Molecular simulation and macroscopic modeling of the diffusion of hydrogen, carbon monoxide and water in heavy n-alkane mixtures

The self-diffusion coefficient of hydrogen (H(2)), carbon monoxide (CO) and water (H(2)O) in n-alkanes was studied by molecular dynamics simulation. Diffusion in a few pure n-alkanes (namely n-C(8), n-C(20), n-C(64) and n-C(96)) was examined. In addition, binary n-C(12)-n-C(96) mixtures with various compositions as well as more realistic five- and six-n-alkane component mixtures were simulated. In all cases, the TraPPE united atom force field was used for the n-alkane molecules. The force field for the mixture of n-alkanes was initially validated against experimental density values and was shown to be accurate. Moreover, macroscopic correlations for predicting diffusion coefficient of H(2), CO and H(2)O in n-alkanes and mixtures of n-alkanes were developed. The functional form of the correlation was based on the rough hard sphere theory (RHS). The correlation was applied to simulation data and an absolute average deviation (AAD) of 5.8% for pure n-alkanes and 3.4% for n-alkane mixtures was obtained. Correlation parameters vary in a systematic way with carbon number and so they can be used to provide predictions in the absence of any experimental or molecular simulation data. Finally, in order to reduce the number of adjustable parameters, for the n-alkane mixtures the "pseudo-carbon number" approach was used. This approach resulted in relatively higher deviation from MD simulation data (AAD of 18.2%); however, it provides a convenient and fast method to predict diffusion coefficients. The correlations developed here are expected to be useful for engineering calculations related to the design of the Gas-to-Liquid process.     

Study of the thermal diffusion behavior of alkane/benzene mixtures by thermal diffusion forced rayleigh scattering experiments and lattice model calculations

In this work the thermal diffusion behavior of binary mixtures of linear alkanes (heptane, nonane, undecane, tridecane, pentadecane, heptadecane) in benzene has been investigated by thermal diffusion forced Rayleigh scattering (TDFRS) for a range of concentrations and temperatures. The Soret coefficient ST of the alkane was found to be negative for these n-alkane/benzene mixtures indicating that the alkanes are enriched in the warmer regions of the liquid mixtures. For the compositions investigated in this work, the magnitude of the Soret coefficient decreases with increasing chain length and increasing alkane content of the mixtures. The temperature dependence of the Soret coefficient depends on mixture composition and alkane chain length; the slope of ST versus temperature changes from positive to negative with increasing chain length at intermediate compositions. To study the influence of molecular architecture on the Soret effect, mixtures of branched alkanes (2-methylhexane, 3-methylhexane, 2,3-dimethylpentane, 2,4-dimethylpentane, 2,2,3-trimethylbutane, and 2,2,4-trimethylpentane) in benzene were also investigated. Our results for the Soret coefficients show that the tendency for the alkanes to move to the warmer regions of the fluid decreases with increasing degree of branching. The branching effect is so strong that for 2,2,4-trimethylpentane/benzene mixtures the Soret coefficient changes sign at high alkane content and that equimolar 2,2,3-trimethylbutane/benzene mixtures have positive Soret coefficients in the investigated temperature range. In order to investigate the effect of molecular interactions on thermal diffusion, we adapted a recently developed two-chamber lattice model to n-alkane/benzene mixtures. The model includes the effects of chain-length, compressibility, and orientation dependence of benzene-benzene interactions and yields good qualitative predictions for the Soret effect in n-alkane/benzene mixtures. For the branched isomers, we find some correlations between the moments of inertia of the molecules and the Soret coefficients. PACS numbers: 66.10.Cb, 61.25.Hq.     

