Nonporous Silica Particles Research Articles

Separation performance of 1-μm silica particles as stationary phase for pressurized electrochromatography

Bare nonporous silica particles with diameter around 1 microm were prepared. A 20 cm section of a total length of 45 cm capillary (100 microm i. d.) was packed electrokinetically and the separation performance of basic compounds was investigated in the pressurized capillary electrochromatography (pCEC) system using 1 microm nonporous bare silica spheres as stationary phase and acetonitrile-water as mobile phase. The effects of the composition of the mobile phase, the concentration of the buffer, pH value, applied voltage and so on on the separation performance were investigated. The experimental results demonstrated that the bare silica column showed a typical reversed phase separation mechanism when it was used for separation of basic compounds. The separation performance changed slightly with changing buffer concentration. Since the degree of dissociation of the basic compounds depended greatly on the pH value, the interaction between the compounds and stationary phase changed with the changing of pH value, thereby changing the resolution of the compounds. The separation performance increased with the increase of applied voltage. A column efficiency of 35 000 for o-toluidine was obtained when a voltage of 1 kV was applied.

Peak parking-moment analysis: A strategy for the measurement of molecular diffusivity in liquid phase

The peak-parking (PP) method permits the measurement of molecular diffusivities ( D m) in solutions. D m is first measured for benzene in pure methanol and acetonitrile (ACN), using an empty open tube. This yields an effective axial diffusion coefficient ( D ax,m) equal to D m because there is no tortuosity nor constriction in the flow channel. The same measurements made for the same combinations of solute and solvents, using a column packed with non-porous silica particles provides the obstructive factor ( Y m), defined as the ratio D ax,m/ D m, which accounts for the influence of tortuosity and constriction of the interparticulate space in packed columns on axial molecular dispersion. The value obtained for Y m, 0.74–0.75, is constant, irrespective of the solvent. Then, PP experiments were made with the same apparatus and column to measure D m for benzene, toluene, and ethylbenzene in aqueous solutions of methanol and ACN. The values of D m obtained by correcting the experimental D ax,m values with Y m were compared with values previously reported and those estimated by several literature correlations. They were in good agreement with each other. The average relative error is estimated at 4.5–10%, demonstrating that the PP method is practically effective for experimental measurements of D m.

Hydrodynamic Chromatography of Silica Colloids on Small Spherical Nonporous Silica Particles

Hydrodynamic chromatography of silica colloids with 5 - 78 nm was investigated for spherical nonporous silica packed columns. Use of phosphate buffer as the eluent improved peak shape of silica colloids and allowed elution of the analytes at times relating to their sizes. The larger the diameter of the silica colloids, the faster the elution time was. Elution volumes of analytes were slightly increased with decreasing eluent flow rate, which may be due to more diffusion to narrower interstitial channels at lower flow rates. The separation column packed with 1.9-microm nonporous silica achieved slightly better resolution than that with 3.1-microm nonporous silica. Silica colloids were monitored based on turbidity using a UV detector, and larger signals were observed for larger silica colloids.

Probing the Conformation of Polyelectrolytes in Mesoporous Silica Spheres

We report a fluorescence-based approach to probing the conformation of a macromolecule, poly(allylamine hydrochloride) (PAH), in bimodal mesoporous silica (BMS) particles. The method involves monitoring the fluorescent properties of the probe, 1,3,6,8-pyrenetetrasulfonic acid tetrasodium salt (4-PSA), upon electrostatic binding to PAH molecules adsorbed in the nanopores of the BMS particles. PAH infiltration into the BMS particles, quantified by thermogravimetric analysis and visualized by confocal laser scanning microscopy, was examined as a function of PAH adsorption time, PAH molecular weight, and the sodium chloride (NaCl) concentration and pH of the PAH adsorption solution. The conformation of PAH molecules in the nanopores was investigated by incubating the PAH-loaded BMS particles in 4-PSA and using the ratio of the excimer to monomer emission intensity to discern differences in the PAH conformation in the nanopores. Control experiments involving nonporous silica (NS) particles were also conducted to determine the extent to which the nanopores within the BMS particles influence the degree of PAH adsorption and the conformation of the adsorbed PAH molecules. The data indicate that PAH molecules adsorbed in the nanopores adopt a more coiled conformation than PAH molecules adsorbed on NS particles over a wide range of conditions. Further, the conformation of PAH molecules in the nanopores can be tuned by adjusting the NaCl concentration and/or pH of the PAH adsorption solution. 4-PSA titration experiments revealed that at saturation binding there are ca. 3.8 PAH monomer units per 4-PSA molecule. This study provides insights into macromolecule infiltration and conformation in nanopores, which are important for the application of mesoporous materials in the fields of adsorption/immobilization, catalysis, delivery, sensing, separations, and synthesis.

