Adsorption of organic molecules on silica surface

The adsorption behaviour of various organic adsorbates on silica surface is reviewed. Most of the structural information on silica is obtained from IR spectral data and from the characteristics of water present at the silica surface. Silica surface is generally embedded with hydroxy groups and ethereal linkages, and hence considered to have a negative charged surface prone to adsorption of electron deficient species. Adsorption isotherms of the adsorbates delineate the nature of binding of the adsorbate with silica. Aromatic compounds are found to involve the pi-cloud in hydrogen bonding with silanol OH group during adsorption. Cationic and nonionic surfactants adsorb on silica surface involving hydrogen bonding. Sometimes, a polar part of the surfactants also contributes to the adsorption process. Styryl pyridinium dyes are found to anchor on silica surface in flat-on position. On modification of the silica by treating with alkali, the adsorption behaviour of cationic surfactant or polyethylene glycol changes due to change in the characteristics of silica or modified silica surface. In case of PEG-modified silica, adsolubilization of the adsorbate is observed. By using a modified adsorption equation, hemimicellization is proposed for these dyes. Adsorptions of some natural macromolecules like proteins and nucleic acids are investigated to study the hydrophobic and hydrophilic binding sites of silica. Artificial macromolecules like synthetic polymers are found to be adsorbed on silica surface due to the interaction of the multifunctional groups of the polymers with silanols. Preferential adsorption of polar adsorbates is observed in case of adsorbate mixtures. When surfactant mixtures are considered to study competitive adsorption on silica surface, critical micelle concentration of individual surfactant also contributes to the adsorption isotherm. The structural study of adsorbed surface and the thermodynamics of adsorption are given some importance in this review.     

Preparation and evaluation of a hydrolytically stable amide-embedded stationary phase

We have developed a new hydrolytically stable amide-embedded stationary phase via a simple and effective synthetic method. The preparation of the new phase involves the synthesis of multifunctional silane ligands and the surface modification of porous silica particles via multiple attachments of these ligands to the silica surface. A hydrolytically stable coating was produced as a result of multiple covalent linkages formed between silane ligands and the silica surface, and cross-linking between adjacent ligands. The resulting amide-embedded stationary phase showed excellent hydrolytic stability over a wide pH range. Like other existing amide-embedded columns, this new stationary phase exhibits higher retention for polar compounds and different selectivity as compared to conventional C18 columns. The new phase is compatible with 100% aqueous mobile phases, and also provides high column efficiency and good peak shapes for both acidic and basic compounds.     

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.     

有机改性二氧化硅及其负载钴催化剂的费托合成反应性能

利用硅烷化作用分别制得了甲基、二甲基和三甲基改性的SiO2载体,采用等体积浸渍法制备了质量分数为5%的一系列负载型钴催化剂.考察了有机改性对催化剂费托合成催化性能的影响.结果表明,SiO2经有机基团改性后,表面硅羟基浓度减小,削弱了钴硅之间的相互作用,促进了催化剂的还原,提高了催化剂的活性,降低了甲烷选择性.由于空间位阻不同,不同有机基团改性的SiO2的表面硅羟基浓度不同,催化剂活性随着表面硅羟基浓度的减小而提高.     

Synthesis and characterization of surface modified SBA-15 silica materials and their application in chromatography

Hexagonally ordered SBA-15 mesoporous silica spheres with large uniform pore diameters are obtained using the triblock copolymer, Pluronic P123, as template with a cosurfactant cetyltrimethylammonium bromide (CTAB) and the cosolvent ethanol in acidic media. A series of surface modified SBA-15 silica materials is prepared in the present work using mono- and trifunctional alkyl chains of various lengths which improves the hydrothermal and mechanical stability. Several techniques, such as element analysis, nitrogen sorption analysis, small angle X-ray diffraction, scanning electron microscopy (SEM), FTIR, solid-state (29)Si and (13)C NMR spectroscopy are employed to characterize the SBA-15 materials before and after surface modification with the organic components. Nitrogen sorption analysis is performed to calculate specific surface area, pore volume and pore size distribution. By surface modification with organic groups, the mesoporous SBA-15 silica spheres are potential materials for stationary phases in HPLC separation of small aromatic molecules and biomolecules. The HPLC performance of the present SBA-15 samples is therefore tested by means of a suitable test mixture.     

