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Knowledge of the nanoscale distribution of proteins in chromatographic resins is critical to our mechanistic understanding of separations performance. However, the nano- to mesoscale architecture of these materials is challenging to characterize using conventional techniques. Small-angle neutron scattering was used to probe (1) the nano- to mesoscale structure of chromatographic media and (2) protein sorption in these media in situ with protein-scale resolution. In particular, we characterize the effect of the architecture of cellulose-based and traditional and dextran-modified agarose-based ion-exchange resins on the nanoscale distribution of a relatively small protein (lysozyme) and two larger proteins (lactoferrin and a monoclonal antibody) at different protein loadings. Traditional agarose-based resins (SP Sepharose FF) can be envisioned as comprising long, thin strands of helical resin material around which the proteins adsorb, while higher static capacities are achieved in dextran-modified resins (SP Sepharose XL and Capto S) due to protein partitioning into the increased effective binding volume provided by the dextran. While protein size is shown not to affect the underlying sorption behavior in agarose-based resins such as SP Sepharose FF and XL, it plays an important role in the cellulose-based S HyperCel and the more highly cross-linked agarose-based Capto S, where size-exclusion effects prevent larger proteins from binding to the base matrix resin strands. Based on the data, we propose that entropic partitioning effects such as depletion forces may drive the observed protein crowding. In general, these observations elucidate the structure and point to the mechanism of protein partitioning in different classes of chromatographic materials, providing guidance for optimizing their performance. [ABSTRACT FROM AUTHOR]

The microstructure of mixtures of deuterated water in the ionic liquid [C4mim][BF4] is investigated by small-angle neutron scattering (SANS) measurement. In the salt-rich region, water dissolves in the ionic liquid up to 0.7 mole fraction, whereupon distinct, nanometer-sized water clusters are observed. These water nanoclusters increase in size with increasing water addition while the mole ratio of water dissolved into the ionic liquid nanostructure increases from 2 to 4. These results provide direct confirmation for recent simulations as well insight into the source of nonidealities in some thermophysical and transport properties (e.g., density and viscosity) of salt-rich aqueous mixtures reported in the literature. [ABSTRACT FROM AUTHOR]

Oppositely charged polyelectrolyte-surfactant mixtures are ubiquitous in biology and the basis of numerous consumer healthcare products. Despite their broad use, however, a rational approach to their formulation remains challenging because of the complicated association mechanisms. Through compilation and analysis of literature reports and our own research, we have developed a semiempirical correlation of the binding strength of surfactants to polyelectrolytes in salt-free mixtures as a function of the polyion linear charge density and the surfactant hydrophobicity. We have found that the cooperative binding strength increases as the square of the polyelectrolyte's linear charge density and in proportion to the surfactant's hydrophobicity, such that a quantitative relationship holds across a broad range of polyelectrolytes. Deviations from the correlation reveal the role of system-specific interactions not considered in the analysis. This engineering relationship aids in the rational design of oppositely charged polyelectrolyte-surfactant formulations for consumer products and biomedicines by enabling the prediction of binding strengths in polyelectrolyte-surfactant mixtures based on mesoscale parameters determined from the chemical composition. [ABSTRACT FROM AUTHOR]

Micellar solutions of hexadecyltrimethylammonium bromide(CTAB)in a protic ionic liquid, ethylammonium nitrate (EAN), are studiedby shear rheology, polarizing optical microscopy (POM), conductivitymeasurements, and small angle neutron scattering (SANS). Three concentrationregimes are examined: A dilute regime (with concentrations [CTAB]< 5 wt %) consisting of noninteracting spherical micelles, a semidiluteregime (5 wt % ≤ [CTAB] ≤ 45 wt %) where micelles interactvia electrostatic repulsions, and a concentrated regime (45 wt % <[CTAB] ≤ 62 wt %) where a reversible, temperature-dependentisotropic (L1) to hexatic (Hex) phase transition is observed.The L1–Hex transition, which has been predictedbut not previously observed, is characterized by (1) a sharp increasein the shear viscosity, (2) the formation of focal conical birefringencetextures (observed by POM), and (3) enhancement of the crystallineorder, evidenced by the appearance of Bragg reflections in the SANSprofiles. Ionic conductivity is not sensitive to the L1–Hex transition, which corroborates the absence of topologicaltransitions. [ABSTRACT FROM AUTHOR]

