Self-diffusion Behavior Research Articles

Homogeneous length scale of shear-induced multilamellar vesicles studied by diffusion NMR

A recently developed protocol for pulsed gradient spin echo (PGSE) NMR is applied for the size determination of multilamellar vesicles (MLVs). By monitoring the self-diffusion behavior of water, the technique yields an estimate of the homogeneous length scale λ hom, i.e. the maximum length scale at which there is local structural heterogeneity in a globally homogeneous material. A cross-over between local non-Gaussian to global Gaussian diffusion is observed by varying the experimentally defined length- and time-scales. Occasional observation of a weak Bragg peak in the PGSE signal attenuation curves permits the direct estimation of the MLV radius in favorable cases, thus yielding the constant of proportionality between λ hom and radius. The microstructural origin of the Bragg peak is verified through Brownian dynamics simulations and a theoretical analysis based on the center-of-mass diffusion propagator. λ hom is decreasing with increasing shear rate in agreement with theoretical expectations and results from 2H NMR lineshape analysis.

A Molecular Dynamics Study on the Effects of Nanostructural Clearances on Thermal Resistance at a Lennard-Jones Liquid-Solid Interface

The effects of the surface structures and the surface structural clearances at the nanometer scale on the thermal resistance at a Lennard-Jones liquid-solid interface, as well as the self-diffusion behaviors of liquid molecules, were investigated directly by the non-equilibrium classical molecular dynamics simulations. When the potential parameter between liquid molecules and nanostructure atoms is equal to that between liquid molecules and solid wall atoms, in other words, in the case of nano-engineered surface, the geometric surface area change depending on the nanostructures as well as their clearances and the self-diffusion coefficient change of the liquid molecules at the interface depending on the nanostructural clearances cause the thermal resistance change depending on the nanostructures at the liquid-solid interface. When the potential parameter between liquid molecules and nanostructure atoms is different from that between liquid molecules and solid wall atoms, in other words, in the case of a modified surface at the nanometer precision, the interfacial thermal resistance is much dependent on the potential parameter between liquid molecules and nanostructure atoms itself rather than the geometric surface area at the molecular scale.

Size dependency of self-diffusion and creep behavior of nanostructured metals

Grain boundary and bulk diffusion are two main governing processes of creep deformation in materials. Since the diffusion activation energy for nanostructured materials is lower than that for bulk, the diffusion is enhanced at the nano-scale, and nanosructured materials are expected to creep at lower temperatures and stresses. In this paper, a model has been developed to explain the effect of grain size on diffusion and creep behavior of nanostructures. Diffusion and creep phenomena have been shown to depend significantly upon size. Comparisons have been made with reported experimental results and related theoretical studies.

Ionic transport mechanism in cation-exchange membranes studied by NMR technique

The ionogenic groups, H + and alkaline metal cations hydration and mobility in perfluorinated sulfocation membrane MF-4SC, carboxyl perfluorinated membrane F-4CF, aromatic mono- and bisulfocontaining polyamides films were investigated by 1H, 7Li, 23Na, and 133Cs high resolution NMR. Water and Li +, Na +, and Cs + ion self-diffusion coefficients were measured by pulsed field gradient NMR. The ionic conductivities calculated from self-diffusion data were compared with experimental values measured by impedance spectroscopy. The cation hydration numbers were calculated from proton magnetic resonance data. In aromatic sulfocontaining polymers, the amide groups solvate the cations, taking part in additional hydrogen bond network formation. Water self-diffusion coefficients increase in the following cation row in the sulfonated membrane H + > Cs + > Na + > Li +. The self-diffusion coefficients of the carboxyl membrane salt forms and in aromatic polyamide membranes are ordered in the same row Cs + > Na + > Li +. The self-diffusion behaviour in acid form of the carboxyl membrane is restricted; the restricted size is about 1 μm. The hindered self-diffusion coefficient is two orders of magnitude low compared with sulfonate membrane acid form. The conductivity data are the same order of magnitude with the values calculated from the self-diffusion results.

