Diffusion Coefficient Of Benzene Research Articles

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.

Permeation of Petroleum-Based Hydrocarbons through PVC Pipe Joints with Rieber Gasket Systems

There is significant concern in water utilities regarding the permeation of petroleum-based hydrocarbons from contaminated sites through potable drinking-water pipes made of polyvinyl chloride (PVC). Previous studies have addressed the impacts of hydrocarbons on the pipe materials, but the impacts of the hydrocarbons on gasket materials joining and sealing the pipes have not been investigated. This study investigated the permeation of benzene, toluene, ethylbenzene, and xylene (BTEX) compounds through PVC pipe joints equipped with 50-mm styrene-butadiene rubber (SBR) and nitrile-butadiene rubber (NBR) Rieber gaskets exposed to various levels of gasoline contamination. Results of permeation experiments with premium gasoline indicated that permeation occurred only through the gaskets and not through the intact pipe materials. For experiments with premium gasoline, benzene was the first compound detected with a breakthrough time of approximately 21 days. However, for experiments with various gasoline concentrations in water, including water saturated with gasoline, no significant amounts of BTEX compounds were detected in the pipe water after 9 months of exposure. The steady-state permeation rates of benzene when exposed to premium gasoline were found to be 0.73±0.29 and 0.38±0.32 mg/joint/day for SBR and NBR gaskets, respectively. The corresponding diffusion coefficients of benzene in SBR and NBR gaskets (which have not been reported before) were determined to be 1.1×10-7 and 6.0×10-8 cm2/s, respectively, by curve fitting the permeation data with a diffusion model. Although PVC pipe joints were found to be permeated by free-product gasoline, water flow in pipes would dilute the benzene in drinking water to below the maximum contamination level (MCL) of 5 μg/L.

Adsorption and Diffusion of Benzene on Chromium-Based Metal Organic Framework MIL-101 Synthesized by Microwave Irradiation

Adsorption equilibrium and diffusion of benzene on mesoporous chromium−terphthalate-based crystals (MIL-101) were experimentally studied by the gravimetric method in the pressure range up to 60.0 mbar. The MIL-101 crystals, whose sizes are about 100 nm, were synthesized by using microwave irradiation method. Adsorption isotherms and kinetic curves of benzene on the MIL-101 were measured experimentally. Results show that the maximum amount adsorbed of benzene on the synthesized MIL-101 was up to 16.5 mmol/g (or 176 wt %) at 288 K and 56.0 mbar. Diffusion coefficients of benzene within the MIL-101 were in the range of (4.25−4.76) × 10−9 cm2/s in 288−318 K with a lower activation energy of 2.41 kJ/mol. Temperature programmed desorption (TPD) curves exhibited two separated peaks corresponding to two types of major adsorption sites for benzene adsorption on MIL-101. Consecutive cycles of adsorption−desorption experiment showed a fast desorption kinetics and high reversibility for benzene adsorption on MIL-101....

How to predict diffusion of medium-sized molecules in polymer matrices. From atomistic to coarse grain simulations

The normal diffusion regime of many small and medium-sized molecules occurs on a time scale that is too long to be studied by atomistic simulations. Coarse-grained (CG) molecular simulations allow to investigate length and time scales that are orders of magnitude larger compared to classical molecular dynamics simulations, hence providing a valuable approach to span time and length scales where normal diffusion occurs. Here we develop a novel multi-scale method for the prediction of diffusivity in polymer matrices which combines classical and CG molecular simulations. We applied an atomistic-based method in order to parameterize the CG MARTINI force field, providing an extension for the study of diffusion behavior of penetrant molecules in polymer matrices. As a case study, we found the parameters for benzene (as medium sized penetrant molecule whose diffusivity cannot be determined through atomistic models) and Poly (vinyl alcohol) (PVA) as polymer matrix. We validated our extended MARTINI force field determining the self diffusion coefficient of benzene (2.27·10⁻⁹m² s⁻¹) and the diffusion coefficient of benzene in PVA (0.263·10⁻¹² m² s⁻¹). The obtained diffusion coefficients are in remarkable agreement with experimental data (2.20·10⁻⁹m² s⁻¹ and 0.25·10⁻¹² m² s⁻¹, respectively). We believe that this method can extend the application range of computational modeling, providing modeling tools to study the diffusion of larger molecules and complex polymeric materials.

