Silica Surface Chemistry Research Articles

Monomolecular Siloxane Film as a Model of Single Site Catalysts.

Achieving structurally well-defined catalytic species requires a fundamental understanding of surface chemistry. Detailed structural characterization of the catalyst binding sites in situ, such as single site catalysts on silica supports, is technically challenging or even unattainable. Octadecyltrioxysilane (OTOS) monolayers formed from octadecyltrimethoxysilane (OTMS) at the air-liquid interface after hydrolysis and condensation at low pH were chosen as a model system of surface binding sites in silica-supported Zn(2+) catalysts. We characterize the system by grazing incidence X-ray diffraction, X-ray reflectivity (XR), and X-ray fluorescence spectroscopy (XFS). Previous X-ray and infrared surface studies of OTMS/OTOS films at the air-liquid interface proposed the formation of polymer OTOS structures. According to our analysis, polymer formation is inconsistent with the X-ray observations and structural properties of siloxanes; it is energetically unfavorable and thus highly unlikely. We suggest an alternative mechanism of hydrolysis/condensation in OTMS leading to the formation of structurally allowed cyclic trimers with the six-membered siloxane rings, which explain well both the X-ray and infrared results. XR and XFS consistently demonstrate that tetrahedral [Zn(NH3)4](2+) ions bind to hydroxyl groups of the film at a stoichiometric ratio of OTOS:Zn ∼ 2:1. The high binding affinity of zinc ions to OTOS trimers suggests that the six-membered siloxane rings are binding locations for single site Zn/SiO2 catalysts. Our results show that OTOS monolayers may serve as a platform for studying silica surface chemistry or hydroxyl-mediated reactions.

Ion–ion correlation, solvent excluded volume and pH effects on physicochemical properties of spherical oxide nanoparticles

One major source of complexity in the implementation of nanoparticles in aqueous electrolytes arises from the strong influence that biological environments has on their physicochemical properties. A key parameter for understanding the molecular mechanisms governing the physicochemical properties of nanoparticles is the formation of the surface charge density. In this article, we present an efficient and accurate approach that combines a recently introduced classical solvation density functional theory for spherical electrical double layers with a surface complexation model to account for ion–ion correlation and excluded volume effects on the surface titration of spherical nanoparticles. We apply the proposed computational approach to account for the charge-regulated mechanisms on the surface chemistry of spherical silica (SiO2) nanoparticles. We analyze the effects of the nanoparticle size, as well as pH level and electrolyte concentration of the aqueous solution on the nanoparticle’s surface charge density and Zeta potential. We validate our predictions for 580Å and 200Å nanoparticles immersed in acid, neutral and alkaline mono-valent aqueous electrolyte solutions against experimental data. Our results on mono-valent electrolyte show that the excluded volume and ion–ion correlations contribute significantly to the surface charge density and Zeta potential of the nanoparticle at high electrolyte concentration and pH levels, where the solvent crowding effects and electrostatic screening have shown a profound influence on the protonation/deprotonation reactions at the liquid/solute interface. The success of this approach in describing physicochemical properties of silica nanoparticles supports its broader application to study other spherical metal oxide nanoparticles.

Atomic Oxygen Recombination at Surface Defects on Reconstructed (0001) α-Quartz Exposed to Atomic and Molecular Oxygen

The surface chemistry of silica is strongly affected by the nature of chemically active sites (or defects) occurring on the surface. Here, we employ quantum mechanical electronic structure calculations to study an uncoordinated silicon defect, a non-bridging oxygen defect, and a peroxyl defect on the reconstructed (0001) surface of α-quartz. We characterized the spin states and energies of the defects, and calculated the reaction profiles for atomic oxygen recombination at the defects. We elucidated the diradical character by analyzing the low-lying excited states using multireference wave function methods. We show that the diradical defects consist of weakly coupled doublet radicals, and the atomic oxygen recombination can take place through a barrierless process at defects. We have delineated the recombination mechanism and computed the formation energy of the peroxyl and non-bridging oxygen defects. We found that key recombination reaction paths are barrierless. In addition, we characterize the electro...