Poly(vinyl phosphonic acid): Hydrodynamic Properties and SEC‐Calibration in Aqueous Solution

Poly(vinyl phosphonic acid) (PVPA) as obtained by free radical polymerization of aqueous vinyl phosphonic acid was studied by light scattering (SLS, DLS) and size exclusion chromatography (SEC) in dilute aqueous solutions containing sufficient salt in order to screen long range electrostatic interactions. Samples of 37<$\overline M _{\rm w}$< 110 × 10(3) were studied. The polymers showed positive A(2) -values in aqueous NaH(2) PO(4) solution (0.04 M), and self-diffusion behavior and R(H) /R(G) -ratios indicative of the structure of random coiled chains. A comparison of the SEC-elugrams of the PVPA-samples with those of commercially available standards of poly(acrylic acid) sodium salt gave a fit to the same calibration curve described by log P(n(PVPA)) = -0.21ν(e) + 7.0(+0.1) which correlates the number average degree of polymerization (P(n) ) with the elution volume ν(e) . This indicates that PVPA and PAA have the same hydrodynamic structure under given solution conditions.

CH/.PI. Interaction between Benzene and Hydrocarbons Having Six Carbon Atoms in Their Binary Liquid Mixtures

Molecular interactions between benzene and hydrocarbons having six carbon atoms, such as hexane, cyclohexane and 1-hexene in their binary liquid mixtures were studied through the measurements of density, viscosity, self-diffusion coefficient, (13)C NMR spin-lattice relaxation time and (1)H NMR chemical shift. CH/pi attraction between hexane and benzene in their binary mixture was observed in a relatively benzene rich region, whereas a special attractive interaction was not observed between cyclohexane and benzene. On the other hand, 1-hexene and benzene in their binary mixtures were characteristic in their self-diffusion coefficient behaviors: 1-hexene more strongly attract benzene not only by the CH/pi attraction but also probably by the p/p interaction between the double bond in 1-hexene and the p-electron in benzene ring.

Self-diffusion behaviors of Pd adatom and dimer on Pd(001) surface

Abstract Using molecular dynamics simulations and a modified analytic embedded atom potential, the self-diffusion dynamics of Pd adatom and dimer on Pd(0 0 1) surface have been studied in the temperature ranges from 600 K to 900 K. The simulation time varies from 25 to 50 ns according to the different substrate temperature. The diffusion mechanism of both the single Pd adatom and dimer is the simple exchange mechanism with the substrate atoms. The diffusion prefactors D 0 and activation energies E a are derived from the Arrhenius relation. The activation energy E a is in good agreement with the result of the quench molecular dynamics and experimental data of the single Pd adatom. The activation energy 0.58 ± 0.02 eV of the dimer is slightly lower than that 0.63 ± 0.02 eV of the single adatom because of the higher coordination numbers. When substrate temperature T = 290 K, the diffusion mobility D of the single adatom is equal to that of the dimer. In addition, the remarkable self-diffusion behaviors in several previous experiments that the diffusion mobility of the dimer is higher than that of the single adatom at low temperature are explained according to our molecular dynamics results.

Energetics and self-diffusion behavior of Zr atomic clusters on a Zr(0 0 0 1) surface

Using a molecular dynamics method and a modified analytic embedded atom potential, the energetic and the self-diffusion dynamics of Zr atomic clusters up to eight atoms on α-Zr(0001) surface have been studied. The simulation temperature ranges from 300 to 1100K and the simulation time varies from 20 to 40ns. It’s found that the heptamer and trimer are more stable comparing to other neighboring non-compact clusters. The diffusion coefficients of clusters are derived from the mean square displacement of cluster’s mass-center and the present diffusion coefficients for clusters exhibit an Arrhenius behavior. The Arrhenius relation of the single adatom can be divided into two parts in different temperature range because of their different diffusion mechanisms. The migration energies of clusters increase with increasing the number of atoms in cluster. The differences of the prefactors also come from the diverse diffusion mechanisms. On the facet of 60nm, the heptamer can be the nuclei in the crystal growth below 370K.