When Size Matters—Near Infrared Reflection Spectroscopy of Nanostructured Materials

This article evaluates the applicability of near infrared (NIR) reflection spectroscopy for the physico-chemical and morphological characterisation of matter on a scale smaller than 1 micrometre (normally between 1 and 100 nanometres). The investigated materials comprise porous and non-porous silica particles, carbon based materials such as C60 fullerenes, nano-crystalline diamond (NCD) and dendrimers, all of them having diameters and/or pore sizes in the nanometre range, respectively. In case of the silica packings and differently derivatised C60 fullerenes, absorbance signals could be clearly assigned to corresponding surface modifications. Identification or classification of the material can be achieved successfully by principal component analysis. Nano crystalline diamond surfaces, whether H- or O-terminated, could be differentiated by a computed partial least squares (PLS) regression model with around 80% precision. Generations 0–7 of poly(amidoamine) (PAMAM) dendrimers with functionalised surface amine groups are characterised in respect of particle diameter and molecular weight. The established PLS models showed a standard error of prediction of only 0.43 nm and 12.30 kDa, respectively. NIR spectroscopy has developed as a flexible analytical method that can be utilized for fast, reliable and highly reproducible screening of matter even in the nanometer range.

Redox-active silica nanoparticles: Part 2. Photochemical hydrosilylation on a hydride modified silica particle surface for the covalent immobilization of ferrocene

The surface of spherical nonporous monodisperse silica particles (diameter: 200 nm) is covalently modified with hydride by means of hydrosilanization. A direct silicon–carbon link between the hydride modified silica surface and 10-undecylenic acid is obtained by photochemical radical initiated hydrosilylation. A ferrocene unit is attached through an amide link to the carboxylic acid modified surface. Solid-state NMR and DRIFT spectroscopy indicate success of the reactions. The electrochemical oxidation and reduction of the ferrocene modified silica particles are observed after their adsorption on a platinum electrode. The cyclic voltammograms indicate, in addition to the charge transfer between ferrocene units on the particle surface, an interparticle charge transfer.

TiO 2 coating on silica particles by deposition of sol-gel-derived nanoparticles

TiO 2 was coated on nonporous transparent silica particles of 3.2 μm diameter by deposition of sol-gel-derived TiO 2 nanoparticles. Effects of water concentration, feed rate of titanium tetraisopropoxide (TTIP) solution and amount of supplied TTIP solution on the amount of TiO 2 coated on the silica particles were examined. Scanning electron microscopy observation confirmed that TiO 2 was coated on the silica particles in the form of ‘nanoparticles’ by using this method. Because of that, even though the TiO 2 surface area decreased due to sintering after calcination at high temperature to change the crystalline phase of TiO 2 to the anatase phase, the final surface area was still much larger than that of the original silica particles. The results also showed that as the water concentration increased, the amount of coated TiO 2 decreased. On the other hand, when the amount of supplied TTIP solution increased, the amount of coated TiO 2 increased. It was also confirmed that the feed rate of TTIP solution had little effect on the amount of coated TiO 2. The photocatalytic activities of the resulting TiO 2-coated silica particles were also evaluated by the photocatalytic decomposition of 2,4-dinitrophenol as a model substance. The results showed that the photocatalytic activity of the particles is not a function of the total surface area, but of the surface area in which anatase phase TiO 2 is exposed to the reaction space. The sedimentation velocity of the TiO 2-coated silica particles becomes about 5 orders of magnitude faster than that of the primary particles of the TiO 2. This indicates that the handling of the TiO 2 was also improved considerably by coating on the silica particles.

