Aggregation and breakage dynamics of alumina particles under shear by coupled Computational Fluid Dynamics - Discrete Element Method.

HYPOTHESIS: Shear affects simultaneous aggregation and fragmentation of fine particles. Understanding the effect of shear on the dynamics of particle aggregation and break-up is important to predict aggregate size and structure. It is hypothesized that there is a transition from pure breakage of large aggregates to regimes where restructuring and aggregation also play a role as aggregates become smaller. SIMULATIONS: Here, aggregation and fragmentation dynamics of alumina particles are investigated under laminar shear flow using Discrete Element Method (DEM) coupled with Computational Fluid Dynamics (CFD). The effect of the shear rate on the aggregation and breakage rates is quantified accounting for particle-particle and particle-fluid interactions. FINDINGS: High shear rates promote the formation of small, compact aggregates. The collision efficiency decreases with increasing shear rate following a power law for shear rates higher than 1250 s-1. The transition from the pure breakage limit to the region dominated by breakage and restructuring has been observed for the first time. The breakage rate decreases significantly as the transition occurs upon decreasing aggregate size. CFD-DEM-derived collision efficiency and breakage rate equations are proposed that can be readily employed in detailed population balance equation models for industrial particle process design.

Solvent Effect on the Nanotextural Formation of Reduced Graphene Oxide Membranes.

Solvent is involved in many wet-chemical synthesis and bottom-up assembly processes. Understanding its influence on the nanotextural formation of the resultant assemblies is essential for the design and control of the properties for targeted applications. With wet chemically reduced graphene oxide (rGO) membranes as a materials platform, this study investigates the solvent effect on nanotexture formation in 2D nanomaterial-based membranes through light scattering and electrochemical characterization. Our finding indicates that the nanotexture of the resultant rGO membrane is largely correlated to the dielectric constant of the solvent. Specifically, solvents with higher dielectric constants yield rGO membranes with more wrinkled, loosely stacked, and less graphitized structures. In contrast, solvents with a lower dielectric constant tend to yield densely stacked structures with larger graphitized domains. Our finding underscores the important role of solvents in wet processing and nanoengineering of 2D nanomaterial-based membranes and provides valuable insights for their controlled synthesis and application.

Subnanometric Stacking of Two-Dimensional Nanomaterials: Insights from the Nanotexture Evolution of Dense Reduced Graphene Oxide Membranes.

Assembling two-dimensional (2D) nanomaterials into laminar membranes with a subnanometer (subnm) interlayer spacing provides a material platform for studying a range of nanoconfinement effects and exploring the technological applications related to the transport of electrons, ions and molecules. However, the strong tendency for 2D nanomaterials to restack to their bulk crystalline-like structure makes it challenging to control their spacing at the subnm scale. It is thus necessary to understand what nanotextures can be formed at the subnm scale and how they can be engineered experimentally. In this work, with dense reduced graphene oxide membranes as a model system, we combine synchrotron-based X-ray scattering and ionic electrosorption analysis to reveal that their subnanometric stacking can result in a hybrid nanostructure of subnm channels and graphitized clusters. We demonstrate that the ratio of these two structural units, their sizes and connectivity can be engineered by stacking kinetics through the reduction temperature to allow the realization of high-performance compact capacitive energy storage. This work highlights the great complexity of subnm stacking of 2D nanomaterials and provides potential methods to engineer their nanotextures at will.

Elastic plastic fracture mechanics investigation of toughness of wet colloidal particulate materials: Influence of saturation.

