Diffusion Of Benzene Research Articles

Experimental diffusivity of energetic compounds determined by peak parking

Diffusivities of several explosive compounds, as well as other complex organic compounds were experimentally derived using a peak parking methodology. High performance liquid chromatography (HPLC) with stop flow ‘parking’ was used to experimentally determine diffusivity based on band broadening associated with chemical diffusion within the HPLC column. This research builds on prior methods by determining an obstruction factor through comparing benzene diffusion in methanol in a C18 column to the previously published capillary column. The method is useful for structures not easily described by computational chemistry methods or for solvents that do not have association coefficients in published diffusivity models. Using the peak parking method, diffusivities (cm2/s) ranged from 6.29 × 10-7 to 1.06 × 10-6 for hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), 4.82 × 10-5 to 1.26 × 10-4 for 3-nitro-1,2,4-triazol-5-one (NTO), 1.63 × 10-5 to 7.20 × 10-6 for nitroguanidine (NQ), 7.07 × 10-6 to 2.09 × 10-5 for pyrimidine-2-carboxylic acid, and 1.02 × 10-6 to 2.27 × 10-6 for streptomycin. Compared to diffusivities estimated computationally, the empirical diffusivities reported herein estimated by two different methods have percent differences averaging 142%, 174%, 27%, 85%, and 33% for RDX, NTO, NQ, pyrimidine-2-carboxylic acid, and streptomycin, respectively. NTO had experimental diffusivities greater than the computational values, while RDX and streptomycin had experimental diffusivities lower than computational values.

Study of the effect of temperature on thermophysical properties of ethyl myristate by dual-beam thermal lens technique

One of the components of biodiesels is ethyl myristate and the study and measurement of its thermal parameters are of great importance. The dual-beam laser thermal lens spectroscopy is used for accurate measurement of thermophysical parameters of ethyl myristate. First, to check the accuracy of our optical setup, thermal lens characteristic time, thermal diffusivity and thermal conductivity of carbon disulfide and benzene were measured at 298 K. There was a reasonable agreement between the measured values and the reference values. Then, the effect of temperature changes on thermal diffusivity and thermal conductivity values of ethyl myristate was studied. The results showed that with increasing temperature from 285 to 358 K, thermal diffusivity decreases from 8.75 to 7.45 × 10−8 m2/s, and thermal conductivity from 0.155 to 0.1432 W/mK. It was also shown that by changing in pump laser energy in the range of 0.06–0.12 joules, both thermal diffusivity and thermal conductivity of ethyl myristate are almost constant.

Full-scale multi-physics numerical analysis of an isothermal chemical vapor infiltration process for manufacturing C/C composites

The internal architecture of a CVI reactor significantly influences the gas flow behavior, as well as the complex time-varying chemical reactions, but has been typically ignored in previous CVI models. Herein we developed, validated, and applied a fully three-dimensional (3D) physicochemical CVI model of an industry-scale reactor to simulate an isothermal CVI process for fabricating bulk carbon-carbon composites using methane as a precursor gas and a multi-layered preform consisting of a non-crimp fabric and felt. The flow inside the reactor was modeled using the Navier–Stokes equation, coupled with the convection-diffusion equation, to simulate the dispersive behaviors of the reactive gases inside the porous preform. The interactive molecular diffusion of methane (CH4), ethylene (C2H4), acetylene (C2H2), and benzene (C6H6) were modeled by considering the multi-step hydrocarbon reactions between the species. The hydrocarbon concentration changes, resulting from the carbon deposition on the preform surface, were computed to predict the evolution of the preform density and porosity. The current surface area of the preform was then determined based on the current porosity. The numerical results for the average preform density agreed well with the experimental data. In addition, the present model can provide detailed simulations of the temporal and spatial evolution of the preform density that cannot be experimentally observed. The effectiveness and utility of the developed model could benefit the design of CVI reactors and processes and minimize the need for test runs when processing conditions change.

