Amorphous Silica Surface Research Articles

Ice-grain impact on a rough amorphous silica surface

Ice-grain impact on a rough amorphous silica surface

Enhanced photocatalytic degradation of antiviral drugs lopinavir and ritonavir by Ni doped ZnO/SiO2 nanocomposites

A series of silica-based Ni doped ZnO nanocomposites were successfully synthesized by the co-precipitation of zinc and nickel acetate salts in the presence of urea on the surface of amorphous silica using thermo-oxidative degradation at 600 °C with a concentration of 3, 6 and 9 mmol ZnO per 1 g SiO2. The effects of the dopants and SiO2 matrix on the structure, morphology and optical properties of the composites were investigated using N2 adsorption-desorption, Raman, XRD, UV-Vis, PL, FT-IR, SEM, and TEM analyses. XRD indicated the formation of a hexagonal ZnO wurtzite structure, while Ni2+ ions substituted Zn2+ sites in the host lattice. The size of nanoparticles was in the range of 10–100 nm and showed a broad fluorescence emission at 442–433 nm. The specific surface area was 110–171 m2 g−1. The materials showed high efficiency in removing the antiviral SARS-CoV-2 drugs lopinavir (LOPI) and ritonavir (RITO) from water. The comparative photocatalytic activity showed 100 % removal of LOPI by 3-, 6- and 9-Ni-ZnO/SiO2 photocatalysts after 60, 30 and 45 min of ultraviolet light irradiation, respectively, and 60 min for RITO for all catalyst samples. The effects of inorganic ions, dissolved organics and reusability on photocatalytic degradation were also described in terms of practical applications.

The structural and electronic configuration of an amorphous silica surface, from its liquid to glassy state

Through classical molecular dynamics computational simulations, the surface of amorphous silica (vitreous silica) was generated for a system comprising 64 SiO2 molecules (192 atoms) via the classical three-body glass potential. Starting from the liquid state of silica at 3400 K, it was cooled in three different ways at intervals of 200, 400, and 2000 K, with a cooling rate of 4.4 K/ps and establishing relaxation at intermediate temperatures, yielding three groups of surfaces from 3400 to 1400 K. Subsequently, these systems underwent sudden cooling from 1400 to 300 K via the Andersen and Nosé‒Hoover thermostats, followed by relaxation at 300 K via a microcanonical ensemble (NVE constant). Each surface that underwent relaxation was analyzed through ab initio molecular dynamics for the following three physical properties, which are temperature dependent. The average electric charges of oxygen and silicon. Microstructures on the outermost part of the surface were observed and comprised nonbridging oxygen atoms, undercoordinated silicon, and N-membered rings (3 ≤ N ≤ 5). The radial distribution functions were analyzed for all surfaces. Below 2000 K, a type of freezing is observed in all the structures because of the glass transition. All analyses were performed at zero pressure.

Interfacial Structure of Epoxy Resins on Amorphous Silica Surface under Dry and Wet Conditions

Interfacial Structure of Epoxy Resins on Amorphous Silica Surface under Dry and Wet Conditions

Hybrid Organic–Inorganic Biomaterials as Drug Delivery Systems: A Molecular Dynamics Study of Quercetin Adsorption on Amorphous Silica Surfaces

Many important drugs in pharmaceutical applications are poorly soluble. Solubilization, which is diffusion through biological barriers, and the control of local administration are crucial steps for bioavailability and to avoid cytotoxic effects. Hybrid organic/inorganic biomaterials can incorporate drugs for in situ release after implantation. Molecular Mechanics (MM) and Molecular Dynamics (MD) simulations are useful tools for investigating intermolecular interactions between drug and biomaterial surfaces at the atomistic level for these applications. This work studies quercetin, a flavonoid drug important for its anti-inflammatory, antioxidant, and anticancer properties, and the amorphous SiO2 surface using a simulation protocol proposed in previous work related to ketoprofen drugs. After adsorption on the amorphous silica surface, the adsorption process of quercetin drug molecules at two different drug concentrations near a hydrated and then dried silica surface is investigated. Interestingly, these theoretical results are compared with experimental data obtained via Fourier Transform Infrared Spectroscopy (FT–IR) spectra related to quercetin molecules homogenously entrapped in a silica matrix obtained via the Sol–Gel method. Favorable H– bonds and some π–π interactions among drug molecules are crucial surface interactions for the new generation of biocompatible materials capable of incorporating anti-inflammatory agents for release into the human body.

