Aggregation Of Silica Nanoparticles Research Articles

Shear Thickening Electrolyte Built from Sterically Stabilized Colloidal Particles.

We present a method to prepare shear thickening electrolytes consisting of silica nanoparticles in conventional liquid electrolytes with limited flocculation. These electrolytes rapidly and reversibly stiffen to solidlike behaviors in the presence of external shear or high impact, which is promising for improved lithium ion battery safety, especially in electric vehicles. However, in initial chemistries the silica nanoparticles aggregate and/or sediment in solution over time. Here, we demonstrate steric stabilization of silica colloids in conventional liquid electrolyte via surface-tethered PMMA brushes, synthesized via surface-initiated atom transfer radical polymerization. The PMMA increases the magnitude of the shear thickening response, compared to the uncoated particles, from 0.311 to 2.25 Pa s. Ultrasmall-angle neutron scattering revealed a reduction in aggregation of PMMA-coated silica nanoparticles compared to bare silica nanoparticles in solution under shear and at rest, suggesting good stabilization. Conductivity tests of shear thickening electrolytes (30 wt % solids in electrolyte) at rest were performed with interdigitated electrodes positioned near the meniscus of electrolytes over the course of 24 h to track supernatant formation. Conductivity of electrolytes with bare silica increased from 10.1 to 11.6 mS cm-1 over 24 h due to flocculation. In contrast, conductivity of electrolytes with PMMA-coated silica remained stable at 6.1 mS cm-1 over the same time period, suggesting good colloid stability.

Towards the development of superhydrophilic SiO2-based nanoporous coatings: concentration and particle size effect

Superhydrophilic nanoporous coatings were prepared from layer-by-layer assembled silica nanoparticles (SiO2) and a polycation. As expected, concentration of silica dipping suspension and particle size are closely related to thickness of the obtained multilayer films. However, wettability might not to be mainly governed by coating thickness as usually reported on literature; instead, superficial homogeneity should be carefully considered. It was found that if a uniform nanoporous structure is obtained, superhydrophilic behavior is observed and, at least for the studied system, increases in coating thickness are not followed by improvements on wetting performance. In turn, for obtaining uniform structures, the usage of dipping suspension with a hypothetical minimum silica nanoparticles concentration is mandatory. Below such critical value, stacking defects along the coating surface are observed and the manifestation of superhydrophilicity is inhibited. As an additional feature, low refractive indices of multilayer coatings resulting from nanoporosity was found to provide good antireflection (AR) properties.

Predicting the DNP-SENS efficiency in reactive heterogeneous catalysts from hydrophilicity.

Identification of surfaces at the molecular level has benefited from progress in dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP SENS). However, the technique is limited when using highly sensitive heterogeneous catalysts due to secondary reaction of surface organometallic fragments (SOMFs) with stable radical polarization agents. Here, we observe that in non-porous silica nanoparticles (NPs) (dparticle = 15 nm) some DNP enhanced NMR or SENS characterizations are possible, depending on the metal-loading of the SOMF and the type of SOMF substituents (methyl, isobutyl, neopentyl). This unexpected observation suggests that aggregation of the nanoparticles occurs in non-polar solvents (such as ortho-dichlorobenzene) leading to (partial) protection of the SOMF inside the interparticle space, thereby preventing reaction with bulky polarization agents. We discover that the DNP SENS efficiency is correlated with the hydrophilicity of the SOMF/support, which depends on the carbon and SOMF concentration. Nitrogen sorption measurements to determine the BET constant (CBET) were performed. This constant allows us to predict the aggregation of silica nanoparticles and consequently the efficiency of DNP SENS. Under optimal conditions, CBET > 60, we found signal enhancement factors of up to 30.

Insight into the stability of hydrophilic silica nanoparticles in seawater for Enhanced oil recovery implications

