Chemistry of dicumyl peroxide-induced crosslinking of linear polyethylene

The course of intermolecular reactions induced by peroxide decomposition in linear polyethylene has been determined and compared with similar reaction systems in which the substrate is a lowmolecular-weight alkane. Efficiency of intermolecular coupling is 40% for an alkane and 25–30% for the polymer system. Competing reactions, which reduce coupling efficiency, are mainly disproportionation of the initially formed radicals and to a minor extent further reactions involving the products from the peroxide decomposition. The low efficiencies found are similar to those well studied when initial radicals are produced by radiolysis in polymer and alkane systems.     

Thermal analysis of normal alkanes

The thermal analysis of n-alkanes and their mixtures has shown that a complicated binary system sometimes exists. This fact makes, it difficult to interpret the heating curves, in certain cases it will not be possible to calculate from these curves, in a simple manner the amount of a second component present in a given alkane. A complete insight into the behaviour of the binary system will then be necessary in order to interpret the heating curve and to estimate the amount of “impurity” accompanying a particular alkane Multi-component systems may even have to be studied if more than one “impurity” is present     

Organic molecule-modulated phase evolution of inorganic mesostructures

The involvement of alkane in the P123-TEOS-NH4F-H3O+ synthesis system alters the phase behavior of the complex emulsion system dramatically. Changing one of the reaction parameters (such as the initial reaction temperature, IRT) will result in diverse solution mesostructures. With subsequent condensation of silicate species, interesting inorganic materials with various mesostructures are obtained. The present work is aimed at understanding the phase evolution behavior of this complex alkane (C6-C12)-P123-TEOS-NH4F-H3O+ emulsion system, with emphasis on the influence of alkane chain number (ACN) and IRT. HREM (high-resolution electron microscopy), XRD (X-ray diffraction), nitrogen sorption, FFEM (freeze-fracture electron microscope), and interfacial tension techniques have been used to investigate the phase behavior of the emulsion system and the structure of the inorganic products. A linear relationship between the phase-transformation temperature (PTT) and ACN has been established, which could be attributed to the modification of alkane with respect to the hydrophobic-hydrophilic properties of the complex emulsion system. Moreover, the right combination of reaction temperature, ACNs, and thus-induced swelling of hydrophobic PPO blocks as well as the modification of hydrophilicity of PEO brushes by silicate oligmers is the driving force in altering the packing parameter/geometry of the copolymers surfactant (P123) aggregates. This leads to the diverse structures of the obtained mesoporous silicas. A temperature-induced phase-transformation mechanism has also been proposed and discussed.     

Flocculation behavior of asphaltenes in solvent/nonsolvent systems

Diffusion coefficients of asphaltenes dissolved in two aromatic solvents, toluene-d(8) or ethylbenzene-d(10), were measured with the pulsed-field gradient spin echo nuclear magnetic resonance (PFG-SE NMR) technique upon addition of flocculant (pentane-d(12) or heptane-d(16)). It was observed that the change in the diffusion coefficients, as a function of amount of added flocculant, was small in the concentration interval studied (up to 30 wt% alkane). Complementary kinetic flocculation studies were made at alkane additions above 55 wt%. The initial change in turbidity upon the addition of alkane was measured with an UV-VIS spectrophotometer. The obtained stability ratio, W, showed that asphaltenes were least stable in the ethylbenzene-pentane system and most stable in the toluene-heptane system. These findings were in agreement with the PFG-SE NMR. When combining the results from the two different techniques it appeared as if there was a dramatic increase in flocculation above a certain "threshold concentration" of added alkane. Furthermore, the flocculation appeared to be reaction controlled until as much as 63 wt% of n-pentane or, alternatively, 68 wt% of n-heptane had been added to the systems, after which the flocculation became primarily diffusion controlled. Finally, careful relaxation measurements showed that the asphaltenes displayed two distinctly different transverse (T(2)) relaxation times (most probably averages), one at 0.6 ms and the other at 7 ms.     