Rational design of original materials for the electrocatalytic hydrogenation reactions: concept, preparation, characterization, and theoretical analysis

Original and versatile new materials for the electrocatalytic hydrogenation of organic compounds were designed. The materials consist of reticulated glassy carbon cathode electrodes in which the modified silica particles (average diameter 40-63 microm) were dynamically circulated. The modification of the silica surface is 2-fold. First, the silica is surface-modified using organic functions such as -OSi(CH3)2(CH2)3OCH2CH-(OH)(CH)2OH (SiO2-Diol), -OSi(CH3)2(CH2)7CH3 (SiO2-C8), and -OSi(CH3)2C6H5 (SiO2-Phenyl). Second, these silica particles were further modified by vapor phase deposition of nickel nanoaggregates (used as sites for hydrogen atoms and electric contacts with the electrode material), which does not destroy or alter the organic functionalization as demonstrated by thermogravimetric analysis-mass spectrometry and Raman, diffuse reflectance IR Fourier transform, and Auger electron spectroscopies. The new concept stems from relative adsorption and desorption properties of the organic molecules and their corresponding reduced products into the organic functionalization of the surface-modified silica. In this work, the electrocatalytic hydrogenation cyclohexanone was used to test the concept. The performances (amount of cyclohexanol vs time of generated electrolysis at constant current) are measured and compared for the various bonded organic functions of the silica surface listed above, along with the unmodified silica particles (but still containing nickel nanoaggregates) and the presence or absence of methanol in solution. The measurements of the adsorption isotherms of cyclohexanone, and the calculations of the interaction energies (MM3 force field) between the chemisorbed organic functions and the substrates, corroborate perfectly the electrocatalysis results.     

Silica-metal core-shell nanostructures

Silica-metal nanostructures consisting of silica cores and metal nanoshells attract a lot of attention because of their unique properties and potential applications ranging from catalysis and biosensing to optical devices and medicine. The important feature of these nanostructures is the possibility of controlling their properties by the variation of their geometry, shell morphology and shell material. This review is devoted to silica-noble metal core-shell nanostructures; specifically, it outlines the main methods used for the preparation and surface modification of silica particles and presents the major strategies for the formation of metal nanoshells on the modified silica particles. A special emphasis is given to the St?ber method, which is relatively simple, effective and well verified for the synthesis of large and highly uniform silica particles (with diameters from 100 nm to a few microns). Next, the surface chemistry of these particles is discussed with a special focus on the attachment of specific organic groups such as aminopropyl or mercaptopropyl groups, which interact strongly with metal species. Finally, the synthesis, characterization and application of various silica-metal core-shell nanostructures are reviewed, especially in relation to the siliceous cores with gold or silver nanoshells. Nowadays, gold is most often used metal for the formation of nanoshells due to its beneficial properties for many applications. However, other metals such as silver, platinum, palladium, nickel and copper were also used for fabrication of core-shell nanostructures. Silica-metal nanostructures can be prepared using various methods, for instance, (i) growth of metal nanoshells on the siliceous cores with deposited metal nanoparticles, (ii) reduction of metal species accompanied by precipitation of metal nanoparticles on the modified silica cores, and (iii) formation of metal nanoshells under ultrasonic conditions. A special emphasis is given to the seed-mediated growth, where metal nanoshells are formed on the modified silica cores with deposited metal nanoparticles. This strategy assures a good control of the nanoshell thickness as well as its surface properties.     

