Self-diffusion Coefficient Of Water Research Articles

Molecular dynamics study of the local structure and diffusivity of partially miscible water/n-alcohols binary mixtures

In this work, we look at partially miscible water/n-alcohol mixtures that have been studied extensively by experiment and, in some cases, by classical simulation methods. As a reference point, we chose Su et al.'s [J. Chem. Phys. 132 (2010), 044506] experimental study of the diffusion of water in its binary mixtures with n-alcohols and n-alkanes. Their measurements showed that diffusivity of water in n-alcohols is substantially lower than in n-alkanes of similar size whereas its decrease with increasing n-alcohol viscosity violates Stokes–Einstein relation, a result attributed by them to the hydrogen bonded water molecules spending most of their time near the n-alcohol hydroxyls. To explore their hypothesis that that is the main determinant of water's dynamic behaviour in those mixtures, we employed the molecular dynamics method to simulate the organic phase of binary mixtures of water and n-alcohols (butan-1-ol, pentan-1-ol, and hexan-1-ol) at ambient conditions. The chosen force fields (SPC/E and TraPPE) were shown to predict accurately the systems' volumetric properties. Also, self-diffusion coefficients of water compared very well with experimental measurements and previous simulations. As the local structure and dynamics of the mixtures are not fully understood so far, special attention was paid to the interaction sites taking part in hydrogen bonding, their pair distribution functions and the local structure in their vicinity. Our results, expressed in terms of specific quantitative descriptors, provide a clear picture of hydrogen-bonding connectivity between n-alcohol molecules, water molecules and each other. In particular, it is shown that water and n-alcohol molecules take part in complex extended structures with a rather stable topology, thus confirming, and at the same time, enriching Su et al.'s initial hypothesis. Both species occupy places in these structures in proportions depending on the mixtures' composition — and that is the main cause of their particular diffusive behaviour.

On the temperature dependence of several physicochemical properties for aqueous solutions of the ionic liquid 1-butyl-3-methylimidazolium methanesulfonate ([C4mim][MeSO3

The viscosity, conductivity, proton T1 relaxation, and the self-diffusion coefficients of cation, anion and water are presented for the binary system water–1-butyl-3-methylimidazolium methanesulfonate ([C4mim][MeSO3]) for the temperature and composition ranges of (298.15–343.14) K and ionic liquid mole fractions 0<xIL<0.5. The temperature dependence of these properties at fixed compositions was analyzed to obtain Arrhenius activation energies, which were found to be linearly dependent to xIL for xIL greater than about 0.15 for the self-diffusion coefficients, viscosity and proton T1 relaxation times (except for the water proton T1 relaxation times). The specific xIL dependencies of the various Arrhenius activation energies are qualitatively different from what was observed in a prior study of the water–1-ethyl-3-methylimidazolium methanesulfonate ([C2mim][MeSO3]) binary system [J. Mol. Liq. 160 (2011) 166–179]. In particular, the T1 relaxation behavior is markedly different und is discussed in detail along with other comparisons between these two water–IL binary systems.

Experimental investigation of water self-diffusion coefficient and tracer diffusion coefficient of tert-butanol in water-based silica nanofluids

Water self-diffusion coefficient and tracer diffusion coefficient of tert-butanol in water-based silica nanofluids were measured by using pulsed field gradient nuclear magnetic resonance (PFG-NMR) method for silica nanoparticle volume fractions ranged from 0.005% to 1% at three different temperatures (15, 25 and 35 °C). In this work, formation of dye–nanoparticle complexes was minimized and undesirable primary advection effects induced by solute injection were also eliminated. Insignificant enhancement (mostly less than 10% increase) in both water self-diffusion coefficient and tracer diffusion coefficient of tert-butanol in nanofluids was observed. These results indicate that probably Brownian motion of nanoparticles and induced micro-convection tend to enhance mass transfer but the obstruction effect of solid nanoparticles limits this enhancement by reducing the free volume face to diffusion path of molecules.

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.

Diffusion of water inside carbon nanotubes studied by pulsed field gradient NMR spectroscopy.