Generation of Amorphous Silica Surfaces with Controlled Roughness.

Amorphous silica (a-SiO2) surfaces, when grafted with select metals on the active sites of the functionalized surfaces, can act as useful heterogeneous catalysts. From a molecular modeling perspective, one challenge has been generating a-SiO2 slab models with controllable surface roughness to facilitate the study of the effect of surface morphology on the material properties. Previous computational methods either generate relatively flat surfaces or periodically corrugated surfaces that do not mimic the full range of potential surface roughness of the amorphous silica material. In this work, we present a new method, inspired by the capillary fluctuation theory of interfaces, in which rough silica slabs are generated by cleaving a bulk amorphous sample using a cleaving plane with Fourier components randomly generated from a Gaussian distribution. The width of this Gaussian distribution (and thus the degree of surface roughness) can be tuned by varying the surface roughness parameter α. Using the van Beest, Kramer, and van Santen (BKS) force field, we create a large number of silica slabs using cleaving surfaces of varying roughness (α) and using two different system sizes. These surfaces are then characterized to determine their roughness (mean-squared displacement), density profile, and ring size distribution. This analysis shows a higher concentration of surface defects (under-/overcoordinated atoms and strained rings) as the surface roughness increases. To examine the effect of the roughness on surface reactivity, we re-equilibriate a subset of these slabs using the reactive force field ReaxFF and then expose the slabs to water and observe the formation of surface silanols. We observe that the rougher surfaces exhibit higher silanol concentrations as well as bimodal acidity.

An Insightful Approach to Understanding the Mechanism of Amino Acid Adsorption on Inorganic Surfaces: Glycine on Silica

The adsorption of glycine on amorphous silica surface has been studied to demonstrate the catalytic activity of silica surfaces towards the formation of peptide bonds on prebiotic earth. Silica nanoparticles were synthesized using a microwave assisted method and the nanoparticles were characterized using SEM. Glycine was adsorbed from aqueous solution on the nanoparticles and the adsorption behavior was characterized using FTIR and TGA analyses. At a glycine concentration of 0.5M and at pH=7, favorable adsorption was observed which obeyed the Langmuir isotherm model. From the FTIR characterization, peptide bond formation was confirmed. It was concluded that the adsorption of glycine occurs via electrostatic interactions as well as hydrogen bonding between the silica surface and glycine molecules.

Ion adsorption and hydration forces: a comparison of crystalline mica vs. amorphous silica surfaces.

Hydration forces are ubiquitous in nature and technology. Yet, the characterization of interfacial hydration structures and their dependence on the nature of the substrate and the presence of ions have remained challenging and controversial. We present a systematic study using dynamic Atomic Force Microscopy of hydration forces on mica surfaces and amorphous silica surfaces in aqueous electrolytes containing chloride salts of various alkali and earth alkaline cations of variable concentrations at pH values between 3 and 9. Our measurements with ultra-sharp AFM tips demonstrate the presence of both oscillatory and monotonically decaying hydration forces of very similar strength on both atomically smooth mica and amorphous silica surfaces with a roughness comparable to the size of a water molecule. The characteristic range of the forces is approximately 1 nm, independent of the fluid composition. Force oscillations are consistent with the size of water molecules for all conditions investigated. Weakly hydrated Cs+ ions are the only exception: they disrupt the oscillatory hydration structure and induce attractive monotonic hydration forces. On silica, force oscillations are also smeared out if the size of the AFM tip exceeds the characteristic lateral scale of the surface roughness. The observation of attractive monotonic hydration forces for asymmetric systems suggests opportunities to probe water polarization.

Emergence of Order in Origin-of-Life Scenarios on Mineral Surfaces: Polyglycine Chains on Silica.

The polymerization of amino acids (AAs) to peptides on oxide surfaces has attracted interest owing to its high importance in biotechnology, prebiotic chemistry, and origin of life theories. However, its mechanism is still poorly understood. We tried to elucidate the reactivity of glycine (Gly) from the vapor phase on the surface of amorphous silica under controlled atmosphere at 160 °C. Infrared (IR) spectroscopy reveals that Gly functionalizes the silica surface through the formation of ester species, which represent, together with the weakly interacting silanols, crucial elements for monomers activation and polymerization. Once activated, β-turns start to form as initiators for the growth of long linear polypeptides (poly-Gly) chains, which elongate into ordered structures containing both β-sheet and helical conformations. The work also points to the role of water vapor in the formation of further self-assembled β-sheet structures that are highly resistant to hydrolysis.

