Determination of impurities in propranolol hydrochloride by high-performance liquid chromatography on dynamically modified silica

A rapid high-performance liquid chromatographic method has been elaborated for the separation and determination of small amounts of impurities in propranolol hydrochloride. The separation was achieved on a column of bare silica (Zorbax SIL) with methanol-water-0.2 M phosphate buffer pH 8.0 (70:25:5) containing 2.5 mM of cetyltrimethylammonium (CTMA) bromide as the eluent. The concentrations of methanol and CTMA as well as the pH of the phosphate buffer were found greatly to affect the separation. The selectivity of the system towards a mixture of propranolol and three possible impurities was investigated using different brands of silica. Only minor variations were found relative to those of a chromatographic system based on chemically bonded ODS silicas from the same sources. The method is also suitable for identification purposes, being able to separate most beta-blocking drugs of structures similar to that of propranolol.     

Ketoimino groups as silica surface modifiers

The presented work is devoted to Porasil C silica, with organic compounds bonded to its surface and capable of electron-donor-acceptor (EDA) interactions. These packings are a good base for studying interactions among stationary phases and the adsorbate molecules showing electron-donor properties. The presented work concentrates on the phases containing ketoimino groups at their surface. Copper and chromium chlorides were bonded through these to the surface. Physicochemical characteristics of the obtained packings were determined by the use of elemental analysis, differential scanning calorimetry (DSC) and inverse gas chromatography. We examined the influence of the surface modification on the retention parameters of the nucleophilic compounds.     

Synthesis of lamellar mesoporous silica materials using LCs as templates

The study focuses on the synthesis of mesoporous silica materials using liquid crystals (LCs) formed in an aqueous mixture of cationic cetyltrimethylammonium bromide (CTAB) and anionic sodium dodecyl sulfate (SDS) as templates and tetrathoxysilane (TEOS) as precursor. For this purpose, the phase behavior and range of LC areas were determined at different temperatures, concentrations, and ratios of CTAB/SDS. It was found that LCs became denser with the increased of concentration of surfactants. The mesoporous materials were synthesized using LCs as templates at various temperatures, surfactant concentrations, and pH values. The mesoporous samples were characterized using SEM and nitrogen sorption analysis. The research results showed that the structure of synthesized samples were lamellar and their surface areas increased significantly with the increase of temperature in the temperature range of LCs, reaching about 900?m²/g at 60°C. The surfactant concentrations affect the thickness of pore wall and thereby the specific surface area of products. The specific surface area and the order of mesoporous sample increased gradually with the decrease of pH.     

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.     

High-performance liquid chromatography on dynamically modified silica : III. Modification of silica with long-chain and symmetrical quarternary ammonium compounds

The influence of the nature of quaternary ammonium compounds on retention in high-performance liquid chromatography on dynamically modified silica was investigated. Adsorption isotherms were determined on bare silica (LiChrosorb Si 60) for four alkyltrimethylammonium bromides and two symmetrical tetraalkylammonium bromides, each containing 15–21 carbon atoms. It was found that only the long-chain quaternary ammonium ions are adsorbed on to the silica surface in appreciable amounts and that the affinity increasing number of carbon atoms in the alkyl chain. The maximum amount that can be adsorbed per gram of silica is of the same order of magnitude for each of the four long-chain quaternary ammonium compounds. This amount, however, is reached at lower concentrations in the eluent the longer is the alkyl chain.The retention of five test compounds was determined over the whole concentration range investigated for each of the modiflying agents. The surfactant concentration that causes maximum retention for four of five test compounds coincides with its critical micellar concentration. The retention mechanisms and the influence of the type of the modifying agent on selectivity are discussed, and compared with published results on related experiments on chemically bonded stationary phases.     

Foaming in aqueous solutions of hexadecyltrimethylammonium bromide and silica nanoparticles: Measurement and analysis of rheological and interfacial properties

The properties of aqueous foams stabilized by a mixture of negatively charged silica nanoparticles and hexadecyltrimethylammonium bromide were studied in this work. Rheological properties of the foams were studied. The interaction between nanoparticles and surfactant molecules in the bulk phase was studied by zeta potential and size measurements of the particles. The interaction at the interface was studied by means of interfacial shear rheology, surface pressure measurement, and atomic force microscopy. It was found that foams were more stable at low surfactant concentrations, though the foamability was low. This was due to the formation of a strong viscoelastic film of surfactant-laden particles at the air–water interface. A suitable mechanism has been proposed to explain the stability of foams in the presence of nanoparticles at different surfactant concentrations.     