Ultrahigh pressure liquid chromatography using elevated temperature

Fast liquid chromatographic (LC) methods are important for a variety of applications. Reducing the particle diameter ( d p) is the most effective way to achieve fast separations while preserving high efficiency. Since the pressure drop along a packed column is inversely proportional to the square of the particle size, when columns packed with small particles (<2 μm) are used, ultrahigh pressures (>689 bar) must be applied to overcome the resistance to mobile phase flow. Elevating the column temperature can significantly reduce the mobile phase viscosity, allowing operation at higher flow rate for the same pressure. It also leads to a decrease in retention factor. The advantage of using elevated temperatures in LC is the ability to significantly shorten separation time with minimal loss in column efficiency. Therefore, combining elevated temperature with ultrahigh pressure facilitates fast and efficient separations. In this study, C 6-modified 1.0 μm nonporous silica particles were used to demonstrate fast separations using a temperature of 80 °C and a pressure of 2413 bar. Selected separations were completed in 30 s with efficiencies as high as 220,000 plates m −1.

Flow Counterbalanced Capillary Electrophoresis Using Packed Capillary Columns: Resolution of Enantiomers and Isotopomers

A method with the ability to increase greatly both the resolution and efficiency of a given capillary electrophoretic system is described. This method differs from traditional capillary electrophoresis (CE) in that a counterflow is induced in the direction opposite to the electrokinetic migration of the analyte. This has the effect of extending not only the time the analytes migrate in the electric field but also the effective length and the effective applied voltage of the system. Previous work in our group with flow counterbalanced capillary electrophoresis has utilized an open tube of small inner diameter to reduce peak broadening caused by hydrodynamic flow. Narrow-diameter capillaries (5-10 microm) restricted analysis to fluorescent analytes and laser-induced fluorescence detection. The method described here uses a capillary of much larger inner diameter (75 microm) that has been packed with nonporous silica particles. The packing material reduces the amount of band broadening caused by pressure-induced flow relative to that experienced in an open tube. A larger diameter capillary allows the detection of analytes by UV absorption, not only eliminating the need to tag analytes with fluorescent tags but also allowing for the detection of a much broader range of analytes. The system was evaluated by studying the separations of several enantiomers using only beta-cyclodextrin as the chiral selector. The system was also used to resolve the two naturally occurring isotopes of bromine and to resolve phenylalanine from phenylalanine-d8. Relative to traditional CE, large improvements in resolution and separation efficiency have been achieved with this method.

Influence of the pressure on the properties of chromatographic columns: I. Measurement of the compressibility of methanol–water mixtures on a mesoporous silica adsorbent

The compressibilities of aqueous solutions of methanol or acetonitrile containing 0, 20, 40, 60, 80 and 100% (v/v) organic solvent were measured with a dynamic chromatographic method. The elution volumes of thiourea samples (2 μ L) in these solutions were measured at different average column pressures, adjusted by placing suitable capillary restrictors on-line, after the detector. The reproducibility of the measurements was better than 0.2%. In the range of average pressures studied (10–350 bar), the maximum change in elution volume of thiourea is 1.3% (in pure water) and 4.0% (in pure methanol). This difference is due to the different compressibilities of these pure solvents. For mixtures, the plots of the elution volume of thiourea versus the pressure are convex downward, which is inconsistent with the opposite curvature predicted by the classical Tait model of liquid compressibility. This difference is explained by the variation of the amount of thiourea adsorbed with the pressure. The deconvolution of the two effects, adsorption of thiourea and solvent compressibility, allows a fair and consistent determination of the compressibilities of the methanol–water mixtures. A column packed with non-porous silica particles was also used to determine the compressibility of methanol–water and acetonitrile–water mixtures. A negative deviation by respect to ideal behavior was observed.

