Self-diffusion Measurements Research Articles

Structural Evolution in the Isotropic Channel of a Water–Nonionic Surfactant System That Has a Disconnected Lamellar Phase: A 1H NMR Self-Diffusion Study

We showed in a previous study that a water-nonionic surfactant system, where the surfactant is a 9:1 mixture of tetraethylene glycol monodecyl ether (C(10)E(4)) and pentaethylene glycol monodecyl ether (C(10)E(5)), forms a disconnected lamellar (L(α)) phase. Thus, the isotropic phase spans the whole concentration range from the water-rich L(1) region to the surfactant-rich L(2) region of the phase diagram. The L(1) and L(2) regions are connected via an isotropic channel that separates the two regions of the L(α) phase. In this letter, we monitored the structural evolution of the isotropic phase along a path through this isotropic channel via (1)H NMR self-diffusion measurements. We used this technique because it enables us to distinguish between discrete and bicontinuous structures by comparing the relative self-diffusion coefficients (obstruction factors) D/D(0) of the solvents (i.e. of water and surfactant in the present case). We found that the obstruction factor of water decreases whereas the obstruction factor of the surfactant increases with increasing surfactant concentration and increasing temperature. This trend is interpreted as the transition from a water-continuous L(1) region, which contains discrete micelles, to a bicontinuous structure, which may extend to very high surfactant concentrations. Although there is good evidence of bicontinuity over a broad concentration range, there is no evidence of inverse micelles or any other microstructure at the highest concentration studied in the surfactant-rich L(2) phase.

Determination of mean droplet sizes of water-in-oil emulsions using an Earth’s field NMR instrument

The use of the Earth’s magnetic field (EF) to conduct nuclear magnetic resonance (NMR) experiments has a long history with a growing list of applications (e.g. ground water detection, diffusion measurements of Antarctic sea ice). In this paper we explore whether EFNMR can be used to accurately and practically measure the mean droplet size (〈a〉) of water-in-oil emulsions (paraffin and crude oil). We use both pulsed field gradient (PFG) measurements of restricted self-diffusion and T2 relaxometry, as appropriate. T2 relaxometry allows the extension of droplet sizing ability below the limits set by the available magnetic field gradient strength of the EFNMR apparatus. A commercially available bench-top NMR spectrometer is used to verify the results obtained using the EFNMR instrument, with good agreement within experimental error, seen between the two instruments. These results open the potential for further investigation of the application of EFNMR for emulsion droplet sizing.

Diffusion and Partitioning of Cations in an Agarose Hydrogel

Self- and mutual diffusion were measured in a low-melt 1.5% agarose hydrogel as a function of ionic strength (0.1-100 mM) and pH (3-7) for Cd(2+) and a charged rhodamine derivative. Self-diffusion was measured by fluorescence correlation spectroscopy, whereas mutual diffusion was evaluated using a diffusion cell. In contrast to the results observed for the diffusion cell, self-diffusion of rhodamine 6G increased from 50 to 90% of that found in water as the ionic strength increased from 0.1 to 100 mM (pH = 6). The combined observations of decreasing diffusive flux in parallel with an increasing diffusion coefficient were attributed to the gel's Donnan potential. Donnan potentials obtained voltammetrically using a Au amalgam microelectrode varied from -30 to 0 mV as the ionic strength increased from 0.1 to 100 mM (pH = 6). At the low ionic strengths, Donnan potentials of this magnitude accounted for a 13× enhancement of Cd(2+) concentrations in the hydrogel, which was consistent with measurements obtained by a nitric acid extraction of the gel (15×) and able to explain the apparent discrepancy between mutual and self-diffusion measurements. The overall diffusion of the positively charged substrates decreased as the pH was decreased from 12 to 3.

