Influence of synthetic routes on the conformational order and mobility of C18 and C30 stationary phases

Silica gels modified with n-alkyl chains (n = 18, 30) are prepared by two different synthetic routes and are examined by variable temperature FTIR and solid-state NMR spectroscopy. HPLC measurements of SRM 869, cis/trans ss-carotene isomers and xanthophylls isomers confirm the dependence of the separation mechanism on the alkyl chain length and the synthetic routes. The determination of the silane functionality and degree of cross-linking of silane ligands on the silica surface is achieved by 29Si CP/MAS NMR measurements. The structural order and mobility of the alkyl chains are investigated by means of variable temperature 13C CP/MAS NMR measurements. Variable temperature FTIR studies are performed where conformational order and flexibility of the alkyl chains in C18 and C30 phases are monitored through conformational sensitive CH2 symmetric, anti-symmetric stretching and wagging modes. In addition, the chromatographic properties of the C18 and C30 phases are determined. The results derived from the FTIR, NMR and HPLC measurements are discussed in the context of the applied synthetic routes and alkyl chain lengths.     

Effect of surface coverage on the conformation and mobility of C18-modified silica gels

C18-modified silica gels with surface coverages of 2 to 8.2 micromol m(-2), were prepared by different synthetic pathways and characterized by Fourier Transform infrared spectroscopy (FTIR), solid-state nuclear magnetic resonance (NMR) spectroscopy, and chromatographic measurements. The effects of temperature and bonding density on the conformational order of C18-modified silica gels were studied in detail by FTIR spectroscopy. The silane functionality and degree of cross-linking of silane ligands on the silica surface were evaluated by 29Si cross-polarization magic-angle spinning (CP/MAS) NMR and the structural order and mobility of the alkyl chains were investigated by 13C CP/MAS NMR spectroscopy. CH2 symmetric and anti-symmetric stretching bands and CH2 wagging bands were used as IR probes to monitor the conformational order and flexibility of the alkyl chains in the C18 phases. Qualitative information about the conformational order was obtained from frequency shifts of the CH2 symmetric and anti-symmetric stretching bands. The relative amounts of kink/gauche-trans-gauche, double-gauche, and end-gauche conformers in the alkyl chains were determined by analysis of CH2 wagging bands. These results indicate that surface coverage plays a dominant role in the conformational order of C18-modified silica gels. The FTIR and NMR data are discussed in the context of the chromatographic shape-selectivity differences.     

Conformational order of n-alkyl modified silica gels as evaluated by Fourier transform infrared spectroscopy

The conformational behaviour of non-deuterated and selectively deuterated alkyl modified silica gels in the dry state is examined by variable temperature FT IR spectroscopy. In the present study, three systems are considered, which are distinguished by the length of the tethered alkyl chains (C9Hl9-, C18H37-, C22H45-). The desired information is obtained by the analysis of various conformational-sensitive IR bands, including CH2 wagging, CD2 stretching and CD2 rocking bands. The analysis of the CH2 wagging bands provides the relative amounts (i.e., integral numbers over the whole chain) of the kink/gauche-trans-gauche, double-gauche and end-gauche conformers in the tethered alkyl chains. From the analysis of the CD2 stretching and CD2 rocking bands information about the conformational order at a specific deuterated methylene segment is available. Here, the CD2 rocking band data are used to determine the amount of gauche conformers at the deuterated carbon positions C-4 and C-6, and C-12. It is found that the conformational order critically depends on the actual alkyl chain length, chain position and sample temperature. Particular emphasis is given to the impact of the external pressure during sample preparation on the alkyl chain conformations, about which so far no information is available. It is observed that the samples prepared as KBr pellets, which experienced a pressure of about 10 kbar, are characterised by a lower amount of gauche conformers. This substantial increase of conformational order is attributed to better alkyl chain packing along with a gain of intermolecular chain interactions.     