Modeling the phase behavior of H2S+n-alkane binary mixtures using the SAFT-VR+D approach

A statistical associating fluid theory for potential of variable range has been recently developed to model dipolar fluids (SAFT-VR+D) [Zhao and McCabe, J. Chem. Phys. 2006, 125, 104504]. The SAFT-VR+D equation explicitly accounts for dipolar interactions and their effect on the thermodynamics and structure of a fluid by using the generalized mean spherical approximation (GMSA) to describe a reference fluid of dipolar square-well segments. In this work, we apply the SAFT-VR+D approach to real mixtures of dipolar fluids. In particular, we examine the high-pressure phase diagram of hydrogen sulfide+n-alkane binary mixtures. Hydrogen sulfide is modeled as an associating spherical molecule with four off-center sites to mimic hydrogen bonding and an embedded dipole moment (micro) to describe the polarity of H2S. The n-alkane molecules are modeled as spherical segments tangentially bonded together to form chains of length m, as in the original SAFT-VR approach. By using simple Lorentz-Berthelot combining rules, the theoretical predictions from the SAFT-VR+D equation are found to be in excellent overall agreement with experimental data. In particular, the theory is able to accurately describe the different types of phase behavior observed for these mixtures as the molecular weight of the alkane is varied: type III phase behavior, according to the scheme of classification by Scott and Konynenburg, for the H2S+methane system, type IIA (with the presence of azeotropy) for the H2S+ethane and+propane mixtures; and type I phase behavior for mixtures of H2S and longer n-alkanes up to n-decane. The theory is also able to predict in a qualitative manner the solubility of hydrogen sulfide in heavy n-alkanes.     

Phase behavior of undecane-tetradecane mixtures confined in SBA-15

Phase behavior of undecane-tetradecane (n-C11H24-C14H30, C11-C14) mixtures in bulk and confined in SBA-15 have been studied using differential scanning calorimetry. The bulk C11-C14 system shows multiple phase regions due to rotator phase. Confined in the pores of SBA-15 (pore diameters 3.8-7.8 nm), the mixtures only show a melting boundary of a straight line and a curve, respectively. In SBA-15 (17.2 nm), phase behavior of themixtures has some similarity to that of the bulk. Under confinement, the phase diagrams of the mixtures vary with the pore size, temperature, and compositions.     

Structures of alkyl benzoate binary mixtures. A Kirkwood-Buff fluctuation theory study using UNIFAC

The structure of the alkyl benzoate + n-alkane, and + alkan-1-ol binary mixtures were analyzed according to the Kirkwood-Buff fluctuation theory on the basis of both the mixture properties measured over a wide temperature range and the activity coefficients calculated with the modified UNIFAC (Dortmund) model as well. Application of this model reveals that both the microheterogeneous structure and the clustering effects are strongly dependent on the chain length of the n-alkane and alkan-1-ol cosolvents. Knowledge of the local composition around each type of molecule is drawn from the Kirkwood-Buff integrals and the excess (or deficit) molecules aggregated around a central one. The rather high values of the integrals evaluated for some of these systems provide first-hand evidence for phase splitting. The conclusions drawn support previous analyses and confirm the adequacy of the methodology put forward for studying liquid mixtures at microscopic level; easily measurable experimental properties can advantageously be used with the fluctuation theory.     

四氯合锌酸十六烷基铵-四氯合锌酸十八烷基铵二元体系亚固相相图

合成了具有层状结构的热致相变材料四氯合锌酸十六铵(n-C16H33NH3)2ZnCl4,C16Zn)和四氯合锌酸十八铵(n-C18H37NH3)2ZnCl4,C18Zn)(在340~370 K的温度范围内存在着焓值较大的可逆固-固相变)。 并将此2种材料在乙醇溶液中结晶出一系列二元体系。 对二元体系利用差热分析(DTA)和X射线衍射技术进行测定,构筑了C16ZnC18Zn二元体系亚固相相图。 根据相图,确定了w(C16Zn)=41.11%处有中间化合物(n-C16H33NH3)(n-C18H37NH3)ZnCl4的存在,并测定在w(C16Zn)=16.19%和w(C16Zn)=63.07% 2处存在着2个不变的共析点,2个共析点温度分别约为356和353 K。 与同类体系的其它相图相比,在此相图的左右边界存在端际固溶体（α和β）和中间区域存在非化学计量相（γ）。     

