Synthesis and structure of grafted layer of silicas modified with alkanesulfonic acid

A convenient method for preparation of silicas modified with alkanesulfonic acid through the reaction of NaHSO₃ with the C₂H₃ groups grafted on the SiO₂ surface was proposed. The influence of the carrier and the structure of the grafted layer on the modification process was studied by DRIFT, MAS NMR, and temperature-programmed desorption mass spectrometry (TPD-MS). It was found that at most 40% of the C₂H₃ groups transform to the acid sites, which mostly have the structure of 2-Si-ethanesulfonic acid. The mechanism of the thermal decomposition of the grafted groups, which are stable up to 240 °C, was proposed. The monomeric structure of the grafted layer leads to partial olygomerization of the C₂H₃ groups through the N=N bonds.     

Synthesis and investigation of chemically modified silicas with the island structure of the grafted layer

Island structures on the chemically modified surface of silica are synthesized by the method of matrix prints. Dichlorosilicon phthalocyanine and copper naphthalocyanine were used as molecular templates. Two types of heterogeneous surfaces are prepared by this method: the islands of nonmodified silica on the trimethylsilylated silica (the first type) and the islands of aminopropyl silica on the esteric groups grafted on silica (the second type). The structure of the surfaces prepared is studied by ESR spectroscopy with the use of paramagnetic probes.     

Comparison of surface states of chemically modified silicas in aqueous acetonitrile and methanol

Summary The thermodynamic behaviour of n-alkylbenzenes on chemically modified silicas in reversed-phase liquid chromatography has been examined in acetonitrile-water and methanol-water systems. Plots of the thermodynamic parameters obtained against the organic solvent compositions indicate an interesting trend which implies that the surface states of the chemically modified silicas under the two eluent systems are different.     

Study of modified silicas by inverse gas chromatography. Influence of chain length on the conformation of n-alcohols grafted on a pyrogenic silica

Summary A fumed silica surface was systematically modified by the grafting of n-alkyl chains with increasing carbon numbers. The samples were characterized by the dispersive component of the surface energy, their specific interaction potential and enthalpies of adsorption of polar, in particular, alcohol probes. It is shown that the variation of the surface properties and adsorption capacities depend on the chain length of the graft. For instance, minimum values are recorded when the surface coverage by methylene groups corresponds either to one or two CH₂ surface layers. This behaviour is related to the mobility of the grafted alkyl chains, mobility which was examined by solid state NMR.     

Characterization by inverse gas chromatography of the surface properties of silicas modified by poly(ethylene glycols) and their models (oligomers,diols)

Summary Silicas are modified by esterification of the surface silanol groups either with poly(ethylene glycol), its oligomers or diols of the same chain length. A whole series of samples carrying grafts differing in chemical nature, molecular weight and number are prepared. These samples are examined by inverse gas chromatography. The London component of the surface energy, the acid/base interaction parameter and the enthalpy of adsorption of alkanes and polar probes are measured. It is apparent for PEG-grafted silicas that the most important variable is the surface coverage, i.e., the number of monomer units of the grafted PEG per unit surface area, and not its molecular weight. The fixation of PEG changes drastically the surface properties of the silicas. According to the surface coverage, silica may acquire base-like properties. The strong PEG-silica interactions cause the polymer to adopt a flat configuration by spreading on the solid surface. Grafted diols behave entirely differently: the conformation of the longer grafts is best described by a sandwich structure in which the outer surface layer is formed by associated alcohol groups, whereas the middle part is formed by the highly organized aliphatic parts of the chains. IR spectroscopy supports these conclusions.     

Determination of thorium by adsorptive type chemically modified electrode with a polycomplex system

A preconcentration and determination method for thorium in aqueous solution with a tri-n-octylphosphine oxide modified glassy carbon electrode is proposed. In the presence of 2-thenoyltrifluoroacetone, thorium in NaAc-HAc supporting electrolyte is preconcentrated on a modified rotating disk electrode, and a highly sensitive reduction peak is obtained by cathodic stripping voltammetry at –1.10 V versus Ag/AgCl. A linear response of reduction peak height and concentration is observed for 1.15×10^–9–1.44×10^–8 mol·1^–1 of thorium and the detection limit is 1.0×10^–9mol·1^–1. It is very selective and sensitive, with a standard deviation of 3.4% and a recovery of 90–110%.     

