Relationship between the gas chromatographic behaviour and the molecular structure of hydrocarbon samples and various stationary phases

Summary Retention parameters of different hydrocarbon classes were determined on non-polar and medium-polar stationary phases. The relationship between the retention indices, the physicochemical properties and the chemical structure of the solutes (samples) and solvents (liquid phases) was investigated.     

The composition of iron-containing microphases in the structure of multiwalled carbon nanotubes

The phase composition of carbon nanotubes (CNTs) with encapsulated iron atoms was studied by ⁵⁷Fe Mössbauer spectroscopy. The synthesis of CNTs was shown to stabilize iron atoms as both thermodynamically stable iron carbide and oxide phases and phases not characteristic under synthesis conditions (γ-Fe and γ-Fe₂O₃). In addition, an analysis of the data obtained led us to suggest the existence of Fe-CNT atomic complexes in interlayer positions. CNTs and iron-containing phases were shown to be stable when subjected to prolonged annealing in air at temperatures up to 670 K.     

Influence of the pore structure on the properties of silica based reversed phase packings for LC

This paper describes the preparation and investigation of new, highly loaded, monomeric, silica based, reversed phase C18 and C30 packings. The influence of pore structure and endcapping on the properties of C18 and C30 packings is described. Using hydrothermal procedures, silicas with predictable pore size (9.3-25.5 nm) and surface area have been prepared. Silylation with long chain silanes substantially alters the pore structure of the silica: pore size and pore volume decrease. A new parameter, the volumetric surface coverage [mm3 x m(-2)] has been introduced. This parameter--calculated from on-column measured porosity data--indicates the pore volume portion occupied by the hydrocarbon chains. Endcapping does not significantly change the pore structure of the bonded phases. The reduced retentions (reduced with respect to unit area: [k/m2])--a good measure for comparing the retention behaviour of packings with different surface areas--are similar for most of the phases, demonstrating good accessibility of the pores for the solutes. Slightly lower retentions were found on the endcapped than on the non-endcapped phases for probes with dense pi-electron system (e.g. polyaromatic hydrocarbons) demonstrating the contribution of silanophilic interactions to the retention. The phases had been successfully used for various demanding separations, e.g. for the separation of flavonoids, carotenoids, resveratrol, and tocopherol isomers, fullerenes, and anions.     

On the structure of the intermediate phases in the praseodymium oxide system

The defect crystal structures for two phases in the praseodymium oxide system (Pr₉O₁₆, ζ phase; and Pr₄₀O₇₂, ε phase) are proposed on the basis of the results of a high-resolution transmission electron microscope (HRTEM) study. Dynamical n-beam electron scattering calculations are used to provide a more accurate and reliable interpretation of the experimental micrographs. In addition, electron diffraction studies reveal the unit-cell dimensions for the n = 11 member in the homologous series of phases in this system. The structural relationships among the related praseodymium oxide phases are discussed.     

On the synthesis and structure of LaCaGaO4

The phase relationship of the La₂O₃-CaO-Ga₂O₃ system at 1150°C has been studied and is expressed in a phase-diagram, in which the presence of two ternary compounds were confirmed. The ternary compound LaCaGaO₄ has two forms, i.e., the orthorhombic (o-) phase and the tetragonal (t-) phase. Both phases are isolators exhibiting ferroelectric behavior, so the structures were reformulated in the polar space groups.     

New stationary phases of conjugated π-electron systems for HPLC—characterization of structure and dynamics by solid state NMR spectroscopy

Four new stationary phases for HPLC were prepared by modifying silica gel with a trifrnctional aminoalkyl silane. A conjugated π-electron system was linked to the amino group either directly or with a carboxylic anhydride as a spacer in between. Characterization and dynamic measurements of the new stationary phases were performed by solid state NMR spectroscopy. The results of the ²⁹si and ¹³C CP/MAS techniques were compared with the recently developed ¹H MAS-only technique. Despite strong homonuclear dipole-dipole interactions it was possible to obtain well resolved ¹H MAS spectra of those stationaru phases with a high degree of crosslinking. Limited mobility of the aromatic ligand fragments is common for all examined stationaryu phases. The chromatographic properties of the new phases were tested by their ability tio sepatate a mixture of eight PAHS. It could be shown that the π-π interaction mechanism is responsible for the separation of the eight PAHs, because the elution order of the PAHs did not changem despite the use of both a nonpolar mobile phase (n-heptane) and a polar mobile phase (methanol/ water mixture).     

