Surface Silanol Groups Of Silica Research Articles

Microwave assisted facile green synthesis of silica coated magnetite nanoparticles for fast removal of methylene blue dye from environmental water samples

ABSTRACT To prepare a magnetic nano-adsorbent for efficient and fast removal of a dye, especially if present in trace concentration, along with a facile green synthetic methodology is a great challenge. The present work realises this task by combining the well-known advantages of both solvent-free microwave approach, nanoscale particle size and the fast magnetic separation. Thus, silica-coated magnetite nanoparticles (Fe3O4 NPs@SiO2) as an important precursor were produced directly via an optimised solid-solid interaction between silica and Fe3O4 NPs at 200 watt of microwave irradiation for only 10 min. The as-developed adsorbent was characterised using Fourier Transform Infrared (FT-IR), X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Energy Dispersive X-ray (EDX) analysis. Its performance with no further modification for removal of Methylene blue (MB) dye from aqueous solution was achieved in a batch mode as a function of pH, contact time, adsorbent dose and initial dye concentration. The results demonstrate excellent removal efficiency (98.9%) within 5 min, as compared with recent adsorbents synthesised through a multi-steps reaction. Modelling of the experimental data denoting well-fitting with both Langmuir sorption isotherm (monolayer capacity 47.98 mg/g) and pseudo-second-order kinetic model. A possible MB binding mechanism has been proposed taking in account values of dissociation constants of the different forms of silica surface silanol groups. The adsorbent after first was successfully regenerated in situe for five consecutive cycles. Moreover, it was proved to be valid for collecting efficiently trace concentrations of MB spiked environmental water samples to be in the range 92.4–98.8%.

In English

The structure and energy characteristics of structures formed during arginine adsorption on silica surface from aqueous solution were studied by the density functional theory (B3LYP) method using a valence-split basis set 6-31++G(d,p) within the continuous solvent model (PCM) and supermolecular approximation. The equilibrium structural and energy parameters of the protonated arginine molecule in the gas phase dependent on the location of the hydrogen atom are considered including those of two possible zwitterions. The structure of the arginine ion Н2А+, which is formed when a proton attaches to a molecule or zwitterion of a given amino acid, has been elucidated. To determine the deprotonation constant of the carboxyl group in an acidic medium, the complexes of the arginine molecule (AH32+) in the state with undissociated and deprotonated carboxyl groups are considered. The simulation of the acid medium was performed by taking into account the interaction with two hydrated HCl ion pairs, which provided the protonation of the a-amino group and the nitrogen atom of amino group within the guanidine group. In the study on the interaction of an arginine molecule with silica surface in an aqueous medium, complexes containing a Si8O12(OH)7O– ion with a deprotonated silanol group, six water molecules, and an arginine molecule with a deprotonated carboxyl group were considered. It has been found that the arginine molecule is most likely to be adsorbed on slica surface with formation of hydrogen bonds between the hydrogen atoms of the a-amino group and the oxygen atom of the deprotonated silanol group. In this case, the formation of a hydrogen bond between the oxygen atom of the carboxyl group and the hydrogen atom of the neighboring silanol group is possible. Slightly less likely is adsorption of arginine molecules due to interaction of the guanidine group with silanol groups of the surface. According to the calculated data, the adsorption of the zwitterionic form of the arginine molecule from the aqueous solution is equally likely to occur due to interaction of silanol groups of silica surface with both the carboxyl group and the guanidine group.

In English

Fumed silica has found widespread application in industry due to variety of fascinating properties. Owing to its specific manufacturing process, it consists of finely dispersed particles and is featured with large specific surface area covered by profoundly reactive silanol groups which are available for chemical grafting. Spherical shape of fumed silica particles and lacking porosity provides a space-filling structure. These characteristics implement the fume silica’s utilization as high-surface-area carriers for various catalysts, i.e. metallic nanometer-sized particles, organic moieties, etc. Currently a great attention is called to on-surface grafting to improve the silica-based carrier. Most of research is carried out in area of liquid phase chemistry involving an abundance of expensive and often toxic solvents while the space-filling properties of silica are favoring reactions in fluidized bed conditions. In current research fumed silica (A-300) was a subject for hydridesilylation with triethoxysilane under fluidized bed conditions. In all synthesis reported in current research the insignificant amount of solvent (1.00 wt. % of the amount used in typical wet-chemical modifications method) was spent for the silica surface silylation. While the mass ratio of silica/TES was kept constant, other conditions, i.e. solvent/catalyst presence, surface pretreatment, additional treatment with water, and the fluidized bed heating mode have been varied. FTIR spectroscopy revealed the interaction between groups of triethoxysilane and silica surface silanol groups and demonstrated the effect of modification conditions on the density of the hydridesilyl groups coverage. The results of FTIR spectroscopic studies have confirmed the presence of grafted silicon hydride groups on the surface of modified silica, as well as the presence of ethoxy and/or silanol groups – either intact or formed due to hydrolysis of the ethoxy groups. Titrimetric and spectrophotometric analysis was performed to estimate the concentration of grafted SiH groups (in all samples prepared under fluidized bed conditions their concentration ranged within about 0.28–0.55 mmol/g as dependent on the reaction conditions). Other important aspects of fluidization such as the presence of solvent and/or hydrolyzing agent, bed heating mode and the effect of the silica sample thermal pre-treatment are also discussed.

