Computer modeling of amorphous silica structures

An algorithm for large cluster construction for quantum-chemical modeling of amorphous silica structures is suggested. The algorithm is based on the conception that different species of disperse amorphous silica, namely aerosil, silica gel and aerogel are different polymorphic forms; that these polymorphic modifications can be classified by Libau's crystallochemical classification of silicates; that each classification mode is determined by the manufacturing method. The structure models presented are obtained in accordance with the algorithm and are in line with the chemical reactions forming the basis of commercial manufacturing of the products.     

Simulation of nonwetting phase entrapment within porous media using magnetic resonance imaging

Models representing the pore structures of amorphous, mesoporous silica pellets have been constructed using magnetic resonance images of the materials. Using magnetic resonance imaging (MRI), maps of the macroscopic (approximately 0.01-1 mm) spatial distribution of porosity and pore size were obtained. The nature and key parameters of the physical mechanism for mercury retraction, during porosimetry experiments on the silica materials, were determined using integrated gas sorption experiments. Subsequent simulations of mercury porosimetry within the structural models derived from MRI have been used to successfully predict, a priori, the point of the onset of structural hysteresis and the final levels of mercury entrapment for the silicas. Hence, a firm understanding of the physical processes of mercury retraction and entrapment in these amorphous silica materials has been established.     

Model studies of colloidal silica precipitation using biosilica extracts from<Emphasis Type="Italic"> Equisetum telmateia</Emphasis>

Structural materials containing silicon are produced in single celled organisms through to higher plants and animals. Hydrated amorphous silica is a colloidal mineral of infinite functionality that is formed into structures with microscopic and macroscopic form. Proteins and proteoglycans are suggested to play a critical role in the catalysis of silica polycondensation and in structure direction during the formation of these magnificent structures. This article extends knowledge on the effect of protein containing biosilica extracts from Equisetum telmateia on the kinetics of silica formation and structure regulation. Utilising potassium silicon catecholate as the source of soluble silicon, bioextracts obtained from plant silica by dissolution of the siliceous phase with aqueous HF following extensive acid digestion of the plant cell wall were found to modify the kinetic rate constants for the formation of small silicic acid oligomers under circumneutral pH conditions and to modify the solubility of silicic acid in solution. Addition of the bioextracts at ca. 1 wt% to the reaction medium reduced the sizes and range of sizes of the fundamental silica particles formed and led to the formation of crystalline polymorphs of silica under conditions of ca. neutral pH, room temperature and in the absence of multiply charged cations, conditions assumed to be relevant to the biological mineralization environment. The ability of biological organisms to regulate the formation of silica structures with prevention of crystallinity is discussed as are the implications of this study in terms of the generation of new materials with specific form and function for industrial application.     

有序的介观尺寸生物SiO2针的合成

生物体利用生物矿化作用,在有机分子模板上协同合成有种间差异的生物SiO2材料。具有精确形态可控的固态SiO2结构是在蛋白和多糖生物分子诱导下,在水相、中性pH和室温等温和反应条件下形成的,然而,利用化学合成方法,使SiO2的前体分子聚合形成具有一定模式结构的SiO2则需要极端的pH值或表面活性剂诱导。在人工培养条件下施用硅酸钠(Na2SiO3)时,在芦荟植物叶刺内,以细胞壁为模板生物矿化合成微米尺寸有序SiO2材料。X-ray(EDX)能谱分析显示一根硅针中含有Si,O和C元素,表明Si(OH)4吸收进入植物体内后Si-OH与细胞壁多糖和糖蛋白上的羟基(OH)组分,通过界面分子识别、细胞水平调控和再加工作用,聚合形成了有序的无定形(电子衍射确定)SiO2针状结构体。     

Micro Raman Spectroscopy of Silica Nanoparticles Treated with Aminopropylsilanetriol

Summary: Micro Raman spectroscopy was used to investigate the structures formed at the surface of amorphous pirogenic silica Aerosil 200. Nanoparticles were treated with 25% water solution of aminopropylsilanetriol and dried in laboratory atmosphere and under vacuum of about 5 mbar. Spectra of pure silica nanoparticles and treated nanoparticles were recorded in the frequency range 20–1700 cm⁻¹ and 2500–3800 cm⁻¹. Comparative analysis reveals the existance of different structures formed at the surface of silica nanoparticles depending on drying conditions.     

From Crystalline to Amorphous Germania Bilayer Films at the Atomic Scale: Preparation and Characterization

A new two‐dimensional (2D) germanium dioxide film has been prepared. The film consists of interconnected germania tetrahedral units forming a bilayer structure, weakly coupled to the supporting Pt(111) metal‐substrate. Density functional theory calculations predict a stable structure of 558‐membered rings for germania films, while for silica films 6‐membered rings are preferred. By varying the preparation conditions the degree of order in the germania films is tuned. Crystalline, intermediate ordered and purely amorphous film structures are resolved by analysing scanning tunnelling microscopy images.     

