Self-diffusion Coefficients Of Methanol Research Articles

Molecular dynamics simulations of nano-confined methanol and methanol-water mixtures between infinite graphite plates: Structure and dynamics.

Molecular dynamics simulations are used to investigate microscopic structures and dynamics of methanol and methanol-water binary mixture films confined between hydrophobic infinite parallel graphite plate slits with widths, H, in the range of 7-20 Å at 300 K. The initial geometric densities of the liquids were chosen to be the same as bulk methanol at the same temperature. For the two narrowest slit widths, two smaller initial densities were also considered. For the nano-confined system with H = 7 Å and high pressure, a solid-like hexagonal arrangement of methanol molecules arranged perpendicular to the plates is observed which reflects the closest packing of the molecules and partially mirrors the structure of the underlying graphite structure. At lower pressures and for larger slit widths, in the contact layer, the methanol molecules prefer having the C-O bond oriented parallel to the walls. Layered structures of methanol parallel to the wall were observed, with contact layers and additional numbers of central layers depending on the particular slit width. For methanol-water mixtures, simulations of solutions with different composition were performed between infinite graphite slits with H = 10 and 20 Å at 300 K. For the nanoslit with H = 10 Å, in the solution mixtures, three layers of molecules form, but for all mole fractions of methanol, methanol molecules are excluded from the central fluid layer. In the nanopore with H = 20 Å, more than three fluid layers are formed and methanol concentrations are enhanced near the confining plates walls compared to the average solution stoichiometry. The self-diffusion coefficients of methanol and water molecules in the solution show strong dependence on the solution concentration. The solution mole fractions with minimal diffusivity are the same in confined and non-confined bulk methanol-water mixtures.

Dynamic properties of methanol–water mixtures at the temperatures up to 476.2 K and at high pressures via molecular dynamics simulation

Viscosities and self-diffusion coefficients of methanol–water mixtures were calculated via molecular dynamics simulation from 298.2 to 476.2 K and at pressures up to 40 MPa. The periodic perturbation method (non-equilibrium molecular dynamics) and the Green-Kubo method with three-site flexible potential models were adopted for evaluating the viscosities and the diffusion coefficients, respectively. The calculated viscosities and diffusion coefficients of methanol–water mixtures agreed qualitatively with literature experimental values. The three-site flexible potential models used in this study provided similar performance to the previous studies that used rigid models. Results correlated with the Stokes–Einstein relation show the possibility that the effective hydrodynamic radii were approximately constant for the studied condition and might be little affected by the clathrate-type structure at lower methanol compositions.

Self diffusion coefficients of organic solvents and their binary mixtures with CO2 in silica alcogels at pressures up to 6 MPa derived by NMR pulsed gradient spin echo

Self diffusion coefficients were determined using NMR (nuclear magnetic resonance) pulsed gradient spin echo (PGSE) in combination with a special high pressure autoclave. The goal was to determine the self diffusion coefficients for different CO2/methanol and CO2/ethanol mixtures within the pore phase of silica alcogels at pressures up to 6MPa. These values are required as input data for the modeling of process steps preceding the supercritical drying of these gels.The self diffusion coefficients derived with the set-up at 15°C for unconfined CO2/methanol and CO2/ethanol mixtures as a function of concentration of the organic solvent as well as of fluid pressure are in line with results from previous publications thus proofing the approach to provide reliable data.Measurements of self diffusion coefficients of methanol at different pressures and methanol/CO2 ratios were performed in tetramethoxysilane based silica alcogels with target densities ranging from 80 to 200kg/m3; the results reveal tortuosities of up to 2.5 depending on the silica fraction in the gel as well as its structure. The experimental self diffusion coefficients are in good agreement with the transport diffusion coefficient determined from time resolved NMR imaging of a fluid exchange within a alcogel of methanol for CO2 at 5.8MPa.

Dynamics of methanol in ionic liquids: validity of the Stokes-Einstein and Stokes-Einstein-Debye relations.

