Structure-Property Relationship in Capillary Foams.

The recently discovered capillary foams are aqueous foams stabilized by the synergistic action of colloidal particles and a small amount of oil. Characteristically, their gas bubbles are coated by a particle-stabilized layer of oil and embedded in a gel network of oil-bridged particles. This unique foam architecture offers opportunities for engineering new foam-related materials and processes, but the necessary understanding of its structure-property relations is still in its infancy. Here, we study the effects of particle wettability, particle volume fraction, and oil-to-particle ratio on the structure and selected properties of capillary foams and use our findings to relate measured foamability, foam stability, and rheological key parameters to the observed foam microstructure. We see that particle wettability not only determines the type of gel network formed but also influences the prevalence of oil droplets included within the foam. Our results further show that the stability and rheology of capillary foams are mainly a function of the particle volume fraction whereas the foamability and observed microstructure are sensitive also to the oil-to-particle ratio. These insights expand our fundamental understanding of capillary foams and will greatly facilitate future work on new foam formulations.

Modeling Amyloid Aggregation Kinetics: A Case Study with Sup35NM.

Understanding the aggregation mechanism of amyloid proteins, such as Sup35NM, is essential to understanding amyloid diseases. Significant recent work has focused on using the fluorescence of thioflavin T (ThT), which undergoes a red shift when bound to amyloid aggregates, to monitor amyloid fibril formation. In the present study, the progression of the total mass of aggregates during fibril formation is monitored for initial monomer concentrations in order to infer the relevant aggregation mechanisms. This workflow was implemented using the amyloid-forming fragment Sup35NM under different agitation conditions and for initial monomer concentrations spanning 2 orders of magnitude. The analysis suggests that primary nucleation, monomeric elongation, secondary nucleation, and fragmentation might all be relevant, but their relative importance could not be determined unambiguously, despite the large set of high-quality data. Discriminating between the fibril-generating processes is shown to require additional information, such as a fibril length distribution. Using Sup35NM as a case study, a framework for fitting the parameters of arbitrary amyloid aggregation kinetics is developed based on a population balance model (PBM), which resolves not only the total aggregate mass (monitored experimentally via ThT fluorescence) but the entire fibril length distribution over time. In addition to the rich new set of ThT fluorescence data, we have reanalyzed a previously published aggregate size distribution using this method. With the size distribution, it was determined that in the reanalyzed in vitro experiment, secondary nucleation generated significantly fewer new Sup35NM fibrils than fragmentation. The proposed strategy of applying the same PBM to a combination of kinetic data from fluorescence monitoring and experimental fibril length distributions will allow the inference of aggregation mechanisms with far greater confidence than fluorescence studies alone.

Rheology of capillary foams.

Aqueous foams are ubiquitous; they appear in products and processes that span the cosmetics, food, and energy industries. The versatile applicability of foams comes as a result of their intrinsic viscous and elastic properties; for example, foams are exploited as drilling fluids in enhanced oil recovery for their high viscosity. Recently, so-called capillary foams were discovered: a class of foams that have excellent stability under static conditions and whose flow properties have so far remained unexplored. The unique architecture of these foams, containing oil-coated bubbles and a gelled network of oil-bridged particles, is expected to affect foam rheology. In this work, we report the first set of rheological data on capillary foams. We study the viscoelastic properties of capillary foams by conducting oscillatory and steady shear tests. We compare our results on the rheological properties of capillary foams to those reported for other aqueous foams. We find that capillary foams, which have low gas volume fractions, exhibit long lasting rheological stability as well as a yielding behavior that is reminiscent of surfactant foams with high gas volume fractions.

Modulation of the Formation of Aß- and Sup35NM-Based Amyloids by Complex Interplay of Specific and Nonspecific Ion Effects.