Interaction of Premicellar States of a PEO-PPO-PEO Triblock Copolymer with Partially Hydrophobic Substances: NMR Study

According to (1)H and (13)C NMR spectra, relaxations, and PFG NMR self-diffusion measurements, partially hydrophobic additives methyl-ethyl ketone (MEK), methyl-isopropyl ketone (MIPK), and methyl-t-butyl ketone (MTBK) facilitate the conformation change and subsequent self-association of the copolymer Pluronic L64. The correlation time (4-9 ms) and activation energy (43-52 kJ/mol) of transition between its conformation states decrease with the increasing hydrophobicity and bulkiness of the additive. The temperature of the first PPO self-association decreases in the same order (by 4 K for MTBK). The interaction of the additives was indirectly proved by the decrease of their rotational and translational mobility in the presence of L64. The rotational correlation time τ(c) is between 3 and 6 × 10(-11) s, whereas that of the same molecules in the absence of L64 is lower than 6 × 10(-12) s. The normalized self-diffusion coefficient decreases to about 0.7 of its original value in the presence of L64. The interaction of the additive with the PPO block is transient but effective enough to facilitate its conformational change and self-association. Its mediation by a water molecule bound to PPO as a possible mechanism is suggested.

Transport Properties Investigation of Aqueous Protic Ionic Liquid Solutions through Conductivity, Viscosity, and NMR Self-Diffusion Measurements

We present a study on the transport properties through conductivity (σ), viscosity (η), and self-diffusion coefficient (D) measurements of two pure protic ionic liquids--pyrrolidinium hydrogen sulfate, [Pyrr][HSO(4)], and pyrrolidinium trifluoroacetate, [Pyrr][CF(3)COO]--and their mixtures with water over the whole composition range at 298.15 K and atmospheric pressure. Based on these experimental results, transport mobilities of ions have been then investigated in each case through the Stokes-Einstein equation. From this, the proton conduction in these PILs follows a combination of Grotthuss and vehicle-type mechanisms, which depends also on the water composition in solution. In each case, the displacement of the NMR peak attributed to the labile proton on the pyrrolidinium cation with the PILs concentration in aqueous solution indicates that this proton is located between the cation and the anion for a water weight fraction lower than 8%. In other words, for such compositions, it appears that this labile proton is not solvated by water molecules. However, for higher water content, the labile protons are in solution as H(3)O(+). This water weight fraction appears to be the solvation limit of the H(+) ions by water molecules in these two PILs solutions. However, [Pyrr][HSO(4)] and [Pyrr][CF(3)COO] PILs present opposed comportment in aqueous solution. In the case of [Pyrr][CF(3)COO], η, σ, D, and the attractive potential, E(pot), between ions indicate clearly that the diffusion of each ion is similar. In other words, these ions are tightly bound together as ion pairs, reflecting in fact the importance of the hydrophobicity of the trifluoroacetate anion, whereas, in the case of the [Pyrr][HSO(4)], the strong H-bond between the HSO(4)(-) anion and water promotes a drastic change in the viscosity of the aqueous solution, as well as on the conductivity which is up to 187 mS·cm(-1) for water weight fraction close to 60% at 298 K.

TiCl4 immobilized on a composite support SiO2/MgCl2·x(1,4-butanediol)/poly[styrene-co-(acrylic acid)] for ethylene polymerization: The barrier effect of poly[styrene-co-(acrylic acid)

By immobilizing titanium-based Ziegler–Natta catalyst on composite support, SiO2/MgCl2·x(1,4-butanediol)/poly[styrene-co-(acrylic acid)] (SiO2/MgCl2·xBD/PSA) and SiO2/MgCl2·xBD/PSA/TiCl4 (SMPT) were synthesized for ethylene polymerization. SiO2/MgCl2·xBD/TiCl4 without PSA was also prepared for comparison. The results of energy-dispersive X-ray analysis, SEM, and thermogravimetric analysis demonstrated that SMPT had a unique core-mantle-shell structure. The PSA layer can be considered as a barrier for the mass-transfer of reactants based on the results of self-diffusion measurement by pulsed field gradient NMR and ethylene polymerization. The polyethylene produced by SMPT showed high molecular weight (MW) and broad molecular weight distribution (MWD). The influences of PSA content, hydrogen, and comonomer on the ethylene polymerization behavior were also investigated. The results further demonstrated that the PSA layer in the composite support had different diffusion capabilities to the reactants. The physical properties of the produced polyethylene implied the possibility to control the MW and MWD of polyethylene by the manipulation of PSA layer. The catalyst fragmentation during ethylene polymerization was also affected by the PSA shell due to its barrier effect. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Measurement of Defect-Mediated Oxygen Self-Diffusion in Metal Oxides