The validity of Stokes-Einstein (SE) and Stokes-Einstein-Debye (SED) relations for methanol in the physical environment of the ionic liquid (IL) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide is studied by means of nuclear magnetic resonance (NMR) relaxation time experiments, viscosity measurements and molecular dynamics (MD) simulations. The reorientational correlation times of the hydroxyl groups of pure methanol and of methanol in the IL/methanol mixtures were determined. For that purpose an approach for estimating NMR deuteron quadrupole coupling constants, presented by Wendt and Farrar (Mol. Phys. 1998, 95, 1077-1081), was confirmed. The self-diffusion coefficients of methanol were taken from the MD simulations. The viscosities of all systems were then measured and the SE and SED relations validated. For pure methanol both relations are valid, whereas they become increasingly invalid with increasing IL concentration, as indicated by effective volumes and radii that are too low. The deviation from the SE and SED relations could be related to dynamical heterogeneities described by the non-Gaussian parameter α(t) obtained from MD simulations. For pure methanol, α(t) is close to zero in accord with the validity of both relations. With increasing IL concentration the dynamical heterogeneities of methanol increase strongly. The times t* at the maximum of α(t) increase linearly with the relative number of methanol monomers in the mixtures. Thus, the dynamical heterogeneities are largest for single methanol molecules fully embedded in the IL environment. In their own environment methanol molecules are highly mobile, whereas in the IL-rich region the mobility is strongly reduced leading to the non-validity of SE and SED relations.

Sorption and transport properties of 2-acrylamido-2-methyl-1-propanesulfonic acid-grafted bacterial cellulose membranes for fuel cell application

This study investigated the sorption and transport properties of proton-conducting membranes based on a bacterial cellulose (BC) biopolymer with 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) grafted using ultraviolet (UV)-induced polymerization. The transport properties of the membranes were characterized according to their self-diffusion coefficients and methanol permeabilities. Using pulsed field-gradient nuclear magnetic resonance (PFG-NMR) technology, the water and methanol self-diffusion coefficients through the AMPS-g-BC membrane were identified as 1.48 × 10−5 cm2 s−1 and 5.30 × 10−6 cm2 s−1, respectively. The methanol permeability of the AMPS-g-BC membrane was 5.64 × 10−7 cm2 s−1, which was approximately 42% of that of Nafion 115. The differences in the transport behaviors of the Nafion 115 and AMPS-g-BC membranes correlated with the sorption characteristics of solvent uptake and lambda (λ) values (number of solvent molecules absorbed per sulfonic acid). The ratio of the water and methanol λ values (i.e., λCH3OH/λH2O) for the AMPS-g-BC membrane was 0.07, which indicated its sorption preference for water compared to methanol. Overall, results indicate that the AMPS-g-BC membrane is an effective methanol barrier and a potential solid electrolyte candidate for direct methanol fuel cells.

An NMR study of sorption-diffusion properties of MF-4SK-carbon composite membranes in aqueous methanol solutions

The distribution of water and methanol and their self-diffusion coefficients (SDCs) in carbon-doped MF-4SK perfluorinated membranes have been studied by NMR. It has been shown that at a concentration of 12–35 wt % in the external solution, the methanol concentration inside the membrane is higher by a factor of 1.4–2 than that in the external solution and increases with increasing the carbon phase content. It has been found that SDC values for water and methanol in the membrane decrease with an increase in the C phase content. In a membrane with 23 wt % C, the SDC of methanol is reduced by a factor of 1.6. It has been shown that the SDC of methanol in these membranes is below that of water by a factor of 1.5. The activation energies of self-diffusion of water and methanol have been determined. It has been found that they remain almost unaffected by the C content and have close values of 19–20 kJ/mol, which are 2–4 kJ/mol above the activation energy of self-diffusion of pure water in these membranes.

Electrokinetic transport of water and methanol in Nafion membranes as observed by NMR spectroscopy

Electrophoretic NMR (eNMR) and pulsed-field-gradient NMR (PFG-NMR) methods were used to study transport processes in situ and in a chemically resolved manner in the electrolyte of an experimental direct methanol fuel cell (DMFC) setup, constituted of several layers of Nafion 117. The measurements were conducted at room temperature for membranes fully swollen by methanol–water mixtures over a wide concentration interval. The experimental setup and the experimental protocol for the eNMR experiments are discussed in detail. The magnitude of the water and methanol self-diffusion coefficients show a good agreement with previously published data while the ratio of the two self-diffusion coefficients may indicate an imperfect mixing of the two solvent molecules. On the molecular level, the drag of water and methanol molecules by protons is roughly of the same magnitude, with the drag of methanol molecules increasing with increasing methanol content. The electro-osmotic drag defined on mass-flow basis increased for methanol from a low level with increasing methanol concentration while that of water remained roughly constant.