Niobium diffusion in niobium-doped titanium dioxide

The present work studied the self-diffusion coefficient of 93Nb in Nb-doped TiO2 single crystal (4.3 at.% Nb) at high oxygen activity [p(O2) = 21 kPa] over the temperature range 1,073 to 1,573 K. The diffusion-induced 93Nb concentration profile was determined by using secondary ion mass spectrometry (SIMS). The subsequently determined self-diffusion coefficient of 93Nb exhibits the following temperature dependence:\(D_{{}^{93}{\text{Nb}}} = 1.77 \times 10^{ - 9} \,{\text{m}}^2 \,{\text{s}}^{ - 1} \,\exp \left( {\frac{{ - 197 \pm 9\,{\text{kJ}}\,{\text{mol}}^{ - 1} }}{{{\text{RT}}}}} \right)\). This study builds upon a similar study performed previously for 93Nb tracer diffusion in undoped TiO2, and identifies the effect of compositional change on self-diffusion behaviour. The obtained activation energy has been considered in terms of migration and formation enthalpies of titanium vacancies.

Atomistic simulation of self-diffusion in Al and Al alloys under electromigration conditions

The effect of alloying elements on the self-diffusion behavior of Al under electromigration conditions was investigated using nonequilibrium molecular dynamics. The corresponding defect structures were also characterized energetically by Mott–Littleton approach. Pd, Cu, Mn, and Sn were found to be the most effective alloying elements that may retard the electromigration failure by increasing the activation energy for self-diffusion of Al. This increase in the activation energy is believed to be either because of the dragging effect that may be experienced in a coupled substitutional-vacancy defect structure or the energy penalty for the separation of this couple.

Comparative study of compact hexagonal cluster self-diffusion on Cu(1 1 1) and Pt(1 1 1)

Self-diffusion behaviors of two-dimensional compact hexagonal Cu 7 and Pt 7 clusters on (1 1 1) surface are studied with molecular dynamics simulation. The diffusion barriers ( E m ) and prefactors ( D 0 ) are determined from the Arrhenius diagram and compared with recent low-temperature field ion microscopy experiment. We find that D 0 of Pt 7 cluster is about 35 times larger than that of Cu 7 cluster. From the calculated phonon spectral densities of adatoms, we suggest that the different diffusion behaviors of the two clusters may result from strong interactions between the Pt adatoms and surface atoms, which make the nonequivalent self-diffusion processes, which can produce a high prefactor, occur more easily for Pt 7 cluster.

Molecular dynamics study of diffusional creep in nanocrystalline UO 2

We present the results of molecular dynamics (MD) simulations to study high-temperature deformation of nanocrystalline UO 2. In qualitative agreement with experimental observations, the oxygen sublattice undergoes a structural transition at a temperature of about 2200 K (i.e. well below the melting point of 3450 K of our model system), whereas the uranium sublattice remains unchanged all the way up to melting. At temperatures well above this structural transition, columnar nanocrystalline model microstructures with a uniform grain size and grain shape were subjected to constant-stress loading at levels low enough to avoid microcracking and dislocation nucleation from the grain boundaries (GBs). Our simulations reveal that in the absence of grain growth, the material deforms via GB diffusion creep (also known as Coble creep). Analysis of the underlying self-diffusion behavior in undeformed nanocrystalline UO 2 reveals that, on our MD timescale, the uranium ions diffuse only via the GBs, whereas the much faster moving oxygen ions diffuse through both the lattice and the GBs. As expected for the Coble-creep mechanism, the creep activation energy agrees well with that for GB diffusion of the slowest-moving species, i.e. the uranium ions.

An improved theory for temperature-dependent Arrhenius parameters in mesoscale surface diffusion

Experiments on metals typically show an abrupt change in the Arrhenius behavior of surface self-diffusion at temperatures near 60–75% of the bulk melting point. To explain this phenomenon, we propose based on correlational evidence that the most common mechanism for surface self-diffusion is one in which adatoms dominate low-temperature transport, while surface vacancies dominate at high temperatures. The high-temperature dominance of vacancies results from their substantially higher entropy of diffusion, which is a consequence of the large vibrational displacements of surface atoms relative to the bulk. This phenomenon may also explain the Arrhenius behavior on some non-metal surfaces.

