Silicon-29 NMR studies of aqueous silicate solutions: Part II. Isotopic enrichment

Silicon-29 NMR spectra have been recorded for aqueous solutions of sodium silicate in the alkaline pH range using silica enriched in the ²⁹Si isotope. Work at low concentration shows that essentially only monomeric orthosilicate ions are present at ca. 0.01 M in silica. At higher concentrations, peaks which lack fine structure indicate species whose silicon nuclei are all equivalent. The experiments have assisted in the difficult process of assigning the spectra of silicate solutions.     

Determination of the changes of the basic structures of silica species in dependence on the concentration of sodium chloride by FAB-MS

The chemical species of silica in NaCl solutions of different concentrations were identified by FAB-MS (fast atom bombardment mass spectrometry). The basic structures of silica species, such as cyclic pentamer (Si5 (OH)9O6-), linear pentamer (Sis(OH)11O5-), cyclic hexamer (Si6(OH)9O8-, Si6(OH)11O7-) and linear hexamer (Si6(OH)14 O6-), were identified, in addition to dimer (Si2(OH)5O2-), trimer (Si3(OH)7O3-) and cyclic tetramer (Si4(OH)7O5-). The patterns of changes of the peak intensities of the silicate complexes relative to the dimer with increasing NaCl concentration were classified into two types: that represented by linear silicate complexes and the other by cyclic silicate complexes. The differences in the type of chemical species and their changes according to the NaCl concentration reflect the number of bonds necessary for polymerization and hydrolysis of the silica complexes. The differences between the linear and the cyclic silicate type have some implications on the dissolution mechanism of silicate complexes, the hydration of the molecules and the equilibrium between solubility, hydrolysis, polymerization and the salting-out effect in NaCl solution.     

L沸石导向剂陈化机制的研究

应用核磁共振和拉曼光谱研究了L沸石导向剂的陈化机制。实验结果表明,当铝源与硅源混合后铝酸根离子与硅酸根离子之间聚合反应很快,同时高聚态硅酸根离子发生了解聚。L沸石导向剂陈化反应的控制步骤是低聚态硅铝酸根离子之间或低聚态硅铝酸根离子与低聚态硅酸根离子之间的定向聚合反应。     

Formation at low surfactant concentrations and characterization of mesoporous MCM-41

At low concentrations of cetyltrimethylammonium bromide,all silica-based mesoporous materials with hexagonal phase have been synthesized via interactions between self-assembled surfactant molecule aggregates and aniomc silicate polymers.The resulting materials are characterized by XRD,FT-IR,solid state 29Si MAS NMR,thermal analysis and N2 adsorption-desorption measurements.After soluble ions are removed,the interactors between surfactant micelles and silicate polymers are reorganized and then form mesostructures 1 he hexagonal framework is sonsistent with amorphous silica gel.The structures of materials depend on the synthesis conditions Hydrothermal process improves the interactions between molecules and increases the degree of framework silicon atom polymerization The.surface area and the mesopore volume of the material prepared at 100℃ increase by 87% and 71 %,respectively,compared with those obtained at room temperature.     

High surface area nanoporous amorphous silica prepared by dodecanol assisted silica formate sol-gel approach

A simple polycondensation of monocarboxylic acids with silicon alkoxides led to transparent silica gels mainly comprised of silicate species of closed structures. This 'sol-gel formic acid' approach was modified by trapping an organic template (dodecanol) inside the silicate network during the polymerization process. Using this templating approach, porous silica of extremely high surface area, was produced in contrast to non-porous silica obtained by non-templating approach. The S(BET) surface areas of the template assisted samples resulting from the entire pores were found to be up to 725 m(2)/g. The total pore volumes of the samples were in the range of 0.40-0.74 cc/g in which micropore volumes were about 0.15-0.25 cc/g; the porosity depending on the reactants molar ratios of dodecanol, silicon alkoxide and formic acid.     

<Superscript>29</Superscript>Si NMR Spectroscopy Investigation of Alcoholic Phenyltrimethyl Ammonium Silicate Solutions

²⁹Si NMR spectroscopy is a powerful tool for studies of the silicate species existing in both aqueous and non-aqueous solutions. In this report ²⁹Si NMR spectroscopy was used to characterize species present in alkaline alcoholic silicate solutions. Phenyltrimethylammonium (PTMA) hydroxide was used as a base. The effects of polymerization/depolymerization of silicate anions in alcoholic alkaline solutions were investigated with different alcohols by ²⁹Si NMR spectroscopy. The esterification of monomeric silicate, Si(OH)₄, in the presence of different alcohols was also studied. Esterification depends on the alkyl chain as well as number of hydroxyl groups in the alcohol.     