MgO/SiO2 prepared by wet-kneading as a catalyst for ethanol conversion to 1,3-butadiene: Prins condensation as the predominant mechanism

MgO/SiO2 catalysts prepared by wet-kneading with various compositions were applied to directly convert ethanol to 1,3-butadiene. The MgO/SiO2 molar ratio significantly affected the structural, textural, and surface properties of the catalysts, leading to different activities toward ethanol conversion. At low MgO loading (51 mol %), magnesium silicate is formed on the amorphous silica surface, resulting in a higher activity for ethanol dehydration to ethylene. Increasing the MgO loading, crystalline magnesium oxide is formed in addition to the magnesium silicate, producing strong basic sites capable of increasing the ethanol conversion to 1,3-butadiene. Interestingly, for the mixed oxides prepared herein, a linear correlation was found between the forward reaction rate for 1,3-butadiene formation as a function of the reaction rate for acetaldehyde and ethylene. This behavior suggests a predominance of the Prins condensation mechanism. Therefore, the paired arrangement and the strength of the acid-basic conjugate sites of these catalysts must be determinant for the dehydrogenation and dehydration of ethanol and the condensation of the respective intermediates acetaldehyde and ethylene to form 1,3-butadiene.

Simulating the diffusion of hydrogen in amorphous silicates: A ‘jumping’ migration process and its implications for solar wind implanted lunar volatiles

We use molecular dynamics (MD) simulations to better explain the movement of atomic hydrogen in amorphous silica and quantify the planetary science implications of these findings. Previous MD simulations had a large range of predicted values and did not agree well with experiment. Our simulations sample atomic motion for a longer duration and consider a wider range of temperatures than previous simulations. In contrast to constant atomic motion, the hydrogen atoms were shown to undergo random intermittent jumps from one oxygen atom to another, the number of which increase with temperature. Predicted diffusion coefficients had a better agreement to experimental values than previous MD simulations, suggesting the importance of longer simulation durations for better statistics. The low activation energy and jumps observed at lunar temperatures do not support the theory of diurnal variations in OH content for an undamaged amorphous silica surface. Instead, we conclude that energetic solar wind impacts can induce two competing atomic hydrogen motion processes in the exposed surface: A prompt effect that induces jumps in the temperature spike volume, but also a long-term effect of damage in the structure that traps atomic hydrogen. We then use SDTrimSP to quantify the damage created during exposure and MD to demonstrate the H retention and trapping near these defects. Damage was shown to be dependent on impact energy, with defects easily retaining implanted hydrogen. MD results like those presented herein on unweathered surfaces are therefore most relevant to magnetic anomalies. As a result, we demonstrate the importance of lunar volatile models to account for the damage state of the substrate when modelling hydrogen diffusion, retention, and subsequent OH/water production.

Silica scaling and boiling effect on corrosion behaviour of nickel-based alloys associated with superheated hydrothermal fluids containing HCl, H2S and CO2

The effect of amorphous silica (AM-silica) surface deposits on the corrosion behaviour of two nickel-based alloys associated with superheated hydrothermal fluid containing sour gases (HCl, H2S and CO2) was investigated experimentally at 350 °C at 10 barG. The results demonstrate that the AM-silica scales do not promote localized corrosion damage whereas some corrosion products were detected in non-scale samples due to fluid boiling from sub- to superheated conditions. This study contributes to more understanding of the effect of amorphous silica scaling and boiling on corrosion behaviour and the selection of materials for future application in superheated geothermal systems.

Slow liquid dynamics near solid surfaces: Insights from site-resolved studies of ionic liquids in silica confinement.

We perform molecular dynamics simulations of ionic liquids that are confined between amorphous silica surfaces and composed of 1-butyl-3-methylimidazolium [C4mim] cations and tetrafluoroborate [BF4], hexafluorophosphate [PF6], or bis(trifluoromethylsulfonyl)imide [TFSI] anions. Near the silica surfaces, we observe strong slowdowns of ion dynamics, which involve mainly two layers of ions and amount to about two orders of magnitude, depending on temperature and anion type. For a detailed investigation of the slow interfacial dynamics, we determine the adsorption sites of the various anion species on the amorphous silica surfaces and ascertain the repopulation dynamics of these sites. The analysis reveals that the mean residence times show a broad distribution, where anions stay longer at sites, which provide better opportunities for hydrogen bonding. Furthermore, the mean residence times follow Arrhenius laws, providing access to site-specific activation energies Ei. The distributions G(Ei) have Gaussian shape with mean values from ∼0.40eV for TFSI to ∼0.48eV for PF6 and standard deviations of about 0.31eV. Thus, the amorphous silica surfaces impose static and disordered energy landscapes to the neighboring liquid, which have considerable ruggedness and, in this way, substantially hinder ion rearrangements. We discuss that qualitatively similar situations are expected for all kinds of confined liquids.

