Concentrated Aqueous Dispersions Research Articles

Noncovalent Functionalization of Graphene Nanoplatelets and Their Applications in Supercapacitors.

We demonstrate a simple noncovalent functionalization technique, which involves graphite exfoliation and subsequent coating of the resulting graphene nanoplatelets (GNPs) with trimellitic anhydride (TMA), using a thermomechanical exfoliation process. TMA adsorbs on the surface of the GNPs, resulting in a reduction of the specific surface area to 312 ± 9 m2/g compared to 410 ± 12 m2/g for the unmodified GNPs. Detailed imaging, thermogravimetric, and X-ray diffraction analysis showed that the modified GNPs (TMA-GNPs) maintain similar structure to the unmodified GNPs. The presence of functional groups, confirmed by X-ray photoelectron spectroscopy analysis, caused an increase in the surface energy from 45.6 mJ/m2 for the GNPs to 57.9 mJ/m2 for TMA-GNPs. The resulting coated TMA-GNPs form stable dispersions in water while maintaining their inherent conductive properties, thus enabling applications, such as the manufacture of conductive films and supercapacitors. As a proof-of-concept, electrodes for supercapacitors are prepared from concentrated aqueous dispersions of the functionalized GNPs. Electrochemical characterization of the supercapacitors using electrochemical impedance spectroscopy, cyclic voltammetry and galvanostatic charge/discharge tests showed a specific capacitance of 22.2 F/cm3 at a scan rate of 1 mV/s from cyclic voltammetry and 17.3 F/cm3 at a current density of 1 A/g from galvanostatic charge/discharge tests, with a 90% capacitance retention after 10,000 cycles.

Mucoadhesive pickering nanoemulsions via dynamic covalent chemistry

Hypothesis. Submicron oil droplets stabilized using aldehyde-functionalized nanoparticles should adhere to the primary amine groups present at the surface of sheep nasal mucosal tissue via Schiff base chemistry. Experiments. Well-defined sterically-stabilized diblock copolymer nanoparticles of 20 nm diameter were prepared in the form of concentrated aqueous dispersions via reversible addition-fragmentation chain transfer (RAFT) aqueous emulsion polymerization of 2,2,2-trifluoroethyl methacrylate (TFEMA) using a water-soluble methacrylic precursor bearing cis-diol groups. Some of these hydroxyl-functional nanoparticles were then selectively oxidized using an aqueous solution of sodium periodate to form a second batch of nanoparticles bearing pendent aldehyde groups within the steric stabilizer chains. Subjecting either hydroxyl- or aldehyde-functional nanoparticles to high-shear homogenization with a model oil (squalane) produced oil-in-water Pickering macroemulsions of 20–30 µm diameter. High-pressure microfluidization of such macroemulsions led to formation of the corresponding Pickering nanoemulsions with a mean droplet diameter of around 200 nm. Quartz crystal microbalance (QCM) experiments were used to examine adsorption of both nanoparticles and oil droplets onto a model planar substrate bearing primary amine groups, while a fluorescence microscopy-based mucoadhesion assay was developed to assess adsorption of the oil droplets onto sheep nasal mucosal tissue. Findings. Squalane droplets coated with aldehyde-functional nanoparticles adhered significantly more strongly to sheep nasal mucosal tissue than those coated with the corresponding hydroxyl-functional nanoparticles. This difference was attributed to the formation of surface imine bonds via Schiff base chemistry and was also observed for the two types of nanoparticles alone in QCM studies. Preliminary biocompatibility studies using planaria indicated only mild toxicity for these new mucoadhesive Pickering nanoemulsions, suggesting potential applications for the localized delivery of hydrophobic drugs.