HYPOTHESIS: Previous use of linear elastic fracture mechanics to estimate toughness of wet particulate materials underestimates the toughness because it does not account for plastic deformation as a dissipation mechanism. Plastic deformation is responsible for the majority of energy dissipated during the fracture of wet colloidal particulate materials. Plastic deformation around the crack tip increases with saturation of the particulate body. The toughness of the body increases with increasing saturation. EXPERIMENTS: Elastic plastic fracture mechanics using the J-integral approach was used for the first time to measure the fracture toughness (JIC) of wet micron sized alumina powder bodies as a function of saturation. The samples were prepared by slip casting. The saturation was controlled by treatment in a humidity chamber. The elastic modulus (E) and the energy dissipated by plastic flow (Apl) were measured in uniaxial compression. The critical stress intensity factor (KIC) was measured using a diametral compression sample with a flaw of known size. The fracture toughness (JIC) was calculated from these measured quantities and the geometry of the specimen. FINDINGS: Elastic plastic fracture mechanics was used for the first time to quantitively account for plastic deformation of wet particulate materials. The linear elastic fracture mechanics approach previously used accounted for less than 1% of the total energy dissipated in fracture. Toughness (JIC) was found to increase with increasing saturation due to plastic deformation that increased with saturation level. The improved understanding of toughness as a function of saturation will aid in providing quantitative analysis of cracking in drying colloidal films and bodies.

Fracture toughness of wet and dry particulate materials comprised of colloidal sized particles: role of plastic deformation.

This work demonstrates a method of measuring the fracture toughness of particulate materials comprised of colloidal sized particles over a wide range of saturation. Diametral compression of cylinders containing flaws of controlled length was used to measure the mode I fracture toughness. The effect of degree of saturation on the fracture toughness of slip cast ceramic grade alumina (d50 = 0.7 µm) was investigated. Dry powder compacts have significantly lower fracture toughness than when the powder compact is nearly fully saturated. All observations are consistent with the fracture mechanism being predominantly brittle for the dry samples but predominantly ductile in the nearly saturated samples. The additional dissipation that occurs during the ductile fracture of the nearly saturated samples is due to plastic deformation in front of the crack tip. This well-known mechanism for toughening in metals has been quantified for the first time in soft matter. Analysis of the results indicates that the size of the plastic dissipation zone is more than an order of magnitude larger in the nearly saturated materials compared to the dry material. Understanding the fracture mechanisms that control the propagation of cracks through saturated, partially saturated and dry particulate materials comprised of colloidal sized particles provides additional insight into understanding drying cracks in paint, other coatings, ceramics and water treatment sludge.

In situ investigation of aggregate sizes formed using thermo-responsive polymers: Effect of temperature and shear.

Temperature-responsive flocculants, such as poly(N-isopropylacrylamide) (PNIPAM), induce reversible particle aggregation upon heating above a lower critical solution temperature (LCST). The aim of this work is to investigate the aggregation of ground iron ore using PNIPAM and conventional polyacrylamide (PAM) flocculants in a continuously-sheared suspension, through in situ chord length measurements using Focused Beam Reflectance Measurement techniques and real-time imaging of the particle aggregates. In the presence of uncharged PNIPAM, particle aggregation occurs only upon heating to the LCST, and the aggregates continue to grow with further heating. Subsequent cooling re-disperses the aggregates, and repeated heating causes reformation. Unlike uncharged PNIPAM, anionic PNIPAM produces aggregates at temperatures below the LCST due to the polymer chains binding to two different particles via attractive interactions between the acrylic acid groups and the hematite surfaces, and can be added at temperatures above the LCST due to the formation of charge-stabilised micelles. Under continuous shear, the flocculant most able to resist aggregate size reduction was anionic PAM, followed by PAM, anionic PNIPAM, PNIPAM (6MDa), and PNIPAM (122kDa). Reversible aggregate breakage was found with all samples, except with PNIPAM (6MDa) after being subjected to shear rates above 550s-1. Furthermore, heating of the PNIPAM-dosed suspensions at shear rates below 200s-1 produced larger and more breakage-resistant aggregates.

Versatile Synthesis of Amino Acid Functional Polymers without Protection Group Chemistry.

