Mixture Of Tetramethoxysilane Research Articles

Cosolvent-free sol–gel synthesis of macroporous silica gels from tetramethoxysilane–tetraethoxysilane mixtures

Tetramethoxysilane (TMOS), tetraethoxysilane (TEOS), and their mixtures were used for the cosolvent-free synthesis of macroporous silica gels as precursors for monolithic silica glasses. The liquid-state 29Si nuclear magnetic resonance (NMR) spectroscopy of precursor solutions indicated that cross-transesterification between TMOS and TEOS was completed in a few hours at 20 °C in the presence of an acid catalyst, whereas it was negligible when the catalyst was absent. In the precursor solutions prepared from TMOS, phase separation occurred after gelation, resulting in translucent gels. In contrast, in the solutions prepared from TEOS or a mixture of TMOS and TEOS at a TMOS mole fraction of 0.8, the phase separation can be induced before gelation, and opaque xerogels were easily obtained without fracture. The average size of macroscopic particles and macroporous structures were uniform over opaque xerogels prepared from TEOS. In contrast, in opaque xerogels prepared from the TMOS-rich mixture of TMOS and TEOS, the average particle size and macroscopic porosity inside them were notably smaller than those of the subsurface, probably because of a large exotherm upon gelation and the resulting temperature gradient in the gelling solutions. Such spatial morphology distribution made the sintering of the opaque gels into clear silica glasses difficult. Opaque gels prepared from TEOS and translucent gels prepared from solutions containing TMOS were transformed to clear silica glasses in high yields of ~99% by sintering in a helium atmosphere at 1050–1350 °C.Graphical abstractPhotographs of xerogels prepared from (left) TEOS or (right) a mixture of TMOS and TEOS with a TMOS molar fraction of 0.8, their SEM images taken at the inside (central) and subsurface (edge) parts, and silica glasses derived from these xerogels. The former xerogel was sintered into a clear glass because of the high uniformity of average particle size and macroscopic porosity, whereas the spatial unevenness of macroscopic morphology made the sintering of the latter one difficult.

Magnetic methylene-based mesoporous organosilica composite-supported IL/Pd: a powerful and highly recoverable catalyst for oxidative coupling of phenols and naphthols

A novel magnetic methylene-based mesoporous organosilica composite-supported IL/Pd complex (Fe3O4@MePMO-IL/Pd) was synthesized and characterized, and its catalytic performance was investigated. The preparation of the Fe3O4@MePMO composite was achieved through coating of Fe3O4 nanoparticles with a mixture of tetramethoxysilane, bis(triethoxysilyl)methane, and (3-chloropropyl)-trimethoxysilane in the presence of cetyltrimethylammonium bromide surfactant. The Fe3O4@MePMO was then modified with alkyl imidazolium ionic liquid and palladium species to deliver the Fe3O4@MePMO-IL/Pd nanocatalyst. This catalyst was characterized using Fourier transform infrared, thermal gravimetric, wide-angle powder X-ray diffraction, low-angle powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, vibrating sample magnetometer, energy-dispersive X-ray, and nitrogen adsorption–desorption analyses. The Fe3O4@MePMO-IL/Pd was effectively used as a highly recoverable and durable catalyst for the selective oxidative coupling of phenols and 2-naphthols under aerobic conditions.

Parallel optimization of a reactive force field for polycondensation of alkoxysilanes.

We have optimized a reactive force field (ReaxFF) in order to model the gelation of alkoxysilanes in bulk precursor solutions. The force field parameter set was refined using a parallelized local search algorithm. Using this approach, each processor is assigned a small list of parameters. At the end of every iteration, all parameters are updated simultaneously after being independently evaluated. In comparison to the serial evaluation of parameters, this results in faster parametrization of ReaxFF, as well as helps to prevent entrapment in local minima. The resulting model is found to reproduce hydrolysis and condensation reaction energies well. By applying the model to the condensation of silicic acid monomers at several temperatures, the activation energy of silane condensation is determined. The expected behavior, a gradual depletion of hydrolyzed silicon and growth of condensed silica clusters is observed over timescales of a few nanoseconds. The new model is also verified by modeling the early stages of clusterization in an alkoxysilane precursor solution. Both hydrolysis and condensation reactions are observed in a system containing a mixture of tetramethoxysilane, methanol, and water.

