Theoretical progress in polymer adsorption, steric stabilization and flocculation

Local Structure of Nonuniform Fluid Mixtures Containing Associating and Chainlike Molecules from a Multilayer Quasichemical Model

In this work, we develop a theory for predicting details of the local structure in nonuniform multicomponent fluids that may contain chainlike and associating components. This theory is an extension─to the fluid interfaces and mesoscopic structures of different geometry─of the multilayer quasichemical model originally proposed by Smirnova to describe liquid solution in the vicinity of a planar solid wall. The basis of the theory is the "cut-and-bond" approach, much in spirit of SAFT, where an infinite attraction between the separated monomeric units of a chainlike molecule mimics the chemical bonds of the chain. We describe the equilibrium structure of the mixture, including the spatial distribution of the monomeric units and the local orientation of the chemical bonds in chainlike molecules, and discuss the contribution of chemical bonds to the local chemical potential in a nonuniform fluid. To test the new theory, we apply it to mixtures containing combinations of model components: a strongly associating solvent, an inert substance of varying chain length, and a chainlike amphiphile. To compare predictions from the multilayer model with the results of continuous description of nonuniform fluids, we also address the square-gradient theory and derive an analytical expression for the influence parameter that takes into account pair correlations in the quasichemical approximation. The multilayer quasichemical model developed in this work predicts formation of aggregates in liquid solution and describes the local structure of the interfaces between the coexisting liquid phases in the mixture. Our theoretical predictions agree on a qualitative level with the accumulated knowledge about the structure of different types of systems studied in this work.

Conformations of polyolefins on platinum catalysts control product distribution in plastics recycling

The design of catalysts for the chemical recycling of plastic waste will benefit greatly from an intimate knowledge of the interfacial polymer-catalyst interactions that determine reactant and product distributions. Here, we investigate backbone chain length, side chain length, and concentration effects on the density and conformation of polyethylene surrogates at the interface with Pt(111) and relate them to experimental product distributions resulting from carbon-carbon bond cleavage. Using replica-exchange molecular dynamics simulations, we characterize the polymer conformations at the interface by the distributions of trains, loops, and tails and their first moments. We find that the preponderance of short chains, in the range of 20 carbon atoms, lies entirely on the Pt surface, whereas longer chains exhibit much broader distributions of conformational features. Remarkably, the average length of trains is independent of the chain length but can be tuned via the polymer-surface interaction. Branching profoundly impacts the conformations of long chains at the interface as the distributions of trains become less dispersed and more structured, localized around short trains, with the immediate implication of a wider carbon product distribution upon C-C bond cleavage. The degree of localization increases with the number and size of the side chains. Long chains can adsorb from the melt onto the Pt surface even in melt mixtures containing shorter polymer chains at high concentrations. We confirm experimentally key computational findings and demonstrate that blends may provide a strategy to reduce the selectivity for undesired light gases.

Shielding behavior of electrokinetic properties of polystyrene latex particle by the adsorption of neutral poly(ethylene oxide)

To understand the shielding of electrokinetics of colloidal particles by polymer coating, we measured the electrophoretic mobility of negatively charged polystyrene sulfate latex (PSL) adsorbed with electrostatically neutral polyethylene oxide (PEO) chains with various molecular weights under different ionic strengths. We confirmed that substantial adsorbed neutral polymer on the particle surface would decrease the absolute value of electrophoretic mobility. Even though the polymer layer is sufficiently thicker compared to the thickness of electric double layer (EDL), the electrophoretic mobility (EPM) never vanishes, which indicates the incompleteness of electrokinetic shielding by an adsorbed neutral polymer. To relate such interesting phenomena, a simple mathematical model has been proposed to evaluate the electrophoretic mobility, assuming the presence of a scaling structure of adsorbed permeable polymer layer does not influence the Poisson-Boltzmann distribution of ions in the electric double layer (EDL). An analytical expression of electrophoretic mobility under Debye-Hu¨ckel approximation has been derived using the method of Ohshima-Kondo theory, which successfully justifies the experimentally obtained data.

Exploring the gelation of aqueous cellulose nanocrystals (CNCs)-hydroxyethyl cellulose (HEC) mixtures

We investigated the gelation and microstructure of cellulose nanocrystals (CNCs) in nonionic hydroxyethyl cellulose (HEC) solutions. Cellulose nanocrystals (CNCs) with a particle length of 90 nm and width of 8 nm currently produced by acid hydrolysis of wood pulp were used in this study. The microstructures of CNCs/polymer suspensions were investigated by performing linear small amplitude oscillatory shear (SAOS) and nonlinear large amplitude oscillatory shear (LAOS), in addition to constructing CNCs phase diagrams and measuring steady-state shear viscosities. Significant viscosity increases at low shear rates coupled with high shear thinning behaviors were observed in CNCs in HEC solutions above the overlapping concentration of HEC. The physical strength of CNCs/HEC solution gels increased with the increase in CNCs concentration and resembled the weakly crosslinked gels according to the scaling of linear dynamic mechanical experiments. According to LAOS analysis, CNCs/HEC mixtures showed type III behavior with intercycle stress softening, while the samples showed stress stiffening in single cycles.