The copolymerization of N-isopropylacrylamide (NiPAm) with aldehyde functional monomers facilitates postpolymerization functionalization with amino acids via reductive amination, negating the need for protecting groups. In reductive amination, the imine formed from the condensation reaction between an amine and an aldehyde is reduced to an amine. In this work, we categorize amino acids into four classes based on the functionality of their side chains (acidic, polar neutral, neutral, and basic) and use their amine groups in condensation reactions with aldehyde functional polymers. The dynamic nature of the imine as well as the versatility of reductive amination to functionalize a polymer with a range of amino acids is highlighted. In this manner, amino acid functional polymers are synthesized without the use of protecting groups with high yields, demonstrating the high functional group tolerance of carbonyl condensation chemistry and the subsequent reduction of the imine. Prior to the reduction of the imine bond, transimination reactions are used to demonstrate dynamic polymers that shuffle from a glycine- to a histidine-functional polymer.

Xanthate-Functional Temperature-Responsive Polymers: Effect on Lower Critical Solution Temperature Behavior and Affinity toward Sulfide Surfaces.

Xanthate-functional polymers represent an exciting opportunity to provide temperature-responsive materials with the ability to selectively attach to specific metals, while also modifying the lower critical solution temperature (LCST) behavior. To investigate this, random copolymers of poly(N-isopropylacrylamide) (PNIPAM) with xanthate incorporations ranging from 2 to 32% were prepared via free radical polymerization. Functionalization with 2% xanthate increased the LCST by 5 °C relative to the same polymer without xanthate. With increasing xanthate composition, the transition temperature increased and the transition range broadened until a critical composition of the hydrophilic xanthate groups (≥18%) where the transition disappeared completely. The adsorption of the polymers at room temperature onto chalcopyrite (CuFeS2) surfaces increased with xanthate composition, while adsorption onto quartz (SiO2) was negligible. These findings demonstrate the affinity of these functional smart polymers toward copper iron sulfide relative to quartz surfaces, presumably due to the interactions between xanthate and specific metal centers.

Assembly-Controlled Permeability of Layer-by-Layer Polymeric Microcapsules Using a Tapered Fluidized Bed.

Nano- and microcapsules engineered through layer-by-layer (LbL) assembly are finding an increasingly large number of applications as catalysts, electrochemical biosensors, bioreactors, artificial cells and drug delivery vehicles. While centrifugation-based LbL assembly is the most common method for coating template particles and preparing capsules, it is a batch process and requires frequent intervention that renders the system challenging to automate and scale up. Here, we report the use of a tapered fluidized bed (TFB) for the preparation of multilayered polymer capsules. This is a significant improvement over our recent approach of fluidizing particles in cylindrical fluidized beds (CFB) for LbL assembly. We demonstrate that TFB is compatible with particles <3 µm in diameter (an order-of-magnitude improvement compared with CFB), which can be fluidized with minimal entrainment. Additionally, layering materials were expanded to include both electrostatic and hydrogen-bonding polymer pairs (e.g., poly(allylamine hydrochloride) (PAH) and poly(styrenesulfonate) (PSS), and thiol-modified poly(methacrylic acid) (PMASH) and poly(N-vinylpyrrolidone) (PVPON), respectively). Finally, differences between capsules prepared via centrifugation-based and TFB LbL assembly were investigated. The obtained TFB microcapsules demonstrate increased film thickness and roughness compared with those prepared using centrifugation-based LbL assembly. Furthermore, PMASH microcapsules exhibit lower swelling and permeability when prepared via TFB LbL assembly compared with centrifugation-based LbL assembly due to enhanced multilayer deposition, entanglement, and cross-linking. Therefore, polymeric capsules fabricated via TFB LbL assembly may be useful for encapsulation and retention of relatively low molecular weight (∼20 kDa) hydrophilic biomacromolecules to passively or responsively release the payload for drug delivery applications.

Modeling the IR spectra of aqueous metal carboxylate complexes: correlation between bonding geometry and stretching mode wavenumber shifts.