Wetting behavior of mixtures of water and nonionic polyoxyethylene alcohol

Five binary water + C4Ej mixtures, water + n-C4E0, water + 2-C4E0, water + iso-C4E0, water + n-C4E1, and water + iso-C4E1, were chosen to perform the surface/interfacial tension measurements over the experimental temperature range from 10 to 85 degrees C at the normal pressure by using a homemade pendent drop/bubble tensiometer. The symbol CiEj is the abbreviation of a nonionic polyoxyethylene alcohol CiH(2i+1)(OCH2CH2)jOH. The wetting behavior of the CiEj-rich phase at the interface separating gas and the aqueous phase is systematically examined according to the wetting coefficient resulting from the experimental data of surface/interfacial tensions measurements. For those systems with a lower critical solution temperature, for example, water + n-C6E2, water + n-C4E1, and water + iso-C4E1, a wetting transition from partial wetting to nonwetting is always observed when the system is brought to close to its lower critical solution temperature. On the other hand, to start with a partial wetting CiEj-rich phase, a wetting transition from partial wetting to complete wetting is always observed when the system is driven to approach its upper critical solution temperature. The effect of hydrophobicity of CiEj on the wetting behavior of the CiEj-rich phase at the interface separating gas and the aqueous phase was carefully investigated by using five sets of mixtures: (1) water + n-C4E0, water + n-C5E0, and water + n-C6E0; (2) water + 2-C4E0 and water + 2-C5E0; (3) water + 2-C4E0 and water + n-C4E0; (4) water + n-C4E1, water + n-C5E1, and water + n-C6E1; (5) water + n-C4E0 and water + n-C4E1. The CiEj-rich phase would tend to drive away from complete wetting (or nonwetting) to partial wetting with an increase in the hydrophobicity of CiEj in the binary water + CiEj system. All the wetting behavior observed in the water + CiEj mixtures is consistent with the prediction of the critical point wetting theory of Cahn.     

Equation of state for square-well chain molecules with variable range II. Extension to mixtures

An equation of state (EOS) developed in our previous work for square-well chain molecules with variable range is further extended to the mixtures of non-associating fluids. The volumetric properties of binary mixtures for small molecules as well as polymer blends can well be predicted without using adjustable parameter. With one temperature-independent binary interaction parameter, satisfactory correlations for experimental vapor–liquid equilibria (VLE) data of binary normal fluid mixtures at low and elevated pressures are obtained. In addition, VLE of n-alkane mixtures and nitrogen + n-alkane mixtures at high pressures are well predicted using this EOS. The phase behavior calculations on polymer mixture solutions are also investigated using one-fluid mixing rule. The equilibrium pressure and solubility of gas in polymer are evaluated with a single adjustable parameter and good results are obtained. The calculated results for gas + polymer systems are compared with those from other equations of state.     

Effect of chain length and substrate temperature on the growth and morphology of n-alkane thin films

Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and microscopy has been used to study the orientational morphology of thin films of the linear alkanes n-C36H74 and n-C60H122, prepared by vacuum deposition onto NaCl (001) surfaces at ambient and elevated substrate temperatures. The orientational morphology, specifically, the nature of domains with lateral and normal orientation, is explored as a function of the chain length and the substrate temperature. It is found that the longer n-C60H122 molecules are laterally oriented on the substrate surface within the investigated substrate temperatures but that the morphology of these thin films varies with substrate temperature. The shorter n-C36H74 molecules are partially laterally oriented at low substrate temperature and are completely normally oriented at high substrate temperature. The relative magnitude of "side-by-side" and "end-to-end" intermolecular interactions leads to the formation of highly ordered alkane structures with a high aspect ratio. The formation of complex, nanoscale orientational morphologies are rationalized by considering kinetic and thermodynamic effects, in particular, the relative enthalpic and entropic contributions to the free energy associated with the different molecular orientations.     