In vitro formation of highly ordered protein aggregates, amyloids, is influenced by the presence of ions. Here, we have studied the effect of anions on amyloid fibril formation by two different amyloidogenic proteins, human amyloid beta-42 (Aß42), associated with Alzheimer disease and produced recombinantly with an N-terminal methionine (Met-Aß42), and histidine-tagged NM fragment of Sup35 protein (Sup35NM-His6), a yeast release factor controlling protein-based inheritance, at pH values above and below their isoelectric points. We demonstrate here that pH plays a critical role in determining the effect of ions on the aggregation of Met-Aß42 and Sup35NM-His6. Further, the electrophoretic mobilities of Met-Aß42 and Sup35NM-His6 were measured in the presence of different anions at pH above and below the isoelectric points to understand how anions interact with these proteins when they bear a net positive or negative charge. We find that although ion-protein interactions generally follow expectations based on the anion positions within the Hofmeister series, there are qualitative differences in the aggregation behavior of Met-Aß42 and Sup35NM-His6. These differences arise from a competition between nonspecific charge neutralization and screening effects and specific ion adsorption and can be explained by the different biochemical and biophysical properties of Met-Aß42 and Sup35NM-His6.

The dynamics of rising oil-coated bubbles: experiments and simulations.

Air bubbles rising through an aqueous medium have been studied extensively and are routinely used for the separation of particulates via froth flotation, a key step in many industrial processes. Oil-coated bubbles can be more effective for separating hydrophilic particles with low affinity for the air-water interface, but the rise dynamics of oil-coated bubbles has not yet been explored. In the present work, we report the first systematic study of the shape and rise trajectory of bubbles engulfed in a layer of oil. Results from direct observation of the coated bubbles with a high-speed camera are compared to computer simulations and confirm a pronounced effect of the oil coat on the bubble dynamics. We consistently find that the oil-coated bubbles display a more spherical shape and straighter trajectory, yet slower rise than uncoated bubbles of comparable size. These characteristics may provide practical benefits for flotation separations with oil-coated bubbles.

Process Principles for Large-Scale Nanomanufacturing.

Nanomanufacturing-the fabrication of macroscopic products from well-defined nanoscale building blocks-in a truly scalable and versatile manner is still far from our current reality. Here, we describe the barriers to large-scale nanomanufacturing and identify routes to overcome them. We argue for nanomanufacturing systems consisting of an iterative sequence of synthesis/assembly and separation/sorting unit operations, analogous to those used in chemicals manufacturing. In addition to performance and economic considerations, phenomena unique to the nanoscale must guide the design of each unit operation and the overall process flow. We identify and discuss four key nanomanufacturing process design needs: (a) appropriately selected process break points, (b) synthesis techniques appropriate for large-scale manufacturing,

Interfacial Activity of Nonamphiphilic Particles in Fluid-Fluid Interfaces.

Surfactants can adsorb in fluid-fluid interfaces and lower the interfacial tension. Like surfactants, particles with appropriate wettability can also adsorb in fluid-fluid interfaces. Despite many studies of particle adsorption at fluid interfaces, some confusion persists regarding the ability of (simple, nonamphiphilic) particles to reduce the interfacial tension. In the present work, the interfacial activity of silica nanoparticles at air-water and hexadecane-water interfaces and of ethyl cellulose particles at the interface of water with trimethylolpropane trimethacrylate was analyzed through pendant drop tensiometry. Our measurements strongly suggest that the particles do significantly affect the interfacial tension provided that they have a strong affinity to the interface by virtue of their wettability and that no energy barrier to adsorption prevents them from reaching the interface. A simplistic model that does not explicitly account for any particle-particle interactions is found to yield surprisingly good predictions for the effective interfacial tension in the presence of the adsorbed particles. We further propose that interfacial tension measurements, when combined with information about the particles' wetting properties, can provide a convenient way to estimate the packing density of particles in fluid-fluid interfaces. These results may help to understand and control the assembly of nonamphiphilic nanoparticles at fluid-fluid interfaces, which is relevant to applications ranging from surfactant-free formulations and food technology to oil recovery.

Bubble Meets Droplet: Particle-Assisted Reconfiguration of Wetting Morphologies in Colloidal Multiphase Systems.

Wetting phenomena are ubiquitous in nature and play key functions in various industrial processes and products. When a gas bubble encounters an oil droplet in an aqueous medium, it can experience either partial wetting or complete engulfment by the oil. Each of these morphologies can have practical benefits, and controlling the morphology is desirable for applications ranging from particle synthesis to oil recovery and gas flotation. It is known that the wetting of two fluids within a fluid medium depends on the balance of interfacial tensions and can thus be modified with surfactant additives. It is reported that colloidal particles, too, can be used to promote both wetting and dewetting in multifluid systems. This study demonstrates the surfactant-free tuning and dynamic reconfiguration of bubble-droplet morphologies with the help of cellulosic particles. It further shows that the effect can be attributed to particle adsorption at the fluid interfaces, which can be probed by interfacial tensiometry, making particle-induced transitions in the wetting morphology predictable. Finally, particle adsorption at different rates to air-water and oil-water interfaces can even lead to slow, reentrant wetting behavior not familiar from particle-free systems.