A widely used principle is that shifts in the wavenumber of carboxylate stretching modes upon bonding with a metal center can be used to infer if the geometry of the bonding is monodentate or bidentate. We have tested this principle with ab initio modeling for aqueous metal carboxylate complexes and have shown that it does indeed hold. Modeling of the bonding of acetate and formate in aqueous solution to a range of cations was used to predict the infrared spectra of the metal-carboxylate complexes, and the wavenumbers of the symmetric and antisymmetric vibrational modes are reported. Furthermore, we have shown that these shifts in wavenumber occur primarily due to how bonding with the metal changes the carboxylate C-O bond lengths and O-C-O angle.

Modeling the antisymmetric and symmetric stretching vibrational modes of aqueous carboxylate anions.

The infrared spectra of six aqueous carboxylate anions have been calculated at the M05-2X/cc-pVTZ level of theory with the SMD solvent model, and validated against experimental data from the literature over the region of 1700 cm(-1) to 1250 cm(-1); this region corresponds to the stretching modes of the carboxylate group, and is often interrogated when probing bonding of carboxylates to other species and surfaces. The anions studied here were formate, acetate, oxalate, succinate, glutarate and citrate. For the lowest energy conformer of each anion, the carboxylate moiety antisymmetric stretching peak was predicted with a mean signed error of only 4 cm(-1) using the SMD solvent model, while the symmetric peak was slightly overestimated. Performing calculations in vacuum and scaling was found to generally over-predict the antisymmetric vibrational frequencies and under predict the symmetric peak. Different conformers of the same anion were found to have only slightly different spectra in the studied region and the inclusion of explicit water molecules was not found to significantly change the calculated spectra when the implicit solvent model is used. Overall, the use of density functional theory in conjunction with an implicit solvent model was found to result in infra-red spectra that are the best reproduction of the features found experimentally for the aqueous carboxylate ions in the important 1700 cm(-1) to 1250 cm(-1) region. The development of validated model chemistries for simulating the stretching modes of aqueous carboxylate ions will be valuable for future studies that investigate how carboxylate anions complex with multivalent metal cations and related species in solution.

In situ particle film ATR FTIR spectroscopy of poly (N-isopropyl acrylamide) (PNIPAM) adsorption onto talc.

The adsorption of poly(N-isopropyl acrylamide) (PNIPAM) onto talc from aqueous solutions has been studied using the in situ methodology of particle film attenuated total reflection (ATR) Fourier transform infrared (FTIR) spectroscopy. PNIPAM was observed to adsorb significantly onto the talc particle film at a temperature below its lower critical solution temperature (LCST). Peak shifts were seen in the adsorbed layer FTIR spectrum that match those observed when PNIPAM solution is heated above its LCST. This observation indicates that adsorption causes a conformational re-arrangement similar to that seen when PNIPAM undergoes a coil-to-globule transition, in this case presumably induced by hydrophobic interactions between PNIPAM and the talc basal plane surface. The kinetics of adsorption are seen to be complex, with potential influences of conformational rearrangement and differential adsorption kinetics for the two dominant surface regions of talc particles. The adsorbed PNIPAM was seen to be exceptionally resistant to removal, with no desorption occurring when a background electrolyte solution was flowed over the adsorbed layer. Spectra acquired of the adsorbed polymer layer heated above the LCST reveal that a further conformational rearrangement takes place for the adsorbed layer, finalizing the transition from coil-to-globule that was initiated by the interaction with the mineral surface.

Fluidized bed layer-by-layer microcapsule formation.

