Pore Structure Simulation of Gels with a Binary Monomer Size Distribution

The pore size distribution in silica gels can be tailored by the addition of silica soot particles during the gel formation. We introduce a numerical model in order to simulate the structure of this “composite gel”. The algorithm is based on Diffusion-Limited Cluster-Cluster Aggregation model with an initial binary distribution of monomer sizes. The textural properties of the simulated gels are calculated using a simple triangulation method. Nitrogen adsorption-desorption experiments show that with the powder addition the mean pore size is shifted towards larger pore size and the specific surface area decreases. Numerical results of the mean pore size, specific surface area, and particles are in good agreement with experimental data. Because of these textural properties this new type of gels and aerogels has larger permeability and interesting properties as host matrix. The composite gels and the numerical model could also be helpful to simulate the natural allophanic gel found in volcanic soils.     

Influence of the Amount of Bonded Non-Polar Phase and of the Length of Attached Alkyl Chains on Retention Characteristics of Silica-Based Sorbents for Reversed-Phase High Performance Liquid Chromatography

Abstract

The retention characteristics of two sets of chemically bonded non-polar silica packings with a high surface concentration of functional groups have been compared in the reversed-phase mode of liquid chromatography: (i) the conventional packings prepared by chemical modification with trimethylsilyl, heptyl, dodecyl and octadecyl groups, and (ii) mixed-phase materials where differences in the amount of organic bonded phase were achieved by bonding octadecyl and trimethylsilyl groups in different proportions. The retention data of two homologous series of solutes show that these two classes of packings are distinctly different; in particular, the capacity factors and selectivities are always higher on the mixed-phase bonded silica packings at the same percentage of carbon.     

Determination of Surface Silanols of Silica Gels and HPLC Bonded Phases

Abstract

A rapid method determining the surface silanols of silica gels and HPLC bonded phases involves a titration of silica gel with sodium hydroxide, in a medium of any of the following 10% aqueous salts: sodium chloride, sodium nitrate, sodium sulfate, potassium chloride, potassium nitrate and potassium sulfate. The silanols are quantified as m eg/g. It is possible to determine cation exchange capacity of a cation-exchanger due to an acid as an end group and surface silanols separately.     

Experimental Study of Model Bonded Stationary Phases for Liquid Chromatography II. Silica/Polystyrene

Abstract

Mechanisms of separations performed on bonded phases in liquid chromatography are investigated by studying model grafted phases.

These phases are prepared by reaction of chlorinated silica with living anionic polystyrene of various molecular weights. Owing to thermodynamic predictions, an expression of the elution volume of various solutes on such packings and conditions for reverse-phase chromatography are proposed.     

Ethanol Washing Effect on Textural Properties of the Sodium Silicate-Derived Silica Xerogel

This study deals with the use of ethanol as washing solvent in the preparation of the silica gels from sodium silicate in order to enhance the textural properties, especially surface area. We here examined the effect of ethanol-washing on surface area, micro- and mesopore volume, and average pore size. The silica xerogels prepared from sodium silicate solution exhibited an extremely high surface area of 1139 m2/g by washing their hydrogels with ethanol. Compared to water-washed xerogels, ethanol-washed xerogels showed higher surface areas, total pore volumes, and larger average pore sizes. Unlike the surface area of water-washed xerogel, that of the ethanol-washed xerogel was not affected by the silica concentration of initial solution. This study indicates that the textural properties of sodium silicate-derived xerogels are further enhanced by using ethanol as washing solvent.     

高效液相色谱快速分离生物大分子的大孔硅质固定相的研制及应用

介绍了不同孔径的大孔硅胶基质的制备、二醇固定相的合成及其对蛋白质的色谱分离；考察了焙烧温度对硅胶孔隙度的影响；探讨了流动相中盐浓度和ｐＨ对蛋白质保留时间、柱效、分离度的影响以及硅胶孔径对键合密度和分离性能的影响。该种大孔固定相相对于常规多孔固定相具有小的比表面积、柱容量低、从而有利于生物大分子的微量分离分析。     

Long-Chain Alkylsilane-Modified Porous Silica Beads as GPC Packings

Abstract

A series of silica gels having various molecular weight exclusion limits has been modified by reaction with octadecyltrichlorosilane. Toluene being used as moving phase and hydroxyl-end blocked polydimethylsiloxane as solute, the chromatographic process on these modified silica packings is mainly controlled by size exclusion; an additional absorptive retention can still be observed with the low molecular species. Thus, calibration in this case should be performed using standard samples of the specific polymer to be determined.     

