Formation and rheology of viscoelastic "double networks" in wormlike micelle-nanoparticle mixtures

Scattering Function for Branched Wormlike Chains

Wormlike or threadlike structures with local cylindrical geometry are abundantly found in nature and technical products. A thorough structural characterization in the bulk for a whole ensemble, however, is difficult. The inherent semiordered nature of the tortuous large-scale structure and especially the quantification of branching renders an assessment difficult. In the present work we introduce a hybrid function expressing the scattering intensities for X-rays, neutrons, or light in the small-angle regime for this system. The function is termed "hybrid" because it employs terms from different approaches. The large-scale structure is described via a Guinier term as well as a concomitant power-law expression in momentum transfer q taken from the so-called unified function. The local cylindrical shape, however, is taken into account through a form factor for cylinders from rigid-body modeling. In principle, the latter form factor can be replaced by an expression for any other regular body so that the new hybrid function is a versatile tool for studying hierarchical structures assembled from uniform subunits. The appropriateness and capability of the new function for cylindrical structures is exemplified using the example of a wormlike micellar system.

Viscoelasticity of smart fluids based on wormlike surfactant micelles and oppositely charged magnetic particles

Novel viscoelastic smart suspensions based on cationic wormlike micelles (WLMs) of erucylbis(hydroxyethyl)methylammonium chloride and oppositely charged submicron magnetite particles in the presence of added low molecular weight salt were prepared and investigated. The suspensions demonstrate remarkable stability against sedimentation, which can be due to the incorporation of particles into the network of entangled WLMs by linking to energetically unfavorable micellar end-caps. Added particles enhance significantly the viscosity, the plateau modulus, and the relaxation time of the system, acting as additional multifunctional physical cross-links in the micellar network. The increase of plateau modulus stops when the concentration of particles reaches ca. 1.5 wt %, indicating that all micellar end-caps available in the system are linked to the particles. Further addition of particles may lead just to the redistribution of micellar ends between the particles without creation of new elastically active chains. The increase of rheological characteristics by added particles is more pronounced in suspensions with a smaller content of low molecular weight salt KCl when the WLMs are shorter in length and therefore contain a larger amount of end-caps responsible for the interaction with the particles. Magnetite particles not only enhance the rheological characteristics but also impart magnetoresponsive properties to the suspension. Upon application of magnetic field, the liquidlike system transforms into a solidlike one, demonstrating a constant value of storage modulus in all frequency range and the appearance of yield stress, which is due to the formation of field-aligned chainlike aggregates of particles opposing the flow. A combination of responsive properties inherent to both the matrix and the particles makes these smart fluids very competitive with other magnetic soft matter materials for various applications.

Hydrogel formed by the co-assembly of sodium laurate and silica nanoparticles

Fibers formed by the co-assembly of sodium laurate and silica nanoparticles.

Online rheological investigation on ion-induced micelle transition for amphiphilic polystyrene-block-poly(acrylic acid) diblock copolymer in dilute solution

The ion-induced micellar transition is online-investigated by the time dependence of the viscosity of the solution under shear flow for the first time. During the morphological transition, the change in the micellar structure can be tracked by the change in viscosity. Adding HCl or CaCl2 into pre-prepared spherical micelle solution from the self-assembly of polystyrene-block-poly(acrylic acid) (PS144-b-PAA22) in the N,N-dimethylformamide (DMF)/water mixture, the micellar structures change into short cylinders, long, entangled cylinders, and then lamellae or vesicles, corresponding to the viscosity increasing first and then declining. When HCl or CaCl2 is added to the pre-prepared spherical micelle solution formed by PS144-b-PAA50 in the dioxane/water mixture, the micellar structures are quickly transformed into cylinders or lamellae before carrying out the rheological measurement and then are turned to vesicles or spheres under the shearing, corresponding to a gradual decline in viscosity. This study shows that the rheology can be a very simple and effective online method on the investigation of the micellization, which plays an important role in understanding the micellization mechanism and micellar transition pathway of block copolymers in dilute solution.

Comparative study on coating CdSe nanocrystals with surfactants

We have synthesized CdSe nanocrystals (NCs) in sizes from 2.2 to 5.1 nm passivated with hydrophobic trioctylphosphine oxide (TOPO) in combination trioctylphosphine (TOP) or tributylphosphine (TBP) to obtain particles of the type CdSe/TOPO/TOP or CdSe/TOPO/TBP. These NCs were then dispersed in aqueous solution of ionic or non-ionic surfactants (such as stearate, oleic acid, Tween) using a biphase (water and chloroform or hexane) transfer method. It is found that both the structure of the surfactant and the native surface of the ligand govern the coating of the NCs with surfactants. More specifically, the hydrophobicity-hydrophilicity balance of the surfactant regulates the coating efficacy, thereby transferring the NC from the organic to the aqueous phase. The type of ligand on the NCs and the kind of coating surfactant also affect photoluminescence (PL). The ratio of PL and absorbance unit (defined as PL per 0.1 AU) was implemented as a tool to monitor changes in PL intensity and wavelength as a function of size, coatings and surface defects. Finally, the distribution of CdSe nanocrystals between pseudophases in cloud point extraction was discussed based on experimental results. It was concluded that the size of CdSe nanocrystal present in an appropriate pseudophase is correlated with the way in which the non-ionic surfactant coats CdSe nanocrystals. FigureCoating of CdSe semiconductor nanocrystals with surfactants impacts nanocrystals' spectral features. Absorbance of first exciton absorption band was used to estimate ability of surfactant to disperse CdSe nanocrystals. Photoluminescence (PL) intensity and position of PL band were analysed in terms of nanocrystal's surface phenomena via surfactants applied for coating.