The stability of nanoparticles in the aqueous phase is a major challenge in the application of nanoparticles in Enhanced Oil Recovery (Nano-EOR) processes. Previous studies evaluated the performance of nanoparticles for EOR purposes; either deionized water or water at very low ionic strength was used. Nanoparticles can be easily dispersed in the deionized or low salinity water, whereas they are extremely unstable in high salinity seawater or formation water. Typically, seawater or formation brine is injected for water-flooding and EOR purposes. If we want to change the fluid–fluid or fluid-rock properties by injecting nanoparticle enhanced water, then, the stability of the nanoparticles in high salinity water is extremely important. In this work, a novel and simple method to stabilize silica nanoparticles in seawater is proposed. First, the aggregation of silica nanoparticles in the presence of different ions is investigated. The results show that the presence of positive multivalent ions in the electrical double layer around nanoparticles can destabilize silica nanoparticles. In order to reduce the concentration of positive multivalent ions around silica nanoparticles, a theory based on “H+ protection” is proposed and its effectiveness is tested by particle size, turbidity, zeta-potential, and pH measurements. The effect of the concentrations of nanoparticles and HCl on the stability of silica nanoparticles in seawater is evaluated. Experimental results show that H+ protection, which can be obtained by adding HCl to the solution, can effectively stabilize silica nanoparticles in seawater. The experiments show that the size of nanoparticles in the seawater directly depends on the concentration of nanoparticles and inversely to the HCl concentration.

Silica nanoparticle aggregation in calcite replacement reactions

Natural nanoparticles are fundamental building blocks of Earth’s bio- and geosphere. Amorphous silica nanoparticles are ubiquitous in nature, but fundamental knowledge of their interaction mechanisms and role in mineral replacement reactions is limited. Here we show how silica nanoparticles replace Cretaceous calcite bivalve shells in a volume- and texture-preserving process. Electron tomography reveals that mineral replacement transfers calcite crystallographic orientations to twinned photonic crystals composed of face-centered cubic silica sphere stacks. During the face-specific replacement process, silica nanoparticles continuously nucleate, aggregate, and form a lattice of uniform spheres parallel to calcite low-energy facets. We explain the replacement process with a new model that unifies recently proposed, probably universal mechanisms of interface-coupled dissolution-precipitation and aggregation-based crystallization; both key mechanisms in geological processes and nanomaterials design and synthesis.

Self‐Healable Organogel Nanocomposite with Angle‐Independent Structural Colors

AbstractStructural colors have profound implications in the fields of pigments, displays and sensors, but none of the current non‐iridescent photonic materials can restore their functions after mechanical damage. Herein, we report the first self‐healable organogel nanocomposites with angle‐independent structural colors. The organogel nanocomposites were prepared through the co‐assembly of oleophilic silica nanoparticles, silicone‐based supramolecular gels, and carbon black. The organogel system enables amorphous aggregation of silica nanoparticles and the angle‐independent structural colors in the nanocomposites. Moreover, the hydrogen bonding in the supramolecular gel provides self‐healing ability to the system, and the structural colored films obtained could heal themselves in tens of seconds to restore storage modulus, structural color, and surface slipperiness from mechanical cuts or shear failure repeatedly.

Self‐Healable Organogel Nanocomposite with Angle‐Independent Structural Colors

Structural colors have profound implications in the fields of pigments, displays and sensors, but none of the current non-iridescent photonic materials can restore their functions after mechanical damage. Herein, we report the first self-healable organogel nanocomposites with angle-independent structural colors. The organogel nanocomposites were prepared through the co-assembly of oleophilic silica nanoparticles, silicone-based supramolecular gels, and carbon black. The organogel system enables amorphous aggregation of silica nanoparticles and the angle-independent structural colors in the nanocomposites. Moreover, the hydrogen bonding in the supramolecular gel provides self-healing ability to the system, and the structural colored films obtained could heal themselves in tens of seconds to restore storage modulus, structural color, and surface slipperiness from mechanical cuts or shear failure repeatedly.

Adenosine Triphosphate-Encapsulated Liposomes with Plasmonic Nanoparticles for Surface Enhanced Raman Scattering-Based Immunoassays.

In this study, we prepared adenosine triphosphate (ATP) encapsulated liposomes, and assessed their applicability for the surface enhanced Raman scattering (SERS)-based assays with gold-silver alloy (Au@Ag)-assembled silica nanoparticles (NPs; SiO2@Au@Ag). The liposomes were prepared by the thin film hydration method from a mixture of l-α-phosphatidylcholine, cholesterol, and PE-PEG2000 in chloroform; evaporating the solvent, followed by hydration of the resulting thin film with ATP in phosphate-buffered saline (PBS). Upon lysis of the liposome, the SERS intensity of the SiO2@Au@Ag NPs increased with the logarithm of number of ATP-encapsulated liposomes after lysis in the range of 8 × 106 to 8 × 1010. The detection limit of liposome was calculated to be 1.3 × 10−17 mol. The successful application of ATP-encapsulated liposomes to SiO2@Au@Ag NPs based SERS analysis has opened a new avenue for Raman label chemical (RCL)-encapsulated liposome-enhanced SERS-based immunoassays.