In this Article, we report the dielectrophoretic assemblyof colloidalparticles and show how the kinetics of assembly and degree of orderingdepend on the particle size, charge, solution ionic strength, andfield strength and frequency. A special dielectrophoresis (DEP) samplecell is constructed and validated to quantitatively measure directedself-assembly via sequential light scattering and optical microscopymeasurements. Our results confirm the recently established scalingfor the order–disorder transition and extend it to higher scaledfrequencies. The limiting scaling of the order–disorder transitionand particle electrophoretic mobility are correctly predicted by thestandard electrokinetic model (SEKM). In particular, the order–disordertransition line is predicted from the particle properties using arecently proposed empirical scaling law and the SEKM over an orderof magnitude in particle size. [ABSTRACT FROM AUTHOR]

Polysorbate 80 (PS80), a nonionic surfactant used in pharmaceutical formulation, is known to be incompatible with m -cresol, an antimicrobial agent for multi-dose injectable formulations. This incompatibility results in increased turbidity caused by micelle aggregation progressing over weeks or longer, where storage temperature, ionic strength, and component concentration influence the aggregation kinetics. Small-angle neutron scattering (SANS) analysis of PS80/ m -cresol solutions over a pharmaceutically relevant concentration range of each component reveals the cause of aggregation, the coalescence mechanism, and aggregate structure. PS80 solutions containing m -cresol concentrations below ≈2.0 mg/mL and above ≈4.5 mg/mL are kinetically stable and do not aggregate over a 50 h period. At 5 mg/mL of m -cresol, the mixture forms a kinetically stable microemulsion phase, despite being well below the aqueous solubility limit of m -cresol. Solutions containing intermediate m -cresol concentrations (2.0-4.5 mg/mL) are unstable, resulting in aggregation, coalescence, and eventual phase separation. In unstable solutions, two stages of aggregate growth (nucleation and power-law growth) are observed at m -cresol concentrations at or below ≈3.6 mg/mL. At higher m -cresol concentrations, aggregates experience a third stage of exponential growth. A single kinetic model is developed to explain the stages of aggregate growth observed in both kinetic mechanisms. This work establishes the phase diagram of PS80/ m -cresol solution stability and identifies component concentrations necessary for producing stable formulations.

An ultrastretchable iono-elastomer with resistance sensitive to both elongation strain and temperature has been developed by hierarchical self-assembly of an end functionalized triblock copolymer in a protic ionic liquid (ethylammonium nitrate) followed by cross-linking. Small-angle X-ray scattering experiments in situ with uniaxial elongation reveal a nanoscale microstructural transition of the hierarchically self-assembled cross-linked micelles that is responsible for the material's remarkable mechanical and ionic conductivity responses. The results show that the intermicelle distance extends along the deformation direction while the micelles organize into a long-range ordered face-centered-cubic structure during the uniaxial elongation. Besides good cyclability and resistance to selected physical damage, the iono-elastomer simultaneously achieves an unprecedented combination of high stretchability (340%), highly linear resistance vs elongation strain ( R 2 = 0.998), and large temperature gauge factor (Δ R/ R = 3.24%/°C@30 °C). Human subject testing demonstrates that the iono-elastomer-based wearable thermomechanical sensor is able to effectively and accurately register both body motion and skin temperature simultaneously.

