Wormlike Surfactant Micelles Research Articles

Metal-Driven Viscoelastic Wormlike Micelle in Anionic/Zwitterionic Surfactant Systems and Template-Directed Synthesis of Dendritic Silver Nanostructures

In this work, metal ion-induced viscoelastic wormlike micelles in anionic/zwitterionic surfactant solutions (sodium dodecylsulfate/tetradecyldimethylammoniumpropanesulfonate, SDS/TPS) are reported. Steady and dynamic rheology and cryogenic transmission electron microscopy (cryo-TEM) are employed to characterize wormlike micelles in the SDS/TPS/Ca(NO(3))(2) system. Moreover, the surfactant mixing ratio and surfactant tail length are varied to reveal the factors that influence wormlike micelle growth and solution viscoelasticity. A series of metal ions such as Na(+), Mg(2+), Zn(2+), and Al(3+) are proven to promote viscoelastic wormlike micelle formation in the SDS/TPS system. The metal-containing wormlike micelles are expected to be good candidates for directing the synthesis of inorganic nanomaterials. In this article, dendritic silver nanostructures have been prepared in the surfactant wormlike micelle by in situ UV irradiation for the first time.

Aging mechanism of unsaturated long-chain amidosulfobetaine worm fluids at high temperature

Both fundamental and technological research interests have highlighted in recent years a remarkable growing focus on surfactant wormlike micelles. Nevertheless, long-term thermal stability, as well as the corresponding aging mechanism of viscoelastic worm fluids was rarely examined. We report for the first time the high-temperature aging effect on wormlike micellar solutions formed by an unsaturated C22-tailed amidosulfobetaine surfactant in the absence and presence of a reducer, by means of rheological methods, cryo-TEM observations, IR and 1H NMR measurements. It was found that the aging effect mainly resulted from the degradation of the carbon–carbon double bond in the surfactant tail rather than its amido head group; and the addition of a small amount of reducing agent could effectively improve thermal stability of the worm fluids.

Rheological and Tribological Properties of Ionic Liquid-Based Nanofluids Containing Functionalized Multi-Walled Carbon Nanotubes

Room temperature ionic liquid-based nanofluids containing functionalized multi-walled carbon nanotubes (F-MWCNTs) were fabricated. Transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM), Fourier transform infrared (FT-IR), and X-ray photoelectron spectra revealed the morphology and chemical structure of the obtained F-MWCNTs. The rheological behaviors of the F-MWCNTs/1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF6]) nanofluids were studied, demonstrating the shear thinning behavior of nanofluids at rather low concentrations, which could be due to the high specific ratio, flexibility of F-MWCNTs, and the formation of a transient network through nanotube−nanotube and nanotube−matrix interactions. It was found that the rheological behavior of the F-MWCNTs (0.1 wt %)/[Bmim][PF6] nanofluid was similar to that of the surfactant worm-like micelle systems. Therefore, the transient network structure of the F-MWCNTs (0.1 wt %)/[Bmim][PF6] nanofluid could be renovate...

Flexibility and Cross-Sectional Structure of an Anionic Dual-Surfactant Wormlike Micelle Explored with Small-Angle X-ray Scattering Coupled with Contrast Variation Technique

The contrast variation technique by adding sucrose to aqueous solvents as an electron density adjusting reagent was applied to small-angle X-ray scattering (SAXS) from a dual surfactant wormlike micelle consisting of sodium lauryl ether sulfate and coconut fatty acid amido propyl betaine in order to evaluate the flexibility and the cross-sectional structure of the micelle. The salt concentration dependence of the zero-shear and dynamic viscosities were found not to be affected by the cation species nor the presence of sucrose in the solution. SAXS showed that the scattering profile [I(q)] can be fitted a double layer cylinder model. By systematically changing the sucrose concentration (CS), a "matching composition" was found in which the shell layer can become invisible in SAXS for each sodium cation concentration ([Na+]). From this composition, the electron densities of the shell (rhoS) and core (rhoC) layers were evaluated to be 370 and 216 e/nm(-3), respectively. These values were consistent with the chemical structure of the surfactant. At these matching compositions, I(q) was measured at low q range (0.03 nm(-1)<q<4 nm(-1)) and the data points were perfectly fitted with a single layer rigid cylinder model, except for [Na+]>1.02 M. In these larger [Na+], the I(q) showed an up-turn deviation from the rigid rod limit (i.e., I(q) proportional, variant q(-1)) at low q, indicating appearance of the flexibility of the cylinder. By applying a wormlike cylinder theory, the Kuhn statistical segment length was evaluated as a function of [Na+]. At the infinite limit of [Na+], the intrinsic Kuhn length (excluding of the electrostatic interactions) can be evaluated to be about 20-40 nm. This value was comparable with that of the wormlike micelles made of nonionic surfactants.