Reusable ω-transaminase sol–gel catalyst for the preparation of amine enantiomers

Heterogeneous ω-transaminase sol–gel catalysts were prepared and characterized in terms of immobilization degree, loading capacity and catalytic behavior in the kinetic resolution of racemic 1-phenylethylamine (a model compound) with sodium pyruvate in phosphate buffer (pH 7.5). The catalyst obtained when ω-transaminase from Arthrobacter sp. was encapsulated from the aqueous solution of the enzyme, isopropyl alcohol and polyvinyl alcohol in the sol–gel matrices, consisting of the 1:5 mixture of tetramethoxysilane and methyltrialkoxysilane, proved to be optimal including the reuse and storage stabilities of the catalyst. The optimized immobilizate was shown to perform well in the kinetic resolution of four structurally different aromatic primary amines in aqueous DMSO (10, v/v-%). The enzyme preparation showed synthetic potential by enabling the catalyst reuse in five consecutive preparative scale kinetic resolutions using 100mM 1-phenylethylamine in aqueous DMSO (10, v/v-%). It was typical to fresh catalyst preparations that the kinetic resolution tended to exceed 50% before the reaction stopped leaving the (S)-amine unreacted while thereafter in reuse the reactions stopped at 50% conversion as expectable to highly enantioselective reactions.

Ionic liquids--pharmaceutical potential.

1. Nature and scope of bio-active ionic liquids Ionic liquids (ILs) are a recently developed class of materials with the property of being both fully ionic as in a conventional salt but also liquid rather than as the familiar crystalline solid. They have been the subject of much interest in the last year, being the subject of two major international conferences and a number of reviews. The latter have covered: (i) their potential as 'green solvents' (1) (they have virtually zero vapour pressure) (ii) their thermochemistry (2); and (iii) their development and applications (3). One area not addressed in these otherwise very full treatments was the exciting prospects of ILs as agents in pharmacology, an area developing as rapidly as those many applications detailed in refs 1-3. The basic strategy in applying the solvent properties of ILs in pharmaceutical materials has been to develop ILs which contain a pharmaceutically active component as one of the two component ions, i.e. the medication exists in a liquid rather than a solid form, thereby facilitating more controllable and/or faster transport within the subject receiving the medication; an early achievement in this field was that of Rogers et al. (4) who patented the concept of 'Multi-functional ionic liquid compositions'. A further advantage of ILs as opposed to crystalline drugs is that the latter can be prone to polymorphic change on storage to give versions with differing solubilities. An illustrative example of an IL that incorporates pharmaceutical activity is that of 1-methyl-3-butylimidazolium ibuprofenate (BMImlbu), synthesised as shown in Scheme 1 (5). [FORMULA NOT REPRODUCIBLE IN ASCII.] In this example, the pharmaceutically active component is present in the anion but it is also possible to place the activity in the cation, or even in both anion and cation as in lidocaine docusate (6) (Structure 1). [FORMULA NOT REPRODUCIBLE IN ASCII.] Lidocaine in its hydrochloride form is used as a local surface anaesthetic in dentistry whilst the docusate anion acts as an emollient. Combinations of different cations and anions provide a wide range of possible ILs, some such combinations being listed in refs 6 and 7; ILs containing biologically-active cations include antibacterials (8), local anaesthetics, anticholinergics and antifungal agents (9,10), while active anions furnish emollients, anti-acne agents, antibiotics, non-steroidal anti-inflammatory drugs and vitamins, among other materials (6,7). 2. Aspects of drug delivery One aim of developing bio-active ILs is to achieve enhanced drug delivery. It was demonstrated in an early paper (6) that lidocaine docusate, a hydrophobic IL, when compared with lidocaine hydrochloride, exhibited modified solubility, increased thermal stability and significant enhancement in the efficacy of topical analgesia in two different models of mouse antinociception; this was attributed not simply to changes in the delivery system but also to the induction of a different activity mechanism. Subsequent approaches have included attempting to control the release of the drug by encapsulating the bio-active IL in the channels of a silica host using a one-step sol-gel process (5). BMImlbu (Scheme I) was synthesised and then employed in sol-gel synthesis using either pure tetramethoxysilane (TMOS) or mixtures of TMOS and methyltrimethoxysilane (MTMOS) in 75/25 or 50/50 proportions in the presence of dilute HCI; the product was a monolithic ionogel. The loading of the drug in the ionogel was found to be ca 50% by weight. The release kinetics of ibuprofen were determined by high performance liquid chromatography (HPLC) and the results are shown in Figure I. Clearly the encapsulation of the IL form of the drug has a major influence on its release kinetics. The authors surmise that the slower release from the more methylated ionogel is related to the more hydrophobic nature of the cavity walls following the gelation. …