Topological differences in a model series of wormlike micelles (WLMs) with varying degrees of branching are investigated via neutron spin echo (NSE) and dynamic light scattering (DLS). Branching was previously verified via microscopy, scattering methods, and rheology. NSE measurements reveal that on the shortest length scales, where WLMs are topologically similar, the dynamic behavior is independent of branching level. However, branching leads to deviations from wormlike chain behavior on length scales relevant to branching. These differences continue into the DLS regime, where two relaxation modes are observed. While measuring micellar branching is challenging, our measurements of segmental dynamics provide a direct dynamic measurement of WLM branching and demonstrate promise for future identification of topological changes in self-assembled materials.

Protein-stabilizer microheterogeneity is believed to influence long-term protein stability in solid-state biopharmaceutical formulations and its characterization is therefore essential for the rational design of stable formulations. However, the spatial distribution of the protein and the stabilizer in a solid-state formulation is, in general, difficult to characterize because of the lack of a functional, simple, and reliable characterization technique. We demonstrate the use of confocal fluorescence microscopy with fluorescently labeled monoclonal antibodies (mAbs) and antibody fragments (Fabs) to directly visualize three-dimensional particle morphologies and protein distributions in dried biopharmaceutical formulations, without restrictions on processing conditions or the need for extensive data analysis. While industrially relevant lyophilization procedures of a model IgG1 mAb generally lead to uniform protein-excipient distribution, the method shows that specific spray-drying conditions lead to distinct protein-excipient segregation. Therefore, this method can enable more definitive optimization of formulation conditions than has previously been possible.

The emerging technologies involving wearable electronics require new materials with high stretchability, resistance to high loads, and high conductivities. We report a facile synthetic strategy based on self-assembly of concentrated solutions of end-functionalized PEO 106 -PPO 70 -PEO 106 triblock copolymer in ethylammonium nitrate into face-centered cubic micellar crystals, followed by micelle corona cross-linking to generate elastomeric ion gels (iono-elastomers). These materials exhibit an unprecedented combination of high stretchability, high ionic conductivity, and mechanoelectrical response. The latter consists of a remarkable and counterintuitive increase in ion conductivity with strain during uniaxial extension, which is reversible upon load release. Based on in situ SAXS measurements of reversible crystal structure transformations during deformation, we postulate that the origin of the conductivity increase is a reversible formation of ion nanochannels due to a novel microstructural rearrangement specific to this material.

Slurry based electrodes have shown promise as an energy dense and scalable storage technology for electrochemical flow batteries. Key to their efficient operation is the use of a conductive additive which allows for volumetric charging and discharging of the electrochemically active species contained within the electrodes. Carbon black is commonly used for this purpose due to the relatively low concentrations needed to maintain electrical percolation. While carbon black supplies the desirable electrical properties for the application, it contributes detrimentally to the rheology characteristics of these concentrated suspensions. In this work, we develop a synthesis protocol to produce inorganic oxide particles with electrostatically adsorbed poly(3,4-ethylenedioxithiophene):polystyrenesulfonate (, Pedot: PSS). Using a combination of small angle neutron scattering (SANS), electron microscopy, and thin-film conductivity, we show that the synthesis scheme provides a flexible platform to form conductive, Pedot: PSS-SiO2 nanoparticle dispersions. Based on these measurements, we demonstrate that these particles are stable when dispersed in propylene carbonate. Using a combination of rheology and dielectric spectroscopy, we show that these stable dispersions facilitate electrical percolation at concentrations below their mechanical percolation threshold, and this percolation is maintained under flow. These results demonstrate the potential for strategies which seek to decouple mechanical and electrical percolation to allow for the development of higher performance conductive additives for slurry based flow batteries.

The microstructure of mixtures of deuterated water in the ionic liquid [C4mim][BF4] is investigated by small-angle neutron scattering (SANS) measurement. In the salt-rich region, water dissolves in the ionic liquid up to 0.7 mole fraction, whereupon distinct, nanometer-sized water clusters are observed. These water nanoclusters increase in size with increasing water addition while the mole ratio of water dissolved into the ionic liquid nanostructure increases from 2 to 4. These results provide direct confirmation for recent simulations as well insight into the source of nonidealities in some thermophysical and transport properties (e.g., density and viscosity) of salt-rich aqueous mixtures reported in the literature.