Solid-state diffusion of native defects in some metal oxides takes place via highly mobile intermediate species that periodically exchange with lattice atoms. A mathematical model has been formulated to determine the key diffusion parameters from mesoscopic gas-solid diffusion measurements in the short-time limit. The model provides a straightforward analytical means to utilize standard isotope gas-solid exchange experiments to obtain parameters connected to diffusion length and net rate of defect formation at the surface. Application of the method to experimental data for oxygen self-diffusion in titanium dioxide suggests that under conditions where gas-solid exchange takes place through an especially clean surface, a fast-moving species mediates oxygen self-diffusion.

高圧高温NMR分光法の開発と超臨界水中のダイナミクス・構造・反応の研究

The present review introduces our recent studies of the title subjects that have led to the JSHPST Award for Young Scientists, 2011. We studied the structure, dynamics, and reactions in water under high-pressure and high-temperature conditions using NMR and computational methods. The NMR apparatus was newly developed and was applied for the self-diffusion measurements of water and nonpolar organic fluids over a wide range of pressure and temperature. Molecular dynamics simulation combined with a new analysis scheme provided new insights into the translational dynamics in the supercritical water in the medium-density region in relation to the structure and lifetime of the solvation shell.

PFG-NMR self-diffusion measurements in the single phase channels of a microemulsion system with an anionic–nonionic surfactant mixture

The single phase channels of a presently reported microemulsion system were investigated by electrical conductivity and pulsed-field gradient nuclear magnetic resonance (PFG-NMR) spectroscopy. The system consists of a mixed anionic–non-ionic surfactant mixture, water and decane. At constant surfactant concentration and temperature, the phase diagram exhibits two single phase microemulsion channels, separated by an anisotropic lamellar channel. The lower microemulsion channel starts from the water side of the phase diagram with a micellar L1 phase and reaches the middle of the phase diagram with increasing mass fraction of decane in the solvent mixture and increasing mass fraction of lipophilic co-surfactant in the surfactant mixture. The upper microemulsion channel passes from the aqueous side with an L3 phase to the oil side of the diagram. Conductivity data and self-diffusion coefficients, obtained by PFG-NMR, support the previously made conclusion that the nanostructure in the upper channel undergoes an abrupt transition from a bicontinuous structure to a water-in-oil High Internal Phase Microemulsion (HIPME) with already less than 10% of oil in the solvent mixture, while the structures in the lower microemulsion channel are oil-in-water droplets. The HIPME structure is a feature of the surfactant mixture and probably formed due to a high interfacial tension between the aqueous diluted surfactant phase and the oil. By the addition of salt, the HIPME structures are obviously disturbed, resulting in an increased conductivity and self-diffusion rate for the water fraction.

ChemInform Abstract: Dynamic Molecular Movements and Aggregation Structures of Lipids in a Liquid State

AbstractReview: 48 refs.

Interaction of hydrated protons with octyl-phenyl-N,N-diisobutylcarbamoylmethyl phosphine oxide (CMPO): NMR and theoretical study