Polymer microcapsules can be used as bioreactors and artificial cells; however, preparation methods for cell-like microcapsules are typically time-consuming, low yielding, and/or involve custom microfluidics. Here, we introduce a rapid (∼30 min per batch, eight layers), scalable (up to 500 mg of templates), and efficient (98% yield) microcapsule preparation technique utilizing a fluidized bed for the layer-by-layer (LbL) assembly of polymers, and we investigate the parameters that govern the formation of robust capsules. Fluidization in water was possible for particles of comparable diameter to mammalian cells (>5 µm), with the experimental flow rates necessary for fluidization matching well with the theoretical values. Important variables for polymer film deposition and capsule formation were the concentration of polymer solution and the molecular weight of the polymer, while the volume of the polymer solution had a negligible impact. In combination, increasing the polymer molecular weight and polymer solution concentration resulted in improved film deposition and the formation of robust microcapsules. The resultant polymer microcapsules had a thickness of ∼5.5 nm per bilayer, which is in close agreement with conventionally prepared (quiescent (nonflow) adsorption/centrifugation/wash) LbL capsules. The technique reported herein provides a new way to rapidly generate microcapsules (approximately 8 times quicker than the conventional means), while being also amenable to scale-up and mass production.

First principles pKa calculations on carboxylic acids using the SMD solvation model: effect of thermodynamic cycle, model chemistry, and explicit solvent molecules.

Aqueous pK(a) values are calculated from first principles for a set of carboxylic acids using the SMD solvation model with various model chemistries, thermodynamic cycles, and treatments of explicit solvation. In all, 108 unique theoretical protocols are examined. The direct (D) and water proton exchange (PX) cycles are trialled along with a new approach, termed the semidirect (SD) cycle. The SD thermodynamic cycle offers some improvements over the D and PX schemes, as it bypasses the gas-phase heterolytic bond dissociation calculation required in the conventional D approach while also avoiding an aqueous OH(-) calculation required by the PX method when using water as the reference acid. With all three cycles, the recommended model chemistry employs M05-2X/cc-pVTZ Gibbs energies of solvation with a single discrete water molecule and a high-level composite method for the gas-phase reaction energies. With the SD cycle, these calculations result in a mean unsigned error of less than 1 pK(a) units, with respective mean signed error and maximum unsigned error of less than 0.5 and 2 pK(a) units. Similar results are obtained with the D and PX cycles, and further improvement is required in both the gas and aqueous phase ab initio energy calculations before we can truly discriminate between the thermodynamic cycles investigated here.

Selective adsorption to particular crystal faces of ZnO.

We examine the hypothesis that selective adsorption to a particular face of ZnO is responsible for the ability of small organic molecules to control the aspect ratio of ZnO crystals during hydrothermal synthesis. Large, single crystals of ZnO were prepared such that the vast majority of a surface consisted of a single crystal plane, as shown by atomic force microscopy, and the adsorption to a single crystal plane was determined by attenuated total reflectance spectroscopy. The results show that citrate strongly and selectively adsorbs to the (0001) face. Similarly, results show that ethylenediamine selectively adsorbs to the (1010) face. Each of these results separately shows a correlation between selective adsorption to and growth of large areas of a particular face, and thus, each result is consistent with the proposed hypothesis.

Differential etching of ZnO native planes under basic conditions.

The in situ dissolution of polished (0001), (101(-)0), and (0001(-)) surfaces of ZnO was studied using Atomic Force Microscopy under alkaline conditions. In aqueous NaOH solution the (0001) plane forms a stepped surface whereas the (0001(-)) plane converts into more stable {101(-)1(-)} planes. Dissolution of the (101(-)0) plane leaves a combination of (0001) and (101(-)1(-)) planes. Dissolution in solutions containing both NaOH and Na(3)citrate causes the (0001) plane steps to increase in number and reduce in height, and cause an overall increase in the rate of dissolution in the [101(-)0] directions. These observations are explained using a mechanism based on edgewise dissolution where the etching rate depends on the number of surface oxygen atoms per zinc atom. Large areas of single index faces (over 50 µm(2)) of (0001) and (0001(-)), suitable for surface chemistry studies, were also generated by chemical dissolution.

Temperature-responsive solid-liquid separations with charged block-copolymers of poly(N-isopropyl acryamide).