Can Mesoporous Silica Speed Up Degradation of Benzodiazepines? Hints from Quantum Mechanical Investigations.

This work reports for the first time a quantum mechanical study of the interactions of a model benzodiazepine drug, i.e., nitrazepam, with various models of amorphous silica surfaces, differing in structural and interface properties. The interest in these systems is related to the use of mesoporous silica as carrier in drug delivery. The adopted computational procedure has been chosen to investigate whether silica–drug interactions favor the drug degradation mechanism or not, hindering the beneficial pharmaceutical effect. Computed structural, energetics, and vibrational properties represent a relevant comparison for future experiments. Our simulations demonstrate that adsorption of nitrazepam on amorphous silica is a strongly exothermic process in which a partial proton transfer from the surface to the drug is observed, highlighting a possible catalytic role of silica in the degradation reaction of benzodiazepines.

Dehydration of methanol and ethanol over ferrierite originated layered zeolites - the role of acidity and porous structure.

Ferrierites and their delaminated (ITQ-6) and silica intercalated (ITQ-36) forms, with the intended molar Si/Al ratios of zeolite layers of 30 and 50, were synthesized and tested as catalysts of methanol to dimethyl ether (DME) as well as ethanol to diethyl ether (DEE) and ethylene dehydration. It was shown that increased content of acid sites, especially of Brønsted type, resulted in more active catalysts of alcohol dehydration. Brønsted acid sites dominate in ferrierites and their delaminated forms (ITQ-6). Contribution of the Lewis type of acid sites increased in silica pillared ferrierites (ITQ-36) possibly by deposition of aluminium species on the surface of amorphous silica. Conversion of methanol to DME was not limited by internal diffusion of reactants in narrow pores of ferrierite. Such limitation was observed for synthesis of larger DEE molecules over ferrierites. The ITQ-6 catalysts with the opened interlayer structure presented better efficiency in ethanol to DEE conversion due to overcoming these diffusional restrictions. Moreover, selectivity to DEE over ITQ-6 was higher than in the presence of three-dimensional ferrierite.

Simulation and analysis of slip flow of water at hydrophobic silica surfaces of nanometer slit pores

Water flow in nanopores is controlled fundamentally by pore wettability and interfacial water features. In the research herein reported, Molecular dynamics (MD) was used to simulate water flow in nanometer slit pores of silica surfaces with variable wetting characteristics. Compared to Couette flow, the simulated slit-pores of the talc (001) and octadecyltrichlorosilane (OTS) monolayer surfaces exhibited water flow enhancement and a positive slip length at the interface, whereas water flow velocity at the quartz (001) and amorphous silica surfaces was zero, adhering to the no slip condition. More intensive interactions between the interfacial water and hydrophilic silica surfaces were confirmed by MD simulation and sum-frequency vibrational spectroscopy (SFVS) results. A water exclusion zone of approximately 3 Å was found at the hydrophobic silica surfaces by quantitative MD interfacial water analysis and validated by SFVS. This exclusion zone dimension was similar in magnitude to the slip length (4–5 Å) as determined from simulated water flow in hydrophobic silica slit pores. The simulated slip lengths at the hydrophobic surfaces (0.2–1 nm) were significantly less than the experimental slip lengths (>40 nm) reported in the literature. In the case of the experimental results, this difference appears to be due to nanobubbles expected to be present at hydrophobic surfaces.

Selective adsorption of glucose towards itaconic acid on amorphous silica surfaces: Insights from density functional theory calculations

A better separation and purification of organic acids in liquid phase is a key for a more performing biorefinery industry. Herein, amorphous silica surfaces with a density of silanol ranging between 1.1 and 7.2 OH·nm−2 are investigated to separate glucose molecules from the fermentation broths. Different interaction modes of glucose and itaconic acid on the considered silica surfaces at various silanol sites are systematically investigated using periodic density functional theory. Our results show that the surfaces with a silanol density of 2.0 to 7.2 OH·nm−2 are potentially selective for glucose adsorption in comparison with itaconic acid.