The effect of hydrothermal treatment on column performance for monolithic silica capillary columns

Monolithic silica capillary columns with i.d. 100 μm and monolithic silica rods were prepared with tetramethoxysilane (TMOS) or a mixture of TMOS and metyltrimethoxysilane (MTMS) using different hydrothermal treatments at T = 80 °C or 120 °C. Nitrogen physisorption was applied for the pore characterization of the rods and inverse size exclusion chromatography (ISEC) for that of the capillary columns. Using nitrogen physisorption, it was shown change of pore size and surface area corresponds to that of hydrothermal treatment and silica precursor. The results from ISEC agreed well with those from nitrogen physisorption regarding the pore size distribution (PSD). In addition, the retention factors for hexylbenzene with the ODS-modified capillary columns in methanol/water = 80/20 at T = 30 °C could also support the results from nitrogen physisorption. Furthermore, column efficiency for the columns was evaluated with alkylbenzenes and three kinds of peptides, leucine-enkephalin, angiotensin II, and insulin. Column efficiency for alkylbenzenes was similar independently of the hydrothermal treatment at T = 120 °C. Even for TMOS columns, there was no significant difference in column efficiency for the peptides despite the difference in hydrothermal treatment. In contrast, for hybrid columns, it was possible to confirm the effect on hydrothermal treatment at T = 120 °C resulting in a different column efficiency, especially for insulin. This difference supports the results from both nitrogen physisorption and ISEC, showing the presence of more small pores of ca. 3–6 nm for a hybrid silica without hydrothermal treatment at T = 120 °C. Consequently, the results suggest that hydrothermal treatment for a hybrid column with higher temperature or longer time is necessary, compared to that for a TMOS column, to provide higher column efficiency with increase in molecular size of solute.

Highly spongy hierarchical structured meso-macroporous aluminosilicates with high tetrahedral aluminium content and 3D interconnectivity from a single-source molecular precursor (sec-BuO) 2–Al–O–Si(OEt) 3: Effect of silicon co-reactant

The effect of tetraethoxysilane (TEOS), tetrapropoxysilane (TPOS), tetrabutoxysilane (TBOS) and a mixture of tetramethoxysilane (TMOS) and TEOS as silicon co-reactant on the formation of hierarchically structured meso-macroporous aluminosilicates and the tetrahedral aluminium content in the framework using a single molecular alkoxide precursor, (sec-BuO) 2–Al–O–Si(OEt) 3, has been intensively investigated. The use of alkoxysilane as a co-reactant and highly alkaline media improves the heterocondensation rates between the highly reactive aluminium-alkoxide part of the single molecular precursor and the added alkoxysilanes, and minimizes the cleavage of the intrinsic Al–O–Si linkage. The very unique hierarchical meso-macroporosity was auto-generated by the hydrodynamic flow of solvents released during the rapid hydrolysis and condensation processes of this double alkoxide and the inorganic silica co-reactant. No external structural agent was required to template these porous structures. The particles obtained featured outstanding macrostructure with regular micrometer-sized macrovoids and displaying 3D interconnections. Importantly, the diameter of the micrometer-sized macrovoids found in the final materials and the thickness of the mesoporous walls separating these voids can be tuned by adjusting the reactivity of alkoxysilanes used as co-reactant. Higher reactivity of alkoxysilanes can improve the tetrahedral aluminium content in the meso-macroporous framework and reduce the cleavage of Al–O–Si linkage of the single molecular precursor. These correlations are of primary importance for targeting advanced materials with well defined meso- and macroporosities and tetrahedral aluminium content.

Silica-based monolithic capillary columns—Effect of preparation temperature on separation efficiency

The temperature effects during the sol–gel process and ageing of the silica-based monolith on the structure and separation efficiency of the capillary columns (100 μm i.d., 150 mm) for HPLC separations were studied. The tested columns were synthesized from a mixture of tetramethoxysilane, polyethylene glycol and urea under the acidic conditions. The temperature was varied from 40 °C to 44 °C and formation of bypass channels between the silica mold and the capillary wall was examined. The temperature of 43 °C was estimated as optimal for preparation of efficient silica capillary columns which were subsequently modified by octadecyldimethyl- N, N-diethylaminosilane or covered by poly(octadecyl methacrylate) and tested using standard mixture of alkylbenzenes under the isocratic conditions.