Interaction of octyl-phenyl-N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO, the 'classical' rare metal extraction agent) with fully ionized hydrated protons (HP) was studied in acetonitrile-d(3) using (1)H, (13)C, (31)P NMR, PFG NMR and magnetic relaxation. The experimental results were confronted with high-precision ab initio DFT calculations. Relative chemical shifts of NMR signals of CMPO (0.01 mol/L) under the presence of HP in the molar ratio β = 0-2.0 mol/mol show binding between CMPO and HP. Self-diffusion measurements using (1)H PFG NMR demonstrate that larger complexes with higher content of CMPO are generally formed at β < 0.75. Analyzing the collective dependence of (13)C and (31)P NMR chemical shifts on β by the use of program LETAGROP, we obtained very good fitting for the assumed coexistence of two complexes (CMPO)(2)·HP (C(2)) and CMPO.HP (C(1)). The logarithms of the respective stabilization constants log K(i) were found to be 7.518 (C(2)) and 4.581 (C(1)). The system dynamics was studied by measuring the transverse (1)H NMR relaxation using CPMG sequence with varying delays t(p) between the π pulses in the mixtures with β = 0.4-0.8. The following exchange correlation times were obtained: τ(10) = 2.35 × 10(-5), τ(20) = 0.82 × 10(-4), τ(21) = 0.45 × 10(-3) s. The DFT calculations support the conclusion that the complexes C(1) and C(2) are the main species in the mixtures of CMPO with HP. They also agree with the NMR and FTIR observation that the main site to which H(3) O(+) is bound is the P=O group, whereas the amide group does not form a strong bond with the ion when excess water molecules are present.

Temperature Dependence of Ion and Water Diffusion in Crown Ether Loaded Nafion Matrix

Temperature dependence study of the self-diffusion coefficient of Cs(+) ion in dibenzo-18-crown-6 (DB18C6) modified Nafion-117 (Cs-Naf-CR) was carried out in the temperature range of 50-65 °C. Temperature dependence of water diffusion in Cs-Naf-CR was also studied to understand the mechanism of cation and water transport in the membrane. Because of the very slow kinetics of isotopic exchange, self-diffusion measurement of Na(+) in Na-Naf-CR was carried out only at 60 °C. The result indicates that self-diffusion behavior is governed by the nature of the cation in which the crown ether was loaded in the membrane matrix. The activation energy of diffusion for Cs(+) ion and water in Cs-Naf-CR was found to be much higher than that in the pure Cs(+) form of Nafion (Cs-Naf). Water uptake of the membrane was also found to have reduced compared to Cs/Na-Naf. The results point to the binding of the ions by DB18C6 and the destruction of the water channels in the crown ether loaded membrane. The differential scanning calorimetry (DSC) data supports these observations.

Convection Effects on PGSE-NMR Self-Diffusion Measurements at Low Temperature: Investigation into Sources of Induced Convective Flows

The effects of convection on the measurement of the diffusion coefficients of liquids by the pulsed gradient spin echo (PGSE) NMR method at low temperature are discussed. To examine the generation of convective flows, we used four different types of sample tubes in the diffusion measurements with temperature variation; a normal 5 mm NMR tube, a Shigemi tube, an ELISE type tube, and a capillary tube. Below room temperature, the calculated diffusion coefficients of chloroform in 5 mm o.d. type tubes increased with decreasing temperature, while those in the capillary tube decreased linearly. The convective flow was found to be significant even at low temperature and it seemed to be mainly induced by the transverse temperature gradient. It was also found that the capillary tube was most appropriate to measure the diffusion coefficients, since its small diameter is effective in suppressing the convective flows at both high and low temperatures.

Interaction of Cesium Ions with Calix[4]arene-bis(t-octylbenzo-18-crown-6): NMR and Theoretical Study

Using (1)H, (13)C, and (133)Cs NMR spectra, it is shown that calix[4]arene-bis(t-octylbenzo-18-crown-6) (L) forms complexes with one (L·Cs(+)) and two (L·2Cs(+)) Cs(+) ions offered by cesium bis(1,2-dicarbollide) cobaltate (CsDCC) in nitrobenzene-d(5). The ions interact with all six oxygen atoms in the crown-ether ring and the π electrons of the calixarene aromatic moieties. According to extraction technique, the stability constant of the first complex is log β(nb)(L·Cs(+)) = 8.8 ± 0.1. According to (133)Cs NMR spectra, the value of the equilibrium constant of the second complex is log K(nb)((2))(L·2Cs(+)) = 6.3 ± 0.2, i.e., its stabilization constant is log β(nb)(L·2Cs(+)) = 15.1 ± 0.3. Self-diffusion measurements by (1)H pulsed-field gradient (PFG) NMR combined with density functional theory (DFT) calculations suggest that one DCC(–) ion is tightly associated with L·Cs(+), decreasing its positive charge and consequently stabilizing the second complex, L·2Cs(+). Using a saturation-transfer (133)Cs NMR technique, the correlation times τ(ex) of chemical exchange between L·Cs(+) and L·2Cs(+) as well as between L·2Cs(+) and free Cs(+) ions were determined as 33.6 and 29.2 ms, respectively.