Efficient Green Synthesis of Highly Concentrated and Stable Carbon Nanotube Aqueous Dispersions for Photothermal Textile Fabrication

To maximize the performance of carbon nanotubes (CNTs) in photothermal conversion textiles, the green and efficient methods for the preparation of highly concentrated and stable aqueous dispersion of CNTs need to be urgently developed. Herein, the aqueous dispersion of CNTs with exceeding high concentration limits (over 100 mg/mL) is synthesized by an eco-friendly one-step hydrothermal method with the assistance of ionic liquid crystals (ILC). CNTs non-covalently wrapped by ILC can exist in various forms, such as well-dispersed dispersion, conductive slurry, thixotropic gel, and deformable dough. Particularly, the highly concentrated dispersion has excellent dispersion stability and remains stable even after 60 days of resting. Due to the cationic head group of ILC, the positively charged dispersion was electrostatically self-assembled to prepare photothermal textiles with excellent mechanical robustness and chemical stability, which can be applied to multiple harsh working conditions. This research shows an efficient and feasible method for the non-destructive dispersion of CNTs at high concentrations and also provides a reference for photothermal conversion applications of CNTs.

Preparation of concentrated stabilizer-free dispersions of uniform silver nanoparticles

Stable concentrated aqueous dispersions of uniform 10 nm silver nanoparticles were prepared in absence of colloid stabilizers by reducing silver nitrate with phloroglucinol at room temperature and elevated pH. The key feature of the precipitation process entails triggering suddenly the electrons transfer between the reacting species coexisting in a homogeneous solution (‘latent reaction mixture’). The ensuing silver nucleation occurs rapidly and uniformly in the entire reaction volume leading to the formation of highly uniform dispersed nanoparticles. The phloroglucinol oxidation byproducts play an important role in preventing the aggregation of nanoparticles and facilitating the preparation of highly stable dispersions containing up to 62 wt% Ag (5.7 mol.dm−3). A mechanism rationalizing the long-term stability of silver sols at such high metal content is proposed. The rheological properties of concentrated dispersions permit the inkjet printing of electrically conductive patterns suitable for incorporation in sensors, RFID tags, solar cells, displays, and other electronic devices.

Efficient transition metal dichalcogenides exfoliation by cellulose nanocrystals for ultrabroad-pH/temp stable aqueous dispersions and multi-responsive photonic films

A major challenge in developing 2D transition-metal dichalcogenides nanosheets (TMD ns) as nanofillers, catalysts and photothermal reagents lies in its colloidal stability and scalable production. Herein, a biomimetic hybrid strategy to efficiently exfoliate and stably disperse TMD ns in a low-cost, environment-friendly manner is demonstrated. Cellulose nanocrystal (CNC) with controllable morphology and surface chemistry is acted as ideal assisted exfoliation reagent and stabilizer for highly concentrated and thin-layer TMD ns aqueous dispersions. Ultrabroad-pH/temperature (pH = 1–13, temperature = 1–95 °C) stable aqueous dispersions of TMD ns/CNC hybrids and the self-assembly of CNC allow the successful construction of flexible, rapid, and multi-stimuli responsive photonic films, e.g., mechanical strain, humidity and photothermal, providing an avenue for artificial skin, flexible sensor, and smart screen & window applications.

Nonionic surfactant stabilized polytetrafluoroethylene dispersion: Effect of molecular structure and topology

Polytetrafluoroethylene (PTFE) is a kind of fluoropolymer with many critical applications. Nonionic surfactant stabilized concentrated PTFE aqueous dispersions are widely used to prepare corresponding products via dipping, spraying, spinning, or film casting, as these surfactants exert a negligible effect on the properties of the products. Although many nonionic surfactants have been used to stabilize PTFE dispersions in industry, the effects of molecular structure and topology are not well-documented; thus, currently, there is no basis for selecting or designing suitable novel surfactants to enhance dispersion stability. In this work, five nonionic surfactants with different topological structures including Tween 20, Tween 60, Tween 80, Brij L23, and Brij O20, were used to prepare concentrated PTFE dispersions. The variation of particle morphology before and after concentration was examined by scanning electron microscopy, and the static stability of the dispersions was quantitatively and objectively characterized using a Turbiscan analyzer based on multiple light scattering. Then the effect of surfactant molecular structure and topology on dispersion stability was discussed. We found that, after the enrichment process, the PTFE microspheres were tightly packed, and some were deformed. Brij surfactants which bears a linear hydrophilic oxyethylene (EO) chain can form a thicker hydration layer on the surface of PTFE particles than those of Tween surfactants which contains a branched hydrophilic EO chain. Accordingly, PTFE dispersions with Brij surfactants show better static stability. Moreover, in comparison with the saturated hydrophobic tail, the unsaturated hydrophobic chain is more flexible, so it can adhere to the surface of PTFE particles better, improving the stability. In addition, the increase of surfactant concentration contributes to the increase of micelles density and viscosity of the bulk dispersion, which can also enhance the stability. These findings are beneficial to design or choose suitable surfactants for high stability and high-quality concentrated PTFE dispersions.

