Sorption recovery of metal ions using silica gel modified with salicylaldoxime

Trace metals in water were preconcentrated with silica gel modified with salicylaldoxime and determined by AAS. Optimum conditions for the maximum recovery of metal ions, viz. Cu(II), Ni(II), Co(II), Zn(II) and Fe(III), for both batch and column methods were developed. The efficiency of the adsorbent with respect to different experimental conditions was established.     

Ion-exclusion chromatography of carboxylic acids on silica gel modified with aluminium

The modification of silica gel with aluminium by a coating method was effective for the preparation of a silica-based stationary phase, which acted as a cation exchanger under strongly acidic conditions. In order to expand the utility of the laboratory-made aluminium-adsorbing silica gel it was applied as a stationary phase to the ion-exclusion chromatography of various carboxylic acids. Good separations for both aliphatic carboxylic acids and benzenecarboxylic acids with a hydrophobic nature under acidic eluent conditions were achieved in 25 min.     

Extraction of copper ions using silica gel with chemically modified surface

The applicability of silica gels for the application in solid-phase extraction was tested. Silica was modified with ketoimine groups. Surface characteristics of the modified silica were determined by elemental analysis, NMR spectra of solid phases (²⁹Si CP MAS NMR), analysis of pore size distribution of the silica support, and nitrogen adsorption-desorption. Newly proposed sorbents with ketoimine groups were applied in the preconcentration of trace amounts of the Cu (II) ions from lake water, post-industrial water, and demineralized water unburdened back to the lake. Presented at the 34th International Conference of the Slovak Society of Chemical Engineering, Tatranské Matliare, 21–25 May 2007.     

Adsorption of midmolecular peptides by fullerene modified silica gel

The adsorption properties of raw and processed silica gels and of silica gels modified with fullerene (fullerene-silica gel nanosystems) toward serum midmolecular peptides of ischemic origin were examined. A high adsorption capacity of the nanosorbents was found. The influence of fullerenes on the condition of silica gel was studied by solid-state NMR, and an increase in the silanol fraction at the adsorbent surface was demonstrated. It was shown that presence of fullerene C₆₀ in nanosorbents enables oxidation reaction of the adsorbed peptides, i.e., sorption of metabolites.     

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.     

Microcolumn liquid chromatography with silica gel dynamically modified with a bile acid derivative

Summary Characteristics of silica gel dynamically modified with a bile acid derivative have been investigated in liquid chromatography. 3-[(3-cholamidopropyl) dimethylamonio]-1-propane-sulfonate (CHAPS) is selected as modifier. CHAPS possesses a quaternary ammonium group which can be attached to the silica gel surface by cation exchange. The stationary phase modified with CHAPS has been applied to the separation of 1,1-binaphthyl-2,2-diyl-hydrogen phosphate enantiomers.Dedicated to Professor Leslie S. Ettre on the occasion of his 70th birthday.     

Chromatographic properties of mesoporous silica gel modified with acetylacetonates of europium and copper

New mesoporous materials prepared from tetraethoxysilane, i.e., sorbents obtained with the use of surfactants, are synthesized. Their chromatographic polarity is assessed quantitatively. Their Kovats retention indices and Rohrschneider polarity coefficients are calculated. Based on physicohemical data, it is demonstrated that modifying supports with metal chelates changes the structure and properties of the initial sorbents: the differential molar Gibbs free energies rise considerably for sorbent modified with copper acetylacetonate with respect to ethanol (?ΔG′ _i = 7.09 kJ/mol) and nitromethane (?ΔG′ _i = 6.75 kJ/mol), relative to the initial silica gel (5.43 and 4.15 kJ/mol, respectively).     

Regularities of the retention of aminopyridines by silica gel modified with gold nanoparticles

New sorbents based on silica gel and gold nanoparticles stabilized by L-cysteine and its methyl ester are synthesized. Regularities of the retention of various substituted aminopyridines by the synthesized sorbents are examined in the normal-phase mode of high-performance liquid chromatography upon elution by a binary mobile phase (hexane-isopropanol). Quantum-chemical simulations of L-cysteine and its derivative adsorptions on the surface of a gold cluster are performed, along with simulations of subsequent adsorptions of substituted pyridines on the modified gold surface. It is shown that the Snyder-Soczewinski and Scott-Kucera displacement models can be used to describe the experimental data on the synthesized sorbents.     