Liquid–Liquid Phase Separation of Viologen Bistriflimide/Benzene Mixtures: Role of the Dual Ionic and Organic Nature of Ionic Liquids

Liquid-liquid phase separation occurs at room temperature when mixing an excess of benzene with solid viologen bistriflimide salts with various alkyl side-chain lengths. A liquid phase composed of (almost) pure benzene is above the other sponge-like liquid phase with salt absorbed in benzene. Nuclear magnetic resonance experiments indicate that the mole ratio of benzene/salt in the sponge-like phase remains unchanged upon varying the amounts of (nonexcessive) salt or benzene. Moreover, the benzene/viologen salt mole ratio in the sponge-like phase increases linearly with respect to the side-chain length of the cation. Similarly, when an excess of viologen salt is added in benzene, a sponge-like liquid phase composed of salt absorbed by benzene is observed in equilibrium with some solid viologen salt neither dissolved nor absorbed by the solvent. The mole ratio of the sponge-like liquid phase again increases linearly with side-chain length, while it remains independent of the relative amount of benzene and viologen salt as long as the latter is in excess. Finally, when appropriate amounts of benzene and viologen salt are mixed, a single sponge-like liquid phase is observed at an intermediate composition between the lower and upper limits. Molecular dynamics simulations reveal that because of their dual ionic and organic nature, when absorbed in benzene, the studied salts form nanoscale segregated liquid structures, akin ionic liquids, with a continuous polar network composed of anions and cationic charged groups, along with nonpolar domains composed of alkyl cationic side chains. Benzene molecules are preferentially absorbed inside the nonpolar region, which effectively expands the nonpolar region to be sponge-like and consequently liquidizes the viologen salt. The linearity of the benzene/salt ratio in the upper and lower phase boundaries comes from the fact that the effective volume of the nonpolar region for accommodating benzene molecules grows linearly with cationic alkyl side-chain length. The occurrence of the above phenomena is attributed to the nonpolar feature of benzene molecules, and there is no evidence of π-π or ion-π interaction between the ions and benzene molecules. Moreover, the diffusion of benzene in the sponge-like phase is found to be close to that in n-alkanes, supporting the idea of nanoscale segregation of polar and nonpolar regions in the sponge-like phase. The revealed mechanism is anticipated to be general for understanding liquid-liquid phase separation observed in mixtures of organic salts (ionic liquids) having relatively long alkyl chains with small organic molecules.

TCE and PCE diffusion through five geomembranes including two coextruded with an EVOH layer

Aqueous diffusion of trichloroethylene (TCE) and tetrachloroethylene (PCE) is examined for high density polyethylene (HDPE), linear low density polyethylene (LLDPE), polyurethane/urea, and two polyethylene (PE) geomembranes coextruded with ethylene vinyl alcohol (EVOH). Additionally, the diffusion of benzene, toluene, ethylbenzene, and xylenes through polyurethane/urea geomembrane is examined. Permeation coefficients for HDPE, LLDPE, and polyurethane/urea range from 0.4-1.2 × 10−10 m2/s for TCE and 1.0-2.5 × 10−10 m2/s and for PCE. Experiments using the coextruded geomembranes have not reached equilibrium at 500 days, however parameters for the EVOH layer are deduced using data from these experiments. Using the parameters of the individual layers, single layer parameters were calculated. These single layer parameters range from 0.37-2.2 × 10−12 m2/s for TCE to 0.28-0.93 × 10−12 m2/s for PCE. Two hypothetical vapour intrusion cases are modelled using the parameters developed for the five geomembranes, and the calculated airspace concentrations decrease depending on the choice of vapour barrier in the following order: no barrier >0.75 mm LLDPE >1.5 mm polyurethane/urea >1.5 mm HDPE >0.75 mm LLDPE/EVOH/LLDPE >1.5 mm HDPE/EVOH/HDPE.

Diffusion of benzene through water film confined in silica mesopores: Effect of competitive adsorption of solvent

To understand the partial hydrogenation of benzene on Ru particles coated with hydrophilic materials, the diffusion of benzene through the water film confined in silica mesopores was studied by molecular dynamics simulation. There is an unconventional hydrogen bond between benzene and the surface hydroxyl group, which was previously supposed to retard benzene diffusion in hydrophilic mesopores. Driven by stronger hydrogen bonding, solvent water, however, is preferentially adsorbed onto the hydroxylated silica surface. Benzene dissolved in the water film has to diffuse away from the interfacial region, thereby weakening its interaction with the surface. Benzene may diffuse faster in hydroxylated mesopores than in bare mesopores, and the difference depends on the pore diameter. With a pore diameter of 4 to 6 nm, the diffusion coefficient of benzene along the pore axis, varying from 0.04 × 10−8 to 0.12 × 10−8 m2/s, could be adjusted by surface hydroxylation.