High-Yield Production of Nano-Lateral Size Graphene Oxide by High-Power Ultrasonication.

Nanographene oxide (GOn) constitutes a nanomaterial of high value in the biomedical field. However, large scale production of highly stable aqueous dispersions of GOn is yet to be achieved. In this work, we explored high-power ultrasonication as a method to reduce particle size of GO and characterized the impact of the process on the physicochemical properties of the material. GOn was obtained with lateral dimensions of 99 ± 43 nm and surface charge of −39.9 ± 2.2 mV. High-power ultrasonication enabled an improvement of stability features, particularly by resulting in a decrease of the average particle size, as well as zeta potential, in comparison to GO obtained by low-power exfoliation and centrifugation (287 ± 139 nm; −29.7 ± 1.2 mV). Remarkably, GOn aqueous dispersions were stable for up to 6 months of shelf-time, with a global process yield of 74%. This novel method enabled the production of large volumes of highly concentrated (7.5 mg mL−1) GOn aqueous dispersions. Chemical characterization of GOn allowed the identification of characteristic oxygen functional groups, supporting high-power ultrasonication as a fast, efficient, and productive process for reducing GO lateral size, while maintaining the material’s chemical features.

Dense nanolipid fluid dispersions comprising ibuprofen: Single step extrusion process and drug properties

Dense nanolipid fluid (DNLF) dispersions are highly concentrated aqueous dispersions of lipid nanocarriers (LNCs) with more than 1015 lipid particles per cubic centimeter. Descriptions of dense nanolipid fluid dispersions in the scientific literature are rare, and they have not been used to encapsulate drugs. In this paper we describe the synthesis of DNLF dispersions comprising ibuprofen using a recently described twin-screw extrusion process. We report that such dispersions are stable, bind ibuprofen tightly and yet provide high transdermal drug permeation. Ibuprofen DNLF dispersions prepared according to the present study provide up to five times greater flux of the pharmacologically active S-ibuprofen isomer through human skin than a commercially available racemic ibuprofen emulsion product. We demonstrate scaling up the twin-screw extrusion method to pilot production for a stable, highly permeating ibuprofen DNLF composition based on excipients approved by the US FDA for use in topical products as a key step towards development of a commercially viable, FDA approvable topical ibuprofen medicine to treat osteoarthritis, which has never before been accomplished.

Long-Term Colloidally Stable Aqueous Dispersions of ≤5 nm Spinel Ferrite Nanoparticles.

Applications in biomedicine and ferrofluids, for instance, require long‐term colloidally stable, concentrated aqueous dispersions of magnetic, biocompatible nanoparticles. Iron oxide and related spinel ferrite nanoparticles stabilized with organic molecules allow fine‐tuning of magnetic properties via cation substitution and water‐dispersibility. Here, we synthesize≤5 nm iron oxide and spinel ferrite nanoparticles, capped with citrate, betaine and phosphocholine, in a one‐pot strategy. We present a robust approach combining elemental (CHN) and thermal gravimetric analysis (TGA) to quantify the ratio of residual solvent molecules and organic stabilizers on the particle surface, being of particular accuracy for ligands with heteroatoms compared to the solvent. SAXS experiments demonstrate the long‐term colloidal stability of our aqueous iron oxide and spinel ferrite nanoparticle dispersions for at least 3 months. By the use of SAXS we approved directly the colloidal stability of the nanoparticle dispersions for high concentrations up to 100 g L−1.

Effect of molecular weight of polcarboxylate surfactant on properties of concentrated aqueous titanium dioxide dispersions

Universal colorants used for point-of-sale tinting or dealer tinting system usually have titanium dioxide dispersion as one of the essential colorants to adjust “chroma” of the shade. It is imperative that this colorant exhibits optimum performance in water-based and solvent-based products. The paper focuses on the effect of varying molecular weight of polycarboxylate-based surfactants on the properties of the aqueous dispersion.