In Ukrainian

Despite the large number of experimental and theoretical works devoted to the structure and properties of silica surface known in the literature, as well as the extensive research on the structure of hydrated ionic pairs, the mechanism of adsorption of hydrated ions of electrolytes on silica surface has not yet been studied sufficiently. The paper presents a review of experimental works devoted to this problem, as well as their analysis at the atomic-molecular level. In order to study the interface "the silica surface - a solution of electrolyte", a number of papers used a method of density functional theory (DFT) using the exchange-correlation functional В3LYP and the basis set 6-31++G(d,p). The environmental impact modeling was taken into account within the framework of the supermolecular approach using a continuum solvent exposure model (PCM). The structure of the double electric layer and the mechanism of proton exchange in the near-surface layer of the silica surface are considered, as well as the value of the Gibbs energy of the silanol group deprotonation reaction, from which, in turn, the value of pK a2 was obtained. The results of calculations have shown that, with increasing size of oligomer molecules of silicic acid, the degree of localization of charges on hydrogen atoms and oxygen of the silanol group increases, the length of the O-H bond increases and the pK a2 decreases. It is shown that when adsorbed on a silica surface, alkali metal cations, in contact with silanol groups, can change the protolytic equilibrium of surface groups, due to redistribution of electron density, thereby increasing their acidity. The processes of ion exchange on silica surface and the dependence of the pK Me value on the size of ions and pH are considered. In a strongly acidic medium on silica surface, the formation of a cationic form of the silanol group is probable due to the proton transfer from the ion of hydroxonium to the oxygen atom of the silanol group. The deprotonation constant pK a1 of the cationic form of the silanol group depends on the nature of the anion and increases as an absolute value with anion radius increase. The models considered allow us to find the theoretical value of the point of zero charge of silica surface, which corresponds to the experimental data. Calculations using the constructed models of adsorption complexes of alkali hydroxides for the molecular state and for the state with separated charges with the participation of water molecules and silanol groups of silica surface indicate a possibility of ionization of the silanol group. The calculated Gibbs free energy changes are used to determine the values of pK Me , which increase in the series Li<Na<K, which correlates with the experimentally found adsorption values.

Hydrogen bonds at silica–CO2 saturated water interface under geologic sequestration conditions

ABSTRACTTo investigate the effects of sequestration condition on hydrogen bonds between mineral and water, molecular dynamics simulations have been performed. The simulations were conducted at conditions related with geologic sequestration sites: pressure (3.1–32.6 MPa), temperature (318 and 383 K), salinity (0–3 M), salt (NaCl and CaCl2) and silica surface models Q2 (geminal), Q3 (isolated) and amorphous Q3. The hydrogen bonds were classified into four types: silica–silica, silica–dissolved CO2, silica–water as donors and silica–water as acceptors. The mean numbers of hydrogen bonds for each type were analysed. The results show that: (1) silica surface silanol groups do not form H-bonds with dissolved CO2 molecules in water (brine); (2) The mean number of hydrogen bonds between silanol groups follows the order: Q2 > amorphous Q3 > Q3; (3) The mean number of hydrogen bonds between silanol and water molecules follows the order: Q3 > amorphous Q3 > Q2.