Ru/TiO2 catalyst for selective hydrogenation of benzene: Effect of surface hydroxyl groups and spillover hydrogen

Three Ru/TiO2 catalysts were prepared via an impregnation-chemical reduction method by supporting Ru on three different TiO2 supports, namely TiO2-A (anatase), TiO2-R (rutile) and TiO2-A-750 (annealing TiO2-A at 750 °C). The amount of surface hydroxyl groups in the TiO2 supports and Ru/TiO2 catalysts is evaluated by the FTIR technique, which is found to be affected by crystal and textural property of TiO2. It is found that, for the Ru/TiO2 catalyst, the presence of more spillover hydrogen, and more hydroxyl groups that lead to stronger surface hydrophilicity and lower benzene diffusion rate, thereby improve its catalytic activity and selectivity towards selective hydrogenation of benzene. For the Ru/TiO2-A catalyst, it has much more spillover hydrogen and hydroxyl groups than the other two, hence showing the best catalytic activity (TOF of 0.8 s−1) and cyclohexene selectivity (66%) at a benzene conversion of 50%.

Quantification of Ostwald Ripening in Emulsions via Coarse-Grained Simulations.

Ostwald ripening is a diffusional mass transfer process that occurs in polydisperse emulsions, often with the result of threatening the emulsion stability. In this work, we design a simulation protocol that is capable of quantifying the process of Ostwald ripening at the molecular level. To achieve experimentally relevant time scales, the dissipative particle dynamics (DPD) simulation protocol is implemented. The simulation parameters are tuned to represent two benzene droplets dispersed in water. The coalescence between the two droplets is prevented via the introduction of membranes, which allow diffusion of benzene from one droplet to the other. The simulation results are quantified in terms of the changes in the droplet volume as a function of time. The results are in qualitative agreement with experiments. The agreement with the Lifshitz-Slyozov-Wagner theory becomes quantitative when the simulated solubility and diffusion coefficient of benzene-in-water are considered. The effect of two different surfactants was also investigated. In agreement with both experimental observations and theory, the addition of surfactants at moderate concentrations decreased the Ostwald ripening rate because of the reduction in the interfacial tension between benzene and water; as the surfactant film becomes dense, other phenomena are likely to further delay the Ostwald ripening. In fact, the results suggest that the surfactant that yields higher density at the benzene-water interface delayed more effectively Ostwald ripening. The formation of micelles can also affect the ripening rate, in qualitative agreement with experiments, although our simulations are not conclusive on such effects. Our simulations show that the coarse-grained DPD formalism is able to capture the molecular phenomena related to Ostwald ripening and reveal molecular level features that could help to understand experimental observations. The results could be useful for predicting and eventually controlling the long-term stability of emulsions.

Polyurethane hybrid membranes with confined mass transfer channels: The effect of functionalized multi-walled carbon nanotubes on permeation properties

Aiming to prepare pervaporation membranes with confined mass transfer channels, multi-walled carbon nanotubes (MWCNTs) with and without functionalization were incorporated into polyurethane (PU) polymer matrices. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), X-ray diffraction (XRD), Fourier-transform infrared (FTIR), and Raman spectroscopy were employed to explore the effect of functionalization on the morphologies of MWCNTs. The changes in the morphologies of the resulting PU hybrid membrane were then characterized by SEM, TEM, and XRD. Swelling-sorption and pervaporation experiments of benzene and cyclohexane were employed to evaluate the separation performances of various hybrid membranes. The results showed that the introduction of many oxygenated functional groups onto the MWCNT surfaces through functionalization not only efficiently inhibited the MWCNT aggregation but also improved the distributions of the MWCNTs in solvents and polymer materials. Due to the 1D tube structures of the MWCNTs with conjugated π bonds, the addition of well-distributed MWCNTs into the hybrid membranes not only increased the benzene sorption abilities of the membranes but also enhanced the benzene diffusion selectivities due to the strong driving force for the molecules in the confined mass transfer channels. A higher content of well-distributed MWCNTs in the membrane yielded a higher separation performance for benzene. Additionally, increasing the operating temperature in the pervaporation experiments accelerated the molecular motion and weakened the affinity between the benzene molecules and the walls of the MWCNTs, increasing the permeation fluxes of the membranes while decreasing the benzene separation factors.