Temperature responsive charged block-copolymers of poly(N-isopropylacrylamide) (PNIPAM) have been used in the solid-liquid separation of alumina mineral particles from aqueous solution. The effects of temperature, polymer charge-sign and fraction of charged segment have been investigated. Batch settling and adsorption studies showed that rapid sedimentation results for suspensions with polymers of opposite charge-sign to the particle surface-charge (counterionic) at 50 °C. Cooling the suspensions after flocculation at 50 °C was found to increase the final solids volume fraction of the sediment beds formed through a mechanism related to partial desorption of polymer and the reduction of the hydrophobic attraction. Suspension stability results after dosing with polymers of similar charge-sign to the particle surface-charge (co-ionic) at both 25 and 50 °C. Increasing the amount of polymer charge increased the influence of polymer charge-sign on the adsorption and solid-liquid separation behavior. The performance of the charged block copolymers are compared to that of the random charged copolymer and neutral homopolymer PNIPAM structures.

Temperature responsive flocculation and solid-liquid separations with charged random copolymers of poly(N-isopropyl acrylamide).

Temperature-responsive random copolymers based on poly(N-isopropyl acrylamide) (PNIPAM) with 15 mol% of either acrylic acid or dimethylaminoethyl acrylate quaternary chloride were prepared. The effect of the charge and its sign were investigated in the solid-liquid separation of silica and alumina mineral suspensions. The results were compared to PNIPAM homopolymer of similar molecular weight. PNIPAM copolymers of the same charge as the particles (co-ionic PNIPAM) act as dispersants at both 25°C and 50°C. Flocculation occurs when counter-ionic PNIPAM facilitates selective aggregation and rapid sedimentation of minerals at both 25°C and 50°C. Adsorption and desorption studies showed that, unlike non-ionic PNIPAM, little desorption of the counter-ionic copolymers from the oxides occurred after cooling a suspension from 50°C to below the lower critical solution temperature. Thus, incorporation of counter-ionic charge into the temperature sensitive polymer PNIPAM was found to reduce the sediment bed consolidation upon cooling when compared to PNIPAM homopolymers. The lack of secondary consolidation upon cooling is attributed to attractive inter-particle forces, such as conventional polyelectrolyte flocculation mechanisms (bridging, charge neutralization or charge patch) which persist at both 25°C and 50°C when counter-ionic PNIPAM is used. On the other hand, it was possible to obtain rapid sedimentation with the counter-ionic PNIAPMs even when they were added to the suspension already at 50°C, a process which has not been possible with neutral PNIPAM homopolymers.

The size of particle aggregates produced by flocculation with PNIPAM, as a function of temperature.

This work investigates the effect of temperature on the size of alumina aggregates formed by flocculation with temperature responsive Poly(N-Isopropylacrylamide)(PNIPAM). The results are discussed in terms of the effects of temperature on particle collision, particle adhesion and aggregate breakage. It was found that the size of alumina aggregates increases with increasing solution temperature. Particle/particle collision and aggregate breakage are largely unaffected by increasing solution temperature and therefore could not account for the change in aggregate size. The dominant factor in aggregate growth with increasing temperature was found to be the increase in the force of adhesion between alumina particles. The appearance of the adhesive force is triggered by the increase in temperature above the lower critical solution temperature of PNIPAM.

Photochromic, metal-absorbing honeycomb structures.

We demonstrate the synthesis and use of a spiropyran functional polymer to form highly ordered honeycomb materials by the breath figure technique, which is based on the self-assembly of water droplets. These materials undergo rapid and intense color changes both in solution and as porous films by irradiation with light (UV or visible). We also demonstrate the metal binding ability of these polymers ultimately to create hybrid organic-inorganic porous structures. Furthermore, by reduction of the metal and calcination of the organic materials, unique palladium microrings can be prepared. The methods described are general techniques that may be applied to a range of heavy metals.