Influence of chemical structure on the optical properties of new side-chain polymers containing cholesterol

Poly[1-(cholesteryloxyhexyloxy)ethylene] (PHET) and poly[1-(cholesteryloxycarbonyl-hexyloxy)ethylene] (PHES) were prepared by reacting poly(vinyl alcohol) with cholesteryloxyhexyloxy bromides (CHB) or cholesteryloxycarbonylhexyloxy bromides (CEHB), and their thermal and optical properties were investigated. PHET and PHES exhibited monotropic cholesteric phases; however, their thermal behaviours depended on the cholesteryl groups and alkylene spacers with different chemical structures. PHET did not display reflection colours over its entire cholesteric range, whereas PHES did display reflection colours. These results suggested that the thermal stability and helical twisting power (HTP) of these polymers strongly depend on the difference in the chemical structures of the flexible spacer via cholesterol. The mesophase properties of PHET and PHES differed substantially from those of poly(cholesteryl-ω-acryloyloxyalkanoates). The results indicate that the mode of chemical linkage between the side-chain group and the main chain as well as that between the alkylene spacer and side chain play important roles in determining the thermal stability, mesophorphic structure and HTP of the cholesteric mesophases.     

The helical structure of highly ordered smectic phases

Optical studies of smectic phases have been performed in homogeneously oriented samples of chiral 4-(2′-methylbutyl) phenyl-4′-n-octylbiphenyl-4-carboxylate (CE8). The helix structure has been found in smectic phases C, I and J, but not in the smectic G phase. Two chiral phases have been found between S_I* and S_G phases. Up to now one of them has not been observed. The pitch of the helix has been measured in all of the twisted smectic phases, including the S_J* phase. The existence of the helix in this phase suggests that the correlations between smectic layers are not very strong.     

Aspects of surface modification, structure characterization, thermal stability and metal selectivity properties of silica gel phases-immobilized-amine derivatives

Four silica gel phases-bound-amine derivatives (I-IV) were prepared based on chemical immobilization technique. The surface modification was identified by determination of the coverage values in mmol g⁻¹ via thermal desorption method (1.463-1.807) and elemental analysis of nitrogen and carbon contents (1.089-2.456). Structure characterization related to immobilization of the amine derivatives was accomplished and evaluated by means of infrared (IR) and secondary ion mass spectrometric (SIMS) technique. The modified silica gel phases (I-IV) along with their interaction products with copper(II) were also examined by electron impact mass spectrometric analysis (EI-MS) as a method for evaluation of their thermal stability and structure elucidation. Potentiometric titration as a method of characterization was applied for the modified silica gel phases (II-IV) and their copper(II)-adduct. A series of bi- and trivalent metal ions were selected to focus more aspects of the selectivity properties incorporated into the modified silica gel phases for binding and interaction with these metals based on determination of the distribution coefficient and separation factor. The results of these evaluation processes were found to prove higher selectivity and preference of these four phases for binding with lead(II) and cadmium(II) compared to other metal ions.     

Influence of the siliceous support structure on the coverage density of the chemically bonded phase

Two different types of silica support, viz. silica gel and porous glass, have been modified by mono- and difunctional octade-cylsilane. The resulting materials were characterized by physicochemical and chromatographic methods. Special attention was paid to the influence of the siliceous support structure and coverage density of chemically bonded phases on the retention data.     

On the structure and conformational composition of 1,1-bis(methylthio)ethylene

1,1-bis(methylthio)ethylene has been studied in the gaseous phase by electron diffraction and in the solid and liquid phases by Raman spectroscopy. While there is apparently only one conformer in the solid, the fluid phases consist of probably three forms, two of these have a non-planar skeleton. Average values for the bond lengths are: r_a(C_eth—S) = 1.767 Å, r_a(C_met—S) = 1.815 Å, r_a(CC) = 1.348 Å.     