Diffusion dynamics of guest molecules in nanopores has been studied intensively because diffusion is center on a number of research fields such as separation, drug delivery, chemical reactions, and sensing. In the present work, we report an experimental investigation of the self-diffusion of water inside carbon nanotube (CNT) channels using a pulsed field gradient (PFG) NMR method. The dispersion of CNTs homogeneously in water and cooling to temperatures below the melting point of bulk water allow us to probe the translational motion of confined water molecules. The results demonstrate that the self-diffusion coefficient of water in CNTs is highly dependent on the diffusion time and CNT diameter. In particular, the diffusivity of water in double-walled carbon nanotubes (DWNTs) with an average inner diameter of 2.3 ± 0.3 nm is twice that in multiwalled carbon nanotubes (MWNTs) with an average inner diameter of 6.7 ± 0.8 nm in the temperature range of 263-223 K. In addition, the effective self-diffusion coefficient in DWNTs is 1 order of magnitude higher than that reported for mesoporous silica materials with a similar pore size. The faster diffusivity of water in CNTs could be attributed to the ordered hydrogen bonds formed between water molecules within the confined channels of CNTs and the weak interaction between water and the CNT walls.

Structure and Dynamics of Carbon Dioxide, Nitrogen, Water, and Their Mixtures in Metal Organic Frameworks

The self-diffusion coefficients of carbon dioxide, nitrogen, and water in metal organic frameworks (IRMOF-1, Cu-BTC, and MIL-47) are calculated using molecular dynamics simulations at various temperatures for pure gases and in mixtures. The structure of the adsorbates with respect to the metal active sites in the metal organic frameworks and their intermolecular structural features are investigated through radial distribution functions and related to the self-diffusivity behavior. It is found that while in IRMOF-1 the three species maintain their mobility when they are adsorbed as pure components or in mixtures, the diffusivities of CO2 and water in Cu-BTC are slower in the ternary mixture than when adsorbed individually, whereas the opposite behavior is observed in MIL-47 where the species diffuse faster in the mixture than as pure components. The behavior can be explained in terms of the strong interactions of water with the framework which slows down diffusion in Cu-BTC; however in MIL-47, the competition between CO2 and water for the active vanadium sites increases the mobilities of both adsorbates.

Structural and dynamical properties of water confined in carbon nanotubes

Structural and dynamical properties of water confined in carbon nanotubes are investigated by classical molecular dynamics simulations. It is found that density and self-diffusion coefficients of water depend on the diameter of confinant nanotube and exhibit anomalous behavior in narrow tubes. It is shown that water forms several different orderings inside the nanotube channel and these arrangements impact on the energetics of hydrogen bonds. Considering the relationship between the geometry of hydrogen bonds and their energetics, a discussion on the dynamics of enclosed water is presented.

Temperature dependence of diffusion coefficient of nitrogen gas in water: A molecular dynamics study

We have carried out the molecular dynamics (MD) simulation to study the structural properties and to estimate the diffusivity of molecular nitrogen ( N 2) gas (solute) in extended simple point charge model (SPC/E) water (solvent) with N 2 mole fraction of 0.018 at different temperatures. For the structural properties of the system, we have determined radial distribution function (RDF). The solute–solute, solute–solvent and solvent–solvent RDF have been evaluated. Self-diffusion coefficient of N 2 was estimated by evaluating mean-squared displacement (MSD) and velocity autocorrelation function (VACF) separately. The diffusion coefficients obtained from the two methods agree within 3%. The results are in agreement with the experimentally determined values within 10%. The self-diffusion coefficient of water ( H 2 O ) was also estimated by evaluating MSD. Mutual diffusion coefficient of the system have also been estimated invoking Darken's relation. The temperature dependance of the diffusion coefficients were found to follow Arrhenius relation.

Water dynamics within a highly rigid sulfonated polyphenylene

Complex water molecule interactions within the confined environments of a sulfonated polyphenylene (sPP) ionomer were studied using 1H nuclear magnetic resonance (NMR), and Fourier transform infrared (FTIR) spectroscopy. Multiple water environments were observed due to variations in hydrophilicity created by its structure, and chemical composition. Confined water properties are strongly dependent upon water content, degree of ionization, and temperature. Increasing the degree of sulfonation (DS) results in the creation of more water states and sites. The chemical shifts of water depend upon its environment with a smaller dependence on temperature. Confined water relaxation time T1 is than significantly lower bulk water and increases with temperature. Pulse field gradient (PFG) NMR studies reveals that water self-diffusion coefficients increase with. Water molecules diffuse faster in sPP than Nafion™, which implies that diffusion is facilitated by bundled hydrophilic pathways. Time-dependent FTIR reveals that bound water evaporates slower than unbound water during drying, which illustrates the difference of bulk and confined water within sPP ionomers. Hindered water evaporation is due to a reduction in the degrees of freedom for ion containing domains and mass transfer limitations at interfacial boundaries between hydrophobic and hydrophilic domains.