The critical role of solvation forces in dispersing and stabilizing nanoparticles and colloids in 1-butyl-3-methylimidazolium tetrafluoroborate [C4mim][BF4] is demonstrated. Stable silica nanoparticle suspensions over 60 wt % solids are achieved by particle surface chemical functionalization with a fluorinated alcohol. A combination of techniques including rheology, dynamic light scattering (DLS), transmission electron microscopy (TEM), and small angle neutron scattering (SANS) are employed to determine the mechanism of colloidal stability. Solvation layers of ∼5 nm at room temperature are measured by multiple techniques and are thought to be initiated by hydrogen bonds between the anion [BF4](-) and the fluorinated group on the surface coating. Inducing structured solvation layering at particle surfaces through hydrogen bonding is demonstrated as a method to stabilize particles in ionic liquids.

The formation of multilamellar vesicles (MLVs) from the lamellar phase of nonionic surfactant system C12E5/D2O under shear flow is studied by time-resolved small angle neutron and light scattering during shear flow. A novel small angle neutron scattering sample environment enables the tracking of the lamellae alignment in the velocity-velocity gradient (1-2) plane during MLV formation, which was tracked independently using flow small angle light scattering commensurate with rheology. During the lamellar-to-multilamellar vesicle transition, the primary Bragg peak from the lamellar ordering was observed to tilt, and this gradually increased with time, leading to an anisotropic pattern with a primary axis oriented at ∼25° relative to the flow direction. This distorted pattern persists under flow after MLV formation. A critical strain and critical capillary number based on the MLV viscosity are demonstrated for MLV formation, which is shown to be robust for other systems as well. These novel measurements provide fundamentally new information about the flow orientation of lamellae in the plane of flow that cannot be anticipated from the large body of previous literature showing nearly isotropic orientation in the 2,3 and 1,3 planes of flow. These observations are consistent with models for buckling-induced MLV formation but suggest that the instability is three-dimensional, thereby identifying the mechanism of MLV formation in simple shear flow.

We measure the spatially resolved microstructure and concentration in the plane of flow for a viscoelastic solution of polymer-like micelles comprised of mass fraction 6.0% (volume fraction 6.6%) solution of 2:1 molar ratio cetylpyridinium chloride/sodium salicylate in 0.5 mol/L NaCl/D 2 O through the shear banding transition. Spatially resolved flow small-angle neutron scattering measurements in the velocity-velocity gradient (1-2) plane of flow establish the local microstructure, and scanning narrow-aperture flow ultrasmall-angle neutron scattering (SNAFUSANS) measurements indicate no flow-induced concentration gradients within measurement accuracy. These results show shear banding in this solution is not associated with an isotropic-nematic transition and are fundamentally important for validating models of shear-banding complex fluids. Improvements in the SNAFUSANS method are also documented.

Oppositely charged polyelectrolyte-surfactant mixtures are ubiquitous in biology and the basis of numerous consumer healthcare products. Despite their broad use, however, a rational approach to their formulation remains challenging because of the complicated association mechanisms. Through compilation and analysis of literature reports and our own research, we have developed a semiempirical correlation of the binding strength of surfactants to polyelectrolytes in salt-free mixtures as a function of the polyion linear charge density and the surfactant hydrophobicity. We have found that the cooperative binding strength increases as the square of the polyelectrolyte's linear charge density and in proportion to the surfactant's hydrophobicity, such that a quantitative relationship holds across a broad range of polyelectrolytes. Deviations from the correlation reveal the role of system-specific interactions not considered in the analysis. This engineering relationship aids in the rational design of oppositely charged polyelectrolyte-surfactant formulations for consumer products and biomedicines by enabling the prediction of binding strengths in polyelectrolyte-surfactant mixtures based on mesoscale parameters determined from the chemical composition.