Agglomerated non-porous silica nanoparticles as model carriers in polyethylene synthesis

Non-porous submicron silica particles (250 and 500 nm) with high monodispersity were agglomerated to form spherical agglomerates via spray drying. As a binder, 25 nm sized monodisperse silica spheres were selected from a variety of colloidal systems including Levasil-type and Aerosil-type silica nanoparticles. The use of such binders led to an increase of the specific surface area of the agglomerated carriers. All materials were characterised by nitrogen sorption, mercury intrusion and scanning electron microscopy. The silica agglomerates, with highly defined geometrical and pore structural parameters, were employed as model carriers in the heterogeneous polymerization of ethylene using a conventional methylaluminoxane (MAO)/η5-dicyclopentadienyl zirconiumdichloride metallocene catalyst system. The activity of these model carriers was evaluated, as was the influence of MAO impregnation on the carriers’ physico-chemical properties. The activity of the presented catalyst systems depended heavily on the percentage of binder. Due to skin formation, high contents of binder resulted in agglomerates with a smoother, more rigid surface. These carriers did not fragment during polymerization, and displayed low catalytic activity. Less binder gave more readily fragmented agglomerates displaying higher catalytic activities. Despite their small specific surface areas in comparison to commercially available polymerization carriers, e.g. SYLOPOL 948 from W.R. Grace, these model carriers possess similar activities, with the polymerization process depending more on the geometrical and structural aspects of the beads rather than on their specific surface areas.

Use of 1.5-microm porous ethyl-bridged hybrid particles as a stationary-phase support for reversed-phase ultrahigh-pressure liquid chromatography.

A new ethyl-bridged hybrid packing material was evaluated in terms of its suitability for ultrahigh-pressure liquid chromatography (UHPLC). The 1.5-microm particles were obtained and packed into 30-microm-i.d. fused-silica capillary columns up to 50 cm in length. The particles were evaluated by isocratic reversed-phase UHPLC at pressures up to 4500 bar (65,000 psi). The chromatographic performance of these particles was found to be similar to the performance of 1.0-microm nonporous silica particles. The mechanical strength of the ethyl-bridged hybrid material was evaluated by running a 15-cm-long column at pressures up to 4500 bar. No breakdown of the particles in the packed bed was observed. The sample loading capacity of the hybrid material was evaluated and compared to 1.0-microm nonporous silica material by observing analyte peak width versus amount injected. The observed improvement in loading capacity for the hybrid material versus nonporous silica was consistent with the improvement predicted by comparing the phase ratios of the two materials.

Synthesis, Characterization, and Evaluation of Divinylbenzene-Coated Spherical Nonporous Silica

Monodisperse nonporous silica particles were coated with different amounts of a divinylbenzene-based polymer resulting in a new class of reversed-phase high-performance liquid chromatographic material. An optimization of the polymer layer, with respect to the thickness, was performed to obtain optimal surface properties of the chromatographic sorbent. The materials were characterized using solid-state 13C CP/MAS NMR spectroscopy, thermogravimetry analysis, BET surface area measurements, scanning electron microscopy, and transmission electron microscopy. The application of the new materials in reversed-phase HPLC revealed extremely short separation times while maintaining a high chromatographic resolution.

Graft polymerization of vinyl acetate onto silica

AbstractThe free‐radical graft polymerization of vinyl acetate onto nonporous silica particles was studied experimentally. The grafting procedure consisted of surface activation with vinyltrimethoxysilane, followed by free‐radical graft polymerization of vinyl acetate in ethyl acetate with 2,2′‐azobis(2,4‐dimethylpentanenitrile) initiator. Initial monomer concentration was varied from 10 to 40% by volume and the reaction was spanned from 50 to 70°C. The resulting grafted polymer, which was stable over a wide range of pH levels, consisted of polymer chains that are terminally and covalently bonded to the silica substrate. The experimental polymerization rate order, with respect to monomer concentration, ranged from 1.61 to 2.00, consistent with the kinetic order for the high polymerization regime. The corresponding rate order for polymer grafting varied from 1.24 to 1.43. The polymer graft yield increased with both initial monomer concentration and reaction temperature, and the polymer‐grafted surface became more hydrophobic with increasing polymer graft yield. The present study suggests that a denser grafted polymer phase of shorter chains was created upon increasing temperature. On the other hand, both polymer chain length and polymer graft density increased with initial monomer concentration. Atomic force microscopy–determined topology of the polymer‐grafted surface revealed a distribution of surface clusters and surface elevations consistent with the expected broad molecular‐weight distribution for free‐radical polymerization. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 300–310, 2003