Surface properties of silica gel samples modified by selected proteins

The adsorption of a polar (water) and a non-polar (n-octane) liquid on silica gels, modified by adsorption of proteins, has been studied by thermal analysis. Silica gels with physically adsorbed BSA and ovalbumin layers were used. Thermodesorption energies were determined from Q-TG and Q-DTG curves recorded under quasi-equilibrium conditions. Significant differences in liquid desorption were observed from the surfaces due to heterogeneous changes (energetic and geometrical) as a result of modification. These results are compared with those obtained for the samples heated at 160°C for 1 h.     

Electrochemical behavior of pyrroloquinoline quinone immobilized on silica gel modified with zirconium oxide

Pyrroloquinoline quinone (PQQ) was immobilized on the silica gel surface modified with zirconium oxide, designated as Si:Zr, by the carboxylic groups of the PQQ molecule and the zirconium oxide on the silica surface. The electrochemistry of PQQ immobilized on the Si:Zr matrix, incorporated in a carbon paste electrode, was evaluated using the cyclic voltammetry technique. The Si:Zr:PQQ-modified electrode showed a redox couple at E(m)=(E(pa1)+E(pc))/2=-0.150 V vs SCE at pH 7, close to that observed in aqueous solution, and another oxidation peak, E(pa2)=-0.100 V vs SCE. Studies in different pH solutions in the range of 3-7 showed that the first oxidation peak, E(pa1), is highly dependent on the solution pH shifting from to -0.175 to 0.100 V vs SCE, while E(pa2) remains practically constant at 0.100 V as the pH decreases from 7 to 3. The immobilized PQQ electrode presented the property to electrocatalyze the NADH at 150 mV vs SCE. The effect of addition of Ca(2+) ions on the electrode electroactivity for the NADH oxidation was also verified. Different from that observed for the PQQ immobilized on other electrode materials, the Ca(2+) ions did not influence the electrocatalytical response; however, the electrode stability was considerably improved in the presence of Ca(2+) ions, indicating that the matrix surface has a great influence on the electrochemical behavior of PQQ.     

Low-temperature sorption-luminescence determination of silver using silica gel chemically modified with mercaptopropyl groups

Silica gel chemically modified with mercaptopropyl groups extracts silver(I) from solutions in the acidity ranges from 4 M HNO₃ to pH 8 and from 0.01 M HCl to pH 7. The time it takes to attain a sorption equilibrium is no more than 2 min; distribution coefficient is 1 × 10⁵ cm³/g. Yellow-orange luminescence was produced in the adsorbent phase upon the UV irradiation of wet adsorbates cooled to the liquid-nitrogen temperature (77 K). This effect was used for the development of a procedure for the low-temperature sorption-luminescence determination of silver. The calibration graph was linear in the range 1–80 µg Ag per 0.1 g of adsorbent. The detection limit was 0.05 µg per 0.02 g of adsorbent. The relative standard deviation was no more than 6% for silver amounts higher than 5 µg.__________Translated from Zhurnal Analiticheskoi Khimii, Vol. 60, No. 4, 2005, pp. 390–393.Original Russian Text Copyright © 2005 by Losev, Elsuf ev, Trofimchuk.     

Chemisorption of vapors of organic substances on a modified silica gel

Experimental data on the equilibrium and dynamics of chemisorption of vapors of organic substances on a silica gel modified with color reagents are presented.     

Spectroscopic studies of the intermolecular interactions of Congo red and tinopal CBS with modified cellulose fibers

The adsorption of Congo red and tinopal CBS dyes on cellulose fibers was investigated using electronic absorption and fluorescence spectroscopies. Hydrogen bonds appear to be relevant for the dye-fiber interactions as indicated by the solvatochromism of Congo red in water, methanol, and dimethyl sulfoxide solutions, and when adsorbed on cellulose fibers. We also demonstrate that electrostatic interactions play an important role in the dye-medium interaction, through the analysis of absorption spectra of Congo red and fluorescence spectra of tinopal in aqueous solutions containing salt and in layer-by-layer nanostructured films with poly(allylamine hydrochloride). For instance, dye adsorption was enhanced when salt was added to the dipping solution, which was explained by the synergistic effect between the conformational changes of the cellulose and changes in the solvation layer around the cellulose chains and around dye molecules. On the basis of the fluorescence results for tinopal CBS, we inferred that dye aggregation is not relevant for adsorption on the fibers. In addition, fluorescence spectroscopy is proven very sensitive for studying the organization of dye molecules in layer-by-layer films, particularly those undergoing irreversible structural changes.     

