Detection of surface silanol groups on pristine and functionalized silica mixed oxides and zirconia

Physico-Chemical Approaches to Investigate Surface Hydroxyls as Determinants of Molecular Initiating Events in Oxide Particle Toxicity

The study of molecular recognition patterns is crucial for understanding the interactions between inorganic (nano)particles and biomolecules. In this review we focus on hydroxyls (OH) exposed at the surface of oxide particles (OxPs) which can play a key role in molecular initiating events leading to OxPs toxicity. We discuss here the main analytical methods available to characterize surface OH from a quantitative and qualitative point of view, covering thermogravimetry, titration, ζ potential measurements, and spectroscopic approaches (NMR, XPS). The importance of modelling techniques (MD, DFT) for an atomistic description of the interactions between membranes/proteins and OxPs surfaces is also discussed. From this background, we distilled a new approach methodology (NAM) based on the combination of IR spectroscopy and bioanalytical assays to investigate the molecular interactions of OxPs with biomolecules and membranes. This NAM has been already successfully applied to SiO₂ particles to identify the OH patterns responsible for the OxPs' toxicity and can be conceivably extended to other surface-hydroxylated oxides.

A simple liquid state 1H NMR measurement to directly determine the surface hydroxyl density of porous silica

Silica is widely used in industrial applications and its performance is partially decided by its surface hydroxyl density α_OH. Here we report a quick, simple liquid ¹H NMR method to determine α_OH using a benchtop ¹H NMR spectrometer. The results show excellent agreement with the literature with an α_OH range from 4.16 to 6.56 OH per nm².

Improvement of the froth flotation of LiAlO2 and melilite solid solution via pre-functionalization

In this work froth flotation studies with LiAlO₂ (lithium-containing phase) and Melilite solid solution (gangue phase) are presented. The system was optimized with standard collectors and with compounds so far not applied as collectors. Furthermore, the principle of self-assembled monolayers was introduced to a froth flotation process for the first time resulting in excellent yields and selectivities.

Interfacially-adsorbed particles enhance the self-propulsion of oil droplets in aqueous surfactant

Understanding the chemo-mechanical mechanisms that direct the motion of self-propulsive colloids is important for the development of active materials and exploration of dynamic, collective phenomena. Here, we demonstrate that the adsorption of solid particles on the surface of solubilizing oil droplets can significantly enhance the droplets' self-propulsion speeds. We investigate the relationship between the self-propulsion of bromodecane oil droplets containing silica particles of varying concentration in Triton X-100 surfactant, noting up to order of magnitude increases in propulsion speeds. Using fluorescently labeled silica, we observe packing of the particles at the oil-water interfaces of the rear pole of the moving droplets. For bromodecane oil droplets in Triton X-100, the highest droplet speeds were achieved at approximately 40% particle surface coverage of the droplet interface. We find particle-assisted propulsion enhancement in ionic surfactants and different oil droplet compositions as well, demonstrating the breadth of this effect. While a precise mechanism for the propulsion enhancement remains unclear, the simple addition of silica particles to droplet oil-water interfaces provides a straightforward route to tune active droplet dynamics.

The assembly of silica species with alkylamines: Mechanism of wastewater-free synthesis and the application of gel as a catalyst

Silica species generated by the hydrolysis and condensation of tetraethyl orthosilicate (TEOS) could assemble with alkylamines to form silica gel. Herein, it was evidenced that part of the added amines, including butylamine (BA), octylamine (OA) or dodecylamine (DA), was protonated in the mixture of water and ethanol. Therefore, besides the hydrogen bonding between neutral silica species and the micelles composed of the non-protonated amines (Tanev and Pinnavaia, 1995), there existed strong electrostatic attraction between negatively charged silica species and the micelles composed of the protonated amines. This coexisting assembly mechanism could explain why the uncalcined BA- and OA-gels were millimeter-sized small blocks with large porosities and synthesized without waste water emission, while the uncalcined DA-gel was almost non-porous and formed via precipitation from its reaction medium. The uncalcined BA gel was proved to be efficient as a solid basic catalyst, replacing the commonly used ammonia solution which is easily volatilized and has a pungent smell, for the hydrolysis and condensation of TEOS to prepare silica microspheres.

Some Critical Insights into the Synthesis and Applications of Hydrophobic Solid Catalysts

The preparation methods of hydrophobic materials such as zeolites, modified silicas and polymers has been reviewed. Particular attention has been paid to the characterization methods classified according to the surface and bulk composition, on one hand, and to the measure of interactions with water or organic solvents, on the other. Some selected applications are analyzed in order to understand the relevance of the reactants/products adsorption to address activity and selectivity of the reaction. Thus, absorption of a non-polar reactant or desorption of a hydrophilic product are much easier on a hydrophobic surface and can effectively boost the catalytic activity.