Рrospects of research development and new applications of concentrated artificial latexes

The phenomenon of strong stabilization of concentrated aqueous dispersions (emulsions) of polymers by colloidal particles of organic nature (microparticles of synthetic latexes), not accompanied by a significant increase in effective viscosity, opened, firstly, the possibility of carrying out the processes of pigmenting them at low energy costs with a maximum yield (dynamic state, meets the conditions of intensification of heterogeneous chemical-technological processes) and, secondly, determined the directions of further research in this area. So, in contrast to the classical method of emulsion stabilization by mineral microparticles adsorbed directly on the surface of their droplets, microparticles of organic origin interact with them through an interlayer of the dispersion medium, localizing in the immediate vicinity of the dispersed phase and thus maintaining a high mobility of the interphase boundaries. In this regard, the proposed method for predicting the type of emulsions of polymers, taking into account the contact wetting angle and the wetting area of the surface of solid particles of a stabilizer with water and a polymer, requires further verification on various systems. The obtained data of fundamental importance also allowed us to propose new applications of stabilized concentrated artificial latexes for solving applied problems.

Uniform shape monodisperse single chain nanocrystals by living aqueous catalytic polymerization

The preparation of polymer nanoparticles with a uniform size and shape, beyond spheres, is an unresolved problem. Here we report a living aqueous catalytic polymerization, resulting in particles grown by a single active site and composed of a single ultra high molecular weight polyethylene (UHMWPE) chain. The control on a molecular level (Mw/Mn = 1.1–1.2) and at the same time on a particle level (PDI < 0.05) together with the immediate deposition of the growing chain on the growing nanocrystal results in a distinct evolution of the particle morphology over time. These uniform nanocrystals are obtained as concentrated aqueous dispersions of > 10 wt-% (N ≈ 1019 particles L−1) polymer content. Key to this robust procedure to single chain nanoparticles are long-lived water-stable Ni(II) catalysts that do not undergo any chain transfer. These findings are a relevant step towards polymer materials based on nanoparticle assembly.

Absorption spectra of nanodiamond aqueous dispersions by optical absorption and optoacoustic spectroscopies

The multispectral modality and technique for optically dense samples of optoacoustic spectroscopy were applied to measure spectra and high absorbances of concentrated aqueous dispersions of undoped nanodiamonds. The data from optoacoustic and optical transmission measurements and DSC data of the mean particle size by the Gibbs–Kelvin equation are compared to estimate the difference in composition of various nanodiamond trademarks. Optoacoustic spectra confirm the contribution of surface dimer chains into the absorption of nanodiamonds in the long wavelength range. Optoacoustic and conventional absorption spectra of aqueous solutions of nanodiamond fractions after centrifugation (15300g) and ultracentrifugation (130000g) revealed a separation of a highly absorbing non-diamond sp2 phase. The two-step separation by ultracentrifugation followed by extra centrifugation made it possible to isolate a highly absorbing and soluble nanodiamond phase with the particle size of 3.6 nm, showing a change in spectra compared to the starting nanodiamond material.

Rheological study of concentrated dispersions. Application to the drilling fluid.

In order to understand the rheological behavior of concentrated aqueous Algerian bentonite dispersions of drilling (sodium bentonite of Mostaganem “m’zila”), rheological tests were carried out. By varying the concentration of bentonite, flow tests have allowed to estimate the yield stress and apparent viscosity for each concentration and to see their influence on the rheological behavior of these dispersions. In addition dynamic tests (oscillatory) are used to define the linear region of our samples, the state of our fluid (elastic solid or viscous liquid) and understanding the mechanisms of structuring of the particles constituting the material. In parallel, other tests coupled with rheological measurements such as x-rays diffraction to know the mineralogical composition and granulometry to estimate the bentonite particle size.

Using Host-Guest Chemistry to Tune the Kinetics of Morphological Transitions Undertaken by Block Copolymer Vesicles.