Influence of κ-Carrageenan Gel Structures on the Diffusion of Probe Molecules Determined by Transmission Electron Microscopy and NMR Diffusometry

The influence of the microstructures of different kappa-carrageenan gels on the self-diffusion behavior of poly(ethylene glycol) (PEG) has been determined by nuclear magnetic resonance (NMR) diffusometry and transmission electron microscopy (TEM). It was found that the diffusion behavior was determined mainly by the void size, which in turn was defined by the state of aggregation of the kappa-carrageenan. The kappa-carrageenan concentration was held constant at 1 w/w%, and the aggregation was controlled by the amount of potassium and/or sodium chloride and, for samples containing potassium, also by the cooling rate. Gels containing potassium formed microstructures where kappa-carrageenan strands are rather evenly distributed over the image size, while sodium gels formed dense biopolymer clusters interspersed with large openings. In a gel with small void sizes, relatively slow diffusion was found for all PEG sizes investigated. Extended studies of the self-diffusion behavior of the 634 g mol(-)(1) PEG showed that there is a strong time dependence in the measured PEG diffusion. An asymptotic lower time limit of the diffusion coefficient was found in all gels when the diffusion observation time was increased. According to the ratio, D/D(0), where D(0) is the diffusion coefficient in D(2)O and D is the diffusion coefficient in the gels, the gels could be divided into three classes: small, medium, and large voids. For quenched kappa-carrageenan solutions with salt concentrations of 20 mM K(+), 100 mM K(+), or 20 mM K(+)/200 mM Na(+) as well as slowly cooled solutions with only 20 mM K(+), D/D(0) ratios between 0.18 and 0.29 were obtained. By quenching a kappa-carrageenan solution with 100 mM K(+), the D/D(0) was 0.5, while D/D(0) ratios between 0.9 and 1 were obtained in a quenched solution with 250 mM Na(+) and slowly cooled samples with 20 mM K(+)/200 mM Na(+) or 250 mM Na(+).

Fluid Self-Diffusion in Scots Pine Sapwood Tracheid Cells

The self-diffusion coefficients of water and toluene in Scots pine sapwood was measured using low field pulsed field gradient nuclear magnetic resonance (PFG-NMR). Wood chips of 8 mm diameter were saturated with the respective liquids, and liquid self-diffusion was then traced in one dimension orthogonal to the tracheid cell walls in the wood's radial direction. The experimental echo attenuation curves were exponential, and characteristic self-diffusion coefficients were produced for diffusion times spanning from very short times to times on the order of magnitude of seconds. Observed self-diffusion coefficients were decaying asymptotically as a function of diffusion time, an effect which was ascribed to the cell walls' restriction on confined liquid diffusion. The observed self-diffusion behavior in Scots pine sapwood was compared to self-diffusion coefficients obtained from simulations of diffusion in a square. Principles of molecular displacements in confined geometries were used for elucidating the wood's cellular structure from the observed diffusion coefficients. The results were compared with a mathematical model for diffusion between parallel planes.

Oxygen Pipe Diffusion in Sapphire Basal Dislocation

The oxygen self-diffusion behavior in deformed sapphire single crystals (α-Al2O3 sapphire) with a high density of unidirectional basal dislocations was examined in the temperature range of 1424-1636°C using 18O isotopes and secondary ion mass spectrometry-depth profiling techniques. The pipe and lattice diffusion kinetics were best described by r2Dp=4.6×10-20 exp (-4.8[eV]/kT) [m4/s] and Dl=2.9×10-1 exp (-5.5[eV]/kT) [m2/s], respectively. Both the magnitude of pre-exponential factor and activation energy for oxygen pipe diffusion were in good agreement with that of indirect measurements of pipe diffusion deduced from the annihilation of dislocation dipoles. The measured bulk diffusion coefficient is also in good agreement with previously reported data having activation energies ranged between 5.5-6.1 eV.