Molecular Dynamics Simulation of Diffusion Behavior of Benzene/Water in PDMS-Calix[4]arene Hybrid Pervaporation Membranes

Molecular dynamics (MD) simulation was employed to investigate diffusion behavior of small penetrants in rubbery-polymer-based hybrid membranes, using pervaporative removal of benzene from its dilute solution by poly(dimethylsiloxane) (PDMS) membranes filled with calix[4]arene (CA) as the model system. In our previous experimental investigation, the normalized permeation rate of benzene (NPRb) and separation factor (benzene/water) through PDMS−CA hybrid membranes did not follow the usual monotonous or single peak/valley change, but accompanied minimum and maximum values instead. In the present study, nonbonding interaction energy between PDMS and CA, mean-square displacement (MSD), free volume characteristics, and diffusion coefficients of benzene and water in pure PDMS and hybrid membranes were analyzed by molecular dynamics simulation. The simulation results revealed that MSD and fractional free volume (FFV) values were closely dependent on interaction energy. Diffusion coefficients of benzene and water at “infinite dilution” and saturated condition displayed the same changing tendency, although the values at saturated condition were a bit larger. Moreover, it was observed that diffusion coefficients were not only related to FFV but also affected by the interaction between CA and the penetrants. Overall, the MD results agreed well with the experimental results.

Diffusion behavior of benzene/cyclohexane molecules in poly(vinyl alcohol)-graphite hybrid membranes by molecular dynamics simulation

Understanding the effects of incorporated inorganic particles in organic–inorganic hybrid membranes at molecular level is crucial to design hybrid membranes with tailored structure and desired properties. In this study, molecular dynamics simulation has been employed to investigate the effects of graphite particle on the diffusion behavior of benzene and cyclohexane in poly(vinyl alcohol)-graphite hybrid membranes. Firstly, the non-bonding interaction energies between PVA polymer and graphite particle with different amount of hydroxyl and carboxyl groups were calculated by DREIDING force field. Next, polymer chain mobility was analyzed by mean-square displacement (MSD) and glass transition temperature ( T g ) , and found that incorporation of graphite particle into PVA increased the polymer chain mobility. Free volume characteristics of membranes were investigated by Connolly surface, and found that free volumes in PVA-graphite hybrid membranes were larger than those in PVA control membranes. This simulation result showed the same tendency with the PALS measurement. Finally, the diffusion coefficients of benzene and cyclohexane in the membranes were calculated by Einstein relation, and found that the diffusion coefficients in hybrid membranes were larger than those in the control membrane. Furthermore, it was found that the diffusion coefficients of benzene and cyclohexane in the membranes exhibited exponential relation with the inverse fractional free volumes and agreed well with the free volume theory.

How Fluorous Is Poly(2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxide-co-tetrafluoroethylene) (Teflon AF)?

We investigated poly(2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxide-co-tetrafluoroethylene (Teflon AF 2400, Tg = 240 degrees C), a stable and permeable fluorous polymer, as a transport/extraction medium for solutes for the first time. From the study of transport behavior of a series of solutes (in chloroform solution) through the film, and detailed measurement of the partitioning and diffusion of benzene in the film, we showed that the Teflon AF film is influenced by the solvent to which it is exposed. In particular, the solvent chloroform is sorbed in the film at a high concentration of 1.13 M. This plasticizes the film, so that the diffusion coefficient of benzene is about 3 orders of magnitude larger in a chloroform-equilibrated film compared to benzene coming from the gas phase into a "dry" film. The partition ratio of the polar solute 3-hydroxypyridine is dramatically higher in the case of partitioning between chloroform and a film (0.02) compared to partitioning between chloroform and the fluorous solvent FC-72 (6.7 x 10-5). Krytox FSH, a carboxylic acid terminated perfluoropolyether, plasticizes films. Tg in a 50% (w/w) film decreases to -40 degrees C. This carboxylic acid is capable of molecular recognition in the film. The noncovalent association between Krytox FSH (0.13 M in the film) and 3-hydroxypyridine increases the distribution ratio of the polar solute into the film by 41 times. In comparison, the partition ratio into a fluorocarbon solvent (FC-72) increases 15 000 times under the same conditions. As a result of imbibing organic solvent, the films of Teflon AF 2400 are not as fluorous as a fluorous liquid.