Structure and interaction in pathway of charged nanoparticles aggregation in saline water as probed by scattering techniques

The evolution of interaction and structure during the aggregation of anionic silica nanoparticles in saline water has been examined. The net attraction between charged nanoparticles with existing of salt ions is found to be significantly higher than that accounted by van der Waals force of the DLVO theory. The increase in the net attraction with salt leads to systematic transformation of nanoparticles into dimers, tetramers and higher-mers which finally results into fractal aggregate. Such multi-step aggregation is consistent with diffusion limited cluster aggregation (DLCA) fractal morphology but the fractal dimension (∼2.6) obtained is much larger than usually comprehended by DLCA.

Spontaneous Imbibition Investigation of Self-Dispersing Silica Nanofluids for Enhanced Oil Recovery in Low-Permeability Cores

A new kind of self-dispersing silica nanoparticle was prepared and used to enhance oil recovery in spontaneous imbibition tests of low-permeability cores. To avoid the aggregation of silica nanoparticles, a new kind of silica nanoparticle was prepared through the surface modification with vinyltriethoxysilane and 2-mercaptobenzimidazole as modified agents. Transmission electron microscopy, Fourier transform infrared spectroscopy, dynamic light scattering, and ζ potential measurements were employed to characterize the modified silica nanoparticles. Dispersing experiments indicated that modified silica nanoparticles had superior dispersity and stability in alkaline water. To evaluate the performance of silica nanofluids for enhanced oil recovery compared to pH 10 alkaline water and 5 wt % NaCl solution, spontaneous imbibition tests in sandstone cores were conducted. The results indicated that silica nanofluids can evidently improve oil recovery. To investigate the mechanism of nanoparticles for enhanced oil...

Assembly of nanoparticles-polyelectrolyte complexes in nanofiber cellulose structures

We report the effect of cationic polyacrylamide (CPAM) addition on the structural assembly of silica nanoparticles (NPs) within a nanocellulose fibre matrix. Paper like composites are fabricated by first forming complexes of NPs with CPAM, then adding those to a suspension of nanocellulose fibres; followed by filtration, pressing and drying of the final suspension. Complementary small angle X-ray scattering (SAXS) and microscopy (SEM, TEM) investigations of these composites showed a lognormal bimodal distribution of NP sizes. Data analysis allows understanding interparticle interactions within assemblies of SiO2 NPs at the nanometer scale with respect to different dosage of CPAM. Increasing CPAM dosage increases retention of NPs within the cellulose matrix with stronger interparticle interactions and produces composites with smaller pores. The correlation length of NPs, indicative of the size of the NP clusters increased from 30 to 70nm as the CPAM dosage increased from 16.5 to 330mg/g NPs. Retention and assembly of SiO2 NPs by varying CPAM dosage results from the balance of different interaction forces between NPs, CPAM and nanocellulose fibres. Understanding the effect of CPAM dosage on the various NP and composite structural conformations enables us to engineer novel hierarchically and functional cellulose based structured materials.

Enhanced hydrophobicity of polyurethane via non-solvent induced surface aggregation of silica nanoparticles

Fabrication of superhydrophobic surfaces from hydrophilic polymers has always been regarded as a challenge. In this study, to achieve superhydrophobic polyurethane (PU) surfaces, silica nanoparticles and ethanol as non-solvent were simultaneously utilized during a solution casting-based process. Such modified version of phase separation process was found to be highly efficient, and also it required much lower concentration of nanoparticles to achieve superhydrophobicity as compared to the previously reported methods in the literature. According to the proposed mechanism, non-solvent induces a more profound aggregation of silica nanoparticles at the surface’s top layer causing the surface energy to be highly diminished, and thus, the water repellency is improved. Morphology and topography results showed that a unique “triple-sized” structure was formed on the surface of superhydrophobic samples. X-ray photoelectron spectroscopy results proved that both PU macromolecules and silica nanoparticles were concurrently present at the surface layer of the superhydrophobic sample. It was concluded that surface composition and roughness could be regarded as competing factors in achieving superhydrophobicity. Based on the obtained results, the proposed method exhibits a promising potential in large-scale fabrication of surface layers with superhydrophobic property. Moreover, a mechanism was also presented to further explicate the physics behind the suggested method.