Nanoporous silica nanoparticles as biomaterials: evaluation of different strategies for the functionalization with polysialic acid by step-by-step cytocompatibility testing.

Nanoporous silica materials have become a prominent novel class of biomaterials which are typically applied as nanoparticles or thin films. Their large surface area combined with the rich surface chemistry of amorphous silica affords the possibility to equip this material with variable functionalities, also with several different ones on the same particle or coating. Although many studies have shown that nanoporous silica is apparently non-toxic and basically biocompatible, any surface modification may change the surface properties considerably and, therefore, the modified materials should be checked for their biocompatibility at every step. Here we report on different silane-based functionalization strategies, firstly a conventional succinic anhydride-based linker system and, secondly, copper-catalyzed click chemistry, to bind polysialic acid, a polysaccharide important in neurogenesis, onto nanoporous silica nanoparticles (NPSNPs) of MCM-41 type. At each of the different modification steps, the materials are characterized by cell culture experiments. The results show that polysialic acid can be immobilized on the surface of NPSNPs by using different strategies. The cell culture experiments show that the kind of surface immobilization has a strong influence on the toxicity of the material versus the cells. Whereas most modifications appear inoffensive, NPSNPs modified by click reactions are toxic, probably due to residues of the Cu catalyst used in these reactions.

Reversible control of pore size and surface chemistry of mesoporous silica through dynamic covalent chemistry: philicity mediated catalysis.

Here, we report the synthesis of adaptive hybrid mesoporous silica having the ability to reconfigure its pore properties such as pore size and philicity in response to the external environment. Decyl chains were reversibly appended to the pore walls of silica through imine motifs as dynamic covalent modules to switch the pore size and philicity in response to pH. This switching of pore properties was used to gate the access of reactants to the gold nanoparticles immobilized inside the nanopores, thus enabling us to turn-on/turn-off the catalytic reaction. The use of such dynamic covalent modules to govern pore properties would enable the realization of intelligent hybrids capable of controlling many such chemical processes in response to stimuli.

Prediction of Specific Biomolecule Adsorption on Silica Surfaces as a Function of pH and Particle Size

Silica nanostructures are biologically available and find wide applications for drug delivery, catalysts, separation processes, and composites. However, specific adsorption of biomolecules on silica surfaces and control in biomimetic synthesis remain largely unpredictable. In this contribution, the variability and control of peptide adsorption on silica nanoparticle surfaces are explained as a function of pH, particle diameter, and peptide electrostatic charge using molecular dynamics simulations with the CHARMM-INTERFACE force field. Adsorption free energies and specific binding residues are analyzed in molecular detail, providing experimentally elusive, atomic-level information on the complex dynamics of aqueous electric double layers in contact with biological molecules. Tunable contributions to adsorption are described in the context of specific silica surface chemistry, including ion pairing, hydrogen bonds, hydrophobic interactions, and conformation effects. Remarkable agreement is found for compute...

Shape controlled spherical (0D) and rod-like (1D) silica nanoparticles in silica/styrene butadiene rubber nanocomposites: Role of the particle morphology on the filler reinforcing effect

Silica/styrene butadiene rubber (SBR) nanocomposites were prepared by blending method using shape-controlled spherical and rod-like nanoparticles with different aspect ratios as filler for the rubber reinforcement. The differently shaped silica particles were synthesized by sol–gel method using tetraethoxysilane (TEOS) and (3-mercaptopropyl) trimethoxysilane (MPTSM) as silica precursors, and cetyltrimethylammonium bromide (CTAB) as structure directing agent. This strategy allowed to study the influence of the particle morphology on the reinforcing effect independently of the silica surface chemistry and considering the aspect ratio as the only geometrical variance. Spherical and anisotropic rod-like particles, dispersed in the nanocomposites, formed a network of particles bridged by thin rubber layers throughout the SBR matrix. Moreover, differently oriented domains of aligned rods are observed when the aspect ratio of particles increases and is ≥2. Dynamic-mechanical properties demonstrated that the rod-like particles with the higher aspect ratio provided stronger reinforcement of the rubber. This was related to the self-alignment of the anisotropic particles and to the consequent larger filler/polymer interface, compared to that of spherical ones.