A Single‐Crystalline Mesoporous Quartz Superlattice

There has been significant interest in the crystallization of nanostructured silica into α‐quartz because of its physicochemical properties. We demonstrate a single‐crystalline mesoporous quartz superlattice, a silica polymorph with unprecedentedly ordered hierarchical structures on both the several tens of nanometers scale and the atomic one. The mesoporous quartz superlattice consists of periodically arranged α‐quartz nanospheres whose crystalline axes are mostly oriented in an assembly. The superlattice is prepared by thermal crystallization of amorphous silica nanospheres constituting a colloidal crystal. We found that the deposition of a strong flux of Li⁺ only on the surface of silica nanospheres is effective for crystallization.     

无定型氧化硅转变为α-方石英的振动光谱

红外和喇曼光谱是分子结构信息的重要来源,也是研究表面和催化问题的强有力手段,常用来表征活性中心及吸附物种的结构．为提高稳定性,催化剂一般都要经过焙烧处理,在焙烧过程中,载体结构可能会发生变化,有时还会形成一定的晶相．载体结构变化时,一般会伴随振动光谱的变化,因此在利用振动光谱研究催化体系时,弄清载体的振动光谱是十分必要的．二氧化硅是最常用的催化剂载体之一,但它具有比较多的结构形式,除无定型氧化硅中硅氧四面体可连接成不同的结构外,它还具有石英、鳞石英、方石英等多个晶相系列．对氧化硅的不同结构形式,…     

Preparation of wall-coated open tubular columns after surface roughening by means of amorphous silica

A new method for the preparation of wall-coated glass capillary columns after surface roughening by means of amorphous silica is described. After the glass capillary has been leached, a small quantity of amorphous silica is deposited evenly on the inner surface. This is done by passing a plug of dilute water glass through the capillary, followed by a flow of gaseous hydrochloric acid. The roughened surface is stabilized, deactivated and finally coated. This paper furnishes details of the technique and information about the performance of a number of columns prepared by the method.     

Conversion of cellulose materials into nanostructured ceramics by biomineralization

Synthesis of hierarchically ordered silica materials having ordered wood cellular structures has been demonstrated through in-situ mineralization of wood by means of surfactant-directed mineralization in solutions of different pH. At low pH, silicic acid penetrates the buried interfaces of the wood cellular structure without clogging the pores to subsequently “molecularly paint” the interfaces thereby forming a positive replica following calcinations. At high pH, the hydrolyzed silica rapidly condenses to fill the open cells and pits within the structure resulting in a negative replica of the structure. Surfactant-templated mineralization in acid solutions leads to the formation of micelles that hexagonally pack at the wood interfaces preserving structural integrity while integrating hexagonally ordered nanoporosity into the structure of the cell walls following thermal treatment in air. The carbothermal reduction of mineralized wood with silica at high temperature produces biomorphic silicon carbide (SiC) materials, which are typical aggregations of β-SiC nanoparticles. To understand the roles of each component (lignin, crystalline cellulose, amorphous cellulose) comprising the natural biotemplates in the transformation to SiC rods, three different cellulose precursors including unbleached and bleached pulp, and cellulose nanocrystals have been utilized. Lignin in unbleached pulp blocked homogeneous penetration of silica into the pores between cellulose fibers resulting in non-uniform SiC fibers containing thick silica layers. Bleached pulp produced uniform SiC rods with camelback structures (80 nm in diameter; ∼50 μm in length), indicating that more silica infiltrates into the amorphous constituent of cellulose to form chunky rather than straight rod structures. The cellulose nanocrystal (CNXL) material produced clean and uniform SiC nanowires (70 nm in diameter; >100 μm in length) without the camelback structure.     

Density of silanol groups on the surface of silica precipitated from a hydrothermal solution

The physicochemical properties of amorphous silica precipitated from a hydrothermal solution were studied. Low-temperature nitrogen adsorption in conjunction with the BET method was used to determine the specific surface area of this silica. Based on thermogravimetry data, the total content of water was estimated. A comparison of the thermogravimetry data with the Zhuravlev physicochemical constants made it possible to determine the temperature dependences of the concentration of surface and internal silanols over a temperature range of from 200 to 1200°C. A new type of amorphous silica with enhanced internal water content was revealed. The distinctions between the mechanisms of the removal of surface and internal water were established.     

Molecularly ordered inorganic frameworks in layered silicate surfactant mesophases.