The self-diffusion behavior of polyethylene glycol in cartilageas studied by pulsed-field gradient NMR

The self-diffusion behavior of the polyethylene glycol (PFG) polymer in bovine nasal cartilage was studied by pulsed-field gradient (PFG) nuclear magnetic resonance (NMR). PFG NMR allows the determination of the mean square displacement of molecules in a given diffusion time (in the range of a few milliseconds up to seconds), monitors distances in micrometer scales and has the advantage of being non-invasive. Moreover the application of pfg nmr does not require concentration gradients In a previous study, PFG NMR was used to investigate the self-diffusion behavior of the PEG polymer in cartilage at very highconcentrations. In this study, much lower PRG concentrations were used in order to detect the effects of the structural composition of the cartilage tissue more efficiently. It will be shown that at very low (<10 wt.-%) PFG concentrations, the effect of restricted polymer diffusion in cartilage is negligible. The self-diffusion coefficients (SDC) are primarily influenced by the water content and the molecular weight (MW) of the appliec. PFG. The problems encountered with PFG NMR diffusion studies using high field gradients as well as in vivo aspects of this study are discussed.

Ultramicroelectrode Voltammetric Investigation of Intermicellar Interaction and Micellar Growth of Sodium Dodecyl Sulfate in Aqueous NaCl Solutions

Ultramicroelectrode voltammetric measurements have been carried out to calculate the micellar self-diffusion coefficient (Ds) and intermicellar interaction parameter (Kd) of sodium dodecyl sulfate (SDS) in aqueous NaCl solutions at 298.15 K. The hydrodynamic radius ( ), the aggregation number, and the molecular weight of SDS micelles at various NaCl concentrations are also estimated. It is found that the Ds value is a function of SDS concentrations and the electrolyte concentration dependence on Ds exhibits two regions of behavior. With increasing NaCl concentration, Ds initially increases because of increasing Coulombic screening and, then, decreases linearly because of a linear spherical expansion of the micelles resulting from the increasing aggregation number. Kd is found to be a function of electrolyte concentrations. The stepwise decrease of Kd reflects that, due to Coulombic repulsion, the electrostatic interaction is gradually screened with increasing NaCl concentration. As the linear interaction theory applies rigorously over the electrolyte concentration range from 0.20 to 1.00 mol dm-3, it suggests that the micellar structure remains effectively spherical in the region, the linear spherical expansion of the micelle is occurring, and no dramatic structural changes have taken place in the micellar solutions. In addition, the micellar interaction energy is calculated, which implies that it is the Coulombic nature of the interaction between micellar particles that controls the micellar self-diffusion behavior in aqueous solutions.

Translational dynamics of antifreeze glycoprotein in supercooled water

Structure and dynamics of biomolecules in supercooled water assume a particular and distinct importance in the case of antifreeze glycoproteins (AFGPs), which function at sub-zero temperatures. To investigate whether any large-scale structural digressions in the supercooled state are correlated to the function of AFGPs, self-diffusion behavior of the AFGP8, the smallest AFGP is monitored as a function of temperature from 243 to 303 K using nuclear magnetic resonance (NMR) spectroscopy. The experimental results are compared with the hydrodynamic calculations using the viscosity of water at the same temperature range. In order to evaluate results on AFGP8, the smallest AFGP, constituting approximately two-thirds of the total AFGP fraction in fish blood serum, similar experimental and computational calculations were also performed on a set of globular proteins. These results show that even though the general trend of translational dynamics of AFGP is similar to that of the other globular proteins, AFGP8 appears to be more hydrated (approximately 30% increase in the bead radius) than the others over the temperature range studied. These results also suggest that local conformational changes such as segmental librations or hydrogen bond dynamics that are closer to the protein surface are more likely the determining dynamic factors for the function of AFGPs rather than any large-scale structural rearrangements.

Role of entropy of fusion in phase transformation and self-diffusion

The present paper discusses the role of the entropy of fusion vis-a-vis the phase transformation characteristics and the self-diffusion behavior of crystalline matrices. The data correlating the entropy of fusion with these properties in metals, alkali halides and some other inorganic compounds are presented and analyzed. It is shown that the occurrence of a solid-solid state phase transformation decreases the magnitude of the entropy of fusion. In addition, the self-diffusion rates within any class of solids scale inversely with the entropy of fusion. The functional relationship of the entropy of fusion vis-a-vis the compressibility and the volume expansion coefficient is also discussed. The conclusion is that the entropy of fusion is not just a physical parameter describing the energy changes associated with the melting. It is, in fact, related in a substantial manner to the bulk properties of the solid and controls the phase transition characteristics and self-diffusion behavior within any group or class of solids in a uniform and consistent manner.