Modification of silica with mixtures of hexamethylcyclotrisilazane and hexamethyldisilazane

Summary For chromatographic purposes the use of mixtures instead of individual silylating agents can produce better deactivated surface than those shown by silicas modified by hexamethylcyclotrisilazane or hexamethyldisilazane separately.     

NIR studies of the surface modification in silica fillers

Methods of estimating the degree of condensation of the surface silanol groups of silica due to its modification by silane coupling agents are reported.Also, a procedure for estimating the surface silanol groups for the pre- and post-modified silicas for the NIR 7326 cm^–1 band is given.Using electron microscope studies and heats of immersion of silica surfaces, the silane effect on agglomeration of silica particles and, thus, on the physicochemical properties of its surface has been demonstrated.     

New procedure of silica gel surface modification preparation of gaseous standard mixtures for calibration purposes

The new type of silica gel surface modification with using the trimethylamine as a reagent is described. The samples of chemically modified silica gel have been used for generation of gaseous standard mixtures (methyl chloride as a measurand) using the technique of thermal decomposition of the surface compound. The main aim of the research was to check the suitability of the new type of silica gel surface modification for obtaining methyl chloride as a measurand of gaseous standard mixture. The gaseous standard mixture obtained with using this technique was used for calibration of a thermal desorber-gas chromatography-flame ionization detector (TD-GC-FID) system. The homogeneity of coverage of silica gel surface with the immobilized compound has been evaluated. The full uncertainty budget of determination of liberated amount of methyl chloride has been calculated. The average amount of methyl chloride liberated from the unit sample of chemically modified silica gel is 3.59 +/- 0.13 mg g(-1). The influence of the modification way on the amount of liberated analyte has also been determined.     

硅溶胶改性及其应用研究进展

二氧化硅在自然界中通常为晶态,而以溶胶-凝胶、水热、模板剂等方法人工合成的二氧化硅纳米颗粒(SiO₂ NPs)通常为无定形态。无定形SiO₂ NPs广泛应用于航空航天、催化剂载体、靶向送药等众多高端领域。生产SiO₂ NPs的方法有很多,但制备结构、形貌简单的SiO₂ NPs的主要方式为离子交换法、硅醇盐水解法、单质硅粉水解法,这些方法可制备粒径在10～800 nm的单分散SiO₂ NPs,同时可根据生产条件控制粒子的单分散性与粒径大小。寻找一种成本低廉且成品单分散性好、粒径可控的生产方法成为目前研究的热点。本文简介了SiO₂ NPs的结构形貌、基础性质、生产方法和最佳合成条件,总结了各方法的优缺点与适用领域。随后介绍了近年来SiO₂ NPs的改性研究与应用,最后在此基础上展望了合成硅溶胶存在的挑战与未来需解决的问题。     

Polymethylhydrosiloxane surface deactivation of silica particles for packed capillary column supercritical fluid chromatography

Summary Spherical porous silica particles (10 μm diameter, 300 ? and 80 ? pores), spherical nonporous silica particles (10 μm diameter), and irregular porous silica particles (≈ 10 μm diameter, 80 ? pores) were deactivated with polymethylhydrosiloxane (PS). The surface activities of the deactivated silica particles were investigated using various polar compounds under supercritical fluid chromatography (SFC) conditions (neat CO₂), and compared with a commercial C₁₈-bonded phase. The small pore (80 ?) silica particles could be more completely deactivated than larger pore (300 ?) and nonporous silica particles. The success of the PS deactivation method is ascribed to the excellent match between the reactive groups on the polymer backbone and the silica surface, and the formation of a highly crosslinked polymeric layer over the surface. Physical processes, such as adsorption and desorption of the deactivation reagent on the surface and diffusion from the surface, were found to have important effects on the deactivation. Using capillary columns packed with PS deactivated silica particles, typical polar organic compounds, including hydroxyl-containing compounds, carbonyl-containing compounds, free amines, and free carboxylic acids, were separated by SFC and compared with results from a commercial C₁₈-bonded phase. While the results clearly show that the PS deactivated particles were more inert than the C₁₈-bonded phase, better deactivation methods are still needed for separation of free acids and alkylamines.     

Novel fused silica capillary columns: Surface modification through controlled doping of the preform-tube

A method has been developed for fabricating fused silica capillary columns which have specific surface properties but still retain the excellent strength, flexibility, and resilience of pure fused silica. By using the Modified Chemical Vapor Deposition process (MCVD), typically used for the production of optical fiber lightguides, inorganic dopants such as Al, Nd, Ge, and P can be introduced into the preform-tube by MCVD. Doped columns have a wide range of specific surface properties, and columns with undoped fused silica prepared by MCVD are more chemically inert and less acidic than columns prepared by conventional methods. This paper describes the method for fabricating capillaries and the initial studies to characterize them.     