Particle Stabilization of Oil-Fluorocarbon Interfaces and Effects on Multiphase Oil-in-Water Complex Emulsion Morphology and Reconfigurability

Stabilization of oil-oil interfaces is important for nonaqueous emulsions as well as for multiphase oil-in-water emulsions, with relevance to a variety of fields ranging from emulsion polymerization to sensors and optics. Here, we focus on examining the ability of functionalized silica particles to stabilize interfaces between fluorinated oils and other immiscible oils (such as hydrocarbons and silicones) in nonaqueous emulsions and also on the particles' ability to affect the morphology and reconfigurability of complex, biphasic oil-in-water emulsions. We compare the effectiveness of fluorophilic, lipophilic, and bifunctional fluorophilic-lipophilic coated nanoparticles to stabilize these oil-oil interfaces. Sequential bulk emulsification steps by vortex mixing, or emulsification by microfluidics, can be used to create complex droplets in which particles stabilize the oil-oil interfaces and surfactants stabilize the oil-water interfaces. We examine the influence of particles adsorbed at the internal oil-oil interface in complex droplets to hinder the reconfiguration of these complex emulsions upon addition of aqueous surfactants, creating "metastable" droplets that resist changes in morphology. Such metastable droplets can be triggered to reconfigure when heated above their upper critical solution temperature. Thus, not only do these bifunctional silica particles enable the stabilization of a broad array of oil-fluorocarbon nonaqueous emulsions, but the ability to address the oil-oil interface within complex O/O/W droplets expands the diversity of oil chemical choices available and the accessibility of droplet morphologies and sensitivity.

In Situ Zirconia: A Superior Reinforcing Filler for High-Performance Nitrile Rubber Composites

Zirconia particles are generated into a nitrile rubber (NBR) matrix via a solution sol-gel method in a controlled manner. Formation of zirconia particles from their precursor (zirconium(IV) propoxide) occurs under optimized reaction conditions. As a result, the nanoparticles are embedded and well dispersed in the NBR matrix that results in a remarkable improvement in mechanical and thermal properties of the composite. Such reinforcement is not realized when the composites are prepared following the conventional technique of filler loading by physical mixing, although the filler content remains the same. Use of a surface active coupling agent TESPT (bis-(3-triethoxysilylpropyl) tetrasulfide) in the reactive sol-gel system is found to further boost the mechanical performance of the composites. In order to ensure the practical application of the developed composites, a series of studies have been performed that consist of dynamic performance, swelling, thermal degradation, and resistance to oil, ozone, and abrasion. Analysis of the results reveals that in situ zirconia could be an excellent filler for the NBR composites to withstand in a harsh and adverse environment.

Quantification of surface functional groups on silica nanoparticles: comparison of thermogravimetric analysis and quantitative NMR

Thermogravimetric analysis (TGA) coupled with evolved gas analysis-FT-IR has been examined as a potential method to study the functional group content for surface modified silica nanoparticles. A comparison with a quantitative solution NMR method based on analysis of groups released after dissolution of the silica matrix is used to provide benchmark data for comparison and to assess the utility and limitations of TGA. This study focused primarily on commercially available silicas and tested whether it was possible to use a correction based on bare silica to account for the significant mass loss that occurs due to condensation of surface hydroxyl groups and loss of matrix-entrapped components at temperatures above ∼200 °C. Although this approach has been used successfully in the literature for in-house prepared samples, it was problematic for commercial silicas prepared by the Stöber method. For these materials the agreement between estimates from qNMR and TGA mass loss was poor in many cases. However much better agreement was observed for samples for which the mass loss above 200 °C is relatively low, such as non-porous silica, or samples for which the mass fraction of functional group is large (e.g., high molecule weight groups or multilayers). FT-IR was useful in identifying the likely structure of the components lost from the surface at various temperatures and in some cases provided evidence of contaminants in the sample. Nevertheless, in other cases correlation of thermograms and FT-IR with NMR data was necessary, particularly for samples where multi-step modification of the silica surface results in incomplete functionalization that gives a mixture of products. Overall the results indicate that TGA provides reliable results for silicas of low porosity or those for which the functional group accounts for a significant fraction of the total sample mass. It is also suitable as a supplementary or screening technique to indicate the presence of coatings or covalent surface modification, prior to applying other techniques or for routine analyses where sensitivity is not critical.