Phase behavior and molecular thermodynamics of coacervation in oppositely charged polyelectrolyte/surfactant systems: a cationic polymer JR 400 and anionic surfactant SDS mixture

Coacervation in mixtures of polyelectrolytes and surfactants with opposite charge is common in nature and is also technologically important to consumer health care products. To understand the complexation behavior of these systems better, we combine multiple experimental techniques to systematically study the polymer/surfactant binding interactions and the phase behavior of anionic sodium dodecyl sulfate (SDS) surfactant in cationic JR 400 polymer aqueous solutions. The phase-behavior study resolves a discrepancy in the literature by identifying a metastable phase between the differing redissolution phase boundaries reported in the literature for the surfactant-rich regime. Isothermal titration calorimetry analyzed within the framework of the simple Satake-Yang model identifies binding parameters for the surfactant-lean phase, whereas a calculation for polymer-bound micelles coexisting with free micelles is analyzed in the surfactant-rich redissolution regime. This analysis provides a preliminary understanding of the interactions governing the observed phase behavior. The resulting thermodynamic properties, including binding constants and the molar Gibbs free energies, enthalpies, and entropies, identify the relative importance of both hydrophobic and electrostatic interactions and provide a first approximation for the corresponding microstructures in the different phases. Our study also addresses the stability and metastability of oppositely charged polyelectrolytes and surfactant mixtures.

Wormlike micelle assisted rod coating: a general method for facile fabrication of large-area conductive nanomaterial thin layer onto flexible plastics

Through combined application of wormlike-micelle and rod-coating technique, a general method was demonstrated for the facile reparation of thin transparent conductive films (TCF) of various nanomaterials and their hybrids on flexible plastics. The cetyltrimethylammonium hydroxide (CTAOH)/p-toluenesulfonic acid (CTAT) wormlike micelle system was selected for both the dispersion of different nanomaterials and the enhancement of viscosities of the coating fluids. With the single-walled carbon nanotubes (SWNTs)/wormlike micelle aqueous dispersions as coating fluid, TCFs of SWNTs on flexible poly(ethylene terephthalate) (PET) substrates made by rod-coating method were demonstrated. After doping by immersion into thionyl chloride solution, the sheet resistance of SWNTs thin films, which had a transmittance of about 78%, was as low as 480Ω/sq. This coating method was extended to the preparation of thin films or networks of other materials such as reduced graphene oxide and Ag nanowires. The obtained TCF from Ag nanowire networks has a low sheet resistance of 17Ω/sq, which is comparable to the value of best indium tin oxide (ITO) coating on plastic substrates. Finally, hybrid thin films of different nanomaterials were demonstrated by this method.

Mesoporous organohydrogels from thermogelling photocrosslinkable nanoemulsions

We report the formation of mesoporous organohydrogels from oil-in-water nanoemulsions containing an end-functionalized oligomeric gelator in the aqueous phase. The nanoemulsions exhibit an abrupt thermoreversible transition from a low-viscosity liquid to a fractal-like colloidal gel of droplets with mesoscale porosity and solid-like viscoelasticity with moduli approaching 100 kPa, possibly the highest reported for an emulsion-based system. We hypothesize that gelation is brought about by temperature-induced interdroplet bridging of the gelator, as shown by its dependence on the gelator chemistry. The use of photocrosslinkable gelators enables the freezing of the nanoemulsion's microstructure into a soft hydrogel nanocomposite containing a large fraction of dispersed liquid hydrophobic compartments, and we show its use in the encapsulation and release of lipophilic biomolecules. The tunable structural, mechanical and optical properties of these organohydrogels make them a robust material platform suitable for a wide range of applications.

Coarse-grained molecular dynamics simulations of the sphere to rod transition in surfactant micelles

Surfactant molecules self-assemble in aqueous solutions to form various micellar structures such as spheres, rods, or lamellae. Although phase transitions in surfactant solutions have been studied experimentally, their molecular mechanisms are still not well understood. In this work, we show that molecular dynamics (MD) simulations using the coarse-grained (CG) MARTINI force field and explicit CG solvent, validated against atomistic MD studies, can accurately represent micellar assemblies of cetyltrimethylammonium chloride (CTAC). The effect of salt on micellar structures is studied for aromatic anionic salts, e.g., sodium salicylate, and simple inorganic salts, e.g., sodium chloride. Above a threshold concentration, sodium salicylate induces a sphere to rod transition in the micelle. CG MD simulations are shown to capture the dynamics of this shape transition and support a mechanism based on the reduction in the micelle-water interfacial tension induced by the adsorption of the amphiphilic salicylate ions. At the threshold salt concentration, the interface is nearly saturated with adsorbed salicylate ions. Predictions of the effect of salt on the micelle structure in different CG solvent models, namely, single-site standard water and three-site polarizable water, show qualitative agreement. This suggests that phase transitions in aqueous micelle solutions could be investigated by using standard CG water models which allow for 3 orders of magnitude reduction in the computational time as compared to that required for atomistic MD simulations.