Effect of acetonitrile on the catalytic decomposition of hydrogen peroxide by vanadium ions and conjugated oxidation of alkanes

The rate of hydrogen peroxide decomposition in acetonitrile in the presence of a vanadate anion and pyrazine-2-carboxylic acid decreases remarkably when alkane (cyclohexane, n-heptane, isooctane) is added to the reaction solution. The alkane added is oxidized by this system to alkyl hydroperoxide. This is explained by the fact that much more hydrogen peroxide molecules are consumed to acetonitrile oxidation with formation of the final products, which is suppressed considerably by additives of necessary amounts of alkane, than those consumed to the oxidation of cyclohexane to form cyclohexyl hydroperoxide. In an organic solvent, H₂O₂ decomposes in a non-chain radical process.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2231–2234, October, 2004     

Cyanex 923-H2SO4体系萃取第3相的形成和应用

三相体系;Cyanex 923-H2SO4体系萃取第3相的形成和应用     

Alkane/Alcohol mixed monolayers at the solid/liquid interface

In this work, we present the behavior of solid monolayers of binary mixtures of alkanes and alcohols adsorbed on the surface of graphite from their liquid mixtures. We demonstrate that solid monolayers form for all the combinations investigated here. Differential scanning calorimetry (DSC) is used to identify the surface phase behavior of these mixtures, and elastic neutron incoherent scattering has been used to determine the composition of the mixed monolayers inferred by the calorimetry. The mixing behavior of the alcohol/alkane monolayer mixtures is compared quantitatively with alkane/alkane and alcohol/alcohol mixtures using a regular solution approach to model the incomplete mixing in the solid monolayer with preferential adsorption determining the surface composition. This analysis indicates the preferential adsorption of alcohols over alkanes of comparable alkyl chain length and even preferential adsorption of shorter alcohols over longer alkanes, which contrasts strongly with mixtures of alkane/alkane and alcohol/alcohol of different alkyl chain lengths where the longer homologue is always found to preferentially adsorb over the shorter. The alcohol/alkane mixtures are all found to phase separate to a significant extent in the adsorbed layer mixtures even when molecules are of a similar size. Again, this contrasts strongly with alkane/alkane and alcohol/alcohol mixtures where, although phase separation is found for molecules of significantly different size, good mixing is found for similar size species.     

Crystallization of alkane melts induced by carbon nanotubes and graphene nanosheets: a molecular dynamics simulation study

The crystallization of alkane melts on carbon nanotubes (CNT) and the surface of graphene nanosheets (GNS) is investigated using molecular dynamics (MD) simulations. The crystallization process of the alkane melts is analyzed in terms of the bond-orientational order parameter, atomic radial distribution for the CNT/alkane, atomic longitudinal distribution for the GNS/alkane, and diffusion properties. The dimensional effects of the different carbon-based nanostructures on the crystallization of alkane melts are shown. It is found that one-dimensional CNT has a stronger ability to induce the crystallization of the polymer than that of two-dimensional GNS, which provides a support at molecular level for the experimental observation [Li et al., J. Am. Chem. Soc., 2006, 128, 1692 and Xu et al., Macromolecules, 2010, 43, 5000]. From the MD simulations, we also find that the crystallization of alkane molecules has been completed with the highly cooperative processes of adsorption and orientation.     

Reactions of atomic transition-metal ions with long-chain alkanes

Understanding metal ion interactions with long-chain alkanes not only is of fundamental importance in the areas of organometallic chemistry, surface chemistry, and catalysis, but also has significant implication in mass spectrometry method development for the analysis of polyethylene. Polyethylene represents one of the most challenging classes of polymers to be analyzed by mass spectrometry. In this work, reactions of several transition-metal ions including Cr+, Mn+, Fe+, Co+, Ni+, Cu+, and Ag+ with long-chain alkanes, C28H58 and C36H74, are reported. A metal powder and the nonvolatile alkane are co-deposited onto a sample target of a laser desorption/ionization (LDI) time-of-flight mass spectrometer. The metal ions generated by LDI react with the vaporized alkane during desorption. It is found that all these metal ions can form adduct ions with the long-chain alkanes. Fe+, Co+, and Ni+ produce in-source fragment ions resulting from dehydrogenation and dealkylation of the adduct ions. The post-source decay (PSD) spectra of the metal-alkane adduct ions are recorded. It is shown that PSD of Ag+ alkane adduct ions produces bare metal ions only, suggesting weak binding between this metal ion and alkane. The PSD spectra of the Fe+, Co+, and Ni+ alkane adduct ions display extensive fragmentation. Fragment ions are also observed in the PSD spectra of Cr+, Mn+, and Cu+ alkane adduct ions. The high reactivity of Fe+, Co+, and Ni+ is consistent with that observed in small alkane systems. The unusually high reactivity of Cr+, Mn+, and Cu+ is rationalized by a reaction scheme where a long-chain alkane first forms a complex with a metal ion via ion/induced dipole interactions. If sufficient internal energy is gained during the complex formation, metal ions can be inserted into C-H and C-C bonds of the alkane, followed by fragmentation. The thermal energy of the neutral alkane is believed to be the main source of the internal energy acquired in the complex. Finally, the implication of this work on mass spectrometry method development for polyethylene analysis is discussed.     