Contributions of the Prion Protein Sequence, Strain, and Environment to the Species Barrier.

Amyloid propagation requires high levels of sequence specificity so that only molecules with very high sequence identity can form cross-ß-sheet structures of sufficient stringency for incorporation into the amyloid fibril. This sequence specificity presents a barrier to the transmission of prions between two species with divergent sequences, termed a species barrier. Here we study the relative effects of protein sequence, seed conformation, and environment on the species barrier strength and specificity for the yeast prion protein Sup35p from three closely related species of the Saccharomyces sensu stricto group; namely, Saccharomyces cerevisiae, Saccharomyces bayanus, and Saccharomyces paradoxus. Through in vivo plasmid shuffle experiments, we show that the major characteristics of the transmission barrier and conformational fidelity are determined by the protein sequence rather than by the cellular environment. In vitro data confirm that the kinetics and structural preferences of aggregation of the S. paradoxus and S. bayanus proteins are influenced by anions in accordance with their positions in the Hofmeister series, as observed previously for S. cerevisiae. However, the specificity of the species barrier is primarily affected by the sequence and the type of anion present during the formation of the initial seed, whereas anions present during the seeded aggregation process typically influence kinetics rather than the specificity of prion conversion. Therefore, our work shows that the protein sequence and the conformation variant (strain) of the prion seed are the primary determinants of cross-species prion specificity both in vivo and in vitro.

Capillary foams: stabilization and functionalization of porous liquids and solids.

Liquid foams are two-phase systems in which a large volume of gas is dispersed as bubbles in a continuous liquid phase. These foams are ubiquitous in nature. In addition, they are found in industrial applications, such as pharmaceutical formulation, food processing, wastewater treatment, construction, and cosmetics. Recently, we reported a new type of foam material, capillary foam, which is stabilized by the synergistic action of particles and a small amount of an immiscible secondary liquid. In this study, we explore in more detail the foam preparation routes. To illustrate some of the potential applications, we create vividly colored wet and dried foams, which are difficult to prepare using traditional methods, and load-bearing porous solids. The combined action of particles and immiscible secondary fluid confers exceptional stability to capillary foams and many options for functionalization, suggesting a wide range of possible applications.

Stabilization of liquid foams through the synergistic action of particles and an immiscible liquid.

Liquid foams are familiar from beer, frothed milk, or bubble baths; foams in general also play important roles in oil recovery, lightweight packaging, and insulation. Here a new class of foams is reported, obtained by frothing a suspension of colloidal particles in the presence of a small amount of an immiscible secondary liquid. A unique aspect of these foams, termed capillary foams, is the particle-mediated spreading of the minority liquid around the gas bubbles. The resulting mixed particle/liquid coating can stabilize bubbles against coalescence even when the particles alone cannot. The coated bubbles are further immobilized by entrapment in a network of excess particles connected by bridges of the minority liquid. Capillary foams were prepared with a diverse set of particle/liquid combinations to demonstrate the generality of the phenomenon. The observed foam stability correlates with the particle affinity for the liquid interface formed by spreading the minority liquid at the bubble surface.

Gauging colloidal and thermal stability in human IgG1-sugar solutions through diffusivity measurements.

Monoclonal antibodies are the fastest growing class of biotherapeutics. Ensuring their colloidal and conformational stability in liquid dispersions is crucial for maintaining therapeutic efficacy and economic viability. Sugars are often added to increase the colloidal and thermal stability of protein; however, determining which sugar is the most stabilizing requires time and sample-consuming stability tests. Here we show for a human IgG1 that the extent of stabilization by different sugars can be gauged by analyzing the proteins' diffusive virial coefficient kD. This protein interaction parameter is measured conveniently in a noninvasive, high-throughput manner using dynamic light scattering. It is found to correlate closely with experimental aggregation rate constants at the onset of aggregation and with melting temperatures for antibodies in different sugar solutions. The proposed analysis thus provides a rapid test of the subtle differences between inherently similar sugar-protein interactions; it should greatly facilitate the formulation of protein therapeutics. For the antibody investigated in this study, circular dichroism spectroscopy also yields clues about the mechanism by which sugars improve the thermal stability.