The effect of step-stretch parameters on capillary breakup extensional rheology (CaBER) measurements

Extensional rheometry has only recently been developed into a commercially available tool with the introduction of the capillary breakup extensional rheometer (CaBER). CaBER is currently being used to measure the transient extensional viscosity evolution of mid to low-viscosity viscoelastic fluids. The elegance of capillary breakup extensional experiments lies in the simplicity of the procedure. An initial step-stretch is applied to generate a fluid filament. What follows is a self-driven uniaxial extensional flow in which surface tension is balanced by the extensional stresses resulting from the capillary thinning of the liquid bridge. In this paper, we describe the results from a series of experiments in which the step-stretch parameters of final length, and the extension rate of the stretch were varied and their effects on the measured extensional viscosity and extensional relaxation time were recorded. To focus on the parameter effects, well-characterized surfactant wormlike micelle solutions, polymer solutions, and immiscible polymer blends were used to include a range of characteristic relaxation times and morphologies. Our experimental results demonstrate a strong dependence of extensional rheology on step-stretch conditions for both wormlike micelle solutions and immiscible polymer blends. Both the extensional viscosity and extensional relaxation time of the wormlike micelle solutions were found to decrease with increasing extension rate and strain of the step-stretch. For the case of the immiscible polymer blends, fast step-stretches were found to result in droplet deformation and an overshoot in the extensional viscosity which increased with increasing strain rates. Conversely, the polymer solutions tested were found to be insensitive to step-stretch parameters. In addition, numerical simulations were performed using the appropriate constitutive models to assist in both the interpretation of the CaBER results and the optimization of the experimental protocol. From our results, it is clear that any rheological results obtained using the CaBER technique must be properly considered in the context of the stretch parameters and the effects that preconditioning has on viscoelastic fluids.

Nonlinear Rheology of Surfactant Wormlike Micelles Bridged by Telechelic Polymers

We have investigated the nonlinear rheology of a soft composite transient network made of a solution of surfactant wormlike micelles (WM) in the semidilute regime that are reversibly bridged by telechelic polymers. The samples are well described, in the linear regime, as two Maxwell fluids components blends, characterized by two markedly different characteristic times. The slow mode is mainly related to the transient network of entangled WM, and the fast mode to the network of telechelic chains. In this paper we investigate the nonlinear viscoelasticity and show that the nonlinear behavior reflects as well the behavior of two coupled networks. On one hand, stress relaxation experiments and time-resolved stress response following the application of a constant shear rate show that, in the weakly nonlinear regime, these novel composite networks stiffen. A fourfold increase of the elastic modulus with respect to the linear value is reached for strain amplitude of about 200%. This strain hardening is due to the nonlinear stretching of the telechelic polymer chains. On the other hand, the samples exhibit shear banding in the highly nonlinear regime, similarly to pure semidilute solutions of WM.

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.

Structure Characterization of Surfactant Assisted Polymer Thickeners by Silica Nanocasting

The aggregation behavior of the hydrophobically modified polyelectrolytes which act in the interplay with special surfactant mixtures as a thickener is examined by replication of the different soft matter nanostructures into silica gels. The inorganic solidified replicas can be easily microtomed and analyzed by gas sorption measurements, quantitative small angle X-ray scattering and transmission electron microscopy. The bare hydrophobically modified polyelectrolytes show a very homogeneous structure, free of larger aggregated moieties. In addition, a transition between a weakly- or unaggregated structure and an aggregated structure of the bare copolymer with hydrophobic moieties of ca. 3 nm diameter was found which depends on the degree of hydrophobic substitution, the concentration, and the salt content. The simultaneous presence of surfactant in the polymer solution leads to nanocasting of both spherical (ionic surfactant) and worm-like (zwitterionic surfactant) micelles, which are essentially unperturbed by the incorporate the hydrophobic moieties of the polymer. Optimal thickening efficiency is obtained when each of those micelles is coupled to at least two of the polymers. The resulting sterical demands are best met by the presence of worm-like surfactant micelles.

DNA capillary electrophoresis in entangled dynamic polymers of surfactant molecules.