Effect of thermal treatment and moisture content on the charge of silica particles in non-polar media

Surfactants, such as Aerosol-OT (AOT), are known to impart charge on particles in non-polar media and make them colloidally stable. Previous studies on the charge of silica particles, which are one of the most widely used and studied colloids in non-polar media, have shown significant inconsistencies in the observed charging behaviour. While some studies have shown that silica becomes positively charged with AOT, others have shown that silica gains a net negative charge with the same surfactant. In addition, we have noticed differences in the charge state of colloidal silica from different suppliers. To determine the possible origin of these discrepancies, the effect of surface chemistry and moisture on the charge of silica in AOT–toluene is examined. The surface chemistry of silica is altered by thermal treatment at temperatures up to 1000 °C, while the amount of water in silica dispersions is controlled by keeping them at three different relative humidities. We use Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and quartz crystal microbalance (QCM) to gain insight into the effect of thermal treatment and moisture on the charging of silica in non-polar media. Changes in the surface chemistry of silica particles are found to have a significant effect on their charge in AOT–toluene solution. In addition, an increase in the moisture content of silica dispersions is shown to adversely affect the charge in AOT–toluene and, in some cases, induce charge inversion. Our results show that the surface chemistry of silica as well as moisture content are crucial parameters that influence the charging of silica in non-polar media and that these parameters should be carefully controlled to enhance the reproducibility of surface charge measurements in non-polar media. Furthermore, both high temperature thermal treatment and moisture content could serve as control parameters to change the charge of silica in non-polar media.

Note: Mechanically and chemically stable colloidal probes from silica particles for atomic force microscopy

In this note we present a novel approach to prepare colloidal probes for atomic force microscopy by sintering. A central element of this procedure is the introduction of an inorganic "fixation neck" between the cantilever and a micrometer-sized silica particle that is acting as probe. This procedure overcomes previous restrictions for the probe particles, which had to be low melting point materials, such as borosilicate glass or latex particles. The here-presented colloidal probes from silica can withstand large mechanical forces. Additionally, they have high chemical resistivity due to the absence of adhesives and the well-studied surface chemistry of colloidal silica.

Evanescent Wave Cavity Ring-Down Spectroscopy (EW-CRDS) as a Probe of Macromolecule Adsorption Kinetics at Functionalized Interfaces

Evanescent wave cavity ring-down spectroscopy (EW-CRDS) has been employed to study the interfacial adsorption kinetics of coumarin-tagged macromolecules onto a range of functionalized planar surfaces. Such studies are valuable in designing polymers for complex systems where the degree of interaction between the polymer and surface needs to be tailored. Three tagged synthetic polymers with different functionalities are examined: poly(acrylic acid) (PAA), poly(3-sulfopropyl methacrylate, potassium salt) (PSPMA), and a mannose-modified glycopolymer. Adsorption transients at the silica/water interface are found to be characteristic for each polymer, and kinetics are deduced from the initial rates. The chemistry of the adsorption interfaces has been varied by, first, manipulation of silica surface chemistry via the bulk pH, followed by surfaces modified by poly(L-glutamic acid) (PGA) and cellulose, giving five chemically different surfaces. Complementary atomic force microscopy (AFM) imaging has been used for additional surface characterization of adsorbed layers and functionalized interfaces to allow adsorption rates to be interpreted more fully. Adsorption rates for PSPMA and the glycopolymer are seen to be highly surface sensitive, with significantly higher rates on cellulose-modified surfaces, whereas PAA shows a much smaller rate dependence on the nature of the adsorption surface.