NMR investigation of water and methanol mobility in nanocomposite fuel cell membranes

Water and methanol transport behavior, morphology, and solvent adsorption of filler-free Nafion membrane, Nafion–SiO2, Nafion–TiO2, and two Nafion–Zr(HPO4)2 composites were investigated using nuclear magnetic resonance methods, including spin-lattice relaxation and pulsed-field-gradient spin-echo diffusion conducted under both variable temperature and variable hydrostatic pressure conditions and scanning electron microscopy analysis. A comparison between water and methanol self-diffusion coefficients reveals that the water mobility is higher than the methanol mobility in all the membranes. Additionally, the inclusion of inorganic fillers improves both the solvent uptakes and the transport properties of the composite membranes relative to filler-free Nafion, with the exception of one of the Nafion–Zr(HPO4)2 composite. Nafion–Zr(HPO4)2 composites were prepared by two different procedures, in situ and ex situ. Although phosphorus-31 magic-angle spinning nuclear magnetic resonance spectra show the same structures of the particles in both kinds of membranes, the morphology, solvent absorption properties, and solvent mobilities are very different.

An NMR spectroscopic study of water and methanol transport properties in DMFC composite membranes: Influence on the electrochemical behaviour

Water and methanol transport properties through bare recast Nafion and composite Nafion membranes containing ceramic fillers characterised by different acid–base behaviour were investigated by nuclear magnetic resonance (NMR) spectroscopy. The water self-diffusion coefficients were determined for the different membranes in the temperature range from 25 to 150 °C; the methanol self-diffusion coefficients were recorded for the most performing composite membrane (Nafion–SiO 2) and compared to the bare Nafion membrane. Water and methanol transport characteristics of these membranes were correlated to the electrochemical behaviour in direct methanol fuel cells. NMR and electrochemical polarization data indicate that both electrical properties and transport mechanism are influenced by the surface properties of the inorganic fillers; there is a clear evidence that acidic fillers such as silica promote the proton transport in the membrane with respect to basic alumina.

Ionic and molecular Self-Diffusion in Ion-Exchange materials for fuel energetics studied by pulsed field gradient NMR

Pulsed field gradient nuclear magnetic resonance technique was applied to investigate the self-diffusion mechanism of water, alcohol molecules and Li− counterions in sulfocation exchangers with different structures of the polymeric matrix. It was shown that in the homogeneous perfluorinated sulfocation exchange membranes the ionic and water translation motions are controled by the hydrogen bond network forming in ionogenic channels at the high water content. At the low solvent content, the self-diffusion coefficients of methanol and ethanol are higher than the water self-diffusion coefficients. The influence of non-ion-exchange sorbed electrolyte on Li+ self-diffusion coefficients was observed in the heterogeneous sulfocation exchanger KU-23.

An NMR and SAXS investigation of DMFC composite recast Nafion membranes containing ceramic fillers

Small angle X-ray scattering (SAXS) and nuclear magnetic resonance (NMR) investigations of recast composite and bare Nafion membranes have been carried out. The self-diffusion coefficients of water and methanol have been determined over a wide temperature range by PFGSE 1H NMR method. The transport mechanism appears to be influenced by surface properties of inorganic fillers. Acidic silica filler appears to promote proton transport in the membrane with respect to basic alumina. An interaction of the silica surface with methanol molecules is also envisaged from the analysis of proton self diffusion coefficients of methanol. The SAXS analysis revealed a modification of the polymer structure immersed in pure methanol or methanol solution with respect to water. A significant increase of the average ion clusters dimension is observed for the composite SiO 2 membrane.