Magnetic-field assisted mixing of liquids using magnetic nanoparticles

Size and magnetic properties of magnetic nanoparticles (MNPs) in fluids allow special remote control of fluid flow using appropriate externally applied magnetic fields, especially when submicronic mixing is critical, inter alia, for catalytic reactions, separation and drug delivery. This work explores MNPs as nanoscale devices to control mixing at microscale by submitting the system of interest to a rotating magnetic field (RMF). Magnetic nanoparticles are harnessed by RMF and converted into nanostirrers thereby generating MNP-pinned localized agitation in the liquid phase. Using this technique, self-diffusion coefficient of water in a static diffusion cell was intensified up to 200 folds. Also, axial dispersion of capillary Poiseuille flows under RMF underwent a reduction prompted by MNP-mediated intensification of lateral mixing relative to that in absence of magnetic field. Finally a multiphase flow case concerned gas–liquid mass transfer from oxygen Taylor bubbles to the liquid in capillaries where dilute MNP solutions led to measurable enhancement of kLa under RMF.

Structure and Dynamics of an Electrolyte Confined in Charged Nanopores

Molecular Dynamics simulations are used to investigate the structure and dynamics of an aqueous electrolyte (NaCl) confined within a nanomembrane, which consists of a nanopore with a diameter 3 nm having a negatively charged surface. Both nanomembranes with a diffuse charge and with local charges are considered (in both cases, two surface charge densities are considered, −0.9 e/nm2 and −1.8 e/nm2). For all nanomembranes, significant layering of water and ions in the vicinity of the nanomembrane surface is observed. While the distribution of water and chloride ions is nearly insensitive to the nanomembrane charge and type, the arrangement of sodium cations within the nanomembrane depends on the system being considered. The water and ion density profiles in the nanomembranes are compared with the predictions of a modified Poisson–Boltzmann equation in which charge image, solvation effects, and dispersion interactions with the surface are taken into account [Huang et al. Langmuir, 2008, 24, 1442]. The self-diffusion coefficient for a given species is smaller than its bulk counterpart and is at most 75% of the bulk value. While the self-diffusion coefficients for water and sodium cations decrease with decreasing the overall negative charge of the nanomembrane, the self-diffusion coefficient for the chloride anions is nearly independent of the nanomembrane type and charge. We also estimate the dynamics of the confined aqueous electrolyte by calculating time correlation functions which allow estimating solvation, ion pairing, and residence times.

Probing membrane and interface properties in concentrated electrolyte solutions

This study deals with the membrane and interface electrical properties investigation by electrochemical impedance spectroscopy (EIS). The EIS is a powerful technique for characterizing electrical behavior of systems in which coupled electrical processes occur at different rates. A systematic study on the effect of solution concentration, temperature and velocity, on the electrical resistance of anion- and cation-exchange membranes (AEMs and CEMs) and their interfaces (electrical double layer and diffusion boundary layer), was carried out. At the best of our knowledge, for the first time electrolyte concentrations up to 4M were used for the study of membranes and interface by EIS.Moreover, Pulsed Gradient Spin Echo Nuclear Magnetic Resonance (PGSE-NMR) technique was used to measure the water self-diffusion coefficients in swelled membrane as a function of the solution concentration and temperature. These measurements gave additional important insights about the effect of the electrolyte solution and fixed charges concentration in membrane, on membrane microstructure and its transport and electrical properties.