Spontaneous Crystalline-to-Amorphous Phase Transformation of Organic or Medicinal Compounds in the Presence of Porous Media, Part 3: Effect of Moisture

Amorphization of crystalline compounds using mesoporous media is a promising technique to improve the solubility and drug release of poorly-soluble compounds. The objective of this paper is to understand the effect of moisture on the capacity and performance of vapor-phase mediated amorphization. Mesoporous silicon dioxide (SiO(2)) and crystalline naphthalene were used as the model system. The effect of moisture on the amorphization capacity of naphthalene was determined using adsorption chambers with various levels of relative humidity. Enthalpy and capacity of water vapor adsorption on SiO(2) were measured using isothermal microcalorimetry and thermogravimetry. Moisture not only suppressed the amorphization capacity of naphthalene, but reversed an already-amorphized formulation as well. On the other hand, through the same competitive interaction, improved drug release and enhanced solubility were obtained. The initial supersaturation was followed by an entropy-driven crystallization. In addition, moisture-induced siloxane bond fracture was found at normal processing conditions, which led to the changes in silica surface chemistry. However, the implication in amorphization has not reached a definitive conclusion. Humidity during processing and storage must be carefully controlled for this type of amorphous formulation. Further investigation is needed to better understand the moisture-induced changes of silica.

Changes in the Silanol Protonation State Measured In Situ at the Silica–Aqueous Interface

Recent nanomedical applications have again highlighted the significance of silica surface chemistry in solution. Here, we report in situ electronic structure measurements at the silica-aqueous interface as a function of pH for nanoparticles (NPs) of 7, 12, and 22 nm using a liquid microjet in combination with synchrotron radiation. The Si K-edge X-ray absorption near-edge spectroscopy (XANES) spectra reveal a change in shape of the Si 1s -> t(2) (Si 2p-3s) absorption brought about by changes in the silanol protonation state at the interface of the NPs as a result of changes in solution pH. Our results are consistent with the number of silanol groups changing the protonation state being inversely correlated with the SiO2 NP size. The importance of in situ studies is also demonstrated by comparing the XANES spectra of aqueous 7 nm SiO2 with the same dehydrated sample in vacuum.

Investigating the effect of pH on the surface chemistry of an amino silane treated nano silica

PurposeThe purpose of this paper is to investigate the effect of different PHS on the surface chemistry of fumed silica treated with aminopropyltrimethoxysilane (APTMS).Design/methodology/approachThe reaction conditions involved variation of pH ranging from acidic to alkaline. Different analytical techniques including FT‐IR spectroscopy, thermogravimetric analysis (TGA), CHN and Zeta potential analyses were employed to investigate the surface chemistry of treated particles. In addition, the stability of silanised silica dispersions were studied using turbidimetric and rheometric measurements.FindingsIt was revealed that in all conditions silica was more or less chemically grafted by the silane. When the pH of treating bath was adjusted to 1‐2 prior and during the reaction, 58 percent grafting was observed, as obtained by CHN and TGA analyses. At very alkaline conditions, however, the grafting content declined to 29 percent. The variations in grafting were dependent on the silane hydrolysis and its further condensation with the silica surface. Zeta potential measurements showed a drastic change from −7.1 mv to +18.01 mv (at pH 7) for the untreated particle and the one with the highest grafting, respectively. The dispersion stability of differently treated particles varied in solvents with different Hansen solubility parameters (HSP). Moreover, due to the variations of surface chemistry of particles, their rheological behaviours were significantly influenced.Originality/valueThe results obtained in this work showed that the surface chemistry of fume silica could be tuned with treating method. The highest content of grafting led to a better dispersion in solvents having greater hydrogen bonding component and to an inferior dispersion in solvents with higher polar component.

Inversion of particle-stabilized emulsions of partially miscible liquids by mild drying of modified silica particles

Using a system of modified silica particles and mixtures of water and 2,6-lutidine to form particle-stabilized emulsions, we show that subtle alterations to the hydration of the particle surface can cause major shifts in emulsion structure. We use fluorescence confocal microscopy, solid state nuclear magnetic resonance (NMR) and thermo-gravimetric analysis (TGA) to explore this sensitivity, along with other shifts caused by modifications to the silica surface chemistry. The silica particles are prepared by a variant of the Stöber procedure and are modified by the inclusion of 3-(aminopropyl)triethoxysilane and the dye fluorescein isothiocyanate. Treatment prior to emulsification consists of gently drying the particles under carefully controlled conditions. In mixtures of water and 2,6-lutidine of critical composition, the particles stabilize droplet emulsions and bijels. Decreasing particle hydration yields an inversion of the emulsions from lutidine-in-water (L/W) to water-in-lutidine (W/L), with bijels forming around inversion. So dependent is the emulsion behavior on particle hydration that microscopic differences in drying within a particle sample can cause differences in the wetting behavior of that sample, which helps to stabilize multiple emulsions. The formation of bijels at emulsion inversion is also crucially dependent on the surface modification of the silica.