Thick-film graphite electrodes in stripping voltammetry

Various types of modified thick-film graphite electrodes are reviewed. Two modification options are available: either in situ modified or beforehand. Electrodes modified in situ by various organic compounds were used for the determination of W, Mo, Cr, Ni and Mn by adsorptive stripping voltammetry. Insoluble inorganic salts, insoluble organic complexes or soluble compounds protected with Nafion were used for modification of the electrode surface beforehand. These pre-conditioned electrodes permit quantification of Pb, Cd, Cu, Zn, Sn, As and Hg ions. The detection limits of these elements were in the sub-μg/L range. The relative standard deviation did not exceed 10–15%. Measurements were made without deaeration of test solutions and normally without destroying small organic matter found in surface waters. A number of characteristics, such as the formation of well-shaped peaks and stable increments of peaks after standard additions, as well as elimination of the mechanical surface regeneration stage and metallic mercury or its soluble salts from the analytical procedure and a long shelf-life of the electrodes make them promising for electroanalysis and for application in portable field instruments. Received: 16 November 1998 / Revised: 16 February 1999 / Accepted: 20 February 1999     

Thick-film graphite electrodes in stripping voltammetry

Various types of modified thick-film graphite electrodes are reviewed. Two modification options are available: either in situ modified or beforehand. Electrodes modified in situ by various organic compounds were used for the determination of W, Mo, Cr, Ni and Mn by adsorptive stripping voltammetry. Insoluble inorganic salts, insoluble organic complexes or soluble compounds protected with Nafion were used for modification of the electrode surface beforehand. These pre-conditioned electrodes permit quantification of Pb, Cd, Cu, Zn, Sn, As and Hg ions. The detection limits of these elements were in the sub-μg/L range. The relative standard deviation did not exceed 10–15%. Measurements were made without deaeration of test solutions and normally without destroying small organic matter found in surface waters. A number of characteristics, such as the formation of well-shaped peaks and stable increments of peaks after standard additions, as well as elimination of the mechanical surface regeneration stage and metallic mercury or its soluble salts from the analytical procedure and a long shelf-life of the electrodes make them promising for electroanalysis and for application in portable field instruments. Received: 16 November 1998 / Revised: 16 February 1999 / Accepted: 20 February 1999     

Surface Modification and Functionalization of Metal and Metal Oxide Nanoparticles by Organic Ligands

Summary. Metal or metal oxide nanoparticles possess unique features compared to equivalent larger-scale materials. For applications, it is often necessary to stabilize or functionalize such nanoparticles. Thus, modification of the surface of nanoparticles is an important chemical challenge. In this survey, various possibilities are discussed how nanoparticles can be protected by organic ligands and how these ligands can be used to introduce functionalities. The preparative possibilities include grafting of an already functionalized ligand on the nanoparticle surface, exchanging part or all existing ligands on the nanoparticle surface, or grafting of a ligand on a nanoparticle followed by modification by organic chemical reactions.     

A review of contamination-resistant antireflective sol–gel coatings

Sol–gel derived silica antireflective (AR) coatings have been widely used as the optical components for high peak power laser systems because of their excellent optical properties and high laser-induced damage thresholds. However, the sol–gel derived coatings have a high surface area that is more susceptible to be contaminated by absorption of trace amounts of water vapor and other volatile organic compounds from the environment. In this paper, the major approaches to fabricate contamination resistant sol–gel derived silica AR coatings have been extensively reviewed. Different approaches, including solution-phase and vapor-phase silanization, ammonia–water vapor treatment and fluorine modification have been discussed. The optical properties and laser-induced damage thresholds of modified coatings have also been evaluated. The improved sol–gel AR coatings have been shown to possess superior contamination resistance to work in vacuum systems compare to the traditional sol–gel AR coatings.     

Improvement of properties of silica‐filled styrene‐butadiene rubber composites through plasma surface modification of silica

The performance of plasma surface modified silica filler in styrene‐butadiene rubber (SBR) matrix has been analyzed. The conditions of plasma modification have been optimized by taking secant modulus as a standard parameter and the occurrence of the modification has been confirmed by surface area determination and Fourier transform infrared spectroscopy. The plasma‐modified surface of silica has been found to be composed of carbon–carbon double bonds and carbon–hydrogen bonds. Silane treatment also has been carried out on silica filler surface for a comparative assessment of its influence in the curing behavior and filler–rubber interaction. The cure reactions of all the rubber compounds have been found to be proceeded according to first‐order kinetics. A reduction in the cure reaction rate constant has been observed with the loading of unmodified and surface modified silica, emphasizing the cure deactivation of the matrix rubber by the silica filler. The filler dispersion, as revealed by scanning electron microscopy, has been found to be greatly improved by the plasma as well as silane treatment. The filler–rubber interaction has been found to be greatly improved by both surface treatments, but the best balance of mechanical properties has been observed with plasma surface modification only. Copyright © 2004 John Wiley & Sons, Ltd.     