Adsorption and Diffusion of Benzene in Mg-MOF-74 with Open Metal Sites.

We performed molecular simulations to investigate the adsorption and diffusion of benzene in metal-organic framework Mg-MOF-74. At 300 K and 20 Pa, the saturated loading of benzene reaches 8.2 mmol/g, almost twice of (12,12) single-walled carbon nanotube with a similar pore size, and 93% of the benzene molecules in Mg-MOF-74 can desorb at 390 K. The energy analysis indicates that the van der Waals contribution still dominates 70-80% of the total fluid-wall interaction energy compared with the Coulombic contribution. We further analyzed the structure of benzene confined in Mg-MOF-74 by the molecular snapshots, pair correlation functions, orientational order parameters, and local density profiles. It is found that low temperature and high pressure make the structure of adsorbed benzene more similar to that of the liquid benzene. Moreover, the benzene molecules in the contact adsorption layer lie flat on the surface of adsorbent, whereas those molecules near the pore center have no particular orientations. Due to the existence of open metal sites, the structures of adsorbed benzene are more compact and ordered than those of bulk liquid benzene. Consequently, the self-diffusion coefficient of saturated benzene in Mg-MOF-74 at 300 K is significantly lower than that of bulk liquid benzene and confined liquid benzene in slit pores and disordered carbons by 4-5 orders of magnitude. We investigated the separation and diffusion of benzene/cyclohexane in the mixture in Mg-MOF-74 and found that the pores almost completely adsorbed benzene, although its self-diffusion coefficient was slightly lower than that of cyclohexane.

Insight into the Contribution of Isolated Mesopore on Diffusion in Hierarchical Zeolites: The Effect of Temperature

In recent years, hierarchical zeolites are becoming more and more attractive as a solution to the diffusion restraint of classical zeolites. There have been many theoretical as well as experimental developments that deepened our understanding of mass transfer phenomena in hierarchical materials. However, there appears to be no consensus on the role that disconnected mesopores played in molecular transport. In this paper, molecular simulation is conducted to study the diffusion of benzene in purely microporous FAU zeolite and hierarchical FAU models which consist of both microporosities of FAU and a cylindrical mesopore with various radiuses (0.74–17.5 nm) at 300–800 K. The usage of the disconnected mesopores at moderate adsorbate loading is testified on the molecular level. In fact, under the reaction temperature of catalytic reactions, the disconnected mesoporous are likely to be fully utilized and beneficial to the reaction process. The temperature dependence illustrated shows a remarkably good agreemen...

Surface diffusion of cyclic hydrocarbons on nickel

Surface diffusion of adsorbates is difficult to measure on realistic systems, yet it is of fundamental interest in catalysis and coating reactions. quasielastic neutron scattering (QENS) was used to investigate the diffusion of cyclohexane and benzene adsorbed on a nickel metal sponge catalyst. Molecular dynamics simulations of benzene on a model (111) nickel surface showed localised motion with diffusion by intermittent jumps. The experimental data was therefore fitted to the Singwi–Sjölander model and activation energies for diffusion of 4.0 kJ mol−1 for benzene and 4.3 kJ mol−1 for cyclohexane were calculated for the two dimensional model. Limited motion out-of plane was seen in the dynamics simulations and is discussed, although the resolution of the scattering experiment is insufficient to quantify this. Good agreement is seen between the use of a perfect crystal as a model for a disordered system over short time scales, suggesting that simple models are adequate to describe diffusion over polycrystalline metal surfaces on the timescale of QENS measurement.