Sorption and preconcentration of copper and cadmium on silica gel modified with 3-aminopropyltriethoxysilane

3-Aminopropyltriethoxysilane, (C₂H₅O)₃ Si(CH₂)₃NH₂, loaded on silica gel was used as a pre-concentration sorbent for copper and cadmium prior to their determination by flame atomic absorption spectrometry (FAAS). Both batch and column methods were used for the separation of the above metals. The analytes are quantitatively retained on the proposed adsorbent at pH 6.5. The complexation capacity of the collector is 0.032 mmol Cu/g silica. In the batch method, the effects of shaking time and the ratio of metal/silica on the retention by the asorbent were investigated. Columns filled with the collector provided quantitative recovery of the above metals from standardized samples as well as from sodium chloride solutions.     

The hydroxyl content of silica gel

Thermogravimetric analysis of silica gel has shown that the loss in weight between 30° and 910°C can be quantitatively explained on the basis of water being lost from three distinct and different populations of sites on the silica gel surface. The results indicate that the site energies of the three different populations are randomly distributed and, consequently, the resulting weight loss steps from each population can be described by the integral of a simple normal distribution with temperature. The calculated weight loss obtained by assuming three different site-groups having randomly distributed adsorption energies is, within experimental error, coincident with the experimental data. It is also shown that the water evolved from the second population of sites originates from strongly bound water and may also contain water generated by the condensation of (geminal) silanol groups contained in the overlapping and neighbouring population.     

Tetracyanoquinodimethanide adsorbed on a silica gel modified with titanium oxide for electrocatalytic oxidation of hydrazine

The electrocatalytical oxidation of hydrazine at low potential using tetracyanoquinodimethanide adsorbed on silica modified with titanium oxide was investigated by cyclic voltammetry and amperometry. The modified electrode was prepared modifying a carbon paste electrode employing lithium tetracyanoquinodimethanide adsorbed onto silica gel modified with titanium oxide. This electrode showed an excellent catalytic activity and stability for hydrazine oxidation. With this modified electrode, the oxidation potential of hydrazine was shifted toward less positive value, presenting a peak current much higher than those observed on a bare GC electrode. The linear response range, sensitivity and detection limit were, respectively, 2 up to 100 μmol l⁻¹, 0.36 μA l μmol⁻¹, and 0.60 μmol l⁻¹. The repeatability of the modified electrode evaluated in term of relative standard deviation was 4.2% for 10 measurements of 100 μmol l⁻¹ hydrazine solution. The number of electrons involved in hydrazine oxidation (4), the heterogenous electron transfer rate constant (1.08 × 10³ mol⁻¹ l s⁻¹), and diffusion coefficient (5.9 × 10⁻⁶ cm² s⁻¹) were evaluated with a rotating disk electrode.     

HPLC separation of nitrogen-containing compounds on silica gel modified with gold nanoparticles stabilized by chitosan

The potentialities of using silica gel modified with gold nanoparticles stabilized by chitosan are evaluated in the normal phase of HPLC as an example of the separation of aniline and pyridine derivatives, as well as triazole fungicides by elution with mixtures of hexane with isopropyl alcohol, isobutanol, or chloroform. The influence of the nature and content of the polar additive on the efficiency and separation selectivity of the column have been studied. The possibility of separating mixtures containing penconazole, propiconazole, diniconazole, difenoconazole, and aminopyridines is shown.     

Thin-layer electrophoresis of hydroxyethyl starches on a modified silica gel support.

The recent developed thin-layer electrophoresis on modified silanized silica gel was applied to the separation of hydroxyethyl starches (HES) and glycogen and of HES with different degrees of substitution. This method permits a rapid qualitative and semi-quantitative determination of HES in animal tissues such as liver, lung, heart and kidney after their disintegration with alkali and precipitation with ethanol.     

Polyaniline-coated silica gel

Silica gel microspheres 7 and 15 μm in diameter were coated with an overlayer of polyaniline camphorsulfonate or hydrochloride during the oxidative polymerization of aniline. Coated silica gel and polyaniline precipitate were separated using a difference in sedimentation rate. In an alternative approach, the microspheres were modified with polyaniline in the presence of 35 nm colloidal silica. This technique prevented the macroscopic precipitation of polyaniline. Coatings of neat, 3-aminopropyl- and octadecyl-modified silica gel with polyaniline hydrochloride were compared. The surface composition of coated microspheres was characterized by X-ray photoelectron spectroscopy. Potential applications of particles in electrorheology, organic catalysis, and in modeling of conductivity behavior in composites are demonstrated.