Experimental Studies with a Bonded N-acetylaminopropylsilica Stationary Phase for the Aqueous High Performance Exclusion Chromatogrpahy of Polypeptides and Proteins

Abstract

The potential of the micoparticulate, chemically bonded N-acetylaminopropylsilica stationary phase of nominal pore diameter of 100 angstroms in the high speed gel permeation chromatography of polypeptides and small proteins has been further investigated. The influence of ionic strength on the elution behaviour of a selected group of polypeptides and proteins on this bonded hydrophilic support has been examined. The results obtained with this porous, microparticulate bonded ‘amide’ phase silica support, packed into standard analytical-size stainless steel HPLC columns, indicate that milligram quantities of polypeptides and protein swith molecular mass up to 45,000-50,000 daltons can be efficiently fractionated with excellent recoveries of biological activities. The role of silica-based sorbents in the gel permeation fractionation of polypeptide and protein hormones, including those of pituitary and hypothalamic origin, is discussed.     

Liquid Chromatographic Behavior of Nitrogen Compounds

Abstract

Chromatographic behavior of nitrogen compounds differed from others. The column efficiency was poor for the compounds and sometimes solutes were not eluted out from a column. Therefore, the elution volume of alkylamines, anilines, pyridines, pyrazines, quinolines and aminopolyaromatic hydrocarbons was measured on a methacrylate gel and octadecyl bonded silica gels in pH controlled acetonitrile/water mixtures. The solvent effect on the dissociation constant differs from that obtained for aromatic acids. The values in acetonitrile/water mixtures are smaller than those obtained in 100% water. The linear relation between log P and log k′ values is obtained in eluents of pH 7 where the retention of these compounds is maximized. Some hydrophobic fragmental constants are proposed from this result. Prediction of retention time of these compounds from their log P values can be done in the individual groups on octadecyl bonded silica gels in pH controlled acetonitrile/water mixtures.     

Dynamics and isotherms of water vapor sorption on mesoporous silica gels modified by different salts

The mesoporous silica gels impregnated with different metal salts were prepared and studied. The pore structure and specific surface area of adsorbents were evaluated using nitrogen adsorption. Then, the sorption isotherms and dynamics of water vapor were carried out at 303 K and different relative humidity (RH). The temperature programmed desorption experiments were conducted to estimate the activation energy (E _d) of water desorption on the silica gels. The results showed that the sorption capacity for water decreased with the increase of the ionic radius (except the calcium ion) and that CaCl₂ and LiCl were particularly suitable for use in modification of the mesoporous silica gel to improve their sorption rates and capacities for water vapor at the lower and medium RH (RH < 80%). The larger the average pore diameter and pore volume of the initial silica gels, higher the accrual rates of the water vapor sorption rate and capacity were after modification with hygroscopic salts. The activation energy of the water desorption on the mesoporous silica gel modified by CaCl₂ were much higher than that on the silica gel modified by LiCl, because the polarizability of the Ca²⁺ was higher than that of Li⁺.     

CP/MAS NMR investigations of silica gel surfaces modified with aminopropylsilane

Summary Aminopropyl chemically bonded phases for high performance liquid chromatography (HPLC) have been prepared using mono- and trifunctional methoxyor ethoxysilanes. Three types of silica gel with different surface characteristics were used as support for the chemically bonded phases (CBPs). Surface characteristics of the packings before and after chemical modification were determined by porosity parameters, elemental analysis and CP/MAS NMR spectroscopy.²⁹Si and¹³C CP/MAS NMR investigations gave informations about different interactions between aminosilyl ligands and/or these ligands and/or water molecules condensed in the pores of the silica gel surface. With decreasing pore diameter of the silica gel the proportion of protonated aminopropyl ligand increases.     