Photografting of polymers onto nanosized silica surface initiated by eosin moieties immobilized onto the surface

The photograft polymerization of various vinyl monomers onto nanosized silica surfaces was investigated. It was initiated by eosin moieties introduced onto the silica surface. The preparation of the silica with eosin moieties was achieved by the reaction of eosin with benzyl chloride groups on the silica surface.These were introduced by the reaction of surface silanol groups with 4‐(chloromethyl)phenyltrimethoxysilane in the presence of t‐butyl ammonium bromide as a phase‐transfer catalyst. The photopolymerization of various vinyl monomers, such as styrene, acrylamide, acrylic acid, and acrylonitrile was successfully initiated by eosin moieties on the silica surface in the presence of ascorbic acid as a reducing agent and by oxygen. The corresponding polymers were grafted from the silica surface. The grafting efficiency (percentage of grafted polymer to total polymer formed) in the photoinitiation system was much larger than that in the radical polymerization initiated by surface radicals; these radicals were formed by the thermal decomposition of azo groups introduced onto the silica surface. It was found that the polymer‐grafted silica gave stable dispersions in good solvents of grafted polymer and the wettability of the surfaces can be easily controlled by grafting of polymers. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 600–606, 2005     

Complexes of transition metals bonded to silica via β-diketonate groups — synthesis, structure, and catalytic activity

Transition metal complexes bonded to silica via silanes with β-diketonate groups can be used as packings for complexation gas chromatography or as immobilized homogenous metal complex catalysts. On basis of elemental analysis and the determination of surface area, possible structures of the complexes formed on the silica surface have been proposed. The possibility of using the immobilized complexes as catalysts has been indicated. Especially nickel complexes were taken into consideration. These immobilized complexes were used previously as packings for complexation gas chromatography.     

Active centers on the surface of thermoactivated silica. Their reactivity toward organoaluminum compounds

Interaction between organoaluminum compounds (triethylaluminum, ethoxydiethylaluminum, and diethoxyethylaluminum) and the surface of silica activated at various temperatures (200–800 °C) was studied by IR spectroscopy, mass spectrometry, and quantum chemistry. Formation of structural silanon defects on the surface of silica activated at 800 °C was considered. It was established that the fraction of terminal silanol groups involved in the interaction with organoaluminum compounds on the surface of silica thermoactivated above 600 °C is low, and siloxane bonds and structural surface defects play a determining role. The thermodynamic favorability of coordination of organoaluminum compounds on these active surface centers is shown. The structure and routes of decomposition of aluminum-containing surface intermediates were studied.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1906–1911, October, 1995.The work was financially supported by the Russian Foundation for Basic Research (Project No. 94-03-08847).     

Interaction between thallium(III) bromide and dicyclohexano-18-crown-6 immobilized on silica

The adsorption of cis-syn-cis-dicyclohexano-18-crown-6 on silica gel from organic solvents was studied. In the adsorption from chloroform solutions, the immobilization of the reagent molecules at the surface was coplanar. Hydrogen bonds between surface silanol groups and oxygen atoms of the polyether ring of the crown ether play an important role in the immobilization of the reagent. The reaction between thallium(III) bromide and the immobilized reagent was studied. The composition of complex compounds formed at the surface of the modified adsorbent was found. A procedure was proposed for the sorption-spectrophotometric determination of thallium(III) (≥0.05 mg/L) in the presence of heavy metal ions.     

Zirconyl-containing microspheric silica gel surfaces

The adsorption properties of two samples of zirconyl-containing silica gels derived from zirconium oxychloride, polyetoxysiloxane oligomer 3% ZrOCl₂/SiO₂ (composite 1) and tetraetoxysilane 5% ZrOCl₂/SiO₂ (composite 2) were investigated by gas chromatography at low surface coverages. n-Alkanes and n-alkenes (C₆–C₈), C₆H₆ were used as test adsorbates, along with polar compounds whose molecules had different donor-acceptor interaction abilities. The dispersion and specific (electron-donor and electronacceptor) components of the energy of intermolecular interactions for the studied systems were determined from the experimental data on chromatographic retention. It was shown that composite 2 had a higher dispersion potential and higher surface energy characteristics of the surface’s electron-donating and electronacceptor centers, as compared to composite 1.     