(R)‐(+)‐Binol‐Functionalized Mesoporous Organosilica as a Highly Efficient Pre‐Chiral Catalyst for Asymmetric Catalysis

(R)-(+)-Binol-functionalized chiral periodic mesoporous organosilicas (PMOs) with different framework compositions were successfully synthesized by cocondensation of (R)-2,2'-di(methoxymethyl)oxy-6,6'-di(1-propyltrimethoxysilyl)-1,1'-binaphthyl (BSBinol) with 1,2-bis(trimethoxysilyl)ethane (BTME) and tetramethoxysilane (TMOS) using triblock copolymer P123 as a template in acidic solution. The mixture of BTME and BSBinol can result in a highly ordered mesostructure in an acidic medium but the mesoporous materials synthesized using a mixture of TMOS and BSBinol can only be obtained in a weak acidic buffer solution. All the materials are efficient catalysts (coordinated with Ti) for asymmetric addition of diethylzinc to aldehydes. The chiral PMO with ethane and (R)-(+)-Binol in the framework exhibits an enantioselectivity as high as 90% with a turnover frequency (TOF) of 104 h(-1), which is even higher than the homogeneous (R)-(+)-Binol catalyst (83% ee with TOF of 96 h(-1)) using toluene as solvent under similar conditions. This work demonstrates the positive effect of the rigid pore wall in increasing the enantioselectivity of the chiral PMOs.

Comparison of hydrophobicity studies of silica aerogels using contact angle measurements with water drop method and adsorbed water content measurements made by Karl Fischer’s titration method

Hydrophobic silica aerogels possesses potential applications as insulating materials for refrigerators, furnaces and thermos flasks. In such applications, aerogel materials may get exposed for longer time to atmosphere and the adsorbed water content from surroundings may deteriorate its properties. Therefore, hydrophobicity of the arogels becomes crucial parameter and needs to be evaluated critically. In the present works, silica alcogels were prepared using the mixture of tetramethoxysilane and methyltrimethoxysilane (MTMS) as precursor chemicals for silica. The concentration of MTMS, which is used as hydrophobic reagent, in the said mixture of silicon alkoxide was varied between 0 and 100% in steps of 25%. After gelation, the alcogels were dried supercritically by solvent extraction method. Resulted aerogels were exposed to relative humidity of 90% for a period of one month which were then characterized to assess hydrophobicity by the contact angle using water drop method and adsorbed water content measurements by Karl Fischer’s Titration method. Observed contact angle and water content measurements were compared and the results are reported in the present research paper.

Hydrophobic silica aerogels prepared via rapid supercritical extraction

Hydrophobic silica aerogels have been prepared using the rapid supercritical extraction (RSCE) technique. The RSCE technique is a one-step methanol supercritical extraction method for producing aerogel monoliths in 3 to 8 h. Standard aerogels were prepared from a tetramethoxysilane (TMOS) recipe with a molar ratio of TMOS:MeOH:H2O:NH4OH of 1.0:12.0:4.0:7.4 × 10−3. Hydrophobic aerogels were prepared using the same recipe except the TMOS was replaced with a mixture of TMOS and one of the following organosilane co-precursors: methytrimethoxysilane (MTMS), ethyltrimethoxysilane (ETMS), or propyltrimeth-oxysilane (PTMS). Results show that, by increasing the amount of catalyst and increasing gelation time, monolithic aerogels can be prepared out of volume mixtures including up to 75% MTMS, 50% ETMS or 50% PTMS in 7.5–15 h. As the amount of co-precursor is increased the aerogels become more hydrophobic (sessile tests with water droplets yield contact angles up to 155°) and less transparent (transmission through a 12.2-mm thick sample decreases from 83 to 50% at 800 nm). The skeletal and bulk density decrease and the surface area increases (550–760 m2/g) when TMOS is substituted with increasing amounts of MTMS. The amount of co-precursor does not affect the thermal conductivity. SEM imaging shows significant differences in the nanostructure for the most hydrophobic surfaces.