Chemical structure of composites derived from poly(silicic acid) and 2‐hydroxyethylmethacrylate

Organic–inorganic hybrids from 2‐hydroxyethylmethacrylate (HEMA) and poly(silicic acid)s were prepared and characterized. The molecular architecture of the inorganic phase was studied with ²⁹Si NMR spectroscopy, and the effect of acid concentration and reaction time on the properties of poly(silicic acid) were elucidated. Interactions between phases were studied with NMR spectroscopy and gel permeation chromatography, and the coexistence and connectivity of the two phases were investigated. Contrary to previous assumptions, evidence was found that indicated that the organic and inorganic phases of this class of sol–gel materials were not covalently linked. The curing process was studied by infrared spectroscopy, by which the disappearance of terminal double bonds of HEMA monomer were monitored. Thermogravimetric analysis profiles showed that the inorganic phase content was about 10 wt %, and it was varied slightly by changes in the molar ratio of the components. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1342–1352, 2001     

The structure and phase diagram of Langmuir films of alcohols on mercury

The coverage-dependent phase behavior of molecular films of alcohols (CH3(CH2)n-2CH2OH, denoted as CnOH) on mercury was studied for chain lengths 8 < or = n < or = 28, using surface tensiometry and surface specific X-ray methods. Phases with surface-normal-oriented molecules are found at high coverage, showing the CS, S, and LS phases found also on water. Phases comprising surface parallel molecules, which do not exist on water, are found here at low coverage. For the lowest coverage a two-dimensional gas phase is found, followed, upon increasing the coverage, by an n-dependent sequence of condensed phases of up to four layers of surface-parallel molecules before converting to the surface-normal phases. In contrast with the surface-normal phases, all of the surface-parallel phases are found to lack long-range order in the surface-parallel direction. Adsorption energies are derived from the phase diagram for the alkyl chain and the alcohol headgroup.     

Dependence of cyano bonded phase hydrolytic stability on ligand structure and solution pH

As part of our program to develop more stable cyano (CN) high-performance liquid chromatography (HPLC) column packings, we have evaluated hydrolytic stability as a function of ligand connectivity, chain length, and side group steric protection and the pH of the mobile phase. Three accelerated tests were used to evaluate stability: (1) A non-HPLC screening test measuring carbon loss in refluxing MeOH-100 mM KH2PO4 pH 4.5 (1:1, v/v) solution; (2) a continuous flow HPLC test measuring capacity factor maintenance in 1% trifluoroacetic acid in water (pH 1.02) at 80 degrees C; and (3) a continuous flow HPLC test measuring column efficiency maintenance in 50 mM triethylamine in water (pH 10.00) at 50 degrees C. The stability of the CN phases was found to be dependent on both ligand chemical structure and the pH of the test conditions. The starting screen test of intermediate pH was least able to differentiate the CN phases based on structure, because two different degradation mechanisms appear to offset each other (acid induced siloxane bond cleavage vs. base induced silica dissolution). A trifunctional and a sterically protected CN phase were notably stable under the acidic test conditions, but had poor stability under basic conditions. Conversely, chain extension afforded poor stability under acidic conditions, but did afford improved stability at higher pH. In total, the data indicate that good CN column stability can be achieved by using a trifunctional or a sterically protected phase in acidic mobile phases. However, as mobile phases of intermediate or higher pH are employed, shorter column lifetimes can be expected due to an accelerated dissolution of the underlying silica substrate. Materials were also compared chromatographically using a mixture of non-polar, polar, and basic analytes under reversed-phase conditions.     

The effect of molecular weight on the structure and temperature behavior of a thermotropic main-chain liquid crystalline copolyester

The structure of a fully aromatic thermotropic liquid crystalline (LC) copolyester poly-[(phenyl-p-phenylene)-co-(terephthalate)-co-(p-hydroxybenzoate)] (PES) prepared from terephthalic acid, phenylhydroquinone, andp-hydroxybenzoic acid at a molar ratio of 45/45/10, respectively, was studied at ambient and elevated temperatures by means of x-ray diffraction and differential scanning calorimetry as a function of molecular weight. On heating of PES fibers with fixed ends an irreversible phase separation process takes place above the glass transition point and two different crystalline phases are formed. A model is proposed where the phases are assumed to contain the constituents of the statistical copolymer in different amounts. The relative volume fraction of the two crystalline modifications depends on the molecular weight of the investigated fibers. At higher temperatures the melting of the two crystalline phases and their transition to a LC nematic mesophase is observed.     

Characteristics of relaxational dynamics and the structure of nematogens

Abstract

Dielectric relaxation of two esters, 4-cyanophenyl-4-n-butyl-(n-pentyl-)cyclohex-1-enecarboxylates, in the frequency range 0-5 to 100 MHz, has been investigated for nematic and isotropic phases. Bifurcation of the loss peak for the nematic phase is observed. For the n-pentyl compound, we conclude that molecular dimerization could play an essential role in mesophase formation.     