Polymer Induced Gelation of Aqueous Suspensions of Cellulose Nanocrystals

We investigated the gelation of cellulose nanocrystals (CNCs) in polyelectrolyte and neutral polymer solutions. Cellulose nanocrystals (CNCs) with half-ester sulfate groups produced by acid hydrolysis of wood pulp were used in this study. The microstructure of CNCs/polymer suspensions was investigated in semidilute concentration regimes by selecting carboxymethyl cellulose (CMC700) as an anionic polymer and poly(ethylene oxide) (PEO600) as a neutral polymer solution. Together with quartz crystal microbalance with dissipation monitoring (QCM-D), rheology, scanning electron microscopy (SEM), and cryo-transmission electron microscopy (cryo-TEM), we characterized CNCs-polymer interactions, the suspension microstructure, and the macroscopic gel flow. Significant viscosity increases at low shear rates coupled with high shear-thinning behaviors were observed in CNC colloid-CMC700 polymer mixtures, but not those CNCs in PEO600 solutions. The apparent differences between CNCs-CMC700 and CNCs-PEO600 mixtures were due to their chain confirmations. On the basis of the evaluations from STEM, cryo-TEM, and polarized optical microscopy, we proposed that the excess CMC700 molecules in solutions result in the depletion of CNCs and the formation of anisotropic domains.

Tuning the bridging attraction between large hard particles by the softness of small microgels

In this study, the attraction between large hard polystyrene (PS) spheres is studied by using three types of small microgels as bridging agents. One is a purely soft poly(N-isopropylacrylamide) (PNIPAM) microgel, the other two have a non-deformable PS hard core surrounded by a soft PNIPAM shell but are different in the core-shell ratio. The affinity for bridging the large PS spheres is provided and thus affected by the PNIPAM constituent in the microgels. The bridging effects caused by the microgels can be indirectly incorporated into their influence on the effective attraction interaction between the large hard spheres, since the size of the microgels is very small in comparison to the size of the PS hard spheres. At a given volume fraction of large PS spheres, they behave essentially as hard spheres in the absence of small microgels. By gradually adding the microgels, the large spheres are connected to each other through the bridging of small particles until the attraction strength reaches a maximum value, after which adding more small particles slowly decreases the effective attraction strength and eventually the large particles disperse individually when saturated adsorption is achieved. The aggregation and gelation behaviors triggered by these three types of small microgels are compared and discussed. A way to tune the strength and range of the short-range attractive potential via changing the softness of bridging microgels (which can be achieved either by using core-shell microgels or by changing the temperature) is proposed.

Gelation of large hard particles with short-range attraction induced by bridging of small soft microgels

In this study, mixed suspensions of large hard polystyrene microspheres and small soft poly(N-isopropylacrylamide) microgels are used as model systems to investigate the static and viscoelastic properties of suspensions which go through liquid to gel transitions. The microgels cause short-range attraction between microspheres through the bridging and depletion mechanism whose strength can be tuned by the microgel concentration. Rheological measurements are performed on suspensions with the volume fraction (Φ) of microspheres ranging from 0.02 to 0.15, and the transitions from liquid-like to solid-like behaviors triggered by the concentration of microgels are carefully identified. Two gel lines due to bridging attraction under unsaturated conditions are obtained. Ultra-small angle neutron scattering is used to probe the thermodynamic properties of suspensions approaching the liquid-solid transition boundaries. Baxter's sticky hard-sphere model is used to extract the effective inter-microsphere interaction introduced by the small soft microgels. It is found that the strength of attraction (characterized by a single stickiness parameter τ) on two gel lines formed by bridging is very close to the theoretical value for the spinodal line in the τ-Φ phase diagram predicted by Baxter's model. This indicates that the nature of the gel state may have the same thermodynamic origins, independent of the detailed mechanism of the short-range attraction. The relationship between the rheological criterion for the liquid-solid transition and the thermodynamic criterion for the equilibrium-nonequilibrium transition is also discussed.

Conformation and adsorption transition on an attractive surface of a ring polymer in solution

Ring polymers have the same critical adsorption point and crossover exponent as linear chains.