Self-assembled lamellar silica-surfactant mesophase composites have been prepared with crystal-like ordering in the silica frameworks using a variety of cationic surfactant species under hydrothermal conditions. These materials represent the first mesoscopically ordered composites that have been directly synthesized with structure-directing surfactants yielding highly ordered inorganic frameworks. One-dimensional solid-state 29Si NMR spectra, X-ray diffraction patterns, and infrared spectra show the progression of molecular organization in the self-assembled mesophases from structures with initially amorphous silica networks into sheets with very high degrees of molecular order. The silicate sheets appear to be two-dimensional crystals, whose structures and rates of formation depend strongly on the charge density of the cationic surfactant headgroups. Two-dimensional solid-state heteronuclear and homonuclear NMR measurements show the molecular proximities of the silica framework sites to the structure-directing surfactant molecules and establish local Si-O-Si bonding connectivities in these materials.     

Molecular dynamics simulation of dense carbon dioxide fluid on amorphous silica surfaces

Molecular dynamics (MD) simulations of dense carbon dioxide on the amorphous dehydroxylated silica surfaces have been carried out. The adsorption potential surfaces of the silica solids have been obtained in order to evaluate the characteristics of the amorphous surfaces. The atom density profiles, adsorption free energy profiles, surface orientation order parameters, and radial distribution functions for the CO2 molecules have been presented in order to study the effect of the amorphous surfaces on the microscopic interfacial structure properties of the CO2 molecules. The translational diffusion and orientation rotation at silica surfaces have also been investigated. It was observed that there is marked hindrance of the translational diffusion and orientation rotation of CO2 molecules near amorphous silica surfaces.     

MCM-48介孔材料相转化合成的形成机制

利用水热合成的方法,使用新型的表面活性剂十六烷基三甲基对苯磺酸盐作为模板剂合成了高质量的MCM-48介孔分子筛,并用X-射线衍射(XRD)、扫描电镜(SEM)、高分辨透射电镜(HRTEM)以及N₂吸附-脱附进行了表征。合成过程的研究表明该合成体系经历了三相,起始相为具有六方对称性的MCM-41,随着加热时间的延长,生成了具有立方对称性的MCM-48,进一步延长加热时间则生成了层状相MCM-50。三相转变发生的核心驱动力来自于表面活性剂有效堆积参数g因子的改变。另外,XRD、傅立叶变换的红外光谱(FT-IR)以及固体魔角自旋核磁共振(²⁹Si MAS NMR)的表征结果证明：随着晶化时间的延长,相转变的同时伴随着介孔材料的孔壁逐渐由原子无序的非晶态向原子有序的晶态结构转变。最终形成的原子有序层状介孔分子筛可以作为扩孔型微孔分子筛合成的有效前驱体。     

Interactions of amines with silicon species in undersaturated solutions leads to dissolution and/or precipitation of silica

The biogeochemical silicon cycle is the focus for many researchers studying the dissolution of silicon species from quartz, amorphous, and biogenic silica. Furthermore, the precipitation of biogenic silica by diatoms, radiolarian, sponges, and plants is also a popular focus for research. The ornate silica structures created by these species has attracted interest from biomaterial scientists and biochemists who have studied mineral formation in an attempt to understand how biogenic silica is formed, often in the presence of proteins and long chain polyamines. This article is at the interface of these seemingly distinct research areas. Here we investigate the effect of a range of amines in globally undersaturated silicon environments. Results are presented on the effect of amine-containing molecules on the formation of silica from undersaturated solutions of orthosilicic acid and globally undersaturated silicon environments. We sought to address two questions: can silica be precipitated/harvested from undersaturated solutions, and can we identify the silicon species that are most active in silica formation? We demonstrate that none of the bioinspired additives investigated here (e.g., poly(allylamine hydrochloride), pentaethylenehexamine, and propylamines) have any influence on orthosilicic acid at undersaturated concentrations. However, under globally undersaturated silicon concentrations, small molecules and polymers containing amine groups were able to interact with oligomers of silicic acid to either generate aggregated materials that can be isolated from solution or increase rates of oligomer dissolution back to orthosilicic acid. Additional outcomes of this study include an extended understanding of how polyelectrolytes and small molecules can promote and/or inhibit silica dissolution and a new method to explore how (bio)organic molecules interact with a forming mineral phase.     

Evolution of the structure of amorphous ice: from low-density amorphous through high-density amorphous to very high-density amorphous ice

We report results of molecular dynamics simulations of amorphous ice for pressures up to 22.5 kbar. The high-density amorphous ice (HDA) as prepared by pressure-induced amorphization of I(h) ice at T=80 K is annealed to T=170 K at various pressures to allow for relaxation. Upon increase of pressure, relaxed amorphous ice undergoes a pronounced change of structure, ranging from the low-density amorphous ice at p=0, through a continuum of HDA states to the limiting very high-density amorphous ice (VHDA) regime above 10 kbar. The main part of the overall structural change takes place within the HDA megabasin, which includes a variety of structures with quite different local and medium-range order as well as network topology and spans a broad range of densities. The VHDA represents the limit to densification by adapting the hydrogen-bonded network topology, without creating interpenetrating networks. The connection between structure and metastability of various forms upon decompression and heating is studied and discussed. We also discuss the analogy with amorphous and crystalline silica. Finally, some conclusions concerning the relation between amorphous ice and supercooled water are drawn.     