Partial Hydrogenation of Benzene to Cyclohexene on Ru@XO2 (X = Ti, Zr, or Si)

The effect of coating Ru particles with a film of hydrophilic materials on the tetra-phase partial hydrogenation of benzene to cyclohexene was examined. By the sol–gel method, amorphous mesoporous TiO2, ZrO2, and SiO2 were coated on hexagonal Ru particles, by which Ru@XO2 (X = Ti, Zr or Si) catalysts were prepared. During the partial hydrogenation of benzene on Ru@XO2, the diffusion of benzene and cyclohexene dissolved in the water phase onto Ru particles is suppressed by the presence of mesoporous coating film. The diffusion of benzene is retarded further by the hydrogen bonding with the structural OH groups contained in hydrophilic materials. Because of the delicate diffusion behavior of benzene and cyclohexene through coating film, a decreasing rate of benzene conversion but a significantly increasing cyclohexene selectivity was observed in the partial hydrogenation on different Ru@XO2 catalysts. A maximum cyclohexene yield of 52.6 mol % occurs on Ru@TiO2 at the benzene conversion of 85.0%.

The dynamics of benzene on Cu(111): a combined helium spin echo and dispersion-corrected DFT study into the diffusion of physisorbed aromatics on metal surfaces.

We use helium spin-echo spectroscopy (HeSE) to investigate the dynamics of the diffusion of benzene adsorbed on Cu(111). The results of these measurements show that benzene moves on the surface through an activated jump-diffusion process between the adsorption sites on a Bravais lattice. Density Functional Theory (DFT) calculations with van der Waals (vdW) corrections help us understand that the molecule diffuses by jumping through non-degenerate hollow sites. The results of the calculations shed light on the nature of the binding interaction between this prototypical aromatic molecule and the metallic surface. The highly accurate HeSE experimental data provide a quantitatively stringent benchmark for the vdW correction schemes applied to the DFT calculations and we compare the performances of several dispersion interaction schemes.

Numerical Modeling on Benzene Dissolution into Groundwater and Transport of Dissolved Benzene in a Saturated Fracture-Matrix System

Dissolution from residual source zones of benzene poses serious threat to the groundwater quality. Proper understanding of fate and migration of dissolved benzene is a prerequisite for planning remediation strategies to reduce groundwater contamination. In the present study, an attempt has been made to numerically model the dissolution of benzene and to investigate the transport of aqueous phase benzene in a saturated fracture-matrix system under steady-state flow condition. In addition to dissolution mass transfer, advection, dispersion and matrix diffusion of aqueous benzene has been considered along the fracture. In the present numerical model, residual phase benzene is considered to be present along the entire length of the fracture and residual phase benzene is assumed to be immobile for the flow conditions considered in the analysis. Transport equations for fracture and rock-matrix are solved using implicit finite difference method. Transport equation for aqueous benzene within the fracture has been solved in a one-dimensional domain and transport equation for aqueous benzene within rock-matrix has been solved in a pseudo-two-dimensional domain. Sensitivity studies have been conducted to investigate the impact of variation of flow velocity, dispersivity, fracture aperture, inlet benzene concentration, rock-matrix diffusion coefficient, and half fracture spacing on transport of aqueous benzene concentration within the fracture. From the present study, it can be concluded that the addition of slow liquid benzene dissolution into aqueous phase influences the benzene breakthrough curves/profiles at different velocity, fracture aperture, initial benzene concentration, mass transfer rate, rock dispersivity and half fracture spacing.

A memory diffusion model for molecular anisotropic diffusion in siliceous β-zeolite

A memory diffusion model of molecules on β-zeolite is proposed. In the model, molecular diffusion in β-zeolites is treated as jumping from one adsorption site to its neighbors and the jumping probability is a compound probability which includes that provided by the transitional state theory as well as that derived from the information about which direction the target molecule comes from. The proposed approach reveals that the diffusivities along two crystal axes on β-zeolite are correlated. The model is tested by molecular dynamics simulations on diffusion of benzene and other simple molecules in β-zeolites. The results show that the molecules with larger diameters fit the prediction much better and that the "memory effects" are important in all cases.