Ellipsometric study of the change in the porosity of silica xerogels after chemical modification of the surface with hexamethyldisilazane

Variable angle spectroscopic ellipsometry (VASE) and ellipsometric porosimetry (EP) have been used to study the effect of treatment with hexamethyldisilazane (HMDS) on the porosity of silica xerogel films. Chemical modification of the surface with HMDS was found to reduce the porosity by approximately 15%. This reduction was connected with changes which occur in the silica network, with further condensation or the reaction between neighbouring trimethylsilyl (TMS) surface groups being possible causes.     

Chemical modification of alumina and silica with alkylphosphonic acids and their esters

Alumina and silica were modified with alkylphosphonic acids and diethyl butylphosphonate. The boundaries of hydrolytic stability of the obtained surface-modified materials were determined.     

Separation studies of amino acids,proteins and enzymes on bonded 1,2-dihydroxy-, 1,2-hydroxylamino-and amino silica packings

Summary 1,2-dihydroxy-3-propoxypropyl (HPPS), 1-amino-2-hydroxy-3-propoxypropyl (AHPS) and 1-aminoethyl-3-aminopropyl (AEAPS) silica were synthesized by means of both a surface modification procedure (I) and a bulk modification procedure (II). Method (I) gave a surface concentration, , of functional groups of 2–3 mole/m², whereas method (II) gave values up to 5 mole/m². Retention times, peak asymmetries and plate heights of thiamine and ascorbic acid eluted with aqueous buffer solutions ranging from pH 5.3 to 9.2 gave only a±5% variation over periods of 12 hours and more.The recoveries of selected enzymes and proteines examined under static and dynamic conditions were between 60% and 100% depending on the functional group of the packing and on the solute. With HPPS silica, which showed the least adsorption and denaturing effects, separation of biopolymers depended on a size exclusion effect in the relative molecular mass range 10,000 to 80,000 and could be achieved within 10 min.A part of the paper was presented at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Febr. 28 to March 4, 1977, Cleveland, Ohio, U.S.A.     

Investigation into the surface modification of aragonite whiskers

Aragonite whiskers (AWs) were treated with several fatty acid surfactants and silane coupling agent in order to determine the optimal modifier by using contact angle measurements. The results revealed that the AWs modified by fatty acids showed more remarkable increase in the contact angle than by silane, suggesting the former were preferentially applied in modifying AWs. While the samples coated with fatty acids exhibited hydrophobicity with contact angles ranging from 104.08° to 137.87° with increasing of carbon chain length. Therefore, the highest contact angle of AWs treated by oleic acid was discussed in detail as an example, which was characterized by field emission scanning electron microscope (FESEM), transmission electron microscopy (TEM), thermo‐gravimetry analyses (TGA), X‐ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR). FESEM and TEM results showed a thin layer coated on the modified sample surface. Both the results of TGA and XPS confirmed organic groups existed in the sample of AWs treated by oleic acid. FTIR demonstrated that calcium dioleate was formed in the modification process. Further, modification mechanism was proposed based on the obtained results. Copyright © 2016 John Wiley & Sons, Ltd.     

A novel and highly efficient method for the silylation of alcohols with hexamethyldisilazane (HMDS) catalyzed by recyclable sulfonic acid-functionalized ordered nanoporous silica

Silylation of alcohols with hexamethyldisilazane (HMDS) in dichloromethane provides the corresponding silyl ethers in almost quantitative yields at room temperature using 1-3 mol % of sulfonic acid-functionalized silica. Additionally, the catalyst displays a high activity and thermal stability (up to 240 °C) and it can be easily recovered and reused for at least 20 reaction cycles without loss of reactivity.     