Influence of particle design on oral absorption of poorly water-soluble drug in a silica particle-supercritical fluid system

The physicochemical characteristics and oral absorption of a poorly water-soluble drug, K-832, adsorbed onto porous silica (Sylysia 350), were compared with those of K-832 adsorbed onto non-porous silica (Aerosil 200). K-832 and silica were treated with supercritical CO(2) (scCO(2)) to produce K-832-Sylysia 350 and K-832-Aerosil 200 formulations. Scanning electron microscopy, polarizing microscopy, powder X-ray diffraction, and differential scanning calorimetry results suggested that K-832 mainly existed in an amorphous state in both formulations. The specific surface area of both formulations was much larger than that of pure K-832 crystals. The dissolution rate of K-832 from both formulations was considerably greater than that from corresponding physical mixtures due to rapid wetting of the hydrophilic carrier surfaces and amorphous state, the dissolution from the K-832-Sylysia 350 formulation being the fastest. In vivo absorption tests on the two formulations indicated no significant differences in their peak concentration (C(max)) and the area under their plasma concentration-time curve (AUC), while the concentrations of K-832 in the K-832-Sylysia 350 formulation were significantly higher than those in the K-832-Aerosil 200 formulation 1 h and 1.5 h after administration of these formulations (p<0.05). This could be attributed to the different dispersion states of K-832 in the formulations due to their different three-dimensional structures (porous and non-porous). In physical stability tests, the amorphous drugs in both formulations were stable at room temperature for at least 14 months. Thus, the absorption of poorly water-soluble drugs could be greatly improved by adsorption onto porous silica using scCO(2).     

Thermal destruction of rice hull in air and nitrogen

In this study, we ashed rice hull in air and nitrogen, respectively, and systematically investigated the effects of ashing temperature and atmosphere on the structures, morphologies, and pore characteristics of rice hull ash (RHA). All RHA samples are amorphous materials with porous structures. IR spectra revealed that RHA that ashed in air (WRHA) exhibit more polar groups on the surface than that of ashed in nitrogen (BRHA). The silica and carbon contents, BET surface area, and pore volume of BRHA increase with ashing temperature. When ashed in air, however, the silica content of WRHA increases and carbon content decreases with temperature. The BET surface area and pore volume of WRHA increase with temperature firstly and decline subsequently due to the closure of pores. Compared with WRHA, BRHA shows higher surface areas, micropore volumes, carbon contents, and lower mesopore fractions and silica contents. This study provides essential information for choosing a suitable thermal treatment of rice hull for a given adsorbate.     

Fungus-mediated biotransformation of amorphous silica in rice husk to nanocrystalline silica

Rice husk is a cheap agro-based waste material, which harbors a substantial amount of silica in the form of amorphous hydrated silica grains. However, there have been no attempts at harnessing the enormous amount of amorphous silica present in rice husk and its room-temperature biotransformation into crystalline silica nanoparticles. In this study, we address this issue and describe how naturally deposited amorphous biosilica in rice husk can be bioleached and simultaneously biotransformed into high value crystalline silica nanoparticles. We show here that the fungus Fusarium oxysporum rapidly biotransforms the naturally occurring amorphous plant biosilica into crystalline silica and leach out silica extracellularly at room temperature in the form of 2-6 nm quasi-spherical, highly crystalline silica nanoparticles capped by stabilizing proteins; that the nanoparticles are released into solution is an advantage of this process with significant application and commercial potential. Calcination of the silica nanoparticles leads to loss of occluded protein and to an apparently porous structure often of cubic morphology. The room-temperature synthesis of oxide nanomaterials using microorganisms starting from potential cheap agro-industrial waste materials is an exciting possibility and could lead to an energy-conserving and economically viable green approach toward the large-scale synthesis of oxide nanomaterials.     

Silicon surface texturing by electro-deoxidation of a thin silica layer in molten salt

A new method of silicon surface texturing is reported, which is based on thin silica layer electrochemical reduction in molten salts. A thermal silica layer grown on p-type silicon was potentiostatically reduced in molten calcium chloride at 850 °C. Typical nano–micro-formations obtained at different stages of electrolysis were demonstrated by SEM. X-ray diffraction measurements confirmed conversion of the amorphous thermal silica layer into crystalline silicon. The proposed approach shows promise in photovoltaic applications, for instance, for production of antireflection coatings in silicon solar cells.