Formation and Characterization of High Surface Area Thermally Stabilized Titania/Silica Composite Materials via Hydrolysis of Titanium(IV) tetra-Isopropoxide in Sols of Spherical Silica Particles

A direct synthetic route leading to titania particles dispersed on nonporous spherical silica particles has been investigated; 5, 10, and 20% (w/w) titania/silica sols mixtures were achieved via hydrolyzation of titanium tetra-isopropxide solution in the mother liquor of a freshly prepared sol of spherical silica particles (Stöber particles). Titania/silica materials were produced by subsequent drying and calcination of the xerogels so obtained for 3 h at 400 and 600°C. The materials were investigated by means of thermal analyses (TGA and DSC), FT-IR, N2 gas adsorption–desorption, powder X-ray diffraction (XRD), and transmission electron microscopy (TEM). In spite of the low surface area (13.1 m2/g) of the pure spherical silica particles calcined at 400°C, high surface area and mesoporous texture titania/silica materials were obtained (e.g., SBET ca. 293 m2/g for the 10% titania/silica calcined at 400°C). Moreover, the materials were shown to be amorphous toward XRD up to 600°C, while reasonable surface areas were preserved. It has been concluded that dispersion of titania particles onto the surface of the nonporous spherical silica particles increase their roughness, therefore leading to composite materials of less firm packing and mesoporosity.

Electroosmotic flow pumps with polymer frits

Electroosmotic flow (EOF) pumps with flow rates on the order of 1 ml/min have been designed and fabricated. These pumps use EOF to propel liquid solutions in a compact system with no moving parts. The pumping chamber is filled with densely packed non-porous silica particles, and the particle bed is held in place using two polymeric porous frits. The pump frame is made of acrylic, the frits are made using UV photopolymerization within the custom-built acrylic frame, and the particles are packed using a novel side-bore packing technology. Platinum wire electrodes on both sides of the pump provide the electric field to drive the flow. Deionized water is used as a working fluid in order to minimize Joule heating and increase thermodynamic efficiency. The maximum flow rates and maximum pressures generated by the pumps are 0.8 ml/min and 2 atm, respectively, at 1.0 kV applied voltage.

Phase-Boundary Catalysis of Alkene Epoxidation with Aqueous Hydrogen Peroxide Using Amphiphilic Zeolite Particles Loaded with Titanium Oxide

A new heterogeneous catalytic system, phase-boundary catalysis, for epoxidation of alkene with aqueous H2O2 is proposed. Amphiphilic titanium-loaded zeolite particles, a part of the external surface of which was covered with hydrophobic alkyl groups and the rest being left hydrophilic, were prepared by deposition of titanium species from titanium(IV) tetra-2-propoxide and attachment of octadecylsilyl groups from n-octadecyltrichlorosilane (ODS) onto an NaY zeolite powder. Due to their amphiphilicity, the catalyst particles lay at the liquid–liquid phase boundary between upper alkene and lower aqueous phases, and they showed catalytic activity for epoxidation of 1-alkenes (e.g., 1-octene) with aqueous hydrogen peroxide. The phase-boundary catalytic system required neither stirring to make an emulsion nor addition of a cosolvent to make a homogeneous solution to drive the epoxidation. The yield of 1,2-epoxyoctane, a sole oxidation product from 1-octene, strongly depended on the apparent interphase area of the aqueous–organic phase boundary. The amphiphilic catalyst exhibited much higher catalytic activity than that of hydrophilic titanium-loaded NaY, without modification by ODS, or of a hydrophobic catalyst with almost full coverage by the alkyl groups. A similar trend in the activities of these catalysts was also observed when the reaction was carried out with vigorous stirring, in the presence of a cosolvent, or in a water-carbontetrachloride (CCl4) mixture, where the aqueous hydrogen peroxide phase lies above the CCl4 phase. The phase-boundary catalytic system could also be applied to epoxidation of other normal alkenes. Compared with nonporous silica particles, the use of microporous NaY with a relatively large surface area had a beneficial effect, probably due to an increase both in the surface contact between the aqueous and organic layer and in the number of effective active sites of titanium species on its external surface. On the basis of these experimental results, a reaction model is proposed.