Imide-siloxane block copolymer/silica hybrid membranes: preparation, characterization and gas separation properties

Imide-siloxane block copolymer/silica hybrid membranes with covalent bonds were prepared via sol–gel reaction. The structural informations of these hybrid membranes were obtained by using Fourier transform-infrared spectrometry (FT-IR), ²⁹Si nuclear magnetic resonance (²⁹Si NMR), XPS and thermogravimetric analysis (TGA). The gas separation properties of the hybrid membranes were also investigated in terms of organosiloxane (PDMS) or silica content at various temperatures. In the hybrids, the addition of PDMS phase increased the permeabilities of gases such as He, CO₂, O₂, and N₂, indicating that the gas transport occurred mainly through rubbery organic matrix. Meanwhile, the PDMS phase contributed the decreased gas selectivities to nitrogen but the reduction in selectivities was very small in comparison with other siloxane containing polymeric membranes. This might be due to the restriction of chain mobility by the existence of inorganic component such as silica network in the hybrids. Additionally, the increase of silica content in these hybrid membranes considerably retarded the falling-off of gas selectivity at elevated temperature. The increase of silica content in hybrid membranes resulted in well-formed silica networks and hence these inorganic components restricted the plasticization of organic matrix by the thermal segmental motion of organic components, leading to preventing the large decrease of the gas selectivity.     

The effect of some modifiers on the retention of aromatic hydrocarbons on untreated silica gel

Summary The retention parameters of benzene, naphthalene; anthracene and phenanthrene on untreated silica gel were determined in the presence of four modifiers: 1-methyl-2-pyrrolidone (MP), 4-formyl morpholine (FM), -butyrolactone (BL) and sulfolane (SF). Better column performance was found in the case of FM and MP only.     

Surface modification strategies for multicomponent polymer systems,Part I: Use of diblock copolymers as surface modifiers of rutile

Various functional diblock copolymers have been used as surface modifiers for rutile pigment in an effort to condition the solid for eventual use in multicomponent polymer systems. Coated surfaces were analyzed by inverse gas chromatography at infinite and finite dilution of the vapor phase, and by XPS. At high coverages (about 10% by weight of the pigment), the diblocks were randomly oriented at the air interface, effectively masking the surface of the rutile. At low diblock concentrations acid/base interactions dominated the orientation of the adsorbed molecule at the rutile interface, thereby also affecting the orientational states at the air interface. In this condition, the performance of the pigment in specified host polymer systems may be expected to vary with the selection of the diblock copolymer modifier. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1793–1805, 1997     

The strength of interaction of highly retentive silanols with hydrocarbons on porasil C

Summary The paper describes how the method of gas-phase titration of the strongest adsorption sites can be utilized to calculate a relative strength of interaction of the sites with chromatographic solutes. An excellent correlation with earlier, theoretical predictions of Giddings is noted. Significant influence of the sites on the width of a chromatographic band is observed when the interactions are an order of magnitude stronger than those of normal sites. Presented at the 17th International Symposium on Chromatography, September 25–30, 1988, Vienna, Austria.     

Synthesis and surface modification of mesoporous mcm-41 silica materials

Surface modification offers a great opportunity to adjust both the pore diameter and surface properties of MCM-41 type organic–inorganic hybrid materials which result in materials of improved hydrothermal and mechanical stability. Therefore, MCM-41 silica, surface modified with organic ligands, are promising systems with engineered properties and attractive for advanced applications. In the present study, after optimization of the reaction conditions highly ordered MCM-41 silica spheres with uniform mesopores were prepared by the pseudomorphic transformation route. The effect of functionality and alkyl chain length of the alkyl ligands during surface modification was probed by using butyl and octylsilanes with two different functionalities. Due to steric hindrance, the longer chains are assumed to bind only on the outer silica surface and near the entrance of the pores, while the shorter chains are also able to bind to the interior mesopore walls. The resulting materials were comprehensively characterized before and after surface modification using nitrogen sorption techniques, XRD, SEM, solid-state NMR spectroscopy and FTIR spectroscopy. From chromatographic test measurements it was found that the separation power primarily depends on surface coverage and alkyl chain length. On the basis of the present data, surface modified mesoporous silica of MCM-41 type are very promising candidates for future chromatographic applications.     

Aspects of surface modification, structure characterization, thermal stability and metal selectivity properties of silica gel phases-immobilized-amine derivatives

Four silica gel phases-bound-amine derivatives (I-IV) were prepared based on chemical immobilization technique. The surface modification was identified by determination of the coverage values in mmol g⁻¹ via thermal desorption method (1.463-1.807) and elemental analysis of nitrogen and carbon contents (1.089-2.456). Structure characterization related to immobilization of the amine derivatives was accomplished and evaluated by means of infrared (IR) and secondary ion mass spectrometric (SIMS) technique. The modified silica gel phases (I-IV) along with their interaction products with copper(II) were also examined by electron impact mass spectrometric analysis (EI-MS) as a method for evaluation of their thermal stability and structure elucidation. Potentiometric titration as a method of characterization was applied for the modified silica gel phases (II-IV) and their copper(II)-adduct. A series of bi- and trivalent metal ions were selected to focus more aspects of the selectivity properties incorporated into the modified silica gel phases for binding and interaction with these metals based on determination of the distribution coefficient and separation factor. The results of these evaluation processes were found to prove higher selectivity and preference of these four phases for binding with lead(II) and cadmium(II) compared to other metal ions.     