Indirect near-infrared spectrophotometric determination of silicate by the 2-amino-4-chlorobenzenethiol method

An indirect near-infrared spectrophotometric method for the determination of silicate, based on the absorbance of an equivalent amount of molybdenum-2-amino-4-chlorobenzenethiol complex in chloroform solution, has been developed. The green chloroform solution has an absorbance maximum at 715 nm. The development of the spectrophotometric method for the determination of silicate included a study of a buffer system for the purpose of maintaining the pH within a narrow optimum pH range, complex stability, effect of diverse ions, and conformity to Beer's law. The limit of detection is 0.034 mug of silicon per ml of aqueous heteropoly acid solution.     

Gelatine/silicate interactions: from nanoparticles to composite gels

The possibility to design new composites associating biopolymers with mineral phases relies on the understanding and control of their mutual interactions. In this work, aqueous solutions of gelatine and sodium silicate were mixed at pH 5, 37 degrees C and left to stand at 20 degrees C for 1 day. At low gelatine and high silicate contents, precipitates were obtained, containing a fixed silicon/polymer molar ratio. Scanning electron microscopy (SEM) reveals that they are formed of large aggregates of platelets, constituted of closely-packed nanoparticles. For high gelatine contents, composite gels were formed consisting of silica particles dispersed in the biopolymer matrix. Swelling studies indicate that the addition of silica decreases the stability of the gels by inducing gelatine depletion in solution. Similar experiments conducted at pH 7 show that at this pH, silicates are more effective at precipitating gelatine. A model is proposed for the formation of the composites, based on the electrostatic interactions arising between silicates and polymer chains. These results are discussed in the context of hybrid biomaterials design and biosilicification processes.     

Solution-phase electronegativity scale: insight into the chemical behaviors of metal ions in solution

By incorporating the solvent effect into the Born effective radius, we have proposed an electronegativity scale of metal ions in aqueous solution with the most common oxidation states and hydration coordination numbers in terms of the effective ionic electrostatic potential. It is found that the metal ions in aqueous solution are poorer electron acceptors compared to those in the gas phase. This solution-phase electronegativity scale shows its efficiency in predicting some important properties of metal ions in aqueous solution such as the aqueous acidities of the metal ions, the stability constants of metal complexes, and the solubility product constants of the metal hydroxides. We have elaborated that the standard reduction potential and the solution-phase electronegativity are two different quantities for describing the processes of metal ions in aqueous solution to soak up electrons with different final states. This work provides a new insight into the chemical behaviors of the metal ions in aqueous solution, indicating a potential application of this electronegativity scale to the design of solution reactions.     

Revisiting the identification of structural units in aqueous silicate solutions by two-dimensional silicon-29 INADEQUATE

(29)Si-(29)Si INADEQUATE experiments have been successfully used on sodium aqueous silicate partially enriched (19% of (29)Si isotope) solutions in order to gain connectivity information and to help in spectral assignments. The structures of almost all known species from the literature were confirmed by this technique. Moreover, accurate scalar J couplings were extracted. The two-dimensional (2D) spectra demonstrated additional cross correlations for some Si-Si bonds never reported before, representing additional oligomeric species. They reveal five silicate cages with the following connectivity sets: Q(2)-Q(2)-Q(3) or Q(3Delta)-Q(3Delta)-Q(3) for one species, Q(2)-Q(2) or Q(3Delta)-Q(3Delta) for three species (Q(3Delta) indicates a three-connected silicon in a three-membered cycle), and Q(3)-Q(3) for one species. All possible molecular graphs for anions containing silicon up to 11 atoms, matching these sequential connnectivities, were enumerated. Additionally, a direct comparison between resonances observed in the 2D experiments with that obtained in the 1D spectrum allows us to assign single-sited silicate anions. Only a few species reported before were not detected, most probably due to their too low concentration in our solutions.     