Host-guest chemistry is exploited to tune the rate at which block copolymer vesicles undergo morphological transitions. More specifically, a concentrated aqueous dispersion of poly(glycerol monomethacrylate-co-glycidyl methacrylate)-poly(2-hydroxypropyl methacrylate) [P(GMA-co-GlyMA)-PHPMA] diblock copolymer vesicles was prepared via polymerization-induced self-assembly (PISA). The epoxy groups in the GlyMA residues were ring-opened using a primary amine-functionalized β-cyclodextrin (NH2-β-CD) in order to prepare β-CD-decorated vesicles. Addition of azobenzene-methoxypoly(ethylene glycol) (azo-mPEG) to such vesicles results in specific binding of this water-soluble macromolecular reagent to the β-CD groups on the hydrophilic P(GMA-co-GlyMA) stabilizer chains. Such host-guest chemistry induces a morphological transition from vesicles to worms and/or spheres. Furthermore, the rate of this morphological transition can be tuned by UV/visible-light irradiation and/or guest molecule competition. This novel molecular recognition strategy offers considerable scope for the design of new stimulus-responsive diblock copolymer vesicles for targeted delivery and controlled release of cargoes.

Bespoke Diblock Copolymer Nanoparticles Enable the Production of Relatively Stable Oil-in-Water Pickering Nanoemulsions.

Sterically stabilized diblock copolymer nanoparticles with an intensity-average diameter of 25 nm are prepared in the form of a concentrated aqueous dispersion using polymerization-induced self-assembly (PISA). The addition of n-dodecane followed by high-shear homogenization produces n-dodecane-in-water Pickering macroemulsions of 22–46 μm diameter. If the nanoparticles are present in sufficient excess, then subsequent processing using a high-pressure microfluidizer leads to the formation of Pickering nanoemulsions with a mean oil droplet diameter below 200 nm. The size of these Pickering nanoemulsions can be tuned by systematically varying the nanoparticle concentration, applied pressure, number of passes, and oil volume fraction. High-internal-phase emulsions can also be achieved by increasing the n-dodecane volume fraction up to 0.80. TEM studies of (dried) n-dodecane droplets confirm the presence of intact nanoparticles and suggest a relatively high surface coverage, which is consistent with model packing calculations based on radius ratios. Such Pickering nanoemulsions proved to be surprisingly stable with respect to Ostwald ripening, with no significant change in the mean DLS droplet diameter after storage for approximately 4 months at 20 °C.

Preparation of water-soluble graphene nanoplatelets and highly conductive films

This paper tackles the challenge of preparation stable, highly concentrated aqueous graphene dispersions. Despite tremendous recent interest, there has been limited success in developing a method that ensures the total dispersion of non-oxidized, defect-free graphene nanosheets in water. This study successfully demonstrates that few-layer graphene nanoplatelets (GNPs) can form highly concentrated aqueous colloidal solutions after they have been pretreated in a low-concentration inorganic sodium-hypochlorite and sodium-bromide salted aqueous solvent. This method retains the graphitic structure as evidenced by nuclear magnetic resonance spectroscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. Vacuum-filtrated freestanding films demonstrate an electrical conductivity as high as 3000 S m−1. This dispersion technique is believed to be applicable not only for GNPs, but also for dispersing other types of graphitic materials, including fullerenes, single/double/multi-walled carbon nanotubes, graphene nanoribbons and etc.

Time-Resolved SAXS Studies of the Kinetics of Thermally Triggered Release of Encapsulated Silica Nanoparticles from Block Copolymer Vesicles.

Silica-loaded poly(glycerol monomethacrylate)-poly(2-hydroxypropyl methacrylate) diblock copolymer vesicles are prepared in the form of concentrated aqueous dispersions via polymerization-induced self-assembly (PISA). As the concentration of silica nanoparticles present during the PISA synthesis is increased up to 35% w/w, higher degrees of encapsulation of this component within the vesicles can be achieved. After centrifugal purification to remove excess non-encapsulated silica nanoparticles, SAXS, DCP, and TGA analysis indicates encapsulation of up to hundreds of silica nanoparticles per vesicle. In the present study, the thermally triggered release of these encapsulated silica nanoparticles is examined by cooling to 0 °C for 30 min, which causes in situ vesicle dissociation. Transmission electron microscopy studies confirm the change in diblock copolymer morphology and also enable direct visualization of the released silica nanoparticles. Time-resolved small-angle X-ray scattering is used to quantify the extent of silica release over time. For an initial silica concentration of 5% w/w, cooling induces a vesicle-to-sphere transition with subsequent nanoparticle release. For higher silica concentrations (20 or 30% w/w) cooling only leads to perforation of the vesicle membranes, but silica nanoparticles are nevertheless released through the pores. For vesicles prepared in the presence of 30% w/w silica, the purified silica-loaded vesicles were cooled to 0 °C for 30 min, and SAXS patterns were collected every 15 s. A new SAXS model has been developed to determine both the mean volume fraction of encapsulated silica within the vesicles and the scattering length density. Satisfactory data fits to the experimental SAXS patterns were obtained using this model.