Aqueous solutions of monomeric nonionic surfactants, n-alkyl polyoxyethylene ethers (C16E6, C16E8, C14E6), can be used as sieving matrixes for the separation of DNA fragments by capillary electrophoresis. Unlike ordinary polymer solutions, these surfactant solutions behave as dynamic polymers. By combining the "reversible gel" theory of DNA electrophoresis and the static and dynamic properties of wormlike surfactant micelles, a model is developed for describing the migration behavior of DNA molecules in these solutions. According to the model, the separation limit can be extended at low surfactant concentrations. Surfactant solutions as a separation medium provide many advantages over ordinary polymers, such as ease of preparation, solution homogeneity, stable structure, low viscosity, and self-coating property for reducing electroosmotic flow. More importantly, the properties of wormlike micelles (micelle size, entanglement concentration) can be adjusted by simply changing the monomer concentration, denaturant, and temperature to allow the separation of different size ranges of DNA fragments. Fast separation is achieved for DNA fragments ranging from 10 bp to 5 kb by using bare fused-silica columns. DNA sequencing fragments of BigDye G-labeled M13 up to 600 bases were separated within 60 min.

Inhomogeneous shear flows of wormlike micelles:mA master dynamic phase diagram

We report on the nonlinear mechanical response of surfactant wormlike micelles subjected to steady shear flow. The system placed under scrutiny is made of cetylpyridinium chloride, sodium salicylate, and salted water at 0.5M NaCl (abbreviated as CPCl-Sal). We have investigated solutions in the intermediate concentration ranges, at \ensuremath{\varphi}=6%, 8% and \ensuremath{\varphi}=12% for temperatures ranging between 20 \ifmmode^\circ\else\textdegree\fi{}C and 50 \ifmmode^\circ\else\textdegree\fi{}C. In these T and \ensuremath{\varphi} ranges, we first confirm that the present system is a true Maxwell fluid. As a consequence, the nonlinear rheology can be formulated in terms of the normalized quantities, \ensuremath{\sigma}=\ensuremath{\sigma}/${\mathrm{G}}_{0}$ and \ensuremath{\gamma}-dot=\ensuremath{\gamma}-dot${\mathrm{\ensuremath{\tau}}}_{\mathrm{R}}$ , where ${\mathrm{G}}_{0}$ is the elastic plateau modulus and ${\mathrm{\ensuremath{\tau}}}_{\mathrm{R}}$ the terminal relaxation time of the Maxwell fluid. Second, we demonstrate that the flow curves of the CPCl-Sal wormlike micelles \ensuremath{\sigma} (\ensuremath{\gamma}-dot) are invariant with respect to relative changes in temperature and concentration. This enables us to define superimposition procedures between flow curves obtained at different temperatures and concentrations and to derive the so-called 'master dynamic phase diagram' of the CPCl-Sal wormlike micelles. One crucial feature of this master phase diagram is the existence of a critical behavior at T=${\mathrm{T}}_{\mathrm{c}}$ . Here, the stress \ensuremath{\sigma} levels off progressively up to a plateau that is reduced to a single flat point of coordinates (\ensuremath{\sigma}=0.9, \ensuremath{\gamma}-dot=3). The low-temperature regime (T${\mathrm{T}}_{\mathrm{c}}$ ) is again identified by a shear stress plateau beginning by a true discontinuity of slope in the \ensuremath{\sigma} (\ensuremath{\gamma}-dot) behavior. At high T (&gt;${\mathrm{T}}_{\mathrm{c}}$ ), \ensuremath{\sigma} increases monotonously at all shear rates. In order to examine the state of shearing at the stress plateau, flow birefringence experiments were undertaken with a rheo-optical device working with polarized light propagating parallel to the vorticity axis of the Couette. In the plateau regime, the flow is clearly inhomogeneous. Within the gap of a cell, two phases of very different birefringence are observed in the steady state of shearing, as well as after cessation of the shearing.

End-evaporation kinetics in living-polymer systems

We study theoretically the process of ‘‘end-evaporation’’ in living polymer systems, such as wormlike surfactant micelles. End-evaporation occurs when single monomers either break away from, or join onto, a chain end, the rates being described by the (mean-field) rate constants k and k′, respectively. Thus the chains can exchange material with one-another via a bath of free monomers. The relaxation of a system of living polymers after a small temperature jump (T-jump) is studied theoretically. The effect of a T-jump is to prepare the system with the wrong mean chain length, which relaxes to its equilibrium value L̄ by end-evaporation. It is found that the number of free monomers in the system relaxes almost completely in a time of order 1/kL̄, while the weight-average chain length, which is the quantity measured in light scattering experiments, relaxes on a time scale τD=4L̄2/k, which is three powers of L̄ longer. We also predict that the stress relaxation after a step strain is dominated by end-evaporation whenever τD≲τrep, where τrep is the reptation (disengagement) time for a chain of length L̄. In this case the stress relaxation is found to be ‘‘stretched exponential’’ for times smaller than τD and single exponential for longer times.