Functionalized Emulsion Styrene-Butadiene Rubber Containing Diethylaminoethyl Methacrylate for Silica Filled Compounds

In this study, diethylaminoethyl methacrylate-styrene-butadiene terpolymer (DEAEMA-SBR), in which diethyl- aminoethyl methacrylate (DEAEMA) was introduced to the SBR molecule as a third monomer, was synthesized by cold emulsion polymerization. It is expected that amine group introduced to a rubber molecule would improve dispersion of silica by the formation of hydrogen bond (or ionic coupling) between the amine group and silanol groups of silica surface. The chemical structure of DEAEMA-SBR was analyzed using proton nuclear magnetic resonance spectroscopy (H-NMR), Fourier transform infrared spectroscopy (FTIR), gel permeation chromatography (GPC) and differential scanning calorime- try (DSC). Then, various properties of DEAEMA-SBR/silica composite such as crosslink density, bound rubber content, abrasion resistance, and mechanical properties were evaluated. As a result, bound rubber content and crosslink density of DEAEMA-SBR/silica compound were higher than those of the SBR 1721 composite. Abrasion resistance and moduli at 300% elongation of the DEAEMA-SBR/silica composite were better than those of SBR 1721 composite due to the high bound rubber content and crosslink density. These results are attributed to high affinity between DEAEMA-SBR and silica. The proposed study suggests that DEAEMA-SBR can help to improve mechanical properties and abrasion resistance of the tire tread part.

Robust amphiphobic coatings from bi-functional silica particles on flat substrates

Robust oil- and water-repellent or amphiphobic particulate coatings were prepared from epoxy glue and bi-functional silica particles bearing poly(2-perfluorooctylethyl methacrylate) (PFOEMA) and poly(acrylic acid) (PAA) coronal chains. To prepare the particles, silica was first co-grafted in a one-pot process by poly[3-(triisopropyloxysilyl)propyl methacrylate]-block-poly(2-perfluorooctylethyl methacrylate) (PIPSMA-b-PFOEMA or P1) and poly[3-(triisopropyloxysilyl)propyl methacrylate]-block-poly(tert-butyl acrylate) (PIPSMA-b-PtBA or P2). P1 and P2 grafted onto silica because of the sol–gel reactions between the surface silanol groups of silica and the PIPSMA blocks of the two polymers. The sol–gel reactions also occurred between the different PIPSMA blocks of P1 and P2, resulting in a crosslinked anchoring PIPSMA layer. This layer was topped by the PFOEMA and PtBA chains. Cleavage of the tert-butyl groups of the PtBA chains yielded PAA corona chains. The particles bearing both PAA and PFOEMA could be dispersed in trifluorotoluene and sprayed onto an epoxy film that was partially cured on a glass plate to yield a rough particulate coating. The curing of the particles with the glue at 120 °C for 30 min bonded the AA units and the glycidyl groups of the epoxy glue and exposed the low-surface-tension PFOEMA chains. At P1 mass fraction of f1 ≥ 80% relative to P1 and P2, the coatings were superhydrophobic and repelled diiodomethane strongly. Furthermore, the coatings resisted trifluorotoluene extraction and abrasion tests if the PAA chains existed in sufficient amounts with P2 weight fraction (f2) ≥ 5%.

Anhydride modified silica nanoparticles: Preparation and characterization

The paper describes modification of colloid silica nanoparticles, using 3-(triethoxysilyl)propylsuccinic anhydride (TESPSA) as a reagent. Anhydride groups were grafted to the silica surface by an interaction of the surface silanol Si–OH groups with reactive ethoxy groups of TESPSA. Elemental analysis, FTIR and 1H NMR spectroscopy were employed to characterize silica nanoparticles before and after surface modification. It was demonstrated that only fifty percent of silica surface silanol groups are involved in the reaction with TESPSA. Size distribution of nanoparticles determined by atomic force microscopy (AFM) technique shows that nanoparticles preserve their size upon modification. TESPSA-modified silica nanoparticles can be used as polyfunctional curing agent for epoxy resin.

Macrocycle–pore network interactions: Aluminum tetrasulfophthalocyanine in organically modified silica

A dependable method was implemented for finding appropriate experimental conditions to attain a disaggregated trapping of aluminum phthalocyanine molecules inside translucent silica pore networks. Aluminum hydroxy-tetrasulfophthalocyanine was chosen as probe species for this task in view of its high colloidal stability and exceptional fluorescence. During early experiments, the fluorescence and stability of the trapped probe species either disappeared or were attenuated due to the interactions with the silanol groups residing on the pore walls. The stability disadvantage was overcome by chemically exchanging the surface silanol groups of silica by longer alkyl or aryl groups proceeding from organo-substituted alkoxides. The interactions between these groups and the aluminum probe induced its aggregation, degradation or the contraction or expansion of the cavities lodging these molecules. Although the red fluorescence of the probe molecule disappears when it was physically trapped in organo-modified silica, the results here presented revealed the possibility of adjusting the shape, pore size, specific surface area, and internal polarity of the pore cavities through the use of organo-substituted alkoxides and templating species, such as phthalocyanines. Hybrid systems generated this way can become important for nanotechnological applications in catalysis, optics, medicine, and gas sensoring.