Photopolymerization on a thiol-ene elastomer is a novel method to spatiotemporally control wrinkle formation gradients and complex patterns. Acrylate-rich elastomers, containing photoinitiator and photoabsorber, were strained and irradiated with UV light to generate wrinkles in as little as five seconds. The presence of the photoabsorber confines the UV light to a thin skin layer, facilitating wrinkle formation upon release of the strain. In conjunction with photomasks, wrinkles are confined and oriented with explicit boundaries defined by the irradiation pattern. Using a photoorthogonal photoiniator, we also demonstrate the ability to independently control the bulk modulus of the wrinkled elastomer. The wide array of thiol and ene monomers, photoinitiators, and photomask designs result in an alternative approach that is rapid, facile, and highly versatile, further enabling the a priori design and engineering of surface topography and shape of the material.

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.

We report an experimental study of the dynamical arrest transition for a model system consisting of octadecyl coated silica suspended in n-tetradecane from dilute to concentrated conditions spanning the state diagram. The dispersion's interparticle potential is tuned by temperature affecting the brush conformation leading to a thermoreversible model system. The critical temperature for dynamical arrest, T*, is determined as a function of dispersion volume fraction by small-amplitude dynamic oscillatory shear rheology. We corroborate this transition temperature by measuring a power-law decay of the autocorrelation function and a loss of ergodicity via fiber-optic quasi-elastic light scattering. The structure at T* is measured using small-angle neutron scattering. The scattering intensity is fit to extract the interparticle pair-potential using the Ornstein-Zernike equation with the Percus-Yevick closure approximation, assuming a square-well interaction potential with a short-range interaction (1% of particle diameter). (1) The strength of attraction is characterized using the Baxter temperature (2) and mapped onto the adhesive hard sphere state diagram. The experiments show a continuous dynamical arrest transition line that follows the predicted dynamical percolation line until ϕ ≈ 0.41 where it subtends the predictions toward the mode coupling theory attractive-driven glass line. An alternative analysis of the phase transition through the reduced second virial coefficient B(2)* shows a change in the functional dependence of B(2)* on particle concentration around ϕ ≈ 0.36. We propose this signifies the location of a gel-to-glass transition. The results presented herein differ from those observed for depletion flocculated dispersion of micrometer-sized particles in polymer solutions, where dynamical arrest is a consequence of multicomponent phase separation, suggesting dynamical arrest is sensitive to the physical mechanism of attraction.

In this study, the phase behavior, structure and properties of systems composed of the cationic, cellulose-based polycation JR 400 and the anionic surfactants sodium dodecylbenzenesulfonate (SDBS) or sodium dodecylethoxysulfate (SDES), mainly in the semidilute regime, were examined. This system shows the interesting feature of a very large viscosity increase by nearly 4 orders of magnitude as compared to the pure polymer solution already at very low concentrations of 1 wt%. By using rheology, dynamic light scattering (DLS), and small-angle neutron scattering (SANS), we are able to deduce systematic correlations between the molecular composition of the systems (characterized by the charge ratio Z=[+(polymer)]/[−(surfactant)]), their structural organization and the resulting macroscopic flow behavior. Mixtures in the semidilute regime with an excess of polycation charge form highly viscous network structures containing rodlike aggregates composed of surfactant and polyelectrolyte that are interconnected by the long JR 400 chains. Viscosity and storage modulus follow scaling laws as a function of surfactant concentration (η~c(s)(4); G(0)~c(s)(1.5)) and the very pronounced viscosity increase mainly arises from the strongly enhanced structural relaxation time of the systems. In contrast, mixtures with excess surfactant charges form solutions with viscosities even below those of the pure polymer solution. The combination of SANS, DLS, and rheology shows that the structural, dynamical, and rheological properties of these oppositely charged polyelectrolyte/surfactant systems can be controlled in a systematic fashion by appropriately choosing the systems composition., (© 2011 American Chemical Society)