Monte Carlo simulation for the adsorption and separation of linear and branched alkanes in IRMOF-1

The adsorption and separation of linear and branched alkanes in the isoreticular metal-organic framework IRMOF-1 have been investigated using Monte Carlo simulation. For pure linear alkanes (C1-nC5), the limiting adsorption properties exhibit linear behavior with the alkane carbon number; the long alkane is preferentially adsorbed over the short alkane at low fugacities, whereas the reverse is found at high fugacities. For pure branched alkanes (C5 isomers), the linear isomer adsorbs more than its branched analogue. The adsorbed amounts of pure alkanes in IRMOF-1 are substantially greater than in a carbon nanotube bundle and in silicalite. For a five-component mixture of C1 to nC5 linear alkanes, the long alkane adsorption first increases and then decreases with increasing fugacity, whereas short alkane adsorption continually increases and progressively replaces the long alkane at high fugacity due to the size entropy effect. For a three-component mixture of C5 isomers, the adsorption of each isomer increases with increasing fugacity until saturation, though there is less adsorption of the branched isomer due to the configurational entropy effect. The adsorption selectivity among the alkanes in IRMOF-1 is smaller than in a carbon nanotube bundle and in silicalite.     

Evidence for alkane coordination to an electron-rich uranium center

A series of five uranium-alkane complexes of the general formula [(ArO)3tacn)U(alkane)].(cy-alkane) has been synthesized and crystallographically characterized. In all cases, X-ray diffraction studies revealed a pseudo-six-coordinate trivalent uranium core structure, [(ArO)3tacn)U], with a coordinated alkane ligand at the axial position. The average U-C bond distance to the bound alkane was determined to be 3.798 A, which is considerably shorter than the sum of the van der Waals radii of the U atom and a CH2 or CH3 unit (3.9 A). In all complexes, the alkane is coordinated in an eta2-H,C fashion.     

Molecular tilting and its impact on frictional properties of n-alkane self-assembled monolayers

Hydrophobic, methyl-terminated self-assembled monolayer (SAM) surfaces can be used to reduce friction. Among methyl-terminated SAMs, the frictional properties of alkanethiol SAMs and silane SAMs have been well-studied. In this research, we investigated friction of methyl-terminated n-hexatriacontane (C36) SAM and compared its friction properties with the alkanethiol and silane SAMs. Alkane SAM does not have an anchoring group. The alkane molecules stand on the surface by physical adsorption, which leads to a higher surface mobility of alkane molecules. We found that C36 SAM has a higher coefficient of friction than that of octadecyltrichlorosilane (OTS) silane. When an atomic force microscope (AFM) tip was swiped across the alkane SAM with a loading force, we found that the alkane SAM can withstand the tip loading pressure up to 0.48 GPa. Between 0.48 and 0.49Ga, the AFM tip partially penetrated the SAM. When the tip moved away, the deformed SAM healed and maintained the structural integrity. When the loading pressure was higher than 0.49 GPa, the alkane SAM was shaved into small pieces by the tip. In addition, we found that the molecular tilting of C36 molecules interacted with the tribological properties of the alkane SAM surface. On one hand, a higher loading force can push the rod-like alkane molecules to a higher tilting angle; on the other hand, a higher molecular tilting leads to a lower friction surface.     