NMR investigation of water and methanol transport in sulfonated polyareylenethioethersulfones for fuel cell applications

We report an investigation of water and methanol transport in polymer electrolyte membranes based on highly sulfonated polyarelenethioethersulfones (SPTES) for direct methanol fuel cell (DMFC) applications. Measurements of both water and methanol self-diffusion coefficients of SPTES polymer as well as in a reference sample of Nafion-117 equilibrated in 2 M methanol solution have been carried out, using the pulsed gradient spin echo technique, over a temperature range of 20–140 °C. The selectivity of the membrane, defined as ( D OH/ D CH3), decreased from 6 to 2.4 as temperature increased from 20 to 140 °C in SPTES sample while in Nafion, the value decreased from 3.2 to 1.4 as temperature increased from 20 to 100 °C. These results indicate significantly lower fuel molecular permeability in SPTES compared to that of Nafion. All results suggest high-temperature stability in these materials, offering the possibility of fuel cell operation at temperatures >120 °C. High pressure NMR diffusion measurements were also carried out for three different water contents (between 20 and 55 wt.%) in a static field gradient in order to get supplemental information regarding water transport in SPTES materials. The calculated activation volume increased from 1.54 to 8.40 cm 3/mol as the water content decreased from 55 to 20%. This behavior is qualitatively similar to previously reported results for Nafion-117.

NMR study of translational mobility of molecules adsorbed on active carbons

The NMR pulsed field gradient technique is applied to measure the self-diffusion coefficients of water, methanol and benzene molecules adsorbed on different active carbons. It is found that under the influence of the adsorbent, molecular mobilities are reduced by up to two orders of magnitude. The observed concentration dependence of the self-diffusion coefficient of water can be explained on the basis of the well-established model of water adsorption by primary adsorption sites. Irrespective of the various nature of the active carbons studied, for both water and methanol the self-diffusion coefficients are found to increase with increasing pore width. For most of the active carbons investigated, molecular mobilities decrease in the sequence water, methanol, and benzene (i.e., in the sequence of increasing molecules size). Only for the active carbons with the largest pore widths are the self-diffusion coefficient of methanol greater than that of water. Moreover, for these adsorbents, the diffusivities of both adsorbates are by the same factor lower than those of the neat liquids. This indicates that for active carbons with large pore widths the reduction of translational molecular mobility is mainly due to the influence of tortousity.

Adsorption and mobility of water molecules and organic substances in carbon adsorbents Communication 3. Dependence of self-diffusion coefficients of adsorbed molecules of water and methanol on size of micropores in active carbons and on degree of pore filling

1. The self-diffusion coefficients (SDCs) of water and methanol molecules, measured by the pulse-gradient NMR method, are independent of the time of observation A. The previously noted dependence of the SDC on A is explained by the large error of SDC measurements at short observation times. 2. The SDC of adsorbed water and methanol molecules increases with increasing size of the micropores. In activated carbons with a mean half-width of the micropores smaller than 0.66 nm, the SDC of adsorbed methanol at 298°K is lower than the SDC of water, but in activated carbon with larger pores, the SDC of methanol is higher than that of water.

Rotational correlation times and self-diffusion coefficients in methanol–ethanol system

It is well known that rotational motion is strongly correlated with translational motion in polar liquids [1] and aqueous solutions of non-electrolytes [2]. The investigation on the correlation of these motions is one of the usefull approach for elucidating the structure in the mixture of polar liquids. In this report we investigate the concentration dependence of the deuteron relaxation rate 1/T 1 of the partially deuterated alcohols CD 3OH, CD 3CH 2OH, and CH 3CD 2OH and the self-diffusion coefficients in methanol–ethanol system at 25 °C. The measurements of the spin-lattice relaxation times and the self-diffusion coefficients have been performed with a spin-echo spectrometer JNM-FSE 60A and a S/N Booster JNM-3B-SB(JEOL). The resonance frequencies for 1H and 2H are 60 and 9.2 MHz, respectively. All samples were degassed and sealed. In Fig. 1 is shown the concentration dependence of the self-diffusion coefficients of methanol and ethanol in the methanol–ethanol mixture. A minimum of self-diffusion coefficient of methanol appears at higher concentration than that of ethanol. The concentration dependence of the deuteron relaxation rate 1/t 1 of CD 3OH in C 2H 5OH is shown in Fig. 2. The deuterion relaxation rate shows a maximum at 11 mol% of methanol. In the case of methanol, therefore, ▪ the self-diffusion coefficient correlates with relaxation rate. In the case of ethanol, however, both the relaxation rates of –CD 3 and –CD 2 did not show the maximum but decreased with increasing the concentration of methanol.