Inverse gas chromatography for determining the dispersive surface energy of porous silica

Inverse gas chromatography (IGC) is a modern technique for the characterization of porous materials, which provides thermodynamic parameters of the solids. Porous glass beads, silica gels and ordered mesoporous materials were used as model systems to study correlations between the dispersive part of the surface energy and the surface chemistry of porous materials. The surface chemistry of silica was controlled via thermal treatment and reaction with 1,1,1,3,3,3-hexamethyldisilazane (HMDS) or 3-amino-propyltriethoxysilane (3-APTS). The dependency of the surface energy on the various modifications was investigated in this study. Furthermore, the impact of microstructure on the dispersive surface energy was determined by a comparative study of the ordered mesoporous materials MCM-41, SBA-15 and SBA-16.

Influence of selected alkoxysilanes on dispersive properties and surface chemistry of spherical silica precipitated in emulsion media

The process of silicas formation in emulsion system in the reaction of precipitation from water solutions of sodium silicate and hydrochloric acid has been studied. The effects of silica surface modifications with silane coupling agents from the group of alkoxysilanes with different functional groups have been determined. Both unmodified and modified silicas have been thoroughly characterised by scanning electron microscopy (SEM) and dynamic light scattering (DLS) and other methods. The effect of alkoxysilanes on the diameter distribution of the modified silica is insignificant; the presence of silane coupling agents causes a small increase in the particle diameter and a small increase in the tendency towards agglomeration, moreover the particles of silicon dioxide precipitated are characterised by spherical shape. The course of the electrokinetic curves depends mainly on the proton affinity of the modifiers. Significant changes in the stability of silica dispersions have been found as a result of modification with N-2-(aminoethyl)-3-aminopropyltrimethoxysilane. This compound causes the greatest increase in the tendency towards agglomeration of the primary particles. The modification mechanism has been established to be chemisorption, as indicated by the 13C and 29Si CP MAS NMR spectra. The products have been shown to have typical mesoporous structure. The surface area of the unmodified silica is 133 m 2/g, while those for the modified silicas vary from 115 to 182 m 2/g.

Development of adsorbent technologies for post-combustion CO2 capture

Increasing awareness of the influence of greenhouse gases on global climate change has led to recent efforts to develop strategies for the reduction of carbon dioxide (CO2) emissions. The strategy that is receiving the most attention involves the capture of CO2 from large point sources, such as fossil fuel-fired power plants, and long-term storage underground or in the ocean. CO2 capture can be achieved either by post or pre-combustion capture at ambient or high pressure respectively. Aqueous solutions of amines have long been used by industry as absorbents for acid gas (CO2,H2S) removal, but have a large number of short comings. As an alternative, adsorption is one of the more promising technologies for capturing CO2 from flue gases, potentially avoiding the shortcomings of aqueous amine systems. In this paper the development of solid adsorbents for post-combustion capture will be described. The generation and testing of a range of adsorbents for post-combustion capture of CO2 at ambient pressure has demonstrated the need for chemical type adsorbents. A range of chemical type adsorbents have been generated by two methods: • Modification of the surface chemistry of both low cost carbons and mesoporous silica by impregnation with a basic nitrogen-containing polymers (i.e., polyethylenimine). • Generation of a range of high nitrogen content carbon matrix adsorbents by carbonisation and subsequent thermal or chemical activation of a range of nitrogen compounds. CO2 adsorption capacities at equilibrium and under dynamic conditions using a simulated flue gas will be described at different temperatures and gas residence times. Regeneration using simulated temperature, pressure swing cycles and steam stripping and the effect of the processes on the lifetime of the adsorbent will be discussed.