Solvent Transport Properties of Er/Ps Thermoplastic Elastomeric Blends

Novel thermoplastic elastomeric blends from Polystyrene (PS) and Exudated Resin (ER) of Ailanthus Malabaricum tree are prepared by solution casting technique. The applicability of the resulting materials to design and fabricate barrier rubber materials for the transport and storage of liquids and gases has been studied in terms of solvent resistivity. It is important to carry out the transport studies to eliminate diffusion of chemicals into such material products. The sorption and diffusion of benzene through blends of Polystyrene and Exudated Resin of different compositions are studied at 35°C, 55°C and 65°C. The effects of blend ratio on diffusion, sorption and permeation coefficients are determined. The sorption data is used to estimate the activation energies of diffusion and permeation parameters. An anomalous behavior is observed for most of the blend compositions. The blend with 60/40 PS/ER combination at 35°C and 65°C exhibited Fickian transport mechanism. Minimum solvent uptake is observed for the blend ratio 60/40 PS/ER. This blend ratio shows better compatibility between the phases among the series of the blends studied.

Mapping the site-specific potential energy landscape for chemisorbed and physisorbed aromatic molecules on the Si(1 1 1)-7 × 7 surface by time-lapse STM

We present a scanning tunnelling microscope study of site-specific thermal displacement (desorption or diffusion) of benzene, toluene, and chlorobenzene molecules on the Si(1 1 1)-7 × 7 surface. Through time-lapse STM imaging and automated image analysis we probe both the chemisorbed and the physisorbed states of these molecules. For the chemisorption to physisorption transition there are distinct site-specific variations in the measured rates, however their kinetic origin is ambiguous. There is also significant variation in the competing rates out of the physisorbed state into chemisorption at the various surface sites, which we attribute to differences in site-specific Arrhenius pre-factors. A prediction of the outcome of the competing rates and pre-factors for benzene over three hours matches experiment.

Benzene diffusion on graphite described by a rough hard disk model

New insight into the nature of diffusion and the origin of friction of a prototype system for weak physisorption – benzene molecules, C6H6, adsorbed on the basal plane (0001) of graphite – has been obtained with quasi-elastic neutron scattering (QENS). Spectra were measured at relative adsorbate coverages between 0.1 and 1.0monolayers (ML) and at sample temperatures between 60K and 140K. Our experimental observations require a substantial modification of the model of interaction between benzene molecules and graphite surfaces: in contrast to recent studies we find only weak kinetic surface friction, but a substantial dissipative interaction during inter-molecular collisions. At coverages of up to 0.5ML the molecular dynamics are successfully modeled by a rough hard disk model, which we derive from a three-dimensional rough hard sphere model. At the full monolayer, three body and higher order collisions are dominant and the rough hard disk model breaks down, as expected.

Effect of free volume redistribution on the diffusivity of water and benzene in poly(vinyl alcohol)

We used molecular dynamics simulation to study the diffusion of two selected penetrants, water and benzene, in poly(vinyl alcohol) (PVA) over a wide range of temperatures and concentrations to gain insights into their diffusion mechanisms under different conditions. To help understand the effect of free volume on the diffusion behavior of water and benzene, we used the technique of Voronoi tessellation to determine key characteristics of free volume redistribution. Unlike the Connolly surface analysis method, Voronoi tessellation yields information not only on the mean fractional free volume but also on the size distribution of the free volume surrounding each penetrant molecule and its time evolution. In the case of water, we showed that it was the free volume redistribution frequency that led to an observation, previously reported in the literature, that the self-diffusion coefficient of water increases with increasing water concentration despite the fact that increase in water concentration decreases the mean fractional free volume in PVA. In the case of benzene (non-polar) diffusing in PVA (polar), we demonstrated that the failure of the Macki–Meares model was not entirely due to the dissimilarity between the intermolecular interactions. Rather, one of the reasons was the inability of the benzene molecules to break the hydrogen bonds between the PVA chains which are essential to increase the polymer segments mobility. Higher polymer segmental mobility is needed to create larger fluctuations in free volume holes for benzene to diffuse. This would in turn prevent the benzene molecules, with a relatively larger size compared to water molecules, from diffusing by a fluid-like streaming mechanism that is required by the model. In fact, the Macki–Meares model also failed for water at low temperatures and concentrations, as water diffused by a hopping mechanism under such conditions.