Functionalized Hybrid Nanoparticles and their Interaction with Spin-Labeled Cyclodextrin

Hybrid nanoparticles of the gold@silica and magnetite@silica type were obtained by embedding gold nanoparticles (protected by captopril, average size 2.3 nm) or magnetite nanoparticles (protected by alanine, average size 10 nm) into larger silica nanoparticles (50–100 nm) using the Stober method. A silica precursor, i.e., a mixture of tetramethoxysilane and 3-aminopropyl-trimethoxysilane (in a 10/1 ratio), was used in order to facilitate the links between inner nanoparticles and the silica layer. These nanoparticles containing free amino-groups were easily functionalized with adamantane, naphtyl and nitrobenzofurazan moieties using appropriate reagents (adamantane-1-carbonylchloride, 1-naphtylisocyanate, and 7-chlor-4-nitrobenzofurazan). The hybrid nanoparticles were characterized by different means (IR and TEM) and their interaction with spin-labeled β-cyclodextrin was also studied (by EPR spectroscopy). EPR spectra show that cyclodextrins are attached to the nanoparticles surface. The hybrid nanoparticles functionalized with nitrobenzofurazan moieties have the largest interaction with the cyclodextrin cavity.

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 mum. 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.

Oxygen and water vapor gas barrier poly(ethylene naphthalate) films by deposition of SiOx plasma polymers from mixture of tetramethoxysilane and oxygen

AbstractSiOx films were deposited from a mixture of tetramethoxysilane (TMOS) and oxygen on poly(ethylene 2,6‐naphthalate) film using ion‐assisted plasma polymerization technique (Method II) and conventional plasma polymerization technique (Method I), and were compared in chemical composition and gas barrier properties. Methods I and II were different in electrical circuit between electrodes (anode and cathode) and electric power supply. In Method I, the anode electrode was grounded, and the cathode electrode was coupled to the discharge power supply. In Method II, the anode electrode was connected with the discharge power supply, and the cathode electrode was grounded. There was not large difference in SiOx deposition rate between the plasma polymerizations by Methods I and II. Plasma polymers deposited from TMOS/O2 mixtures by Method II possessed smaller C/Si and O/Si atomic ratios than those deposited by Method I and showed advantage in gas barrier properties. The oxygen and water vapor permeation rates were 0.08–0.13 cm3 m−2 day−1 atm−1 at 30°C at 90% RH and 0.244–0.276 g m−2 day−1 at 40°C at 90% RH, respectively. From these results, it can be concluded that the ion‐assisted plasma polymerization is a useful technique for deposition of gas barrier SiOx thin films. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 915–925, 2007

Structure and properties of multilayered siloxane–organic hybrid films prepared using long-chain organotrialkoxysilanes containing CC double bonds

Oriented multilayered films composed of alternating siloxane layers and organic layers were prepared from mixtures of tetramethoxysilane [Si(OMe)4] and unsaturated organotrimethoxysilane [RSi(OMe)3, where R is CH2CH(CH2)8– or CH2CH(CH2)2CHCH(CH2)4–], and the structures and macroscopic properties of the films were studied. Hydrolysis and partial condensation of the precursors led to the formation of amphiphilic organosiloxane species which self-assemble into lamellar phases. Polymerization of the organic phase occurred by UV irradiation, as evidenced by substantial decreases of the IR absorption bands due to –CHCH2 or –CHCH– groups. The hardness of the films was remarkably increased by the irradiation, due to the covalent linking of adjacent siloxane layers by organic polymerization. The films after organic polymerization had a much higher resistance to an alkaline solution, which enabled the patterning of the films on the micrometer length scale. These results provide an important insight into the structure–property relationships of nanostructured hybrid materials prepared by sol–gel chemistry.

Preparation of high efficiency and highly retentive monolithic silica capillary columns for reversed-phase chromatography by chemical modification by polymerization of octadecyl methacrylate

High efficiency and highly retentive monolithic silica capillary columns were obtained by polymerization of octadecyl methacrylate using α,α′-azobis-isobutyronitrile (AIBN) as a free radical initiator. Hybrid type monolithic silica columns (25 cm total length × 200 μm I.D.) prepared from a mixture of tetramethoxysilane and methyltrimethoxysilane were used as a support. The effects of the monomer and the radical initiator concentrations in the reaction mixture were examined. The performance of the columns was tested in terms of column efficiency and retention behavior by using alkylbenzenes and a few other compounds as solutes and compared with that of hybrid monolithic silica columns modified with octadecylsilyl-( N, N-diethylamino)silane (ODS-DEA). Highly retentive monolithic silica columns were obtained by polymerization at high monomer concentrations. Although a decrease in column efficiency was observed with the increase in the monomer concentration in a feed mixture, an improvement in efficiency was achieved (a plate height value lower than 10 μm) by increasing an initiator concentration without significant variations in column retention properties. Results obtained by polymerization using other monomers are also presented to demonstrate the applicability of the preparation method.