Global fluid phase equilibria and critical phenomena of selected mixtures using the crossover soft-SAFT equation

We present here the extension of the crossover soft-statistical associating fluid theory (soft-SAFT) equation of state to mixtures, as well as some illustrative applications of the methodology to mixtures of particular scientific and technological interest. The procedure is based on White's work (White, J. A. Fluid Phase Equilib. 1992, 75, 53) from the renormalization group theory, as for the pure fluids, with the isomorphism assumption applied to the mixtures. The equation is applied to three groups of mixtures: selected mixtures of n-alkanes, the CO2/n-alkane homologous series, and the CO2/1-alkanol homologous series. The crossover equation is first applied to the pure components of the mixtures, CO2 and the 1-alkanol family, while an available correlation is used for the molecular parameters of the n-alkane series (Llovell et al. J. Chem. Phys 2004, 121, 10715). A set of transferable molecular parameters is provided for the 1-alkanols series; these are accurate for the whole range of thermodynamic conditions. The crossover soft-SAFT equation is able to accurately describe these compounds near to and far from the critical point. The theory is then used to represent the phase behavior and the critical phenomena of the selected mixtures. We use binary interaction parameters xi and eta for dissimilar mixtures. These parameters are fitted at some particular conditions (one subcritical temperature or binary critical data) and used to predict the behavior of the mixture at different conditions (other subcritical conditions and/or critical conditions). The equation is able to capture the continuous change in the critical behavior of the CO2/n-alkane and the CO2/1-alkanol homologous series as the chain length of the second compound increases. Excellent agreement with experimental data is obtained, even in the most nonideal cases. The new equation is proved to be a powerful tool to study the global phase behavior of complex systems, as well as other thermodynamic properties of very challenging mixtures.     

Solid-fluid and solid-solid phase equilibrium in a model of n-alkane mixtures

Solid-fluid and solid-solid phase equilibrium for binary mixtures of hard sphere chains modeling n-hexane, n-heptane, and n-octane has been calculated using Monte Carlo computer simulations. Thermodynamic integration was used to calculate the Gibbs free energy and chemical potentials in the solid and fluid phases from pure component reference values. A multiple stage free energy perturbation method was used to calculate the composition derivative of the Gibbs free energy. Equation of state and free energy data for the fluid phase indicate ideal solution behavior. Nonideality is much more significant in the solid phase with only partial solubility of shorter chains in the longer chains and essentially no solubility at the other end of the composition range. The miscibility decreases with increasing chain length difference between the components. For the model of n-hexane/n-octane mixtures solid--solid phase separation has been observed directly in some of the simulations, with the components segregating between the layers of the solid structure. The behavior is similar to that seen in some binary n-alkane mixtures with longer chain lengths but comparable chain length ratios between the components. Such phase separation, although indicated thermodynamically, is not seen directly in the simulations of the n-heptane/n-octane mixture due to the difference in the pure component crystal structures.     

烷烃和胺类分子对电子激发态CH(A~2Δ和B~2Σ~-)自由基猝灭

用266nm激光光解CHBr_3分子产生CH(A,B)态自由基,通过测量CH(A,B→X)自发辐射的时间分辨信号测定室温下(CH_3)_2NH、(C_2H_5)_2NH、(C_2H_5)_3N、n-C_5H_(12)、n-C_6H_(14)和n-C_7H_(16)对CH(A,B,v'=0)的猝灭速率常数.发现猝灭速率常数与猝灭剂烷烃分子中的C-H键数近似成线性关系,但对大的烷烃分子,这种增加逐渐趋缓.用碰撞络合物模型计算胺类分子及烷烃分子与CH形成碰撞络合物时的生成截面,结果表明,在电子激发态CH自由基的猝灭过程中,碰撞对子间的多极相互吸引势和色散力作用势可能起重要作用.     

苯乙烯-氧乙烯-苯乙烯三嵌段聚合物的受限结晶行为的反气相分析

 采用反气相法研究了苯乙烯 氧乙烯 苯乙烯三嵌段结晶聚合物 (PS PEO PS)的结晶熔融相变 ,测定了PS PEO PS的结晶度、熔点以及熔程 ,探讨了正构烷烃探针分子的碳链长度对测定结果的影响。研究结果表明 :PS PEO PS的微相分离对PEO链段的结晶行为有较大的影响 ,其晶体结构中存在由多种不完善PEO结晶和PS非结晶构成的中间层 ;正构烷烃探针分子的碳链长度对测定PS PEO PS的熔点和熔程无影响 ,但对结晶度测定和PEO结晶熔融相变的检测影响较大 ,所测得PS PEO PS的结晶度随正构烷烃探针分子碳链的增长而降低。     