Visible wavelength-independent anti-reflection coatings generated from assembled SiO2 particles modified with tetraethoxysilane

The objective of this research was the fabrication of visible wavelength-independent anti-reflection coatings by deposition and aggregation of silica nanoparticles. This approach to anti-reflection coatings is important for technologies which avoid sophisticated process and calcination at high temperature. The goal of this research was achieved by modification of a commercially available SiO2 dispersion with tetraethoxysilane (TEOS) and spin coating of the TEOS–SiO2 (T–SiO2) dispersion. The modification resulted in aggregation of T–SiO2 particles upon evaporation of ethanol aqueous solution and thus assembled into many enlarged bulges at the coating surface. The resulted coating surface morphology resembled moth-eye-like nanostructure that exhibited visible wavelength-independent transmittance enhancement for substrate. More promisingly, owing to this unique behavior, the coated colored substrates display deepened colors without sacrificing any color hues. Such coating is very attractive in many fields such as color displaying and color imaging.

Semitransparent, durable superhydrophobic polydimethylsiloxane/SiO2 nanocomposite coatings on varnished wood

Abstract A superhydrophobic surface on wood can prevent water adhesion and absorption. However, the practical applications of such a non-wetting surface are hindered by the fragility of the microscopic roughness features that are essential for superhydrophobicity. In this study, semitransparent, mechanically durable superhydrophobic coating was successfully deposited on varnished wood via a simple dip-coating method based on polydimethysiloxane (PDMS)/silica (SiO2) nanocomposites. The surface microstructure and roughness of the hybrid coatings on wood are governed by the mass ratio of SiO2 to PDMS. When the nano-SiO2 particle content reaches a critical level, dual-scale features with micro-scale roughness superimposed with a nanostructure can be formed in the surface coatings, resulting in excellent water-repellency with a contact angle above 150° and a sliding angle below 10°. The coated wood surface maintained essentially the original colour and aesthetic appearance despite slightly declined optical transparency of the hybrid coating caused by aggregation of silica nanoparticles. Sandpaper abrasion tests revealed that the superhydrophobic PDMS/SiO2 hybrid coating on wood retained its microstructure and superhydrophobicity after being repeatedly abraded.

Superhydrophobic Surfaces with Very Low Hysteresis Prepared by Aggregation of Silica Nanoparticles During <I>In Situ</I> Urea-Formaldehyde Polymerization

We present a new method for the preparation of superhydrophobic materials by in situ aggregation of silica nanoparticles on a surface during a urea-formaldehyde (UF) polymerization. This is a one-step process in which a two-tier topography is obtained. The polymerization is carried out for 30, 60, 120, 180, and 240 min on silicon shards. Silicon surfaces are sintered to remove the polymer. SEM and AFM show both an increase in the area covered by the nanoparticles and their aggregation with increasing polymerization time. Chemical vapor deposition of a fluorinated silane in the presence of a basic catalyst gives these surfaces hydrophobicity. Deposition of this low surface energy silane is confirmed by the F 1s signal in XPS. The surfaces show advancing water contact angles in excess of 160 degrees with very low hysteresis (< 7) after 120 min and 60 min polymerization times for 7 nm and 14 nm silica, respectively. Depositions are successfully demonstrated on glass substrates after they are primed with a UF polymer layer. Superhydrophobic surfaces can also be prepared on unsintered substrates.

Effect of nanoparticles on the morphology and properties of PET/PP in situ microfibrillar reinforced composites

In this work, the effect of SiO2 nanoparticles on the morphology and properties of PET/PP in situ microfibrillar reinforced composites (MFC) obtained from slit die extrusion and hot stretching‐quenching was investigated. The scanning electron micrographs revealed that the nanoparticles could have different effects on microfibril formation of PET/PP MFC depending on their concentration. Addition of appropriate content of nanoparticles (e.g., 1.6 and 8 wt%) facilitated the PET droplet‐fibril transition during stretching due to the presence of nanosilica in the PP matrix, which increased the viscosity of PP matrix and then improved the droplet deformability. However, at higher loading (e.g., 12 wt%), the aggregation of silica nanoparticles around the PET droplets prevented disperse phase coalescence during drawing and then reduced the fibrillation ability of PET minor phase. The dynamic rheological test performed at 190°C showed that the increased spatial restriction of the nanoparticles improved the viscoelastic moduli and complex viscosity of PET/PP MFC. DSC results indicated that nanosilica had little heterogeneous nucleation effect on the PP matrix of PET/PP MFC. Additionally, nanoparticles could present drastic improvement in the degradation behavior of PET/PP MFC under thermo‐oxidative conditions and increased the modulus of PET/PP MFC. POLYM. COMPOS., 38:2718–2726, 2017. © 2015 Society of Plastics Engineers

From static micrographs to particle aggregation dynamics in three dimensions.