Raman spectral conformational order indicators in perdeuterated alkyl chain systems

Conformational order indicators for perdeuterated alkyl chain systems are identified in the Raman spectra of nonadecane-d40, polyethylene-d4, and stearic acid-d35. Six spectral indicators are identified: I[nu(a)(CD2)2196]/I[nu(a)(CD2)2172], I[nu(C-C)G]/I[nu(C-C)T], I[nu(s)(CD3)]/I[nu(a)(CD2)2172], I[nu(s)(CD2)]/I[nu(a)(CD2)2172], and the full width at half-maximum (fwhm) and frequency of the nu(s)(CD2) mode. Among these indicators, the ratio of I[nu(a)(CD2)2196]/I[nu(a)(CD2)2172] is considered a primary indicator of conformational order, since it responds to alkyl chain conformational changes in a manner similar to the I[nu(a)(CH2)]/I[nu(s)(CH2)] primary indicator for hydrogenated systems. Other indicators are correlated to this primary indicator to derive a better understanding of the effect of structural attributes on conformational order in perdeuterated alkyl chain systems. These Raman spectral order indicators are applicable to any perdeuterated alkyl-containing system including lipids, biological membranes, alkylsilane-based chromatographic stationary phases, and self-assembled monolayers.     

Characterization of chemically modified silica gels by DRIFT spectroscopy

Chemically modified silica gels are investigated with respect to identity, purity and degree of surface coverage using diffuse reflectance FTIR (DRIFT) Spectroscopy. We report the results for two bonded HPLC phases, pyrene butyric acid propylamido- and octadecyl-groups grafted on irregular porous silica gel. For quantitative determination calibration standards are prepared by adsorbing structurally similar ligand compounds onto the surface of the silica gel; these coated materials give linear calibration curves up to a concentration of 1 mmol/g modified silica gel.     

The investigation of the pore structure of silica gels with long-chain alkyl grafts by thermal analysis

The total pore volumes and the core size distributions were calculated for the silica gel Si-100 and chemically modified silica gels RP-8 and RP-18 on the basis of the thermal desorption data of various liquids. The results obtained are compared with those from the nitrogen adsorption/desorption method. Good consistency between the parameters obtained by using different methods is observed in the case of unmodified silica gel. The presence of long alkyl chains on the silica surface strongly influences the pore diameter calculated from thermogravimetric data.     

Influence of the specific surface area on crystallization process kinetics of some silica gels

The influence of the specific surface area on the crystallization processes of two silica gels with different specific surface areas has been investigated in non-isothermal conditions using DTA technique. The activation energies of the crystallization processes were calculated using four isoconversional methods: Ozawa-Flynn-Wall, Kissinger-Akahira-Sunose, Starink and Tang. It has been established that, the decrease of the surface area from S=252.62 m² g⁻¹, in the case of sample GS2, to S=2.52 m² g⁻¹, in the case of sample GS1, has determined a slight increase of the activation energy of the crystallization process of the gels. Regardless of the isoconversional method used, the activation energy (E _α) decreases monotonously with the crystallized fraction (α), which confirms the complex mechanism of gels crystallization. It has been proved that the Johnson-Mehl-Avrami model cannot be applied for the crystallization processes of the studied silica gels.     

Oxidation of alcohols using cerium(IV) alkyl phosphonate modified silica

The oxidation of a range of alcohols to ketones or carboxylic acids proceeds in good yield using catalytic quantities of cerium(IV) phosphonate modified silica and sodium bromate as the re-oxidant.     

An improved methodology for adsorption characterization of unmodified and modified silica gels

An improvement in the adsorption characterization of the surface and structural properties of unmodified and modified mesoporous silica gels is presented. This improvement was achieved by selection of proper macroporous silica as the reference solid for adsorption characterization of porous silica gels. Experimental illustration is provided for unmodified and n-octyl-modified silica gels of different bonding density. The surface and structural properties of these silica gels were characterized by utilizing the standard adsorption data for both unmodified and octyl-modified LiChrospher Si-1000 macroporous silica gels. It was shown that the standard nitrogen adsorption data have an appreciable influence on the analysis of the pore size and surface properties of silica gels. This analysis can be improved by selecting the reference solid of the surface properties close to those of the silica gel studied.     