Ion-specific effects on prion nucleation and strain formation.

Ordered, fibrous, self-seeding aggregates of misfolded proteins known as amyloids are associated with important diseases in mammals and control phenotypic traits in fungi. A given protein may adopt multiple amyloid conformations, known as variants or strains, each of which leads to a distinct disease pattern or phenotype. Here, we study the effect of Hofmeister ions on amyloid nucleation and strain generation by the prion domain-containing fragment (Sup35NM) of a yeast protein Sup35p. Strongly hydrated anions (kosmotropes) initiate nucleation quickly and cause rapid fiber elongation, whereas poorly hydrated anions (chaotropes) delay nucleation and mildly affect the elongation rate. For the first time, we demonstrate that kosmotropes favor formation of amyloid strains that are characterized by lower thermostability and higher frangibility in vitro and stronger phenotypic and proliferation patterns effectively in vivo as compared with amyloids formed in chaotropes. These phenomena point to inherent differences in the biochemistry of Hofmeister ions. Our work shows that the ionic composition of a solution not only influences the kinetics of amyloid nucleation but also determines the amyloid strain that is preferentially formed.

Surfactant mediated charging of polymer particles in a nonpolar liquid.

We study the charging behavior of polystyrene and polymethyl methacrylate particles with different functional surface groups in water and in decane containing either ionic (AOT) or nonionic surfactant (Span 85). Electrophoretic mobilities in the nonpolar media are measured as a function of surfactant concentration and the applied electric field strength by phase analysis light scattering (PALS); acid-base characteristics of the particles and the surfactant are investigated via contact angle measurement and interfacial tensiometry, and the residual water content of the non-aqueous dispersions is assessed by Karl Fischer titration. The results suggest a competition of several mechanisms for particle charging in nonpolar media. At high concentrations of the nonionic surfactant, particle charging becomes insensitive to the functional surface groups responsible for charging in aqueous dispersions, but consistent with a charge transfer between the polymer surface and the surfactant due to acid-base interactions, which can be rationalized in terms of measurable acid-base parameters. By contrast, particle charging in nonpolar solutions of the ionic surfactant (with significantly larger amounts of residual water) suggests a strong influence of surface headgroup ionization, and of dissociated surfactants adsorbed to the particle surface.

Salt-induced aggregation of a monoclonal human immunoglobulin G1.

Physical stability is critical for any therapeutic protein's efficacy and economic viability. No reliable theory exists to predict stability de novo, and modeling aggregation is challenging as this phenomenon can involve orientation effects, unfolding, and the rearrangement of noncovalent bonds inter- and intramolecularly in a complex sequence of poorly understood events. Despite this complexity, the simple observation of protein concentration-dependent diffusivity in stable, low ionic-strength solutions can provide valuable information about a protein's propensity to aggregate at higher salt concentrations and over longer times. We recently verified this notion using two model proteins, and others have shown that this strategy may be applicable to antibodies as well. Here, we expand our previous study to a monoclonal human immunoglobulin G1 antibody and discuss both merits and limitations of stability assessments based on the diffusional virial coefficient k(D). We find this parameter to be a good predictor of relative protein stability in solutions of different chaotropic salts, and a telling heuristic for the effect of kosmotropes. Both temperature and glycosylation are seen to have a strong influence on k(D), and we examine how these factors affect stability assessments. Protein unfolding is monitored with a fluorescence assay to assist in interpreting the observed aggregation rates.

Influence of nanoscale particle roughness on the stability of Pickering emulsions.