Common strategies and technologies for the ecosafety assessment and design of nanomaterials entering the marine environment

The widespread use of engineered nanomaterials (ENMs) in a variety of technologies and consumer products inevitably causes their release into aquatic environments and final deposition into the oceans. In addition, a growing number of ENM products are being developed specifically for marine applications, such as antifouling coatings and environmental remediation systems, thus increasing the need to address any potential risks for marine organisms and ecosystems. To safeguard the marine environment, major scientific gaps related to assessing and designing ecosafe ENMs need to be filled. In this Nano Focus, we examine key issues related to the state-of-the-art models and analytical tools being developed to understand ecological risks and to design safeguards for marine organisms.

Directed assembly of bifunctional silica-iron oxide nanocomposite with open shell structure

The synthesis of nanocomposite with controlled surface morphology plays a key role for pollutant removal from aqueous environments. The influence of the molecular size of the polyelectrolyte in synthesizing silica-iron oxide core-shell nanocomposite with open shell structure was investigated by using dynamic light scattering, atomic force microscopy, and quartz crystal microbalance with dissipation (QCM-D). Here, poly(diallydimethylammonium chloride) (PDDA) was used to promote the attachment of iron oxide nanoparticles (IONPs) onto the silica surface to assemble a nanocomposite with magnetic and catalytic bifunctionality. High molecular weight PDDA tended to adsorb on silica colloid, forming a more extended conformation layer than low molecular weight PDDA. Subsequent attachment of IONPs onto this extended PDDA layer was more randomly distributed, forming isolated islands with open space between them. By taking amoxicillin, an antibiotic commonly found in pharmaceutical waste, as the model system, better removal was observed for silica-iron oxide nanocomposite with a more extended open shell structure.

Dispersions of nanocrystalline cellulose in aqueous polymer solutions: structure formation of colloidal rods

The steady-state shear and linear viscoelastic deformations of semidilute suspensions of rod-shaped nanocrystalline cellulose (NCC) particles in 1.0% hydroxyethyl cellulose and carboxymethyl cellulose solutions were investigated. Addition of NCC at the onset of semidilute suspension concentration significantly altered the rheological and linear viscoelastic properties of semidilute polymer solutions. The low-shear viscosity values of polymers solutions were increased 20-490 times (depending on polymer molecular weight and functional groups) by the presence of NCC. NCC suspensions in polymer solutions exhibited yield stresses up to 7.12 Pa. Viscoelasticity measurements also showed that NCC suspended polymer solutions had higher linear elastic moduli than the loss moduli. All of those results revealed the gel formation of NCC particles and presence of internal structures. The formation of a weak gel structure was due to the nonadsorbing macromolecules which caused the depletion-induced interaction among NCC particles. A simple interaction energy model was used to show successfully the flocculation of NCC particles in the presence of nonadsorbing polymers. The model is based on the incorporation of the depletion interaction term between two parallel plates into the DLVO theory for cubic prismatic rod shaped NCC particles.

Superparamagnetic iron oxide nanoparticles (SPIONs): from synthesis to in vivo studies--a summary of the synthesis, characterization, in vitro, and in vivo investigations of SPIONs with particular focus on surface and colloidal properties

In this work, we present a short summary of the synthesis and characterization of superparamagnetic iron oxide nanoparticles and their behavior in vitro and in vivo. Therefore, we have used various characterization techniques to deduce the physical particle size as well as magnetic properties. It is shown that the particle properties were significantly improved by a thermochemical treatment and dialysis, obtaining weakly interacting particles with a clear blocking temperature. We also present the interaction of polyvinyl alcohol and vinyl alcohol/vinyl amine copolymer-coated SPIONs with HELA cells. It is shown that the uptake increased significantly in the presence of a magnetic field and that surface functional groups had an impact on particle uptake and metabolic activity. Furthermore, the influences of the varied parameters (polymer type and therefore surface charge, cell medium, and serum) on the agglomeration rate and the cell uptake are presented and discussed. Finally, we briefly describe the intraarticular application of SPIONs in sheep, their uptake by synovial membrane, and their systemic distribution and elimination.

Novel long-acting crystal formulation of human growth hormone

PURPOSE

The aim of the study is to solve a significant challenge of extending the half-life of therapeutic proteins using crystalline biopharmaceuticals and without redesigning the molecules.

METHODS

Crystals of recombinant human growth hormone were coated with a monomolecular layer of positively charged poly(arginine). The pharmacokinetics and pharmacodynamics of this poly(arginine)-coated human growth hormone crystalline formulation were determined in hypophysectomized rats and monkeys.

RESULTS

Here we have demonstrated for the first time that crystals of human growth hormone coated with positively charged poly(arginine) allowed for in vivo pharmacokinetic release profiles of over several days in animal models. The efficacy of this crystalline formulation injected subcutaneously once a week was found to be equivalent to seven daily soluble injections in the standard weight gain assay using the hypophysectomized rat model and in measurement of serum insulin-like growth factor in monkeys. The nonviscous nature of the suspension facilitated easy administration through a fine, 30-gauge needle and should provide for improved patient convenience and compliance.