Molecular diffusion coefficients in ethanol/water/carbon dioxide mixtures.

Supercritical and liquid mixtures of ethanol/H2O/CO2 are increasingly used to extract solutes from solid or semi-solid matrixes when nontoxic solvents and fast extraction kinetics are desired. Accordingly, to better understand the mass transport capabilities of these mixtures, the diffusion coefficients of benzene, anthracene, m-cresol, and p-nitrophenol in enhanced-fluidity liquid mixtures of ethanol/H2O/CO2 were studied. The effect of mixture composition and temperature variation on the measured diffusion coefficients was studied. In a mixture containing 0.61/0.39 mole ratio ethanol/H2O, the diffusion coefficients of the four solutes increased comparably either by adding 30 mol % CO2 or by changing the temperature of the mixture from 25 to 60 degrees C. The experimental data were compared to that predicted by the Stokes-Einstein and the Wilke-Chang relations. Often, the experimental diffusion coefficients were greater than those predicted by these mass transport relations. However, the Eyring relationship was useful in describing the variation of diffusion coefficients as a function of temperature change for all of the ethanol/H2O/CO2 mixtures tested.

Calculation of Mutual Diffusion Coefficients near the Critical Region from the Peng−Robinson Equation of State

We observed that the mutual diffusion coefficients of benzene in supercritical carbon dioxide exhibited a maximum at pressures above the critical pressure of the mixture, and a strong concentration dependence near the critical point. These surprising experimental results can be explained by replacing the concentration gradient of diffusion with a chemical potential gradient as suggested from irreversible thermodynamics. Then, it can be understood that diffusion coefficients have a maximum value at the critical point of a mixture. The Peng-Robinson equation of state was used to illustrate these relationships.

Kinetic Study of Membrane Extraction with a Sorbent Interface for Air Analysis

This paper presents the theory and experimental implementation of the technique of membrane extraction with a sorbent interface for air analysis. A mathematical model was derived to predict the extraction process and the significant factors, such as membrane length and carrier gas flow rate. A benzene/nitrogen standard gas mixture was generated by the permeation tube method. Several parameters that affect the extraction efficiency are discussed. Among them, the membrane length, the flow rate of the carrier gas, and temperature strongly affected the extraction rate of analyte. The results demonstrated that the extraction rate of benzene was decreased with increase of the extraction temperature. Toluene, hexane, ethylbenzene, and trichloroethylene were selected to test the theory, and good agreement between theoretical predictions and experimental results was observed. The estimation of air concentration without experimental calibration is discussed. In the proposed estimation method, the diffusion coefficient of benzene in the membrane and the distribution constant between air and the membrane were measured experimentally.

Evaluating organic compound migration in poly(ethylene terephthalate): a simple test with implications for polymer recycling.

The safety of recycled plastics for food contact use is largely dictated by the ability of post-consumer organic contaminants to absorb into recycled materials and later diffuse from containers made from recycled plastics into the food supply. Diffusion and solubility data for organic contaminants in poly(ethylene terephthalate) (PET) are scarce. An approach for determining permeability constants, diffusion coefficients and solubility constants of slowly migrating contaminants in PET is described. Compounds (neat or in admixture) were heat-sealed in packets made from thin (0.00127 cm) PET films. Packets were placed in containers with GC sampling closures. Headspace (volatile compounds) or an external liquid medium (non-volatile compounds) was analysed for emergence of the compound. Diffusion coefficients were determined from non-steady state diffusion equations, permeability constants were determined from steady state permeation, and solubility constants were calculated from diffusion and permeability values. Diffusion coefficients (25 degrees C) ranged from 10(-9) to < 10(-16) cm2/s. The diffusion coefficient of benzene increased dramatically with concentration. Compounds with high permeation increased the transport rate of slower permeating volatiles.