Determination of the changes of the basic structures of silica species in dependence on the concentration of sodium chloride by FAB-MS

The chemical species of silica in NaCl solutions of different concentrations were identified by FAB-MS (fast atom bombardment mass spectrometry). The basic structures of silica species, such as cyclic pentamer (Si₅¶(OH)₉O₆ ^–), linear pentamer (Si₅(OH)₁₁O₅ ^–), cyclic hexamer (Si₆(OH)₉O₈ ^–, Si₆(OH)₁₁O₇ ^–) and linear hexamer (Si₆(OH)₁₄O₆ ^–), were identified, in addition to dimer (Si₂(OH)₅O₂ ^–), trimer (Si₃(OH)₇O₃ ^–) and cyclic tetramer (Si₄(OH)₇O₅ ^–). The patterns of changes of the peak intensities of the silicate complexes relative to the dimer with increasing NaCl concentration were classified into two types: that represented by linear silicate complexes and the other by cyclic silicate complexes. The differences in the type of chemical species and their changes according to the NaCl concentration reflect the number of bonds necessary for polymerization and hydrolysis of the silica complexes. The differences between the linear and the cyclic silicate type have some implications on the dissolution mechanism of silicate complexes, the hydration of the molecules and the equilibrium between solubility, hydrolysis, polymerization and the salting-out effect in NaCl solution.     

Effect of formulation of silica‐based solution on corrosion resistance of silicate coating on hot‐dip galvanized steel

Several silica‐based solutions with 50 g/l of SiO₂ were prepared from sodium silicate solutions and silica sol; the silicate conversion coatings were obtained by immersing hot‐dip galvanized steel sheets in these solutions. These solutions were characterized using high‐resolution transmission electron microscopy and ²⁹Si nuclear magnetic resonance; the morphology of the coatings was observed by SEM and atomic force microscopy while the corrosion resistance was evaluated by electrochemical measurements as well as neutral salt spray tests. The results show that the coatings obtained from the single silica sol solution had poor adhesion and the coating obtained from the sodium silicate solution with low SiO₂/Na₂O molar ratio was uneven. By adding the silica sol to the silicate solution with low molar ratio, uniform coatings with better protection property were obtained. According to the results of ²⁹Si nuclear magnetic resonance spectra, the effects of the distribution of silicate anions with various polymerization degrees in the silica‐based solutions on the microstructure and corrosion resistance of the silicate coatings are discussed. Copyright © 2016 John Wiley & Sons, Ltd.     

Adsorption of Copper-Amino Acid Complexes on the Surface of Highly Dispersed Silica

The adsorption of the copper ions and some amino acids on the surface of silica from aqueous solutions is interpreted in terms of the surface complexation model. The stability constants of ternary surface complexes, in which silanol groups of silica act as one of the ligands, are determined. It is shown that the values of the constants of the binding of amino acid anions to copper ions on the surface are changed in accordance with a series of increasing hydrophobicity of substituents in the side chains of amino acids.     

Atom transfer radical polymerization of n‐butyl acrylate from silica nanoparticles

This article reports the synthesis of atom transfer radical polymerization (ATRP) of active initiators from well‐defined silica nanoparticles and the use of these ATRP initiators in the grafting of poly(n‐butyl acrylate) from the silica particle surface. ATRP does not require difficult synthetic conditions, and the process can be carried out in standard solvents in which the nanoparticles are suspended. This “grafting from” method ensures the covalent binding of all polymer chains to the nanoparticles because polymerization is initiated from moieties previously bound to the surface. Model reactions were first carried out to account for possible polymerization in diluted conditions as it was required to ensure the suspension stability. The use of n‐butyl acrylate as the monomer permits one to obtain nanocomposites with a hard core and a soft shell where film formation is facilitated. Characterization of the polymer‐grafted silica was done from NMR and Fourier transform infrared spectroscopies, dynamic light scattering, and DSC. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 4294–4301, 2001     

Non-thermal synthesis of mesoporous zirconium silicate and its characterization

A simple and rapid method for synthesis of mesoporous zirconium silicate with high thermal stability has been developed using sodium silicate in place of costly silicon alkoxides as a silica source. The product was characterized by means of X-ray diffraction, Nitrogen sorption isotherms, FT-IR spectroscopy, transmission electron microscopy (TEM) and thermogravimetry analysis (TGA).     

Selective Adsorption of Arsenic Ions on Silica Gel Impregnated with Ferric Hydroxide

Abstract

Selective adsorption of trace arsenite- and arsenate anions in an aqueous solution by ferric hydroxyde supported on silica gel particles was investigated. Silica gel particles were loaded with ferric hydroxide of the range of 1 – 3 wt. % in terms of Fe based on the dry gel, and the extent of adsorption of arsenite or arsenate ion measured by batch- and column processes, being the highest at pH 6 in the presence of diverse foreign ions. With the use of silica gel containing 3.3 wt. % Fe, as much as 0.07 m mol of arsenic per gram of dry gel was adsorbed.     

