of the correlation function coincided very well with the shear stress relaxation times of the three-dimensional, transient networks. We observed that the temperature dependence of the slowest mode followed Arrhenius laws. This result provides experimental evidence for thermally activated topological relaxation processes of random fluid phases. We obtained activation energies of approximately 30 kcal/mol, and these data coincided well with previously reported literature values, which were determined in similar surfactant solutions. Characteristic "screening lengths", over which viscous effects became important, could also be determined from the activation energy. The elastic modulus G0, calculated from the slowest mode of the correlation function, was in pretty good agreement with rheological data. The light-scattering spectra were consistent with the theoretical model of dynamical coupling of the concentration fluctuations to viscoelasticity. Since only minute sample volumes are required for advanced DLS experiments, this method to extract viscoelasticity is well suited for advanced studies of gellike biomaterials.     

Strain-stiffening response in transient networks formed by reverse wormlike micelles

Strain-stiffening, that is, an increase in material stiffness at large deformations, is a property of many biological materials. Currently, model systems for the study of this phenomenon are elastic networks (gels) of semiflexible filamentous biopolymers such as actin, keratin, or fibrin. Here, we demonstrate strain-stiffening in a class of viscoelastic solutions, comprising reverse wormlike micelles. These structures are formed by the coassembly of the physiological surfactants, lecithin and bile salt, in an organic solvent, cyclohexane. In contrast to the biopolymer gels, the networks here are transient and are formed by the physical entanglement of relatively flexible worms. Our results suggest that neither a permanent network nor a high filament rigidity is required for strain-stiffening. We suggest a different origin, based on a temporary strain-induced increase in the volume fraction of entangled worms. Our system can also serve as a convenient synthetic model for future studies into this phenomenon.     

Impacting the length of wormlike micelles using mixed surfactant systems

The effect of adding an alcohol ethoxylate nonionic surfactant (C(18)E(18)) to aqueous solutions of a cationic surfactant, erucyl bis(hydroxyethyl) methylammonium chloride (EHAC,CH(3)(CH(2))(7)(CH)(2)(CH(2))(12)N(+)-(CH(2)CH(2)OH)(2)CH(3)Cl(-)), was studied using small-angle neutron scattering (SANS), steady-state rheology, and cryo-transmission electron microscopy (Cryo-TEM). This cationic surfactant has the ability to self-assemble into giant wormlike micelles in the presence of an electrolyte, such as KCl. In salt-free solutions, the mixture of the two surfactants gave rise to spherical micelles. The scattering curves obtained were fitted with a polydisperse core-shell model combined with a Hayter Penfold potential. The inner and outer radii were found to be dependent on the surfactant ratio. In the presence of KCl, mixed wormlike micelles were formed. However, further addition of C(18)E(18) promoted the breaking of the micellar worms with the appearance of a structure peak in the scattering curves. In addition, it was found that the low shear viscosity is decreased upon addition of the alcohol ethoxylate nonionic surfactant. These findings are in good qualitative agreement with the Cryo-TEM images. The results show that the addition of the nonionic surfactant to the system is a method of controlling the worm length.     

Assembling wormlike micelles in tubular nanopores by tuning surfactant-wall interactions

Threadlike molecular assemblies are excluded from narrow pores unless attractive interactions with the confining pore walls compensate for the loss of configurational entropy. Here we show that wormlike surfactant micelles can be assembled in the 8 nm tubular nanopores of SBA-15 silica by adjusting the surfactant-pore-wall interactions. The modulation of the interactions was achieved by coadsorption of a surface modifier that also provides control over the partitioning of wormlike aggregates between the bulk solution and the pore space. We anticipate that the concept of tuning the interactions with the pore wall will be applicable to a wide variety of self-assembling molecules and pores.     

Self-assembly of a hexagonal phase of wormlike micelles containing metal nanoclusters

Stable nanoclusters (approximately 2 nm in diameter) of copper, silver, gold, palladium, and ruthenium coated with hydrophobic coronas are easily trapped in self-assembled "soft crystal" hexagonal phase gels made of water and surfactants. The system's crystal structure and phase behavior are studied in detail. A partial phase diagram showing the hexagonal phase region for the water/SDS/toluene region is presented. High-energy X-ray scattering and cross-polarized optical microscopy experiments show that the clusters are tightly confined within the tubes. The thermal gel-fluid transitions of the hexagonal phase are investigated, and it is shown that the hexagonal phase can melt and recrystallize repeatedly. The melt/gel cycles enable easy trapping of various metal clusters in pre-prepared hexagonal phases. In contrast to spherical micelles, the hexagonal phase doped with metal clusters can grow without limit, basically up to the container walls (Ru-doped soft crystals grew to 0.5 mm over 2 months, forming wormlike tubes that are more than 50 microm long but only 7-10 nm in diameter).