Fast generic-gradient reversed-phase high-performance liquid chromatography using short narrow-bore columns packed with small nonporous silica particles for the analysis of combinatorial libraries.

The extremely large number of samples generated for the quality control analysis of combinatorial libraries that are developed in the pharmaceutical research for drug discovery requires fast generic methods such as rapid-gradient reversed-phase high-performance liquid chromatography (HPLC). These methods are necessary as standard procedures to produce up to several hundreds of analytical results per day and should be optimized in order to be applied to library products of widely differing polarities. This work presents an optimized generic method using a narrow-bore column packed with 1.5-microm nonporous particles and a completely automated HPLC workstation configured for the best efficiency, throughput, and robustness with this column. A test mix of 12 compounds with a wide polarity range is separated within 1.5 min with a cycle time of 3.5 min. The throughput is further enhanced using a Gilson 233XL dual-injection sampler to feed two parallel HPLC systems in order to perform 34 analyses per hour.

Fabrication and characterization of electroosmotic micropumps

Electroosmotic flow (EOF) micropumps which use electroosmosis to transport liquids have been fabricated and used to achieve pressures in excess of 20 atm and flow rates of 3.6 μl/min for 2 kV applied potentials. These pumps use deionized water as working fluids in order to reduce the ion current of the pump during operation and increase thermodynamic efficiency. EOF pumps are fabricated by packing the 3.5 μm diameter non-porous silica particles into 500–700 μm diameter fused-silica capillaries and by using a silicate frit fabrication process to hold the particles in place. The devices have no moving parts and can operate as both open (high flow rate) and closed (high pressure) systems. Pressure versus flow rate performance data are presented and combined with measurements of physical dimensions, dry and wet weight, and ion current to calculate the pump structure porosity, tortuosity, effective pore radius, and zeta potential.

Very high pressure gradient LC/MS/MS.

A very high pressure liquid chromatography (VHPLC) system was constructed by modifying a commercially available pump in order to achieve pressures in excess of 1,200 bar (17,500 psi). A computer-controlled low-pressure mixer was used to generate solvent gradients. Protein digests were rapidly analyzed by reversed-phase VHPLC with linear solvent gradients coupled to either a tandem mass spectrometer using electrospray ionization or a UV/visible detector. The separations were performed at pressures ranging from 790 (11,500 psi) to 930 bar (13,500 psi) in 22-cm-long capillary columns packed with C18-modified 1.5-microm nonporous silica particles. A digest of bovine serum albumin (BSA) was analyzed by the VHPLC system connected to a mass spectrometer in MS mode. An analysis of 12.5 fmol of sample gave signal-to-noise ratios of tryptic peaks greater than 10:1 in the base peak plot mass chromatogram. This system was also used to analyze a proteolytic digest of a rat liver protein excised from a 2-D gel separation of a liver tissue lysate. For this analysis, the mass spectrometer was set up to perform data-dependent scanning (automated switching from MS mode to MS/MS mode when a peak was detected) for peptide sequencing and protein identification by database searching. The results of this analysis are compared to an analysis performed on the same sample using the nanoelectrospray-MS/MS technique. Though both techniques were able to identify the unknown protein, the VHPLC method gave twice as many sequenced peptides as nanoelectrospray and improved the signal-to-noise ratio of the spectra by at least a factor of 10. Direct comparisons with nanoelectrospray for MS and MS/MS data acquisition from a BSA digest were made. These comparisons show enhancements of greater than 20-fold for VHPLC over nanoelectrospray. In addition, the VHPLC/MS/MS data acquisition was accomplished in an automated manner.