CONCLUSIONS

The approach described here offers an exciting possibility of being broadly applicable to other therapeutic proteins.

A kinetic investigation of the flocculation of alumina with polyacrylic acid

Using a model colloidal system of alumina and polyacrylic acid (PAA), the kinetics of flocculation was investigated at low polymer concentrations and short durations (on the order of seconds). The polymer-induced flocculation processes obeyed Von Smoluchowski's bimolecular rate equation. Increases in the concentration of the polymer resulted in higher rate constants for the flocculation process. At a fixed concentration (say 50 ppb, parts per billion), the rate constant values showed a maximum value for 250,000 g mol(-1) polyacrylic acid. At this polymer concentration, calculations of the surface coverage of alumina by PAA molecules of different molecular weights show that for all the cases the coverage is nearly the same, approximately 1x10(-3), but the flocculation response and the rates are significantly different. This trend in flocculation characteristics is attributed to the critical polymer number density requirement for effective flocculation (at least partial charge neutralization and initiation of flocculation). The mechanism governing the flocculation at ultralow concentrations (50 ppb) is the synergistic effect of partial patch neutralization and bridging.

Electrophoretic mobility of human erythrocytes in the presence of poly(styrene sulfonate)

The adsorption and depletion of the anionic polymer poly(styrene sulfonate) (PSS) on fresh human red blood cells (RBC) were investigated by measuring RBC electrophoretic mobility as a function of polymer molecular mass (48-2610 kDa), ionic strength (15 and 150 mM NaCl) and polymer concentration (<or= 1.5 g/dL). A subset of studies was carried out using fixed and PSS-coated cells. Our results indicate a marked increase of mobility with molecular mass and polymer concentration. Adsorption of PSS onto fresh RBC was weak, with normal mobility restored following washing cells in PSS-free buffer. Calculated zeta potentials based upon mobility and medium viscosity rose up to 618 mV for 2610 kDa PSS compared to 13 mV for control cells, thus suggesting significant polymer depletion at the cell surface; fixed and PSS-coated RBC were insensitive to medium viscosity, also validating this depletion layer hypothesis. Calculated values of increased RBC surface charge were used to estimate polymer adsorption per cell; these estimates indicated linear adsorption isotherms and binding levels consistent with studies employing neutral polymers. In overview, our results suggest the usefulness of microelectrophoresis methods for studies of RBC interactions with charged polymers or proteins, and the value of this approach for future studies using proteins known to affect RBC-RBC interactions.

Kinetics of Adsorption of Diethylene-triaminomethylated Polyacrylamide on Dispersed Kaolin Accompanied by Flocculation

The kinetics of the adsorption of diethylene-triaminomethylated polyacrylamide on kaolin dispersed in water has been investigated. An influence of the flocculation of kaolin dispersion on polymer adsorption has been found. The kinetics of particle aggregation under the influence of dissolved polymer has been studied. Polymer adsorption and particle aggregation proceed simultaneously, accompanied by a steady decrease in the amount of adsorbed polymer per unit mass of kaolin. A mathematical model of the adsorption process, consistent with the experimental data, is described. The rate constants and their ratios have been determined. Copyright 1999 Academic Press.

Surface properties of superparamagnetic iron oxide MR contrast agents: ferumoxides, ferumoxtran, ferumoxsil

The surface properties of the active ingredients in AMI colloidal, superparamagnetic iron oxide magnetic resonance (MR) contrast agents are described. Scanning electron microscopy/energy dispersive X-ray elemental analyses and diffuse reflectance Fourier transform infrared spectroscopy (FTIR) spectra of ferumoxsil (AMI-121 drug substance) were consistent with the presence of a monolayer of H2NCH2CH2NHCH2CH2CH2Si(O-)3 siloxane monomer or dimer. The X-ray photoelectron spectra (XPS) of ferumoxsil are also consistent with complete coverage of the iron oxide surface with a monolayer of siloxane. The static secondary ion mass spectra (SSIMS) of ferumoxsil showed that the siloxane film is covalently bonded (i.e., Si-O-Fe bonds) to the iron oxide surface. The FTIR of ferumoxides (AMI-25) and Ferumoxtran (AMI-227) showed only adsorbed dextran. The XPS spectra of the dextran-coated colloids showed that Ferumoxtran has a thicker layer of dextran than ferumoxides iron oxide particles (approximately 5 and approximately 3 nm, respectively). The SSIMS spectra of these dextran-coated colloids showed only low mass fragments due to the adsorbed dextran. The nature of the interactions of the dextran coating with the iron oxide surfaces of ferumoxides and Ferumoxtran is discussed.