Effect of anionic structure on the phase formation and hydrophobicity of amino acid ionic liquids aqueous two-phase systems

Compared with the conventional ionic liquids, amino acid ionic liquids are more biodegradable and biocompatible, and can enhance stability of biomaterials. In this work, amino acid ionic liquids 1-butyl-3-methylimidazolium L-serine ([C(4)mim][Ser]), 1-butyl-3-methylimidazolium glycine ([C(4)mim][Gly]), 1-butyl-3-methylimidazolium L-alanine ([C(4)mim][Ala]) and 1-butyl-3-methylimidazolium L-leucine ([C(4)mim][Leu]) have been synthesized. These ionic liquids are found to form aqueous two-phase systems (ATPSs) by the salted-out of K(3)PO(4) in aqueous solutions. Phase diagram of the ATPSs and the Gibbs energies of transfer of methylene group from the bottom salt-rich phase to the top ionic liquid-rich phase have been determined at 298.15K and pH 14, and the effect of anionic structure of the ionic liquids on phase formation of the ATPSs and the relative hydrophobicity between the top and the bottom phases are then examined. In order to understand the effect of relative hydrophobicity of the phases in equilibrium in the ATPSs on the extraction/separation capability of biomolecules, the partition coefficients of cytochrome-c (as a model biomolecule) in the ATPSs are measured by spectrophotometry. It is suggested that hydrophobic interactions are mainly responsible for the higher partition coefficients of cytochrome-c in aqueous two-phase systems at pH 14, and the extraction and separation capacity of biomolecules can be improved by the modulation of the relative hydrophobicity of the phases and/or the pH of the system.     

The preparation,structure, and conductivity of scandium-substituted NASICONs

A series of NASICON type compounds was prepared in which Sc(III) partially replaced Zr(IV). By simultaneously correlating the extent of this substitution with the amount of Si(IV) replacement of P(V) it was possible to obtain a series with a constant amount (3 mole) of sodium ion. All of the rhombohedral members of this series exhibited similar conductivities and activation energies over the temperature range 30–350°C. Transformation to the monoclinic phase occurred at a composition of Na₃Sc_0.5Zr_1.5Si_1.5P_1.5O₁₂ and was accompanied by a threefold increase in conductivity. Activation energies of the monoclinic (room temperature) phases in the temperature range 200–300°C were 0.2–0.23 eV. Crystal structures of the rhombohedral phase of composition Na₃ScZrSiP₂O₁₂ and the monoclinic phase, Na₃Sc_0.5Zr_1.5Si_1.5P_1.5O₁₂, were carried out by Rietveld refinement of their X-ray powder patterns. The structures confirmed those presented by Hong, and also confirmed the stoichiometry of these phases as evidenced from site occupancy refinement. These results were compared with the structure of a nonstoichiometric phase, Na_3.20Zr_1.68Si_1.84P_1.16O_11.54, prepared by a hydrothermal route. In the latter case it was shown that the low Zr content is due to Na⁺ substitution for Zr⁴⁺. No such substitution occurred in the scandium containing phases, and it is suggested that when Sc³⁺ substitution for zirconium takes place it prevents the sodium ion from occupying these sites.     

Simple metal and binary alloy phases based on the hcp structure: Electronic origin of distortions and superlattices

Crystal structures of simple metals and binary alloy phases based on the close-packed hexagonal (hcp) structure are analyzed within the model of Fermi sphere – Brillouin zone interactions to understand distortions and superlattices. Examination of the Brillouin-Jones configuration in relation to the nearly-free electron Fermi sphere for several representative phases reveals significance of the electron energy contribution to the phase stability. This approach may be useful for understanding high pressure structures recently found in compressed simple alkali and alkali-earth metals.     

Raman spectra and the dynamic structure of the high pressure phase of NH4Br (V) and ND4Br (V)

Similar Raman spectra are observed at high pressures for phases II and V of ND₄Br and NH₄Br. Deuteration lowers the II–V phase transition from 20 to 9 kbar at 296 K. ND₄Br V and NH₄Br V are interpreted as mixed phases, and their spectra as the superposition of the spectra of two other phases, III (antiparallel arrangement of the NH₄⁺ ions) and IV (parallel arrangement). The phonons which become Raman inactive at the V–IV phase transition are assigned to clusters or domains of phase V which have antiparallel arrangement.