Preparation and evaluation of zirconia-coated silica monolith for capillary electrochromatography

Silica monoliths were fabricated inside fused-silica capillaries. Then the monolithic columns were coated with membrane-like zirconia. The zirconia-coated silica monoliths exhibited different EOF behavior comparing with that of bare silica monoliths. The magnitude and direction could be manipulated by changing the running buffers. Due to the amphoteric characteristic of zirconia, the silica monoliths with zirconia surface facilitate the separation of basic compounds. Aromatic amines and alkaloids were separated without obvious peak tailing. The zirconia surface was easily modified with octadecylphosphonic acid for the separation of neutral compounds. Column efficiency as high as 90,000 and 80,000 m⁻¹ was obtained for beberine and naphthalene, respectively. Furthermore, the zirconia coating increased the stability of the monolithic columns. Even after being exposed to severe condition, there was no apparently efficiency decrease for the test samples.     

The influence of mixed cationic–anionic surfactants on the three-phase contact parameters in silica–solution systems

The formation of thin wetting films on silica surface from aqueous solution of (a) tetradecyltrimetilammonium bromide (C₁₄TAB) and (b) surfactant mixture of the cationic C₁₄TAB with the anionic sodium alkyl- (straight chain C₁₂–, C₁₄– and C₁₆–) sulfonates, was studied using the microscopic thin wetting film method developed by Platikanov. Film lifetimes, three-phase contact (TPC) expansion rates, receding contact angles and surface tension were measured. It was found that the mixed surfactants caused lower contact angles, lower rates of the thin aqueous film rupture and longer film lifetimes, as compared to the pure C₁₄TAB. This behavior was explained by the strong initial adsorption of interfacial complexes from the mixed surfactant system at the air/solution interface, followed by adsorption at the silica interface. The formation of the interfacial complexes at the air/solution interface was proved by means of the surface tension data. It was also shown, that the chain length compatibility between the anionic and cationic surfactants controls the strength of the interfacial complex and causes synergistic lowering in the surface tension. The film rupture mechanism was explained by the heterocoagulation mechanism between the positively charged air/solution interface and the solution/silica interface, which remained negatively charged.     

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.     

Synthesis of nanoporous silica aerogel by ambient pressure drying

A crack-free silica aerogel monolith was fabricated from a cheap water glass derived silicic acid solution by adding glycerol, which served as a drying control chemical additive (DCCA). The OH surfaces of the wet gel with glycerol were modified using a TMCS/n-hexane mixture followed by solvent exchange from water to n-hexane. The obtained surface modified wet gel was dried at 75 °C under ambient pressure. The addition of glycerol appears to give the wet gel a more homogeneous microstructure (larger pore size and uniform size distribution) as well as enhanced stiffness. However, glycerol also retards surface modification and solvent exchange. The aerogel synthesized with glycerol added to the silica sol maintained a relatively low bulk density compared with the aerogels aged in a mixed ethanol (EtOH)/TEOS solution. The reproducibility of aerogel production was further improved in the aerogel synthesized with glycerol added to the silica sol and aged in a 70%EtOH/30%TEOS solution.     

ESR probe study of acid sites on the silica surface modified by organotin compounds

Bronsted acid sites formed due to the interaction of coordinately unsaturated tin atoms with water on the silica surface modified by organotin compounds were found and studied using an ESR spin probe technique. The half-life period of a probe at room temperature can serve as an additional characteristic of the acidity of the surface.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 144–146, January, 2005.     

High-performance ligand-exchange chromatography of amino acids on chiral stationary phases

Three chiral stationary phases, obtained by grafting silica gel with (-)-trans-1,2-cyclohexanediamine, were studied for the resolution of α-amino acids by ligand-exchange chromatography. The packings were prepared by bonding the chiral ligand to silica gel via different hydrocarbon spacers. Separation of the optical isomers was accomplished by eluents containing a constant concentration of copper(II) acetate (0.05mM). The elution sequence of amino acids was found to be dependent on the grafting reaction selected to prepare the chiral packings.     