Column packings for high-performance liquid chromatography and positron annihilation lifetime spectroscopy

Positron annihilation measurements as a function of temperature and the length of bonded alkyl groups have been carried out on silica gel samples. Silica gel samples were bare and bonded with alkyl group from C1 up to C18. The diameters of pores were deduced from the lifetime of trapped ortho-positronium (o-Ps), and it was found that o-Ps lifetime provides reasonable information on the pore sizes for both bare and alkyl bonded silica gels.     

Effect of the structure and density of chemically bonded C18 phase on the recovery of tryptophan and its metabolites from human urine

A series of materials with chemically bonded C18 phase for use as the packings in clean-up columns for solid-phase extraction were prepared. The effects of the monomeric and/or polymeric structure of the chemically bonded phase and of the porous structure of the silica gel support on the recovery of tryptophan and two of its metabolites used as test substances were considered. It appeared that the best recoveries of at least 60% of the three test substances were obtained on material of the "monomer" type containing chemically bonded C18 phase characterized by a high coverage density of alpha RP approximately 3.8 mumol/m2. The use of a silica gel support with a larger pore size and volume permits not only the effective isolation of individual substances, e.g., from urine, but also their 5-fold concentration.     

Characterization of Amino-, Cyano-alkyl Bonded Silica Columns in Normal-Phase Liquid Chromatography by Using Steroids

Abstract

In order to characterize the chemically bonded phases in HPLC analysis, the retention behavior of fifteen steroids including estrogen, androgen, progestogen and corticoid were systematically examined using dioxan as the stronger component in an n-hexane-binary system. A linear relationship between the logarithm of the capacity ratio and logarithm of the molar concentration of the binary solvent was confirmed for amino- and cyano-type bonded as well as non-bonded silica gel columns. Based on the retention indices of these phases, the retentivity of the packing materials was determined as follows: the amino-type is similar to and the cyano-type is weaker (0.7 times) than bare silica gel when using dioxan as the stronger component. The specific retentivity of an amino column for polar steroids containing phenolic and alcoholic hydroxyl groups suggests a molecular interaction associated with hydrogen bonding between the polar packing surface and solute compounds. The selectivity of amino packing was found to be larger than cyano packing whose retention selectivity is similar to a bare silica gel.     

A New Packing for Aqueous Size Exclusion Chromatography Polyvinylpyrrolidone-Coated Silica

Abstract

Adsorption of polyvinylpyrrolidone on silica in aqueous medium is studied. Adsorption isotherms, thermogravimetry and elementary analysis show that a complete surface coverage is reached with large pore microbeads. The coating stability and its ability to prevent non-steric effects when used as packing in exclusion chromatography are demonstrated. Four different pore size packings were prepared for chromatographic evaluations. It is shown that water soluble polymers like polyethylene oxide, polyvinylalcohol, polyacrylamide and hydroxyethylcellulose can be chromatographied in pure water according to the steric exclusion mechanism.     

Influence of the pore structure on the properties of silica based reversed phase packings for LC

This paper describes the preparation and investigation of new, highly loaded, monomeric, silica based, reversed phase C18 and C30 packings. The influence of pore structure and endcapping on the properties of C18 and C30 packings is described. Using hydrothermal procedures, silicas with predictable pore size (9.3-25.5 nm) and surface area have been prepared. Silylation with long chain silanes substantially alters the pore structure of the silica: pore size and pore volume decrease. A new parameter, the volumetric surface coverage [mm3 x m(-2)] has been introduced. This parameter--calculated from on-column measured porosity data--indicates the pore volume portion occupied by the hydrocarbon chains. Endcapping does not significantly change the pore structure of the bonded phases. The reduced retentions (reduced with respect to unit area: [k/m2])--a good measure for comparing the retention behaviour of packings with different surface areas--are similar for most of the phases, demonstrating good accessibility of the pores for the solutes. Slightly lower retentions were found on the endcapped than on the non-endcapped phases for probes with dense pi-electron system (e.g. polyaromatic hydrocarbons) demonstrating the contribution of silanophilic interactions to the retention. The phases had been successfully used for various demanding separations, e.g. for the separation of flavonoids, carotenoids, resveratrol, and tocopherol isomers, fullerenes, and anions.     

