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

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.

Nonnative protein polymers: structure, morphology, and relation to nucleation and growth.

Thermally induced aggregates of alpha-chymotrypsinogen A and bovine granulocyte-colony stimulating factor in acidic solutions were characterized by a combination of static and dynamic light scattering, spectroscopy, transmission electron microscopy, and monomer loss kinetics. The resulting soluble, high-molecular weight aggregates (approximately 10(3)-10(5) kDa) are linear, semiflexible polymer chains that do not appreciably associate with one another under the conditions at which they were formed, with classic power-law scaling of the radius of gyration and hydrodynamic radius with weight-average molecular weight (M(w)). Aggregates in both systems are composed of nonnative monomers with elevated levels of beta-sheet secondary structure, and bind thioflavine T. In general, the aggregate size distributions showed low polydispersity by light scattering. Together with the inverse scaling of M(w) with protein concentration, the results clearly indicate that aggregation proceeds via nucleated (chain) polymerization. For alpha-chymotrypsinogen A, the scaling behavior is combined with the kinetics of aggregation to deduce separate values for the characteristic timescales for nucleation (tau(n)) and growth (tau(g)), as well as the stoichiometry of the nucleus (x). The analysis illustrates a general procedure to noninvasively and quantitatively determine tau(n), tau(g), and x for soluble (chain polymer) aggregates, as well as the relationship between tau(n)/tau(g) and aggregate M(w).

Elucidating the assembled structure of amphiphiles in solution via cryogenic transmission electron microscopy.

For the past twenty years, significant progress has been made in both developing cryogenic transmission electron microscopy (cryo-TEM) technology and understanding assembled behavior of amphiphilic molecules. Cryo-TEM can provide high-resolution images of complex fluids in a near state. Samples embedded in a thin layer of vitrified solvent do not exhibit artifacts that would normally occur when using chemical fixation or staining-and-drying techniques. Cryo-TEM has been useful in imaging biological molecules in aqueous solutions. Cryo-TEM has become a powerful tool in the study of -assembled structures of amphiphiles in solution as a complementary tool to small-angle X-ray and neutron scattering, light scattering, rheology measurements, and nuclear magnetic resonance. The application of cryo-TEM in the study of assembled behavior of amphiphilic block copolymers, hydrogels, and other complex soft systems continues to emerge. In this context, the usage of cryo-TEM in the field of amphiphilic complex fluids and self-assembled nano-materials is briefly reviewed, and its unique role in exploring the nature of assembled structure in liquid suspension is highlighted.

A systematic study of equilibrium structure, thermodynamics, and rheology of aqueous CTAB/NaNO(3) wormlike micelles.

We present a systematic study of the self-assembly of wormlike micelles (WLMs) comprised of cetyltrimethylammonium bromide (CTAB) and sodium nitrate (NaNO(3)) in aqueous solution as a function of CTAB concentration, NaNO(3) concentration, and temperature throughout the dilute and semi-dilute regions of the phase diagram where linear micelles are observed. Combining measurements using isothermal titration calorimetry, rheometry, flow-birefringence, cryo-transmission electron microscopy (cryo-TEM), and small angle neutron scattering (SANS) enables complete characterization of the structure, thermodynamics, and rheology of CTAB/NaNO(3) micelles. The addition of NaNO(3) is found to increase the micellization enthalpy as well as the micellar scission energy, resulting in the elongation and growth of WLMs. We find quantitative agreement between the scission energy determined from rheology and the enthalpy of micellization determined from ITC, as well as for contour lengths extracted from rheology and SANS. At fixed molar ratio of NaNO(3) and CTAB, the solution rheology exhibits scaling consistent with dilute, semi-dilute overlapping, and semi-dilute entangled regimes typically found in polymer solutions, as confirmed by cryo-TEM and SANS. The transition between these scaling regimes coincides with the structural transitions identified by SANS. The results validate the relationship between structural parameters and rheological behavior underlying theories for ionic WLMs.

Influence of nanoparticle addition on the properties of wormlike micellar solutions.

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.