The phase behavior of model polymer-colloid mixtures is measured for solutions approaching the "protein limit", that is, when the radius of gyration of the polymer (R(g)) is greater than or approximately equal to the radius of the colloid (R). Cationic nanoparticles are mixed with poly(ethylene oxide) (PEO) or poly(vinyl pyrolidone) (PVP) at size ratios of R(g)/R = 0.7 and 1.8. The addition of PEO to stable nanoparticle dispersions leads to depletion flocculation in both deionized water and buffer solutions. The instability mechanism for the PVP-nanoparticle system depends on the suspension medium. In water, bridging occurs below the saturation adsorption of PVP, whereas depletion phase separation is evident at concentrations exceeding those necessary to saturate the particle surface. In acidic buffer, PVP addition results in depletion phase separation. The difference between bridging and depletion is distinguished by both visual appearances and rheological measurements. There is no trend (within error bars) in the polymer concentration required to induce instability with increasing R(g)/R in contrast with theoretical predictions. This is most likely due to adsorption of polymer onto the particle surface.

We present a systematic study of thermodynamics, structure, and rheology of mixtures of cationic wormlike micelles and like-charged nanoparticles. Structural and thermodynamic measurements in dilute surfactant-nanoparticle mixtures show the formation of micelle-nanoparticle junctions that act as physical cross-links between micelles. The presence of these junctions is shown to build significant viscosity and viscoelasticity in dilute and semidilute WLMs, even in cases where the fluid is Newtonian in the absence of nanoparticles. Increases in viscosity, shear modulus, and relaxation time, as well as decreases in entanglement concentration, are observed with increasing particle concentration. As such, nanoparticle addition gives rise to a so-called "double network" comprised of micellar entanglements and particle junctions. A simple model for such networks is proposed, where the elasticity can be tuned through two energetic scales, the micellar end-cap energy and micelle-nanoparticle adsorption energy. As a practical application, the results demonstrate that nanoparticle addition provides formulators a unique method to tailor surfactant solution rheology over a wide range of conditions.

The adsorption of two polymers, poly(ethylene oxide) (PEO) and poly(vinyl pyrrolidone) PVP, onto cationic nanoparticles suspended in both water and a buffer solution is studied via isothermal titration calorimetry (ITC). These are model systems studied previously to understand polymer-induced phase separation and bridging flocculation in the protein limit. ITC measurements provide critical information for rationalizing the effects of polymer type and added buffer solution on the loss of stability of nanoparticle-polymer solutions. For PEO, weak segmental adsorption energies of approximately 0.2k(B)T for PEO in water and buffer are consistent with depletion phase separation. For PVP in water, segmental adsorption energies on the order of approximately 1.6k(B)T support the observed bridging flocculation, whereas a weaker adsorption energy of approximately 0.7k(B)T for PVP in buffer is consistent with depletion phase separation. Multilayer adsorption is observed in buffer solutions, which corroborates a measured increase in the hydrodynamic size of the polymer-nanoparticle complexes with added buffer. The entropy of adsorption is calculated from equilibrium constants determined by combining ITC and adsorption isotherms.

The phase behavior of a molecular brush-C(18) grafted to the surface of both a silicon wafer and SiO(2) nanoparticles was investigated as a function of temperature using neutron reflectometry (NR) and small-angle neutron scattering (SANS), respectively. The experiments demonstrate a phase change in the brush layer characterized by a straightening of the molecular configuration, increase in shell thickness, and increase in solvent concentration with decreasing temperature that corresponds to gelation in the nanoparticle dispersion.

The penetration behavior of Kevlar fabric intercalated with dry particles and shear thickening fluids (STF), highly concentrated fluid-particle suspensions, is presented. In particular, the role of particle hardness is explored by comparing fabric treatments containing SiO(2) particles, which are significantly harder than Kevlar, to treatments containing softer poly(methyl methacrylate) (PMMA) particles. The fabric testing includes yarn pull-out, quasi-static spike puncture, and ballistic penetration resistance, performed on single fabric layers. It was found that both dry particle and STF treatments resulted in improvements in fabric properties relative to neat or poly(ethylene glycol) (PEG) treated fabrics. On comparison of treatments with different particle hardness, the SiO(2) materials performed better in all tests than comparable PMMA materials, although the SiO(2) treatments caused yarn failure in pull-out testing, reducing the total pull-out energy. In addition, resistance to yarn pull-out was found to be substantially higher for STF-treated fabrics than for dry particle treated fabrics. However, both dry particle addition and STF treatments exhibited comparable enhancements in puncture and ballistic resistance. These observations suggest that viscous stress transfer, friction, and physical entrainment of hard particles into filaments contribute to the demonstrated improvements in the properties of protective fabrics treated with shear thickening fluids.