乳液原子转移自由基聚合*

原子转移自由基聚合(ATRP)应用于乳液聚合体系的主要挑战在于如何同时保证乳液的稳定性和聚合反应的可控性。本文主要对乳液ATRP体系中影响聚合反应可控性和乳液稳定性的各种因素、乳液ATRP的机理和乳液ATRP的应用等方面进行了综述。表面活性剂亲水亲油性及其亲水亲油基团的化学性质、催化剂/配体在油/水两相之间的分配行为、引发剂的溶解性、反应温度以及各组分的浓度是影响反应可控性和乳液稳定性的主要因素。各组分在油/水两相中的分配行为使得乳液ATRP的机理比传统乳液聚合更加复杂。乳液原子转移自由基聚合结合了活性自由基聚合和乳液聚合的优点,在理论研究和工业生产上具有很大的应用前景。     

CNDO/2 calculations of relaxation and reconstruction of diamond and silicon [111] surfaces

CNDO /2 calculations have been performed on the clusters X₄H₉ and X₄Y₉ modeling the [111] diamond and silicon surfaces. The X is either carbon or silicon atom and the Y is a pseudoatom containing one sp³ hybrid orbital. It is shown that in the CNDO /2 approximation in the foregoing pseudoatom models, the charge distribution of the cluster is better than the hydrogen atom, because the electronegativity of the hydrogen differs significantly from the electronegativity of the sp² orbital of the silicon atom. Using the CNDO /2 parametrization, the electronegativity of the hydrogen is very near to the electronegativity of the sp³ orbital of the carbon atom, thus the hydrogen can be used for the saturation of the carbon clusters.     

Zum Anionenaufbau von Tetra-n-butylammoniumsilicaten und ihren wäßrigen Lösungen

On the Anion Constitutions of Tetrabutylammonium Silicates and their Aqueous Solutions The anion distribution of tetra-n-butylammonium-(TBA)-silicate solutions with molar TBA/SiO₂ ratios between 0.6 and 4 and silica concentrations between 0.1 M and 2.2 M has been investigated by trimethylsilylation and ²⁹Si NMR techniques. In contrast to concentrated tetramethylammonium- and tetraethylammonium silicate solutions in TBA silicate solutions a preference of double ring silicate anions does not occur. In TBA silicate solutions a broad distribution of silicate anions consisting of monomeric, oligomeric chain and ring, as well as polymeric silicate anions has been observed. Crystalline TBA silicates with TBA/SiO₂ ratios of 0.78 to 1 contain mainly double five-ring silicate anions Si₁₀O₂₅^10? whereas for TBA/SiO₂ ratios higher than 1.4 the double three-ring anion Si₆O₁₅^6? predominates. A recently prepared TBA silicate with low TBA content (TBA/SiO₂ = 0.23) has been found to consist of double four-ring silicate anions with 6 SiOH groups per double four-ring.     

Neutral hydrophilic coatings for capillary electrophoresis prepared by controlled radical polymerization

In the present study, porous silica particles as well as impervious fused-silica wafers and capillary tubes were modified with hydrophilic polymers (hydroxylated polyacrylamides and polyacrylates), using a surface-confined grafting procedure based on atom transfer radical polymerization (ATRP) which was also surface-initiated from α-bromoisobutyryl groups. Initiator immobilization was achieved by hydrosilylation of allyl alcohol on hydride silica followed by esterification of the resulting propanol-bonded surface with α-bromoisobutyryl bromide. Elemental analysis, IR and NMR spectroscopies on silica micro-particles, atomic force microscopy, ellipsometry and profilometry on fused-silica wafers, as well as CE on fused-silica tubes were used to characterize the chemically modified silica substrate at different stages. We studied the effect of monomer concentration as well as cross-linker on the ability of the polymer film to reduce electroosmosis and to prevent protein adsorption (i. e., its non-fouling capabilities) and found that the former was rather insensitive to both parameters. Surface deactivation towards adsorption was somewhat more susceptible to monomer concentration and appeared also to be favored by a low concentration of the cross-linker. The results show that hydrophilic polyacrylamide and polyacrylate coatings of controlled thickness can be prepared by ATRP under very mild polymerization conditions (aqueous solvent, room temperature and short reaction times) and that the coated capillary tubes exhibit high efficiencies for protein separations (0.3–0.6 million theoretical plates per meter) as well as long-term hydrolytic stability under the inherently harsh conditions of capillary isoelectric focusing. Additionally, there was no adsorption of lysozyme on the coated surface as indicated by a complete recovery of the basic enzyme. Furthermore, since polymerization is confined to the inner capillary surface, simple precautions (e.g., solution filtration) during the surface modification process are sufficient to prevent capillary clogging.