An Analytic Expression for the Dependence of the Apparent Acid Constant on the Degree of Dissociation of Surface Silanol Groups of Silica with Different Sizes

Using the surface charge density/surface potential relationship for a spherical colloidal particle with high surface potential, a simple expression for the relationship of the apparent acid constant with the degree of dissociation of surface silanol groups of silica with different sizes is derived. The present formula has good agreement with literature values and is quite simple for practical use.

Allyltrimethoxysilane as the Reagent of Double Bond Introduction to Porous Silica for Preparation of a Chiral Stationary Phase in Comparison with Allyltriethoxysilane

Chiral separation has been extensively used not only to produce pure enantiomers but also to analyze mixtures of enantiomers. Many different types of chiral stationary phases have been developed such as brush or ligand-exchange type phases, cyclodextrin phases, protein phases, polysaccharide phases, etc. Optically active polymers compose a great portion of the chiral phases and have been thoroughly investigated in a recent review. Among them, polysaccharide chiral phases have been known to be very useful for their versatility in chiral separation. Derivatives of polysaccharides adsorbed on porous silica particles are now very popular chiral stationary phases and are commercially available, but they are not compatible with some common LC solvents such as tetrahydrofuran and chloroform for their solubility in the solvents. Such a problem can be solved by securing chemical bonding between the silica surface and the polysaccharide ligand. Such phases (Diacel) have been commercially available only very recently. Introduction of double bonds to both the silica surface and the polysaccharide ligand followed by cross polymerization between the silica and the ligand has been the method of securing chemical bonding. The surface silanol groups of silica are used in the reaction of introducing double bonds, and two different reagents have been used to incorporate double bonds: 3-(trimethoxysilyl) propyl methacrylate and allyltriethoxysilane. Improvements in double bond introduction to the silica surface and consequent cross polymerization seem to be important for commercial polysaccharide chiral phases with chemically bonded polysaccharide ligands. In this study, we have found that an improvement of the process can be achieved by simply employing allyltrimethoxysilane instead of allyltriethoxysilane(traditional reagent) as the reagent of double bond introduction.

Immobilized Metal Ion‐Containing Ionic Liquids: Preparation, Structure and Catalytic Performance in Kharasch Addition Reaction.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Immobilized metal ion-containing ionic liquids: preparation, structure and catalytic performance in Kharasch addition reaction

Immobilized metal ion-containing ionic liquid catalysts were prepared by the reaction between silyl-functionalized imidazolium ionic molecules and surface silanol groups of silica, followed by addition of MnCl2, FeCl2, CoCl2, NiCl2, CuCl2, or PdCl2; only the immobilized copper catalyst, which has a sandwiched CuCl4(2-) moiety, was very active for the Kharasch reaction between styrene and CCl4.

FTIR spectroscopic and ab initio evidence for an amphipathic character of CO bonding with silanol groups

FTIR spectroscopy provides evidence that carbon monoxide interacts with surface silanol groups of silica and silicalite to form both SiOH⋯CO and SiOH⋯OC hydrogen-bonded complexes. The C-bonded adduct shows a characteristic IR absorption band which is blue-shifted as compared to free CO (2143 cm −1), while the O-bonded adduct is characterized by a red-shifted band. Variable temperature IR spectroscopy has shown that these two hydrogen-bonded adducts are in a temperature-dependent equilibrium which involves an enthalpy change of about 2–3 kJ mol −1. Spectroscopic data are supported by ab initio cluster model calculations of different complexity levels.

Preparation of the hydrophilic/hydrophobic silica particles

Abstract Silicas of high dispersion degree were obtained. The process included formation of silica particles and their aggregation. Glycerin solution was used in precipitation process, resulting in a partial blocking of the silica surface hydroxyl groups (silanol groups) and, thus, in a decreased hydrophilicity of silica. Studies on the surface modification of silicas using silane coupling agents are described. The best modifiers were selected, which induced a change of the silica surface from the hydrophilic to the hydrophobic one. Basic physicochemical analyses of the obtained silicas were performed. Near infrared spectroscopy (NIR) was used to determine the degree of condensation of silica surface silanol groups. The degree of hydrophobization of silica surface was determined by a calorimetric method. Moreover, zeta potential, size distribution of primary particles, aggregates and agglomerates structures were determined by ZetaPlus instrument using electrophorectic (ELS) and dynamic (DLS) light scattering techniques.