Surface Characterization of TiO2 Polymorphic Nanocrystals through 1H-TD-NMR

Nanocrystals (NCs) surface characterization is a fundamental step for understanding the physical and chemical phenomena involved at the nanoscale. Surface energy and chemistry depend on particle size and composition, and, in turn, determine the interaction of NCs with the surrounding environment, their properties and stability, and the feasibility of nanocomposites. This work aims at extracting more information on the surface of different titanium dioxide polymorphs using ¹H-TD-NMR of water. Taking advantage of the interaction between water molecules and titanium dioxide NCs, it is possible to correlate the proton transverse relaxation times ( T₂) as the function of the concentration and the specific surface area (δ_p· C_m) and use it as an indicator of the crystal phase. Examples of three different crystals phase, rutile, anatase, and brookite, have been finely characterized and their behavior in water solution have been studied with TD-NMR. The results show a linear correlation between relaxivity ( R₂) and their concentration C_m. The resulting slopes, after normalization for the specific surface, represent the surface/water interaction and range from 1.28 g m^-2 s^-1 of 50 nm rutile nanocrystals to 0.52 for similar sized brookite. Even higher slopes (1.85) characterize smaller rutile NCs, in qualitative accordance with the trends of surface energy. Thanks to proton relaxation phenomena that occur at the NCs surface, it is possible to differentiate the crystal phase and the specific surface area of titanium dioxide polymorphs in water solution.

In Situ Measurement of Surface Functional Groups on Silica Nanoparticles Using Solvent Relaxation Nuclear Magnetic Resonance

In situ analysis and study on the surface of nanoparticles (NPs) is a key to obtain their important physicochemical properties for the subsequent applications. Of them, most works focus on the qualitative characterization whereas quantitative analysis and measurement on the NPs under their storage and usage conditions is still a challenge. In order to cope with this challenge, solvation relaxation-based nuclear magnetic resonance (NMR) technology has been applied to measure the wet specific surface area and, therefore, determine the number of the bound water molecules on the surface of silica NPs in solution and the hydrophilic groups of various types grafted on the surface of the NPs. By changing the surface functional group on silica particles, the fine distinction for the solvent-particle interaction with different surface group can be quantitatively differentiated by measuring the number of water molecules absorbed on the surface. The results show that the number of the surface hydroxyl, amine, and carboxyl group per nm² is 4.0, 3.7, and 2.3, respectively, for the silica particles with a diameter of 203 nm. The method reported here is the first attempt to determine in situ the number of bound solvent molecules and any solvophilic groups grafted on nanoparticles.

The silanol content and in vitro cytolytic activity of flame-made silica

HYPOTHESIS

The surface chemistry of synthetic amorphous silicas is essential for their applicational performance and for understanding their interactions with biological matter. Synthesis of silica by flame spray pyrolysis (FSP) allows to control the content and type of hydroxyl groups which also affects the cytolytic activity.

EXPERIMENTS

By controlling the FSP process variables, silica nanoparticles with the same specific surface area but different surface chemistry and content of internal silanols are prepared by combustion of hexamethyldisiloxane sprays, as characterized by Raman and infrared spectroscopy, thermogravimetric analysis, and titration with lithium alanate. Cytolytic activity is assessed in terms of membrane damage in human blood monocytes in vitro.

FINDINGS

Unlike commercial fumed silica, FSP-made silicas contain a significant amount of internal silanol groups and a high surface hydroxyl density, up to ∼8OH/nm², similar to silicas made by wet-chemistry. Increasing the residence time of particles at high temperature during their synthesis reduces the internal and surface hydroxyl content and increases the relative amount of isolated silanols. This suggests incomplete oxidation of the silica matrix especially in short and "cold" flames and indicates that the silica particle formation pathway involves Si(OH)₄. The surface chemistry differences translate into lower cytolytic activity for "cold-" than "hot-flame" silicas.

Metal deposition by electroless plating on polydopamine functionalized micro- and nanoparticles

A novel approach for the fabrication of metal coated micro- and nanoparticles by functionalization with a thin polydopamine layer followed by electroless plating is reported. The particles are initially coated with polydopamine via self-polymerization. The resulting polydopamine coated particles have a surface rich in catechols and amino groups, resulting in a high affinity toward metal ions. Thus, they provide an effective platform for selective electroless metal deposition without further activation and sensitization steps. The combination of a polydopamine-based functionalization with electroless plating ensures a simple, scalable, and cost-effective metal coating strategy. Silver-plated tungsten carbide microparticles, copper-plated tungsten carbide microparticles, and copper-plated alumina nanoparticles were successfully fabricated, showing also the high versatility of the method, since the polymerization of dopamine leads to the formation of an adherent polydopamine layer on the surface of particles of any material and size. The metal coated particles produced with this process are particularly well suited for the production of metal matrix composites, since the metal coating increases the wettability of the particles by the metal, promoting their integration within the matrix. Such composite materials are used in a variety of applications including electrical contacts, components for the automotive industries, magnets, and electromagnetic interference shielding.