Coupling between Polymer Conformations and Dynamics Near Amorphous Silica Surfaces: A Direct Insight from Atomistic Simulations.

The dynamics of polymer chains in the polymer/solid interphase region have been a point of debate in recent years. Its understanding is the first step towards the description and the prediction of the properties of a wide family of commercially used polymeric-based nanostructured materials. Here, we present a detailed investigation of the conformational and dynamical features of unentangled and mildly entangled cis-1,4-polybutadiene melts in the vicinity of amorphous silica surface via atomistic simulations. Accounting for the roughness of the surface, we analyze the properties of the polymer chains as a function of their distance from the silica slab, their conformations and the chain molecular weight. Unlike the case of perfectly flat and homogeneous surfaces, the monomeric translational motion parallel to the surface was affected by the presence of the silica slab up to distances comparable with the extension of the density fluctuations. In addition, the intramolecular dynamical heterogeneities in adsorbed chains were revealed by linking the conformations and the structure of the adsorbed chains with their dynamical properties. Strong dynamical heterogeneities within the adsorbed layer are found, with the chains possessing longer sequences of adsorbed segments (“trains”) exhibiting slower dynamics than the adsorbed chains with short ones. Our results suggest that, apart from the density-dynamics correlation, the configurational entropy plays an important role in the dynamical response of the polymers confined between the silica slabs.

Structure of the Water Molecule Layer between Ice and Amorphous/Crystalline Surfaces Based on Molecular Dynamics Simulations.

The structure of the water layer between the ice interface and the hydroxylated amorphous/crystalline silica surfaces was investigated using molecular dynamics simulations. The results indicate that the density profile in the direction perpendicular to the surface has two density peaks in the water layer at the ice-silica interface, which are affected by the silanol group density on the wall and the degree of supercooling in the system. In the two density peaks, the one facing the ice interface side has the same structure as the ice crystal, while the other density peak facing the silica surface has an icelike structure. In the solidification process, the ice and icelike structures in the layer progress more on the amorphous silica surface where the density of the silanol groups is low. The relationship between the ice crystallization and the thickness of the layer has been studied in detail; the lower the temperature, the more the ice crystallization progresses and the thinner the layer becomes.

The pH dependence of the isotopic composition of boron adsorbed on amorphous silica

The use of the boron content and isotopic composition of secondary silicate minerals and siliceous organisms to trace weathering reactions and past ocean pH requires characterizing the fundamental reactions that govern the incorporation and subsequent isotope fractionation of this element in these materials. Toward this goal we have investigated boron adsorption on the surface of amorphous silica (SiO2·0.32H2O) and quantified its isotopic fractionation. Boron adsorption envelopes and corresponding isotope fractionation factors were measured in dilute aqueous solutions (0.01 M) of NaCl and CaCl2. B maximum adsorbed fraction was found to be about 2 times higher in CaCl2 solutions than in NaCl. The modelling of chemical and isotopic data in NaCl solutions allowed to identify the formation of two main B surface species at the SiO2(am)/water interface: a neutral trigonal (B3) inner-sphere complex, >SiOB(OH)20, characterized by a fractionation factor of ~−16‰ relative to aqueous boric acid, and a negatively charged tetrahedral (B4) inner-sphere complex, >SiOB(OH)3–, with fractionation factors of −5‰ with respect to aqueous borate. In CaCl2 solutions the data modelling indicates the presence of B4 inner-sphere complexes with a fractionation factor of −6.5‰ relative to aqueous borate, but excludes the presence of trigonal surface species and suggests instead the formation of a Ca-B(OH)4– complex that could partly account for the observed increase of boron adsorption in these solutions. These observations indicate that changes in the surface charge density and interfacial water structure due to different background electrolytes can induce changes in concentration and both chemical and isotopic composition of B adsorbed on silica surfaces.Although this study suggests that the B adsorption reaction on SiO2(am) plays a minor role in the B incorporation and isotopic signature of clay minerals and siliceous cements forming during weathering reactions, the acquired data should allow for an improved knowledge of the biomineralization reactions. The observed B isotopic fractionations between adsorbed and aqueous B species should help determining the relative contribution of various processes, such as cellular transport, biological mediation and silicification reactions, on the amount of B incorporated in siliceous microorganisms and its isotopic signatures. In addition, the presence in NaCl solutions of both trigonal and tetrahedral boron complexes at the silica surface could explain the observed weak pH dependence of the boron isotopic composition of marine diatoms.