Development and Characterization of a Silica Monolith Immobilized Enzyme Micro-bioreactor

Several 10-cm-long capillary tubes [made of poly(ether ether ketone) (PEEK)] with inside diameters of 0.1-2.0 mm were filled with silica monolith-immobilized protease derived by in situ sol-gel transition from a 1:4 mixture of tetramethoxysilane and methyltrimethoxysilane. Transesterification between 20 mM (S)-(-)-glycidol and 0.4 M vinyl n-butyrate in an organic solvent was used as the test reaction. The substrate solution flowed through the column at a flow rate of 0.0004-5.0 mL.min - 1 . The conversion in the micro-bioreactor was higher than that in the batch reactor at a high liquid flow rate. When three tubes were connected in series, the conversion at a fixed ratio of the mass of the enzyme to the liquid flow rate was increased by approximately 50%, because of the tripling of the flow rate as compared to the case with a single tube. Changes in the tube diameter had no influence on the conversion at a fixed superficial liquid velocity. Further, the conversion increased with a decrease in the enzyme content. These results were ascribed to the apparent effect of liquid-solid mass transfer and were analyzed quantitatively using a simple mathematical model.

Synthesis of silica films on a polymeric material by plasma-enhanced CVD using tetramethoxysilane

Silica films have been deposited on a polymeric material, polyethylene terephthalate (PET), by low-temperature plasma-enhanced CVD using a mixture of tetramethoxysilane (TMOS) and oxygen as the source. In the early stages of deposition, silica domes were formed on the treated PET surface. In the TMOS/oxygen plasma, these silica domes then grew larger and fused together. On the contrary, when no active oxygen species were present in the vapor phase, the silica domes simply agglomerated but were hardly fused. The silica films were found to grow on the treated PET substrate in an island-like and discontinuous manner.

Synthesis of poly(diallyl phthalate) and silica gel polymer hybrids utilizing –π interactions

We report here the synthesis of homogeneous polymer hybrids of poly(diallyl phthalate) (PDAP) and silica by utilizing π–π interactions. Use of arylalkoxysilanes such as phenyltrimethoxysilane (PhTMOS), phenethyltrimethoxysilane (PhenethylTMOS) and mesityltrimethoxysilane (MesTMOS) as sources for inorganic phases resulted in optically transparent PDAP-silica polymer hybrids in a wide range of organic and inorganic content ratios. On the other hand, alkoxysilanes such as tetramethoxysilane (TMOS), methyltrimethoxysilane (MTMOS) and i-butyltrimethoxysilane (iBuTMOS) resulted in phase separated, turbid solids. A mixture of tetramethoxysilane (TMOS) and PhTMOS was also studied for the synthesis of PDAP-silica gel polymer hybrids to control the cross-linking density in the inorganic phase. Homogeneity was found to be improved with an increase in PhTMOS content. These homogeneous PDAP polymer hybrids were found to have high thermal stability which wasachieved by nano-scale dispersion of PDAP in silica through extensiveinterface interactions. The homogeneity of the polymer hybrids was confirmed by SEM and TEM, which demonstrate a nanometer level integration of the organic polymer and the inorganic phase.

Monolithic silica columns with various skeleton sizes and through-pore sizes for capillary liquid chromatography

Reduction of through-pore size and skeleton size of a monolithic silica column was attempted to provide high separation efficiency in a short time. Monolithic silica columns were prepared to have various sizes of skeletons (∼1–2 μm) and through-pores (∼2–8 μm) in a fused-silica capillary (50–200 μm I.D.). The columns were evaluated in HPLC after derivatization to C 18 phase. It was possible to prepare monolithic silica structures in capillaries of up to 200 μm I.D. from a mixture of tetramethoxysilane and methyltrimethoxysilane. As expected, a monolithic silica column with smaller domain size showed higher column efficiency and higher pressure drop. High external porosity (>80%) and large through-pores resulted in high permeability ( K=8·10 −14–1.3·10 −12 m 2) that was 2–30 times higher than that of a column packed with 5-μm silica particles. The monolithic silica columns prepared in capillaries produced a plate height of about 8–12 μm with an 80% aqueous acetonitrile mobile phase at a linear velocity of 1 mm/s. Separation impedance, E, was found to be as low as 100 under optimum conditions, a value about an order of magnitude lower than reported for conventional columns packed with 5-μm particles. Although a column with smaller domain size generally resulted in higher separation impedance and the lower total performance, the monolithic silica columns showed performance beyond the limit of conventional particle-packed columns under pressure-driven conditions.