Temperature dependence of diffusion coefficient of carbon monoxide in water: A molecular dynamics study

In the present work, molecular dynamics simulations of a mixture of carbon monoxide (CO) gas in SPC/E water (H2O) with CO as a solute and water as a solvent have been performed to understand the self-diffusion coefficient of the components, i.e. CO and water along with the mutual diffusion coefficients and structural properties of the system by studying the radial distribution function (RDF) of the components present in the system at different temperatures of 293K, 303K, 313K, 323K, and 333K. The mole fraction of CO in the system is 0.018 and that of water is 0.982. The solvent–solvent, solute–solute and solute–solvent radial distribution functions have been estimated to study the structural properties of the system. The self-diffusion coefficient of CO is calculated using mean square displacement (MSD) and velocity autocorrelation function (VACF) and that of water is estimated using MSD method only. The self-diffusion coefficient of CO agrees within around 20% of the experimental results and that of water agrees within around 10% of the experimental results. The mutual diffusion coefficient of solute–solvent is calculated using Darken's relation. The temperature dependence of self-diffusion coefficients of CO and water and mutual diffusion coefficients of CO in water all follow the Arrhenius behavior from where we estimate the activation energy of the diffusion process. Thus estimated activation energies also agree to experimental values.

Osmolyte Effects: Impact on the Aqueous Solution around Charged and Neutral Spheres

We have performed atomistic molecular dynamics simulations to study the solvation characteristics of model spheres for low concentrations of urea and hydroxyectoine in aqueous solution. The spheres are either positively or negatively charged with a valency of one or charge neutral. Our results illustrate that the presence of osmolytes influences the solvation properties of the spheres significantly. We have conducted a detailed investigation of water properties like the mean dipolar relaxation times, water orientation parameters around the spheres, dielectric constants, preferential binding behavior, water self-diffusion coefficients, and free energies of solvation by thermodynamic integration to study the influence of osmolytes in detail. Our findings indicate that several factors like the charge of the spheres as well as the characteristics of the osmolytes significantly influence the thermodynamic and dynamic properties of the local water shell and the solvation process with regard to varying enthalpic and entropic contributions.

Hemoglobin S Polymerization Effect on Water Self-Diffusion Coefficient

The transversal relaxation time, the effective transversal relaxation time and the water self-diffusion coefficient are evaluated during hemoglobin S polymerization. One homogeneous permanent magnet and one inhomogeneous and portable unilateral magnet with a very strong and constant static magnetic field gradient were utilized. The Carr-Purcell-Meiboom-Gill method was used before and after placing the studied samples 24 hours at 36°C to guarantee the polymerization. The transversal relaxation shows two exponents after polymerization supporting the concept of partially polymerized hemoglobin. The effective transversal relaxation time decreases around 40%, which can be explained by the increase of water self-diffusion coefficient 1.8 times as a main value. This result can be explained considering the effects of the agglutination process on the obstruction and hydration effects in a partially polymerized solution.

A Combined Theoretical and Experimental Investigation of the Transport Properties of Water in a Perfluorosulfonic Acid Proton Exchange Membrane Doped with the Heteropoly Acids, H3PW12O40 or H4SiW12O40

The 3M 825EW perfluorosulfonic acid (PFSA) ionomer was doped with the heteropoly acids (HPAs), H3PW12O40 (HPW) and H4SiW12O40 (HSiW). Dynamic vapor sorption measurements at 95% RH showed a decrease in water content as a result of HPA doping from λ = 8.05 for the undoped 825EW 3M ionmer to λ = 4.40 for a 5% HSiW doped film. FTIR measurement revealed strong interactions between the HPAs, ionomer, and H2O. Irrespective of hydration level, it was found that the PFSA films showed tortuous proton diffusion behavior. At maximum hydration and 25 °C, the self-diffusion coefficient of water was found to decrease upon addition of HPA from 4.97 to 2.19 (×10–6 cm2/s) for the undoped 825EW 3M ionomer and 1% HPW, respectively, in excellent agreement with computation. The model at low HPA loadings revealed the decreased diffusion coefficient was due to the water preferentially residing near the HPA as opposed to the SO3– groups. The addition of HPA generally improved overall conductivity due to additional formation of hydrogen-bonding networks between the HPA particles. At high RH, it was observed that the proton conductivity increased while the water diffusion coefficient decreased as HPA was incorporated. In addition, a mismatch between the conductivity values and those calculated from the Nernst–Einstein equation using the self-diffusion coefficient of water in the system indicated an enhancement in Grotthuss hopping mechanism upon addition of HPA.