Structural and dynamic study of chemically modified polyHIPE by solid‐state 13C NMR spectroscopy

The structure of laboratory‐made polyHIPEs was successfully characterized by cross‐polarity/magic‐angle spinning, solid‐state ¹³C NMR experiments. The signals of vinyl groups appeared in the spectrum of the polyHIPE precursor PH? CH?CH₂, which was prepared by the polymerization of the divinylbenzene continuous phase from a highly concentrated reverse emulsion. This material was chemically modified by the regioselective free‐radical addition of thiols to the pendant vinyl groups. Spectra of materials modified by the grafting of C₈ and C₁₂ alkyl chains, PH? SC₈ and PH? SC₁₂, respectively, were produced. The signals of the vinyl groups disappeared in favor of methylene groups. This experiment clearly established that the alkyl chains were covalently bound to the polymer. To elucidate the dynamic aspect of long chains in polyHIPE, we measured the ¹³C spin–lattice relaxation times (T₁) of PH? SC₁₂ from 25 to 100 °C with variable‐temperature, solid‐state, high‐resolution ¹³C NMR spectroscopy, revealing a strong variation in T₁ along the alkyl side chain. Furthermore, the crystallinity of a wide range of chemically modified polyHIPEs, including PH? SC₁₂, was studied with pulse ¹H NMR. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 956–963, 2001     

Synthesis and characterization of imine-modified silicas obtained by the reaction of essential oil of Eucalyptus citriodora, 3-aminopropyltriethoxysilane and tetraethylorthosilicate

Imine-modified silicas were synthesized by the reaction of the essential oil of Eucalyptus citriodora, 3-aminopropyltriethoxysilane and tetraethylorthosilicate by different methods. The imine supported on silica was characterized by FT-IR-spectroscopy using the attenuated total reflection (ATR) technique, the Brunauer–Emmett–Teller (BET) method and thermogravimetric analysis (TGA). These silicas have shown a large amount of organic matter incorporated on the surface, which affects their textural properties. The hydrolysis reaction of the imine-modified silica leads to the formation of products different from those expected.     

Molecular recognition studies on supramolecular systems (XXIV)——Conformations and complexation abilities of chemically modified cyclodextrins in aqueous solution

Circular dichroism spectral and fluorescence decay methods have been employed to determine the conformations of mono[6-(p-tolylseleno)-6-deoxy]-p-CD(1), mono(6-anilino-6-deoxy) β -CD (2) and mono[6-(L-tryptophan)-6-deoxy]-β-CD (3) in phosphate buffer solution (pH 7.2, 0.1 mol dm-3) at 298.15 K. The results indicate that compounds 2 and 3 formed self-inclusion complexes in aqueous buffer solution, while the substituent of compound 1 was not included into cyclodextrin cavity at all. Furthermore, the complex stability constant (logKs) and Gibbs free energy change (-ΔG° ) of these three cylcodextrin derivatives with several cycloalkanols have been determined by circular dichroism spectral titration in phosphate buffer solution at 298.15 K. It is found that the location of the substituent affects the stability of host-guest complex in aqueous solution.     

Characterization of specific interactions capacity of solid surfaces by adsorption of alkanes and alkenes. Part II: Adsorption on crystalline silica layer surfaces

Summary Adsorption of branched octanes and linear hydrocarbons on crystalline lamellar silica surfaces has been studied by inverse gas chromatography at infinite dilution. Taking the adsorption of the n-alkanes as a reference, the influence of the double bond on the hydrocarbon adsorption phenomena has been demonstrated. Thermodynamical parameters have been calculated which permit conclusions to be made on the adsorption mechanisms of lamellar materials.     