Comparison of the complexation abilities of bifunctional polysiloxane xerogels and chemically modified silica gels

Complexation of copper(II) on the surface of chemically modified silica gels and polysiloxane xerogels both containing the same functional groups, 3-aminopropyl and 3-mercaptopropyl, was studied. The composition of the complexes formed on the surface of these two classes of sorbents was determined by ESR and electronic spectroscopies.     

Some aspects of estimation of extraction selectivity under the conditions of competitive sorption on modified silica gels

Systematic studies of silica gels with covalently immobilized thiosemicarbazide and formazan groups under the conditions of competitive sorption from multicomponent systems were conducted. A methodological approach to determine the selectivity of the modified sorption material with regard to Cu(II), Ni(II), Co(II), Cd(II), and Zn(II) was proposed. Solid-phase extraction in equilibrium conditions of Cu(II), Zn(II), Co(II), Cd(II), and Ni(II) on a silica gel with covalently immobilized thiosemicarbazide and formazan groups in the conditions of competitive sorption was studied. The possibility to use the pseudo-second-order kinetic equation for assessment of mutual influence at competitive sorption has been shown. We found that sorption from multicomponent solutions proceeds as a non-additive process under the conditions of an excess of functional groups.

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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⁺.     

Porous silicon oxycarbide glasses from organically modified silica gels of high surface area

High surface area alkyl-substituted silica aerogels and xerogels were successfully prepared by sol-gel processing and supercritical drying. The gels were further heat treated in inert atmosphere to temperatures as high as 1000°C. Surface areas and pore structure of the gels and gels pyrolyzed at high temperatures were determined by multipoint BET surface area measurement. The aerogels and xerogels exhibited surface areas of about 1100 m²/g. No significant effect of pH was found on the surface areas of gels in the two step sol-gel process, but gels of low pH showed smaller pore diameter and higher density. Xerogels showed smaller surface area, pore size, and pore volume compared to aerogels. Upon pyrolyzing in inert atmosphere, the surface areas of all the gels decreased with temperature as a result of collapse of micropores and shrinkage of mesopores. Unlike pure silica gel, which loses almost all surface area and densifies at 1000°C, the advantage of the alkyl-substituted gels is that they maintained a high surface area of 400 m²/g at 1000°C.Also with the Department of Agronomy.     

Colloidal crystallization of colloidal silica modified with ferrocenyl group-contained polymers in organic solvents

Surface modification of colloidal silica with ferrocenyl-grafted polymer and colloidal crystallization of the particles in organic solvent were studied. Poly(methyl methacrylate-co-vinylferrocene)-grafted silica never formed colloidal crystals in polar solvent, such as acetone, acetonitrile, ethanol and N,N-dimethylformamide (DMF), while poly(methyl methacrylate-co-ferrocenyl acrylate)-grafted silica gave colloidal crystallization in DMF. The particles prepared by grafting of poly(N,N-dimethylacrylamide-co-vinylferrocene), with vinylferrocene (Vfc) mole fraction of 1/13 and 1/23, were observed to give the crystallization in ethanol and DMF over particle volume fraction of 0.058. Further, silica modified with copolymer of Vfc and N-vinyl-2-pyrrolidone, N-vinylcarbazole or N-isopropylacrylamide formed colloidal crystals in ethanol and DMF. Especially, poly(N-isopropylacrylamide-co-Vfc)-grafted silica, which was composed of the highest mole fraction of vinylferrocene, 1/3, afforded colloidal crystallization in ethanol over particle volume fraction of 0.053. Relatively high polar vinylferrocene copolymer grafting of silica resulted in colloidal polymerization in organic solvents.     