Pervaporation separation of alkane/thiophene mixtures with PDMS membrane

Worldwide concerns over environment have stimulated increasing interest both in academic and industry for deep desulfurization of gasoline. Polydimethylsiloxane (PDMS) composite membrane was used to separate the binary and multicomponent alkane/thiophene mixtures by pervaporation. Effect of carbon number and concentration of alkane, and of feed temperature, on the separation efficiency of alkane/thiophene mixtures was investigated experimentally. Experimental results of binary mixtures indicated that the total fluxes for different alkane/thiophene mixtures decrease with increase of carbon number in the alkanes. Corresponding activation energies of permeation for alkanes in PDMS membrane increase with increase of carbon number in the alkanes. Differences of molecular size and structure of the alkanes lead to various selectivities thereof within PDMS membrane. In addition, the permeability and activation energy of thiophene in various systems differ from each other due to coupling effect which should be taken into consideration when dealing with multicomponent systems. Pervaporation results of ternary systems indicated that, the increase of content of lighter alkane in feed would result in a larger total flux, but a smaller selectivity to thiophene simultaneously. A quaternary system, the mixture of n-heptane, n-octane, n-nonane and thiophene, was employed to simulate the desulfurization process of gasoline. With the membrane having a PDMS layer of 11 μm, the total flux was measured to be about 1.65 kg/m² h, with the corresponding enrichment factor of thiophene 3.9 at 30 °C.     

THE EFFECT OF ALKANES ON THE FORMATION OF ULTRAFINE SILVER BROMIDE PARTICLES IN IONIC W/O MICROEMULSIONS

ABSTRACT

Ultrafine AgBr particles formed in w/o microemulsions with different chain length of alkanes were studied by UV-VIS spectrophotometry, stopped-flow spectrophotometry and transmission electron microscopy. The ultrafine particles of AgBr (5-7?nm in diameter) were obtained by mixing two separately prepared microemulsion systems both containing AOT, n-hexane and an aqueous solution of either AgNO³ or NaBr.The size distribution of the nearly spherical AgBr particles broadened from 5 to about 10?nm as chain length of alkane was increased from n-hexane to n-octane. When n-dodecane or a higher alkane was used, the microemulsion system became increasingly unstable causing precipitation of AgBr.     

Engineered complex emulsion system: toward modulating the pore length and morphological architecture of mesoporous silicas

In the complex alkane/P123/TEOS/H2O emulsion system, an emulsion engineering method to modulate pore length and morphological architecture of mesoporous materials has been built. With fine tuning of the synthetic parameters (e.g., the composition of the synthetic mixtures, temperature, stirring, etc.), a series of chemically significant mesostructures (i.e., short-pore SBA-15 materials) with tunable pore length and morphological architecture have been successfully constructed. The effects of alkane solubilizates on pore length and particle morphology are discussed. The resulting short-pore materials would have potential applications in the fields of adsorption/separation of biomolecules and inclusion chemistry of guest species, etc.     

Relation between distance-based topological indices

The pi-index of the line graph of the molecular graph of an alkane is shown to be related to the pi, Harary, and reciprocal Wiener indices of that alkane. Thus a mathematical relation between several distance-based topological indices is obtained.     

Alkane Shape- and Size-Recognized Selective Vapor Sorption in “Channel-Like” Crystals Based on Thiacalixarene Assemblies

Alkanes composed of C−C and C−H show a low electric polarization, and therefore, there is only very weak interaction between alkanes and adsorbents. Thus, it is difficult to separate a specific alkane from a mixture of alkanes by adsorption. Here, two activated “channel-like” crystals generated from brominated thiacalix[4]arene propyl ethers, which adopt 1,3-alternate and partial cone conformations, recognize specific alkane vapors depending on alkane-shape and -size, sorting in three-type alkane guests such as linear, branched, and cyclic alkanes. Two activated crystals, which are prepared by removal of solvent upon heating under reduced pressure, incorporate branched and/or cyclic alkane vapors by a unique “gate-opening” mechanism via a crystal transformation in the process. Linear alkane vapors do not trigger gate opening and are not taken up by the activated crystals. The shape and size molecular-recognition properties of the activated crystals promises considerable usefulness for the separation of linear, branched, and cyclic alkanes.     

Transition metal-alkane σ-complexes with oxygen donor co-ligands

A new family of long-lived alkane σ-complexes of the type (L(OEt))Re(CO)(2)(alkane) [alkane = cyclopentane, cyclohexane, pentane; L(OEt) = cyclopentadienyltris(diethylphosphito)cobaltate(III)] has been observed using both IR and NMR spectroscopies and computationally interrogated with DFT methods. The oxygen-rich coordination spheres makes these complexes perhaps more relevant as models for intermediates in metal oxide mediated hydrocarbon transformations than other known alkane σ-complexes.