Effect of fly ash silica and precipitated silica fillers on the viscosity,cure, and viscoelastic properties of natural rubber

The viscosity, cure properties, storage, and loss moduli and tan δ of natural rubber (NR) filled with the same amounts of precipitated silica (PSi) and fly ash silica (FASi) fillers were measured. The fillers were treated with bis[3‐triethoxysilylpropyl‐]tetrasulfide (TESPT), or, used in the rubber untreated. TESPT is a sulfur‐containing bi‐functional organosilane that chemically adheres silica to rubber and also prevents silica from interfering with the reaction mechanism of sulfur cure. The dispersion of PSi and FASi in the rubber was investigated using scanning electron microscope (SEM). The effects of silica type and loading and surface treatment on the aforementioned properties were of interest. The SEM results showed that the FASi particles were larger in size and had a wider particle size distribution when compared with the PSi particles. The viscosity of the compounds decreased progressively with mixing time, and the compounds with FASi had a lower viscosity than those filled with PSi. The treatment with Si69 had no beneficial effect on the dispersion of the fillers in the rubber matrix. At low temperatures, the type and loading of the filler had no effect on the storage and loss moduli of the compounds, but the effect was more pronounced at high temperatures. There was also evidence from the tan δ and glass transition temperature (T_g) measurements that some limited interaction between the filler particles and rubber had occurred because of TESPT. Copyright © 2008 John Wiley & Sons, Ltd.     

Study of the organic carbon content of silica gel carbonised by pyrolysis of alcohols

Silica gel partially carbonised by pyrolysis of a mixture of C₇ aliphatic and aromatic alcohols at 773 K for 6 h was studied by means of transmission electron microscopy, temperature-programmed desorption (TPD), reaction gas chromatography, and quantum chemical methods. Pyrolysis of n-heptanol and benzyl alcohol at the silica gel surface results in formation of the carbon deposit (globules of 100–500 nm on the outer surfaces of silica gel and smaller carbon particles distributes on the external and internal surfaces of the oxide matrix) and grafted different organic compounds, which begin to desorb at relatively low temperatures (350–500 K) and activation energies of ≈70–120 kJ mol⁻¹. These compounds are not entirely removed from the surface on washing in solvents and subsequent drying at 470 K, as they can be packed in the pyrocarbon deposit between graphene sheets or in silica gel pores partially filled or blocked by pyrocarbon. Some of these compounds have also the second high-temperature TPD maximum at T close to the temperature of alcohol pyrolysis.     

Anion exchange silica monolith for capillary liquid chromatography

An anion exchange monolithic silica capillary column was prepared by surface modification of a hybrid monolithic silica capillary column prepared from a mixture of tetramethoxysilane (TMOS) and methyltrimethoxysilane (MTMS). The surface modification was carried out by on-column copolymerization of N-[3-(dimethylamino)propyl]acrylamide methyl chloride-quaternary salt (DMAPAA-Q) with 3-methacryloxypropyl moieties bonded as an anchor to the silica surface to form a strong anion exchange stationary phase. The columns were examined for their performance in liquid chromatography (LC) and capillary electrochromatography (CEC) separations of common anions. The ions were separated using 50 mM phosphate buffer at pH 6.6. Evaluation by LC produced an average of 30,000 theoretical plates (33 cm column length) for the inorganic anions and nucleotides. Evaluation by CEC, using the same buffer, produced enhanced chromatographic performance of up to ca. 90,000 theoretical plates and a theoretical plate height of ca. 4 μm. Although reduced efficiency was observed for inorganic anions that were retained a long time, the results of this study highlight the potential utility of the DMAPAA-Q stationary phase for anion separations. Figure Micro-LC performance evaluation of a strong anion exchange silica monolith column, 100H-MOP-DMAPAA-Q, 33 cm in length, with a mobile phase of 50 mM phosphate buffer, pH 2.8; linear velocity: u = 1.8 mm/s; UV-Vis detection at 254 nm. Sample solution (5 mg/mL of each component, 4 mL) was injected in split flow injection mode at a split ratio of ca. 1:1900 with a pump flow rate of 1.5 mL/min