Synthesis and reactivity of diphosphino Pt(II) silanolate complexes as molecular models of the interaction of platinum particles with silica

A series of di- and monosubstituted cis-platinum(II) silanolate complexes, Pt(OSiR 3) 2(dppe) (R=Et, 1; R=Me, 2) and Pt(OSiR 3)Cl(dppe) (R=Et, 3; R= i Pr, 4) where dppe is 1,2-bis(diphenylphosphino)ethane, have been isolated and characterised spectroscopically. Complex 1 does not react with CO and H 2 under anhydrous conditions, but the complexes Pt{C(O)OCH 3)} 2(dppe) ( 6) and Pt(CO 3)(dppe) ( 7) have been isolated bubbling CO in methanol and CO 2 in moist benzene solutions of 1, respectively. The behaviour of 1 towards water or methanol is discussed on the basis of 1H, and 31P{ 1H} NMR spectroscopic data. The new complex Pt{S 2C(OSiEt 3) 2}(dppe) ( 8) has been isolated by reaction of 1 with CS 2 in benzene solution. This reactivity would suggest a high sensitivity towards water, but not towards H 2 or CO, of the bonding of slightly oxidised platinum particles with silanol groups of silica surface.

Effect of amorphous precipitated silica on the properties and structure of poly( p -phenylene sulfide)

Using a precipitation technique, silicas were obtained from sodium metasilicate solution employing an acidic agent. Alcohol solutions were used in the process of production of highly dispersed silicas, which resulted in partial blocking of the silica surface silanol groups. Moreover, studies on morphology and microstructure using transmission electron microscopy and scanning electron microscopy were performed. The size distributions of primary particles and aggregate and agglomerate structures were determined using a ZetaPlus instrument using the dynamic light scattering method. The structure and molecular dynamics of the nanocomposite, consisting of poly (p-phenylene sulfide) (PPS) and of the precipitated silica, were studied using atomic force microscopy and nuclear magnetic resonance. It was proved that during annealing the fragmentation of PPS agglomerates takes place. This phenomena probably resulted from repulsion forces existing between agglomerates and aggregates. Fragmentation in the polymer network probably resulted from repulsion forces between agglomerates and smaller aggregates.

Influence of silane coupling agents on surface properties of precipitated silicas

Silicas of highly dispersion degree were obtained. This process includes formation of silica particles and their aggregation. The ethylene glycol solution was used in precipitation process, resulting in a partial blocking of the silica surface hydroxyl groups (silanol groups) and, thus, in a decreased hydrophilicity of silica. Studies on the surface modification of silicas using silane coupling agents are described. The best modifiers were selected, which inducted a change of the silica surface from hydrophilic to hydrophobic. Basic physicochemical analyses of the obtained silicas were performed. The methods of evaluating the degree of surface modification of the silica were presented. Near infrared spectroscopy (NIR) was used to determine the degree of condensation of the silica surface silanol groups. The degree of hydrophobization of silica surface was determined by a calorimetric method. Moreover, studies on morphology and microstructure using transmission electron microscopy (TEM) were performed. Size distribution of primary particles and aggregates and agglomerates structures were determined by ZetaPlus instrument using dynamic light scattering (DLS) method.

Brønsted acidity of silica silanol groups induced by adsorption of acids

Surface silanol groups of silica, which never revealed any Bronsted acidity, are shown to donate protons to adsorbed basic molecules, such as ammonia, pyridine or 2,6-dimethylpyridine, after addition of acids such as SO2 or NO2. The latter, when coadsorbed with bases, interact with the oxygen atoms of silanols leading to OH acidity increase and to protonation. Bases, in turn, enhance chemisorption of SO2 or NO2, and strongly held coadsorption products are formed as a result. The proposed mechanism of induced Bronsted acidity could account for the promoting effect of acidic gases in reactions catalysed by metal oxides.

PROPERTIES OF HIGHLY DISPERSED SILICAS PRECIPITATED IN AN ORGANIC MEDIUM

ABSTRACT Silicas were obtained using a precipitation technique from metasilicate solution with an acidic agent. The precipitation process includes formation of silica particles and their aggregation. Alcohol solutions were used in the production process of the highly dispersed silicas, resulting in a partial blocking of the silica surface silanol groups and, thus, in a decreased hydrophilicity of silica. The precipitated silicas were subjected to physicochemical, structural, and microscopic evaluation, and their surface properties were examined. The effect of alcohols on the silica particle size distributions and on hydrophobicity of silica surface was tested.