Aspects of the synthesis, Characterization and metal–ligand stoichiometry of aminopropyl, nitrobenzamide, and 8-quinolinol silica gels

Surface modification of several silica gels with a number of silanes was studied, as well as further reactions of the modified surfaces. Complete reaction of aminopropylsilane was easily achieved if surface silanols were in excess. Triethoxysilylpropyl-p-nitrobenzamide (TESPN) reacts with difficulty, and complete reaction occured only with long reaction time (at least 12h). The maximum coverage of 100-Å pore silica gel by TESPN was found to be about 1.7 μmol/m², considerably less than the 3–4 /gmmol/m² of available silanol sites, probably due to steric effects of the large silane. Even at this maximum coverage, further reaction of the modified silica with trimethylsilyl capping groups occured, with additional coverage of about 1.0 μmol/m². The maximum amount of silylpropylamido-p-phenylazo-8-quinolinol that could be bound to silica gel was found to be about 1.0 μmol/m², again probably limited by steric effects of the large 8-quinolinol (oxine) moiety. Virtually complete conversion of parent nitrobenzamide silica gel (NBSG) to 8-quinolinol silica gel (QSG) could be achieved if NBSG coverage was less than about 0.7 μmol/m². QSG materials were found to have a 2 oxine to 1 copper(II) stoichiometry, even with surface coverages as low as 0.16 μmol/m².     

Surface structural analysis of fine silica powder modified with butyl alcohol

The surface structure of modified silica powder has been studied by various experiments and simulations. In addition, the effect of surface structure on wettability has also been investigated. Nonporous silica powder was modified with n-and t-butyl alcohol. Two series of the modified silica surfaces were characterized by fractal dimension analysis from isotherms with some kinds of adsorptives. The fractal dimensions of the two series of modified surfaces were different from each other with an increase in modified ratio. The fractal dimension of the surface modified with t-butyl alcohol (t-modified surface) increased monotonously with butoxy group density. It is thought that the structure of the t-butoxy group is rigid and that the t-butoxy group cannot change its conformation. On the other hand, the variation of the surface fractal dimension value for the surface modified with n-butyl alcohol (n-modified surface), whose structure is flexible, was unique compared with the t-modified surface. Such discrepancy was assumed to be caused by the difference in the structure of the modifier and the assembled state of modifiers between the t- and n-modified surfaces. In order to investigate the variation of surface structure of the surface modified by the butoxy group with an increase in modified ratio, molecular dynamics simulations were performed. By comparing the results of these simulations with experimental results, it has been clarified that the variation in the mobility of the methyl group in the n-butoxy groups was closely related to the change in the surface fractal dimension value for the n-modified surface. It was then elucidated that this mobility change was caused by steric hindrance among the groups. Furthermore, the variation of conformation in the n-butoxy groups, which was obtained from molecular dynamics simulations, was in good agreement with the change in the wettability of the n-modified surface. It is suggested that the surface density of the modifier, the covering structure and the bulkiness significantly influence the wettability of the modified surface. Received: 6 April 1999 /Accepted in revised form: 24 August 1999     

Effect of surface modification of colloidal silica nanoparticles on the rigid amorphous fraction and mechanical properties of amorphous polyurethane–urea–silica nanocomposites

Colloidal silica nanoparticles (NPs) modified with eight different silane coupling agents were incorporated into an amorphous poly(tetramethylene oxide)‐based polyurethane–urea copolymer matrix at a concentration of 10 wt % (4.4 vol %) in order to investigate the effect of their surface chemistry on the structure–property behavior of the resulting nanocomposites. The rigid amorphous fraction (RAF) of the nanocomposite matrix as determined by differential scanning calorimetry and dynamic mechanical analysis was confirmed to vary significantly with the surface chemistry of the NPs and to be strongly correlated with the bulk mechanical properties in simple tension. Hence, nanocomposites with an RAF of about 30 wt % showed a 120% increase in Young's modulus, a 25% increase in tensile strength, a 15% decrease in elongation at break with respect to the neat matrix, which had no detectable RAF, whereas nanocomposites with an RAF of less than 5% showed a 60% increase in Young's modulus, a 10% increase in tensile strength and a 5% decrease in the elongation at break. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 2543–2556