Application of the Wilson—Wegner expansion to represent vapor—liquid equilibrium surfaces of binary and ternary systems from the critical locus to the vapor pressure of the heavier component

The Torquato—Stell modification of the Wilson—Wegner expansion has been used to present and to represent the vapor—liquid equilibrium surfaces of several binary methane—alkane mixtures and one ternary system from the critical locus to the heavier component's vapor pressure within the experimental accuracy of the data. Inconsistent phase compositions are revealed without recourse to equations of state computations. Since the critical point of the mixtures are not precisely known, an isothermal regression analysis of the data is first used to obtain an improved estimate of the critical point of mixtures using the scaled expressions. Then, along isotherms, the phase compositions are analyzed in terms of the reduced proximity to the critical pressure. Subsequently, the entire compositional surfaces are presented in terms of both the reduced proximity to the critical pressure of the mixture and the reduced proximity to the critical temperature of the heavier component.     

EFFECT OF DILUENTS ON CRYSTALLIZATION OF POLY(VINYLIDENE FLUORIDE) AND PHASE SEPARATED STRUCTURE IN A TERNARY SYSTEM via THERMALLY INDUCED PHASE SEPARATION

The phase diagram of a ternary system of PVDF,dibutyl phthalate (DBP) and di(2-ethylhexyl) phthalate (DEHP) was determined in terms of a pseudo binary system with the same polymer concentration and different DBP content in diluent mixture.The experimental results showed that as the DBP content increased in diluent mixture,the phase separation changed from liquid-liquid phase separation to solid-liquid phase separation,and both the cloudy point for L-L phase separation and crystallization temperature shif...     

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.     

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.     

蒸气相中笼形类沸石材料MTN单晶的合成规律

采用蒸气相合成法,以正丙胺作为笼形类沸石材料MTN合成的新模板剂,合成出MTN单晶。通过改变F-/Si、氟源种类、液相中胺/水、结晶温度等指标,考察了MTN的合成规律。并采用XRD、SEM、FT-IR对样品进行了物化性能表征。实验表明,在凝胶组成为(0.5~1.0)NH4F-SiO2-6.7H2O,液相组成为PrNH2/ (PrNH2+H2O)(摩尔比) =16.7%~100%的反应体系中,453K~493K晶化反应15d,能够合成MTN。 MTN的相对结晶度分别随着正丙胺的摩尔比、氟化铵与二氧化硅的摩尔比以及晶化温度的升高而升高。合成的MTN晶体粒度达到250μm。     

Ellipsometric and neutron diffraction study of pentane physisorbed on graphite

High-resolution ellipsometry and neutron diffraction measurements have been used to investigate the structure, growth, and wetting behavior of fluid pentane (n-C(5)H(12)) films adsorbed on graphite substrates. We present isotherms of the thickness of pentane films adsorbed on the basal-plane surfaces of a pyrolytic graphite substrate as a function of the vapor pressure. These isotherms are measured ellipsometrically for temperatures between 130 and 190 K. We also describe neutron diffraction measurements in the temperature range 11-140 K on a deuterated pentane (n-C(5)D(12)) monolayer adsorbed on an exfoliated graphite substrate. Below a temperature of 99 K, the diffraction patterns are consistent with a rectangular centered structure. Above the pentane triple point at 143.5 K, the ellipsometric measurements indicate layer-by-layer adsorption of at least seven fluid pentane layers, each having the same optical thickness. Analysis of the neutron diffraction pattern of a pentane monolayer at a temperature of 130 K is consistent with small clusters having a rectangular-centered structure and an area per molecule of approximately 37 A(2) in coexistence with a fluid monolayer phase. Assuming values of the polarizability tensor from the literature and that the monolayer fluid has the same areal density as that inferred for the coexisting clusters, we calculate an optical thickness of the fluid pentane layers in reasonable agreement with that measured by ellipsometry. We discuss how these results support the previously proposed "footprint reduction" mechanism of alkane monolayer melting. In the hypercritical regime, we show that the layering behavior is consistent with the two-dimensional Ising model and determine the critical temperatures for layers n = 2-5.