The wetting behavior of solid surfaces can be altered dramatically by introducing surface roughness on the nanometer scale. Some of nature's most fascinating wetting phenomena are associated with surface roughness; they have inspired both fundamental research and the adoption of surface roughness as a design parameter for man-made functional coatings. So far the attention has focused primarily on macroscopic surfaces, but one should expect the wetting properties of colloidal particles to be strongly affected by roughness, too. Particle wettability, in turn, is a key parameter for the adsorption of particles at liquid interfaces and for the industrially important use of particles as emulsion stabilizers; yet, the consequence of particle roughness for emulsion stability remains poorly understood. In order to investigate the matter systematically, we have developed a surface treatment, applicable to micrometer-sized particles and macroscopic surfaces alike, that produces surface coatings with finely tunable nanoscale roughness and identical surface chemistry. Coatings with different degrees of roughness were characterized with regard to their morphology, charging, and wetting properties, and the results were correlated with the stability of emulsions prepared with coated particles of different roughness. We find that the maximum capillary pressure, a metric of the emulsions' resistance to droplet coalescence, varies significantly and in a nonmonotonic fashion with particle roughness. Surface topography and contact angle hysteresis suggest that particle roughness benefits the stability of our emulsions as long as wetting occurs homogeneously (Wenzel regime), whereas the transition toward heterogeneous wetting (Cassie-Baxter regime) is associated with a loss of stability.

The cellulose-binding domain of cellobiohydrolase Cel7A from Trichoderma reesei is also a thermostabilizing domain.

The thermostability of cellobiohydrolase I Cel7A from Trichoderma reesei was investigated using dynamic light scattering. While the whole enzyme displayed a melting point of 59°C, the catalytic domain obtained via papain-catalyzed proteolysis was shown to denature at 51°C and the cellulose-binding domain (with linker attached) melted at 65-66°C. This variation in individual melting temperatures is proposed to account for the full retention of binding capacity of Cel7A at 50°C, along with a loss of catalytic activity observed for the catalytic domain alone. Thus, the cellulose-binding domain of Cel7A acts as a thermostabilizing domain for the enzyme. The effect of reducing agents on the protein melting behavior was also investigated.

Particle charging and charge screening in nonpolar dispersions with nonionic surfactants.

The electrostatic stabilization of colloidal dispersions is usually considered the domain of polar media only because of the high energetic cost associated with introducing electric charge in nonpolar environments. Nevertheless, some surfactants referred to as "charge control agents" are known to raise the conductivity of liquids with low electric permittivity and to mediate charge stabilization of nonpolar dispersions. Here we study an example of the particularly counterintuitive charging and electrostatic interaction of colloidal particles in a nonpolar solvent caused by nonionic surfactants. PMMA particles in hexane solutions of nonionic sorbitan oleate (Span) surfactants are found to exhibit a field-dependent electrophoretic mobility. Extrapolation to zero field strength yields evidence for large electrostatic surface potentials that decay with increasing surfactant concentration in a fashion reminiscent of electrostatic screening caused by salt in aqueous solutions. The amount of surface charge and screening ions in the nonpolar bulk is further characterized via measurements of the particles' pair interaction energy. The latter is obtained by liquid structure analysis of quasi-2-dimensional equilibrium particle configurations studied with digital video microscopy. In contrast to the behavior reported for systems with ionic surfactants, we observe particle charging and a screened Coulomb type interaction both above and below the surfactant's critical micelle concentration.

Correlating aggregation kinetics and stationary diffusion in protein-sodium salt systems observed with dynamic light scattering.

This paper compares two manifestations of electrolyte-mediated interaction between globular proteins. Salt-induced protein aggregation is studied with dynamic light scattering (DLS) in solutions of lysozyme and bovine serum albumin (BSA) containing different types of sodium salts. The same types of ions are used in a second measurement series assessing the effect of more dilute electrolytes on protein diffusivity in non-aggregating protein dispersions. Both aggregation and stable diffusion exhibit strong ion specificity along the lines of the Hofmeister series: chaotropic counterions act as the strongest coagulants and, in stable protein solutions, lead to the lowest "protein interaction parameter", evaluated as the slope of protein diffusivity versus protein concentration. Within this common qualitative trend, lysozyme and BSA solutions show marked differences, including the sign of the interaction parameter for most of the tested solution compositions. Despite the different nature of lysozyme and BSA, a strong correlation is found in both cases between the ion-specific interaction parameter and the proteins' aggregation tendency as indicated by the salt concentration required for fast aggregation. The interaction parameter, available via quick and easy DLS measurements on stable protein solutions, may thus serve as a predictor of ion-specific aggregation trends.