Complex Formation of d‐Metal Ions at the Interface of TbIII‐Doped Silica Nanoparticles as a Basis of Substrate‐Responsive TbIII‐Centered Luminescence

The complex formation of d‐metal ions at the interface of Tb^III‐doped silica nanoparticles modified by amino groups is introduced as a route to sensing d‐metal ions and some organic molecules. Diverse modes of surface modification (covalent and noncovalent) are used to fix amino groups onto the silica surface. The interfacial binding of d‐metal ions and complexes is the reason for the Tb^III‐centered luminescence quenching. The regularities and mechanisms of quenching are estimated for the series of d‐metal ions and their complexes with chelating ligands. The obtained results reveal the interfacial binding of Cu^II ions as the basis of their quantitative determination in the concentration range 0.1–2.5 μM by means of steady‐state and time‐resolved fluorescence measurements. The variation of chelating ligands results in a significant effect on the quenching regularities due to diverse binding modes (inner or outer sphere) between amino groups at the interface of nanoparticles and Fe^III ions. The applicability of the steady‐state and time‐resolved fluorescence measurements to sense both Fe^III ions and catechols in aqueous solution by means of Tb^III‐doped silica nanoparticles is also introduced.     

硅胶基质高效液相色谱填料研究进展

高效液相色谱(HPLC)不仅是一种有效的分析分离手段,也是一种重要的高效制备分离技术。色谱柱是HPLC系统的核心,不同性能的填料是HPLC广泛应用的基础。硅胶是开发最早、研究最为深入、应用最为广泛的HPLC固定相基质,其制备方法主要有喷雾干燥法、溶胶-凝胶法、聚合诱导胶体凝聚法及模板法等。近年来,亚2μm小粒径硅胶、核-壳型硅胶、双孔径硅胶、介孔性硅胶、有机杂化硅胶等新型硅胶应用于HPLC并取得了色谱分离技术的飞速发展,例如基于亚2μm填料的超高压液相色谱技术、基于核-壳型填料的快速分离技术、基于杂化硅胶填料的高温液相色谱技术等。硅胶经表面化学键合、聚合物包覆等有机改性可制得先进的大分子限进填料、温敏性填料、手性填料等,大大扩展了HPLC的应用范围。本文对液相色谱用硅胶的制备方法、改性与修饰方法以及硅胶基质固定相的评价方法加以系统综述,概述了新型硅胶在HPLC中的应用进展,并对硅胶基质填料的发展方向与应用前景进行了展望。     

Functionalized SiO2 with N-, S-containing ligands for Pb(II) and Cd(II) adsorption

Five novel organosilicas SiO₂-L have been prepared by grafting organic ligands (L) bearing thiazolidine (TAMS), thiazole (TIMS), imidazole (DTIMS), mercaptopropyl (DETATS) or pyridine (DETAPS) functional groups. The SiO₂-L materials have been evaluated for their capacity for Pb²⁺ and Cd²⁺ uptake from aqueous solution as a function of the pH. The present data demonstrate that the grafting of the silica may result in significant enhancement of metal uptake by the SiO₂-L adsorbent. Noticeably, multifunctional ligands such as DTIMS and DETAPS, have exhibited the highest metal capacity between the studied materials. Significant metal uptake was observed for both Pb and Cd on those materials independent of pH. The adsorption data have been analyzed based on a surface complexation model (SCM) with FITEQL by considering the grafted ligands as additional surface sites. FT-IR data are provided in support of this approach. Based on the theoretical speciation results, we may conclude that the determining factors for the metal uptake by the modified silica are the following three: (a) the presence or absence of protonable groups on the ligand determines the pH-edge profile, (b) the complexing affinity of the particular ligand towards each metal, and (c) the amount of grafted ligand on the silica surface. The interplay of these factors will determine the final maximum loading capacity of the sorbing material. Both the nature of the ligand as well as its surface concentration are of equal importance for the sorbing capacity of the material.     