We present the first neutron spin echo (NSE) measurements of self-assembling peptide hydrogel networks to study the fibril dynamics on the nanometer and nanosecond length and time scales. MAX1 and MAX8 are synthetic beta-hairpin peptides that undergo triggered self-assembly at the nanoscale to form a physically cross-linked network of fibrils with a defined cross-section. When subjected to physiological pH and ionic strength (pH 7.4, 150 mM NaCl), the soluble peptides fold into a beta-hairpin and, subsequently, self-assemble to form a structurally rigid hydrogel stabilized by noncovalent cross-links. The sequence of MAX8 is identical to MAX1 with the exception of one single amino acid substitution that reduces the net charge on the peptide. As a result, faster folding and self-assembly kinetics are observed for MAX8 at the same peptide concentration and identical buffer conditions, and gels with a larger storage modulus are formed. NSE measurements of the peptide hydrogels demonstrate that the self-assembled peptide fibrils can be described as semiflexible chains on nanolength and time scales. Alteration of the peptide sequence affected the nanoscale dynamics of the hydrogels but not to an extent comparable to the large difference observed in the bulk viscoelasticity. Small angle neutron scattering (SANS) of the hydrogels reveals increased scattering for MAX8 at low wavevectors, an indication of a heterogeneous network with a tighter mesh size. Therefore, we conjecture that the difference in elastic modulus arises from differences in assembly kinetics that result in increased fibrillar branching and physical cross-links rather than a change in the fibril nanostructure or persistence length.

The addition of positively charged, 30 nm diameter silica nanoparticles to cationic wormlike micellar solutions of cetyltrimethylammonium bromide and sodium nitrate is studied using a combination of rheology, small angle neutron scattering, dynamic light scattering, and cryo-transmission electron microscopy. The mixtures are single phase up to particle volume fractions of 1%. The addition of like-charged particles significantly increases the wormlike micelle (WLM) solution's zero shear rate viscosity, longest relaxation time, and storage modulus. The changes are hypothesized to originate from a close association of the particles with the micellar mesh. Small angle neutron scattering measurements with contrast matching demonstrate associations between particles mitigated by the WLMs. The effective interparticle interactions measured by SANS can explain the observed phase behavior. Dynamic light scattering measurements confirm the dynamic coupling of the particles to the micellar mesh.

We present the first measurements of the fast dynamics of cationic wormlike micelles (WLM) using neutron spin echo (NSE). The comparison with theory [Zilman, A.; Granek, R. Phys. Rev. Lett. 1996, 77, 4788. Granek, R. J. Phys. II 1997, 7, 1761]1,2 enables coarse grained parameters to be identified. We propose and validate a calibration procedure to extract the bending constant kappa from NSE measurements.

Using molecular simulation, the adsorption and self-diffusion of diatomic nitrogen molecules inside a single wall carbon nanotube have been studied over a range of nanotube diameters (8.61-15.66 A) and loadings at temperatures of 100 and 298 K. Nitrogen adsorption energy is found to increase as the nanotube diameter is reduced toward the molecular diameter of nitrogen. A discrete organization of the nitrogen into adsorbed layers is observed at high loadings that follows a regular progression determined primarily by geometric considerations. The formation of an adsorbate core at the center of the nanotube is found to increase the self-diffusion of nitrogen. A "wormlike" phase is found for the adsorbed nitrogen in the (15, 0) carbon nanotube at high loadings and at 100 K., (Copyright 2004 American Chemical Society)

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.