pi-Electron interaction of PAHs with anion-exchange silica gels modified with anionic metal-porphine and -phthalocyanine derivatives as HPLC stationary phase for preparative column in organic solvents

To develop easy-to-prepare stationary phases for HPLC, we investigated anion-exchange silica gels, Nucleosil 5SB (Nuc), modified with metal-porphines and -phthalocyanines (M-P). The modified silica gels (M-P_N) were evaluated for the availability as a stationary phase of HPLC for the separation of π-electron-rich polyaromatic hydrocarbons (PAHs) in polar and non-polar eluents. Separation ability of silica gels modified with Cu-phthalocyanine derivative (Cu-PCS_N) was comparable to that of the silica gels binding Cu-PCS through sulfonamide bonds; however, the latter requires troublesome procedures for the preparation. The PAHs tested interact with Cu-PCS_N in non-polar organic eluents through their π-electrons similarly as in the case of the PYE column^®, in which interaction with PAHs was reported to be only the π-π-electron interaction.     

Allylic and benzylic oxidation using cobalt(II) alkyl phosphonate modified silica

The allylic and benzylic oxidation of a range of substrates proceeds in good yield using catalytic quantities of cobalt(II) alkyl phosphonate modified silica and tert-butyl hydroperoxide.     

Simultaneous separation of common mono- and divalent cations on silica gels modified with aluminium and zirconium

The modification of silica gel with aluminium and zirconium can be used for the preparation of advanced silica-based cation-exchange stationary phases for use in ion chromatography with conductimetric detection (IC-CD) for cations. Silica gels modified with aluminium (Al-silica) and zirconium (Zr-silica) act as cation-exchangers under strongly acidic conditions. Highly sensitive indirect conductimetric detection and excellently simultaneous separation for common mono- and divalent cations (Li⁺, Na⁺, NH₄⁺, K⁺, Mg²⁺ and Ca²⁺) can be achieved on the Al-silica and Zr-silica columns in IC-CD by using acidic eluents containing 15-crown-5 (1,3,7,10,13-pentaoxacyclopentadecane). The Al-silica and Zr-silica can also be applied successfully as cation-exchange stationary phases in ion-exclusion chromatography for the separation of various aliphatic and benzenecarboxylic acids.     

Rigidity of colloidal crystals of silica spheres modified with polymers on their surfaces in organic solvents

Rigidity (G) of colloidal crystals in organic solvents of acetonitrile and nitrobenzene has been measured by reflection spectroscopy in sedimentation equilibrium. The colloidal spheres used are the silica spheres (136 nm in diameter) modified on their surfaces with polymers, poly(maleic anhydride-co-styrene) [P(MA-ST)], poly(methyl methacrylate) (PMMA), or polystyrene (PST). Log G increases linearly with the slope of unity as log N (number density of colloidal spheres) increases. The mean values of the b-factor, which is the fluctuation parameter in crystal lattices and should be smaller than 0.1 according to the Lindeman's rule, are 0.045±0.003, 0.039±0.007, and 0.038±0.003 for P(MA-ST)/SiO₂, PMMA/SiO₂, and PST/SiO₂, respectively. These values are larger than that of colloidal crystals of mother silica spheres in the deionized aqueous suspension, 0.028. These results support the important role of the excluded volume effects from the polymer layers formed around the silica surfaces. However, contribution of the excluded volume effects from the electrical double layers formed around the spheres in the organic solvents is also effective in the colloidal crystallization. Electronic Publication     

Dependence of the conformational equilibrium of alkyl azidoformates on the length of the carbon chain of the alkyl group

Conclusions An empriical dependence of the conformational equilibrium constant of alkyl azidoformates on the length of the alkyl group carbon chain was derived on the basis of an experimental study.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 452–455, February, 1989.