Molecular Modeling of Hydronium Ion and Water Distribution in Water-Filled Pt Nanochannels with Corrugated Walls

Classical molecular dynamics was employed to study the impact of nanoconfinement and surface nanostructure on electrostatic phenomena and transport properties in catalyst layer pores of polymer electrolyte fuel cells. The porous structure of an ionomer-free ultrathin catalyst layer was represented by a water-filled nanochannel with corrugated and negatively charged Pt walls. The equilibrium hydronium ion distribution in the channel was used to assess local reaction conditions. We rationalized the effects of nanostructuring and metal surface charge on the electrostatic effectiveness factor of the channel. Furthermore, we investigated the water dynamics in the nanochannel, calculating the self-diffusion coefficients of surface and bulk-like water.

Effects of Dissolved Water on Li+ Solvation in 1-Ethyl-3-methylimidazolium Bis(trifluoromethanesulfonyl)amide Ionic Liquid Studied by NMR

(1)H and (7)Li NMR chemical shifts of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide-water solutions in the presence and absence of lithium bis(trifluoromethanesulfonyl)amide were determined at 293.2 K over a wide range of water concentrations from 0.0156 to 1.16 mol kg(-1). These results revealed the attractive interaction between water molecule and Li(+) as well as the hydrogen bonding among water molecules. Moreover, self-diffusion coefficients of water, 1-ethyl-3-methylimidazolium cation, Li(+), and bis(trifluoromethanesulfonyl)amide anion in the ionic liquid solutions at various water contents were determined by (1)H, (7)Li, and (19)F NMR techniques. It was found that Li(+) is averagely hydrated by eight water molecules in the ionic liquid solutions. Furthermore, (7)Li longitudinal relaxation times of Li(+) in the ionic liquid solutions at 293.2 K were measured with two different magnetic fields and various water contents. The mean one-jump distances of Li(+) in the ionic liquid solutions were estimated from the correlation times and the self-diffusion coefficients. A comparison between the hydrodynamic radius and the mean one-jump distance of Li(+) suggested the formation of water channels in the ionic liquid solutions.

An NMR study of a water-methanol solution sorbed in MF-4SK sulfonated cation-exchange membranes

Concentration dependences of self-diffusion coefficients (SDCs), self-diffusion activation energies for water and methanol, and chemical shifts of the protons of the hydroxyl groups δOH simultaneously in an external water-methanol solution and the solution sorbed in MF-4SK membranes have been studied by NMR. It has been revealed that the SDC of pure methanol and pure water sorbed in an MF-4SK membrane is 3–5 times lower than that outside the membrane. It has been found that, in the presence of a small amount of methanol, the SDC of water in the membrane is 1.5–2 times higher than the SDC of pure sorbed water. At a solution concentration of 0.1–0.5 mole fraction, the SDC values of water and methanol in the membrane vary only slightly and are about 6 × 10−6 and 4 × 10−6 cm2/s, respectively. It has been determined that the δOH value in the membrane is 100–200 Hz higher than that in the external solution. The observed increase in δOH and decrease in SDC in the membrane suggest that the state of the solution in the MF-4SK sulfonated cation-exchange membrane has significantly changed compared to the external solution. The effect of the implanted carbon phase (CP) on the SDC of water and methanol and δOH of the solution sorbed in the MF-4SK membranes containing the CP has been studied. It has been revealed that at a methanol mole fraction of up to 0.5, the introduction of 23 wt % CP decreases the SDC of the solution components by no more than 10–20%. At a methanol mole fraction of 0.25–0.5, the self-diffusion activation energies for methanol and water in the external and membrane solutions decrease by 5–7 kJ/mol.

Quasi-elastic neutron scattering in the multi-GPa range and its application to liquid water

We present a method which allows quasi-elastic neutron scattering (QENS) at multi-GPa pressures, i.e., pressure conditions which are an order of magnitude higher than possible with QENS till now. The technique uses an opposed-anvil geometry with a sample chamber in the form of a toroid which minimizes multiple scattering effects. This method has been applied to measurements of the self-diffusion coefficient of water at 400 K up to 3 GPa. The results demonstrate the potential of this method for studying diffusion over a large pressure and temperature range.