Microstructured templates produced using femtosecond laser pulses as templates for the deposition of mesoporous silicas

A combination of laser-based top-down and chemical bottom-up processes to prepare hierarchical (pore) structures is presented. Polymer templates with dimensions in the micrometer regime are fabricated by two-photon polymerization of an acrylate monomer and are subsequently infiltrated with mesostructured silica using the liquid crystal templating approach. Calcination, which removes both the macro-templating polymer and the nano-templating surfactant molecules, can be carried out so that the morphology of the sample is preserved, delivering SBA-15-type mesoporous constructs which correspond to a replica (or an inverse copy) of the initial polymer structure. This is shown here on simple line structures and on woodpile structures as examples. As the two-photon polymerization allows the controlled construction of various imaginable shapes, mesoporous materials can be constructed with practically any desired morphology on the micrometer scale. The dual pore system of macro- and mesopores classifies these materials as hierarchical pore systems with applications in catalysis and sorption. In addition, the versatility of silica surface chemistry opens new venues for the fabrication of photonic crystals and the general biocompatibility of silica allows the construction of scaffolds for biomedical applications.

The Local Microenvironment Surrounding Dansyl Molecules Attached to Controlled Pore Glass in Pure and Alcohol-Modified Supercritical Carbon Dioxide

We report on the local microenvironment surrounding a free dansyl probe, dansyl attached to controlled pore glass (D-CPG), and dansyl molecules attached to trimethylsilyl-capped CPG (capped D-CPG) in pure and alcohol-modified supercritical CO2. These systems were selected to provide insights into the local microenvironment surrounding a reactive agent immobilized at a silica surface in contact with pure and cosolvent-modified supercritical CO2. Local surface-bound dansyl molecule solvation on the CPG surface depends on the dansyl molecule surface loading, the surface chemistry (uncapped versus capped), the bulk fluid density, and the alcohol gas phase absolute acidity. At high dansyl loadings, the surface-bound dansyl molecules are largely "solvated" by other dansyl molecules and these molecules are not affected significantly by the fluid phase. When the dansyl surface loading decreases, dansyl molecules can be accessed/solvated/wetted by the fluid phase. However, at the lowest dansyl loadings studied, the dansyl molecules are in a fluid inaccessible/restrictive environment and do not sense the fluid phase to any significant degree. In uncapped D-CPG, one can poise the system such that the local concentration of an environmentally less responsible cosolvent (alcohol) in the immediate vicinity of surface-immobilized dansyl molecules can approach 100% even though the bulk solution contains orders of magnitude less of this less environmentally responsible cosolvent. In capped C-CPG, the surface excess is attenuated in comparison to that of uncapped D-CPG. The extent of this cosolvent surface excess is discussed in terms of the dansyl surface loading, the local density fluctuations, the cosolvent and surface silanol gas phase acidities, and the silica surface chemistry. These results also have implications for cleanings, extractions, heterogeneous reactions, separations, and nanomaterial fabrication using supercritical fluids.

Ultrathin Silica Films Immobilized on Gold Supports: Fabrication, Characterization, and Modification

A novel method for covalent attachment of ultrathin silica films (thickness <10 nm) to gold substrates is reported. Silica layers were prepared using spin-coating of sol-gel precursor solutions onto gold substrates that were cleaned and oxidized using UV photo-oxidation in an ozone atmosphere. The gold oxide layer resulting from this process acts as a wetting control and adhesive agent for the ultrathin silica layer. Control of silica layer thickness between approximately 6 and 60 nm through modification of precursor solution composition or by repetitive deposition is demonstrated. Films were characterized using infrared spectroscopy, ellipsometry, atomic force microscopy, and cyclic voltammetry. For the standard deposition parameters developed here, films were determined to be 5.5 +/- 0.75 nm thick, and were stable in aqueous solutions ranging in pH from 2 to 10 for at least 30 min. Films contained nanoscopic defects with radii of <or=2 microm in all cases, at a density of 1.6 defects per 10 mum x 10 mum region, with a weighted average defect radius of 200 nm. This array of defects was shown to exhibit a voltammetric response consistent with an ultramicroelectrode array. Finally, ultrathin silica films were also modified with self-assembled monolayers of octadecylsilane, demonstrating the surface chemistry of bulk silica.