Organically modified silica: synthesis and applications due to its surface interaction with organic molecules

Silica gels can be chemically modified using organic precursors producing organically modified silica (ORMOSIL), a class of novel materials for hosting varieties of organic and inorganic substrates. Ormosil matrixed materials show an enhanced activity during catalysis, photochemical activities like absorption and emission, electrochemical sensitivities, sensing of gases, solvents, pH of solution and biomolecules, etc. Ormosil-based materials can be used as efficient protective coatings and can be utilized in designing wave guides that can carry out excellent photonic transmission of information. The synthetic and chemical modification of ormosil and some of the above applications have been discussed.     

Upper-rim calixarene phosphines consisting of multiple lower-rim OH functional groups: synthesis and characterisation

The synthetic approaches to mono-isopropoxytrihydroxycalix[4]arenes bearing two or three diphenylphosphino groups at the upper macrocycle rim were elaborated. Due to reactive hydroxyl groups at the lower rim capable of binding with silica gel surface, the calixarenes, especially the enantiomerically pure inherently chiral diphosphinocalixarene, are promissing ligands for creation of hybride organic–inorganic metallocomplexing catalysts of organic (asymmetric) reactions.     

On some aspects of the silica surface modification with hexamethylcyclotrisilazane,hexamethyldisilazane and their mixtures

Summary The paper is concerned with the structure of an organic layer bonded to the silica surface modified with silazanes. The changes of the retention volumes of hydrocarbons is studied for the silicas modified with hexamethylcyclotrisilazane, hexamethyldisilazane and their mixtures. It is shown that most of the carbon in the layer after hexamethylcyclotrisilazane modification is bonded to the surface by Si−O−Si bonds. This fact supports the assumption of a fragmentation of the cyclosilane used for modification.     

Properties and Application of a Chelating Sorbent Prepared by Modification of LiChroprep-NH<Subscript>2</Subscript> with Calcon

A new chelating sorbent for metal ions was prepared by modification of chemically modified silica – LiChroprep-NH₂ with Calcon. The molecular mechanism of binding this reagent to the surface of the applied carrier is presented. The properties of this sorbent were compared to analogous sorbents with a plain silica carrier and chemically modified silicas – LiChroprep-RP containing Calcon. The advantages of the new sorbent compared to the silica and LiChroprep-RP chelating sorbents are demonstrated. The sorbent obtained was applied as stationary phase in solid-phase extraction (SPE) for separations of some chosen mixtures of metal ions and for additional purification of aqueous solutions of salts of alkali metals from trace amounts of heavy metals. The multiple use of the sorbent based on LiChroprep-NH₂ in sorption-desorption processes of metal ions without deterioration of its sorption capacity is demonstrated.     

New solid-phase-nanoscavenger for the analytical enrichment of mercury from water

A nanoscavenger of mercaptopropyl-modified silica microparticles has been developed for the determination of trace levels of mercury(II) in water. The synthesis of silica microparticles nanoscavengers is carried out by modification of pre-formed silica particles with mercaptopropyltrimethoxysilane or by incorporating the thiol modification agent during the growth of the silica particles. The silica nanoscavengers were characterized by SEM, TGA, particle analyzer, IR and the parameters influencing the extraction and recovery phases of the preconcentration process were performed by AAS. The results show that careful choice of particle size and surface characteristics produce a new mercapto-silica-nanoscavenger that disperses in water as a quasi-stable sol. This permits the facile recovery of the mercury-loaded nanoscavenger particles. No agitation is required during the mercury extraction as the dispersion is maintained by Brownian motion and slow gravitational sedimentation. The technique overcomes a number of problems, such as cross-contamination and decreases in mercury concentration during sample transportation to the laboratory. Recovery achieved reaches >97 ± 4% over a wide range of preconcentration factors. At a preconcentration factor of 50 the limit of detection (3σ) was 0.19 ng mL(-1). The method is environmentally friendly as only 300 mg of mercapto-nanoscavenger is used, no organic solvent is required for the extraction and the experiment is performed without the need for manual or mechanical agitation.