Sonochemical synthesis and stabilization of concentrated antimicrobial silver‐chitosan nanoparticle dispersions

ABSTRACTThis work reports on a green synthetic route to produce concentrated aqueous dispersions of silver nanoparticles (AgNP) employing high‐intensity ultrasound (US) and chitosan (CS) as a nontoxic reducing agent for Ag+ salts and AgNP stabilizer. The sonication simultaneously boosted the synthesis and improved the stability of the AgNP, capping them with CS. Hybrid AgNP‐CS antimicrobial dispersions, stable for at least 6 months, were synthesized in a simple single step process. The use of US allowed for applying relatively mild processing temperatures (60 °C) and reaction time between 30 min and 3 h to obtain concentrated dispersions of AgNP that otherwise could not be obtained even after 72 h under mechanical stirring at the same reaction conditions. Upon sonication spherical AgNP‐CS with a size between 60 and 100 nm were generated, in contrast to the average diameter of ∼200 nm of the particles obtained by stirring. The antibacterial efficiency of the AgNP‐CS hybrids was evaluated against the medically relevant pathogens Staphylococcus aureus and Escherichia coli. The US‐synthesized AgNP‐CS showed more than three fold higher antibacterial activity compared to the particles obtained under stirring, due to their higher concentration and smaller size. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45136.

Preparation and Cross-Linking of All-Acrylamide Diblock Copolymer Nano-Objects via Polymerization-Induced Self-Assembly in Aqueous Solution.

Various carboxylic acid-functionalized poly(N,N-dimethylacrylamide) (PDMAC) macromolecular chain transfer agents (macro-CTAs) were chain-extended with diacetone acrylamide (DAAM) by reversible addition–fragmentation chain transfer (RAFT) aqueous dispersion polymerization at 70 °C and 20% w/w solids to produce a series of PDMAC–PDAAM diblock copolymer nano-objects via polymerization-induced self-assembly (PISA). TEM studies indicate that a PDMAC macro-CTA with a mean degree of polymerization (DP) of 68 or higher results in the formation of well-defined spherical nanoparticles with mean diameters ranging from 40 to 150 nm. In contrast, either highly anisotropic worms or polydisperse vesicles are formed when relatively short macro-CTAs (DP = 40–58) are used. A phase diagram was constructed to enable accurate targeting of pure copolymer morphologies. Dynamic light scattering (DLS) and aqueous electrophoresis studies indicated that in most cases these PDMAC–PDAAM nano-objects are surprisingly resistant to changes in either solution pH or temperature. However, PDMAC40–PDAAM99 worms do undergo partial dissociation to form a mixture of relatively short worms and spheres on adjusting the solution pH from pH 2–3 to around pH 9 at 20 °C. Moreover, a change in copolymer morphology from worms to a mixture of short worms and vesicles was observed by DLS and TEM on heating this worm dispersion to 50 °C. Postpolymerization cross-linking of concentrated aqueous dispersions of PDMAC–PDAAM spheres, worms, or vesicles was performed at ambient temperature using adipic acid dihydrazide (ADH), which reacts with the hydrophobic ketone-functionalized PDAAM chains. The formation of hydrazone groups was monitored by FT-IR spectroscopy and afforded covalently stabilized nano-objects that remained intact on exposure to methanol, which is a good solvent for both blocks. Rheological studies indicated that the cross-linked worms formed a stronger gel compared to linear precursor worms.