Studies on colloidal aggregation have brought forth theories on stability of colloidal gels and models for aggregation dynamics. Still, a complete link between developed frameworks and obtained laboratory observations has to be found. In this work, aggregates of silica nanoparticles (20 nm) are studied using diffusion limited cluster aggregation (DLCA) and reaction limited cluster aggregation (RLCA) models. These processes are driven by the probability of particles to aggregate upon collision. This probability of aggregation is one in the DLCA and close to zero in the RLCA process. We show how to study the probability of aggregation from static micrographs on the example of a silica nanoparticle gel at 9 wt%. The analysis includes common summary functions from spatial statistics, namely the empty space function and Ripley's K-function, as well as two newly developed summary functions for cluster analysis based on graph theory. One of the new cluster analysis functions is related to the clustering coefficient in communication networks and the other to the size of a cluster. All four topological summary statistics are used to quantitatively compare in plots and in a least-square approach experimental data to cluster aggregation simulations with decreasing probabilities of aggregation. We study scanning transmission electron micrographs and utilize the intensity-mass thickness relation present in such images to create comparable micrographs from three-dimensional simulations. Finally, a characterization of colloidal silica aggregates and simulated structures is obtained, which allows for an evaluation of the cluster aggregation process for different aggregation scenarios. As a result, we find that the RLCA process fits the experimental data better than the DLCA process.

Bioinspired Nanoparticle‐Assembly Route to a Hybrid Scaffold: Designing a Robust Heterogeneous Catalyst for Asymmetric Dihydroxylation of Olefins

AbstractA hierarchical polyamine‐silica structure is shown to encase osmate ions resulting in robust and reusable catalysts for asymmetric dihydroxylation of olefins with a high TON (&gt;700). Whilst the polypeptide and related polyamines resembling a proteinaceous matrix in biosilica nanostructures preserve the osmate as the active centre, the surrounding assembly of silica nanoparticles imparts stability and textural properties to the catalyst. This confinement of osmate in the microsphere leads to higher catalytic activity and enantioselectivity for a variety of olefins as the substrate. The hybrid nature of the catalyst not only allows for the usage of various solvent systems, but also provides a means to be integrated with multi‐components/functions for tuning the activity, catalysis separation and reusability. Interestingly, the presence of polyamine makes the catalyst retain its activity even in the presence of chelating acids like citrate that are known to diminish enantioselectivity.

The evaluation of physical properties and in vitro cell behavior of PHB/PCL/sol–gel derived silica hybrid scaffolds and PHB/PCL/fumed silica composite scaffolds

PHB/PCL/sol–gel derived silica hybrid scaffolds (P5S1S) and PHB/PCL/fumed silica composite scaffolds (P5S1N) with a 5:1 organic/inorganic ratio were fabricated through a combination of electrospinning and sol–gel methods and dispersion electrospinning, respectively. In contrast to the silica nanoparticle aggregates appearing on the fiber surface of P5S1N, smooth and uniform fibers were obtained for P5S1S. The fiber diameter distribution, tensile strength, thermal gravimetric analysis (TGA), and cellular behavior of both types of scaffolds were characterized and studied. The tensile strength results and TGA indicated that the interfacial interaction between the organic and the inorganic phase was enhanced in P5S1S over the nanocomposite scaffolds, and cells exhibited significantly higher alkaline phosphate activity (ALP) for P5S1S, which makes P5S1S hybrid scaffolds candidate materials for bone tissue engineering applications.

Formation of Highly Ordered Spherical Aggregates from Drying Microdroplets of Colloidal Suspension

The formation of highly ordered spherical aggregates of silica nanoparticles by the evaporation of single droplets of an aqueous colloidal suspension levitated (confined) in the electrodynamic quadrupole trap is reported. The transient and final structures formed during droplet evaporation have been deposited on a silicon substrate and then studied with SEM. Various successive stages of the evaporation-driven aggregation of nanoparticles have been identified: formation of the surface layer of nanoparticles, formation of the highly ordered spherical structure, collapse of the spherical surface layer leading to the formation of densely packed spherical aggregates, and rearrangement of the aggregate into the final structure of a stable 3D quasi-crystal. The evaporation-driven aggregation of submicrometer particles in spherical symmetry leads to sizes and morphologies of the transient and final structures significantly different than in the case of aggregation on a substrate. The numerical model presented in the article allows us to predict and visualize the observed aggregation stages and their dynamics and the final aggregates observed with SEM.