Infinite Dilution Diffusion Coefficients of Several Aromatic Hydrocarbons in Octane and 2,2,4-Trimethylpentane

Diffusion coefficient measurements are required in a number of engineering applications and also in testing transport property theories. The diffusion coefficients of benzene, toluene, p-xylene, o-xylene, ethylbenzene, and mesitylene at infinite dilution in octane and in 2,2,4-trimethylpentane in the temperature range 303.2--333.2 K were determined by the Taylor dispersion technique. A correlation based on a free-volume-type expression represented the results to within experimental uncertainty.

Determination of Infinite Dilution Diffusion Coefficients of Polycyclic Aromatic Hydrocarbons in Heptane and in Octane

AbstractThe diffusion coefficients of benzene, naphthalene, anthracene, phenanthrene, and diphenyl at infinite dilution in heptane and in octane in the temperature range 303.2 to 333.2 K were determined by the Taylor dispersion technique. Two empirical estimation methods for diffusion coefficient were tested against our experimental data and a free‐volume‐type expression was used to correlate the diffusion coefficients.

Diffusion mechanism of hydrocarbons in zeolites—II. Analysis of experimental observations

Abstract The influence of the properties of molecule-zeolite systems on the diffusional and equilibrium behavior is experimentally investigated. Sixteen hydrocarbons, including paraffins, two aromatics and a naphthene, were studied in ZSM-5. The activation energy for diffusion is mainly responsible for the different diffusivities of various compounds. The diffusion coefficients of benzene, toluene, and 2-methylbutane in ZSM-5 are insensitive to the concentration up to about four molecules/unit cell, and the isotherms within this concentration range are of the Langmuir type. At higher concentrations, the diffusivities show strong concentration-dependent trends, and the Langmuir adsorption parameter sharply decreases. The predictions of the mechanistic models proposed in Part I agree, in general, with the experimental results of the diffusion measurements here, both with regard to the order of magnitude and the concentration dependence. Good agreement is also found for 5A between the theoretical predictions and literature data on the diffusion and equilibrium behavior.

The binary diffusion coefficient of benzene at high dilution in the dense gas CO2

The binary diffusion coefficient D 12 of various substances measured at high dilution in dense CO2 is not describable by the Enskog-Thorne approach. A simplified model potential is presented here, which allows the simulation of the D 12 and the thermodynamic properties of the system CO2/C6H6 by molecular dynamics calculations over a wide range of density of CO2. The failure of the Thorne equation is attributed mainly to the neglect of the attractive interactions in dense gases.

A pulsed field gradient system for a fourier transform spectrometer

The construction and performance of a pulsed field gradient system for use with a commercial high-resolution pulse-Fourier transform NMR spectrometer is described. The diffusion coefficient of benzene as measured by the system lies within 0.9% of the current literature value. The use of an external 2H lock in conjunction with signal averaging facilitates the measurement of diffusion coefficients for solution components present in small concentrations and data are presented for a 0.5% (w/v) solution of polystyrene ( M r = 110,000) in carbon tetrachloride. The ability of the system to investigate slow diffusion is demonstrated by measurements made on a 10% (w/v) solution of polystyrene ( M r = 230,000) in carbon tetrachloride. Homogeneity coils in the 1H probe enable the self-diffusion of single components in multicomponent systems to be investigated, and results for the binary system butanol-benzene are presented.

NMR spin echo studies on mobility and diffusion of benzene adsorbed on silica

Diffusion coefficients of benzene in porous plugs compressed from Aerosil were measured by the spin echo technique with the use of constant and pulsed field gradients. In order to obtain surface self-diffusion coefficients D s ∗ the contribution from molecular mobility in the pore volume has been evaluated. D s ∗ shows a maximum near θ = 1 in agreement with earlier results obtained from rates of adsorption. Differences between the limiting experimental diffusion coefficients in the region of capillary condensation and the bulk liquid are attributed to the tortuosity of the pore system. Activation energies for surface self-diffusion are larger than for T 2.