The structure of poly(carbonsuboxide) on the atomic scale: a solid-state NMR study

In this contribution we present a study of the structure of amorphous poly(carbonsuboxide) (C3O2)x by 13C solid-state NMR spectroscopy supported by infrared spectroscopy and chemical analysis. Poly(carbonsuboxide) was obtained by polymerization of carbonsuboxide C3O2, which in turn was synthesized from malonic acid bis(trimethylsilylester). Two different 13C labeling schemes were applied to probe inter- and intramonomeric bonds in the polymer by dipolar solid-state NMR methods and also to allow quantitative 13C MAS NMR spectra. Four types of carbon environments can be distinguished in the NMR spectra. Double-quantum and triple-quantum 2D correlation experiments were used to assign the observed peaks using the through-space and through-bond dipolar coupling. In order to obtain distance constraints for the intermonomeric bonds, double-quantum constant-time experiments were performed. In these experiments an additional filter step was applied to suppress contributions from not directly bonded 13C,13C spin pairs. The 13C NMR intensities, chemical shifts, connectivities and distances gave constraints for both the polymerization mechanism and the short-range order of the polymer. The experimental results were complemented by bond lengths predicted by density functional theory methods for several previously suggested models. Based on the presented evidence we can unambiguously exclude models based on gamma-pyronic units and support models based on alpha-pyronic units. The possibility of planar ladder- and bracelet-like alpha-pyronic structures is discussed.     

Stacking Structure of Confined 1‐Butanol in SBA‐15 Investigated by Solid‐State NMR Spectroscopy

Understanding the complex thermodynamic behavior of confined amphiphilic molecules in biological or mesoporous hosts requires detailed knowledge of the stacking structures. Here, we present detailed solid‐state NMR spectroscopic investigations on 1‐butanol molecules confined in the hydrophilic mesoporous SBA‐15 host. A range of NMR spectroscopic measurements comprising of ¹H spin–lattice (T₁), spin–spin (T₂) relaxation, ¹³C cross‐polarization (CP), and ¹H,¹H two‐dimensional nuclear Overhauser enhancement spectroscopy (¹H,¹H 2D NOESY) with the magic angle spinning (MAS) technique as well as static wide‐line ²H NMR spectra have been used to investigate the dynamics and to observe the stacking structure of confined 1‐butanol in SBA‐15. The results suggest that not only the molecular reorientation but also the exchange motions of confined molecules of 1‐butanol are extremely restricted in the confined space of the SBA‐15 pores. The dynamics of the confined molecules of 1‐butanol imply that the ¹H,¹H 2D NOESY should be an appropriate technique to observe the stacking structure of confined amphiphilc molecules. This study is the first to observe that a significant part of confined 1‐butanol molecules are orientated as tilted bilayered structures on the surface of the host SBA‐15 pores in a time‐average state by solid‐state NMR spectroscopy with the ¹H,¹H 2D NOESY technique.     

Synthesis of SiO2 thin films by sol-gel method using photoirradiation and molecular structure analysis

The SiO₂ thin films were prepared by a process which combines a sol-gel method and photoirradiation. The HF etch rate and microhardness of a film prepared by this process were better than those of a film furnace-fired at same temperature. The Raman and ²⁹Si solid state NMR spectra of film prepared by this process were similar to those of a film furnace-fired at higher temperature. There are many unstable folded non-linear SiO₂ species in the film prepared at low temperature. On treatment at higher temperature, unstable folded non-linear Si-O-Si rearranges to the stable linear Si-O-Si bond. Photoirradiation enhances this structure change. The process provided denser and harder SiO₂ thin films, even at low temperature, than the conventional furnace-firing method did.     

Understanding the Synthesis of Supported Vanadium Oxide Catalysts Using Chemical Grafting

The complexity of variables during incipient wetness impregnation synthesis of supported metal oxides precludes an in-depth understanding of the chemical reactions governing the formation of the dispersed oxide sites. This contribution describes the use of vapor phase deposition chemistry (also known as grafting) as a tool to systematically investigate the influence of isopropanol solvent on VO(OⁱPr)₃ anchoring during synthesis of vanadium oxide on silica. The availability of anchoring sites on silica was found to depend not only on the pretreatment of the silica but also on the solvent present. H-bond donors can reduce the reactivity of isolated silanols whereas disruption of silanol nests by H-bond acceptors can turn unreactive H-bonded silanols into reactive anchoring sites. The model suggested here can inform improved syntheses with increased dispersion of metal oxides on silica.