Elution Behaviors and Rationalized Calibration for Low-Molecular-Weight Polymers in Size Exclusion Chromatography

Abstract

Molecular weight relationships among oligostyrene, n-hydrocarbon, epoxy resin, p-cresol novolak resin, and oligoethylene glycol having the same retention volume were discussed using SEC gels of different pore sizes in chloroform and tetrahydrofuran. Gel capacity and the maximum number of components resolvable increased with the use of chloroform except the case of epoxy resin. Different elution behaviors of oligomers in different eluents make it difficult to use molar volumes or effective chain lengths as calibration parameters. The influence of the pore size and shape of the gel on the elution order among oligomers was negligible except some cases. Molecular weight conversion equations for several oligomers based on molecular weight of oligostyrene or n-hydrocarbon were derived. These equations make it possible to use oligostyrene or n-hydrocarbon as a reference standard when molecular weights of oligomers are measured.     

Synthesis of mesoporous Stöber silica nanoparticles: the effect of secondary and tertiary alkanolamines

The effect of secondary (diethanolamine) and tertiary (triethanolamine) alkanolamines as catalysts on the formation of mesoporous Stöber silica nanoparticles by sol–gel method was studied. The particles were characterized by thermogravimetry and differential thermal analysis, Fourier transform infrared spectroscopy, N₂ physisorption measurements, and field emission scanning electron microscopy. By using ammonia and different alkanolamines as catalysts, the Brunauer–Emmet–Teller (BET) surface area and pore volume increased in the order of ammonia < diethanolamine < triethanolamine. A maximum BET surface area of 140.1 m² g^?1 and pore volume of 0.66 cm³ g^?1 were obtained from triethanolamine catalyzed silica particles. The average particle size of silica prepared by ammonia and different alkanolamines as catalysts decreased in the order of ammonia > diethanolamine > triethanolamine. The role of different alkanolamines on the textural properties and particle size of silica is explained in terms of their relative steric hindrance and basicity.     

Effect of pore size on the performance of composite adsorbent

Three kinds of commercial silica gels with pore size of 2–3, 4–7 and 8–10 nm respectively are used for preparing composite adsorbents by soaking them into the aqueous solution of calcium chloride. The test result indicates that both the water uptake and adsorption rate of composite adsorbents prepared from 4–7 and 8–10 nm silica gels improve greatly compared to pure silica gels, but they do not for 2–3 nm silica gels. The silica gel with pore size of 2–3 nm is not suitable for preparing the composite adsorbent by impregnation method due to the pore blockage because of the small pore size. The SCP and COP of the adsorption chiller with sample SA50 are 128.3 Wkg^?1 and 0.27 respectively at the hot source temperature of 90 °C, which are largely superior to that of SA0. Hence using the composite adsorbent instead of the pure silica gel can reduce the size of the adsorption chiller.     

Changes in the pore character of silicas caused by surface bonding and polymer coating

The pore character of packings for liquid chromatography, especially reversed-phase (RPLC) packings, has been studied by means of the nitrogen adsorption method (BET method). Micro-spherical silica gels with 9, 12, 30, 40 and 50 nm average pore diameter have been used as carriers. These silica gels have been modified with monochlorodimethyloctadecylsilane and several polymers (polyoctadecylmethacrylate-methylmethacrylate co-polymer, polyacrylamide gel, polyvinyl alcohol, poly-2-hydroxyethyl-methacrylate). A larger decrease in the specific surface area values was observed in the case of the polymer coating with the polyoctadecylmethacrylate-methylmethacrylate co-polymer compared with the derivatization by silanes or the modification with polymers without C18-groups. A new approach has been suggested to explain some questions concerning the interpretation of the data obtained during the measurements of the pore characteristics of the derivatized packings. An attempt has been made to reveal peculiarities connecting values of the measured surface of RP-packings with the specific surface area values of the initial silica, as well as with the chromatographically accessible surface.