Measurement of photostimulated self-diffusion in silicon

Photostimulated diffusion within solid semiconductors has been examined for many years, but its existence above room temperature has not been unambiguously confirmed. Here, diffusion rates for silicon self-diffusion are shown to change by factors of up to 25 in response to optical fluxes on the order of 1 W/cm2. Results depend on doping type; the rates of both interstitial formation and migration are affected in the case of n-type material. A model based on photostimulated changes in defect charge state explains the primary results, and the basic outlines should apply to a wide variety of semiconductors.

Hydration modes of an amphiphilic molecule 2: NMR, FTIR and theoretical study of the interactions in the system water–1,2-dimethoxyethane

Using 1H and 13C NMR spectra, PFG NMR self-diffusion measurements, 1H and 13C NMR relaxations and 1H NOESY NMR spectra, FTIR spectra and quantum-chemical DFT and MP2 calculations, the interaction of 1,2-dimethoxyethane (DME) with water (W) was re-examined. It was confirmed that, primarily, one W molecule forms two O···H hydrogen bonds with DME in tgt conformation. At medium and higher W contents, however, larger hydrates of DME are formed, predominantly with five W molecules. The compact structure of the hydrate is warranted by O⋯H hydrogen bonds, some of them perceptibly tighter than those in the primary hydrate, and by non-classical CH 3⋯O hydrogen bonds.

Comparison of the structure and the transport properties of low-set and high-set curdlan hydrogels

Curdlan, a bacterial polysaccharide, can form different types of thermogels, having the very same chemical composition, but whose structures depend on the incubation temperature. Structural characterization of 10% (w/v) low-set and high-set curdlan gels was carried out by Fourier transformed infrared (FT-IR) imaging and environmental scanning electron microscopy (eSEM) in the hydrated state. Considerable differences were observed between the two gels, the high-set one being overall more homogeneous. The self-diffusion coefficients of a series of analytes of different sizes (water, phosphate, glucose-6-phosphate, polyphosphate, polyethylene glycol, and dextran labelled with rhodamine B) were measured in aqueous solution (Dssln) and in both types of curdlan gels (Dsgel) using 1H and 31P pulsed field gradient nuclear magnetic resonance (PFG NMR) spectroscopy. The mutual-diffusion coefficients (Dmgel) of dextran in the curdlan gels were determined from release experiments based on fluorescence spectroscopy. The dependence of the relative diffusion coefficient (DsgelDssln) on the size of the analyte, expressed by its hydrodynamic radius (Rh), could be expressed by DsgelDssln∝exp(-Rh0.46), valid for both types of gels. The self-diffusion measurements for the largest investigated analytes were not compatible with a single diffusion coefficient and, therefore, were analysed using an approach based on a normal distribution of self-diffusion coefficients. In the hydrogels, broadening of the self-diffusion coefficient distribution increased as a function of the analyte size. This phenomenon was associated with the limited distance travelled by the analytes during the measurements, and it is inferred that the distribution of diffusion coefficients is representative of the distribution of local environments of the individual analyte. It was found that the structural differences observed between both types of curdlan gels are not correlated with the gel transport properties, highlighting the complexity of the relationship between structural details and transport properties in gels.

Impact of chemical treatments on the behavior of water in Nafion® NRE-212 by 1H NMR: Self-diffusion measurements and proton quantization

In this article, we have studied by 1H NMR the behavior of water in Nafion under acid, sodium and potassium forms which arises from different chemical treatments with or without EDTA. Self-diffusion coefficients were measured by PGSTE (Pulsed Gradient STimulated Echo) whereas integration of the proton spectrum provides a quantitative information about water content. Examination of the self-diffusion coefficients of water in Nafion shows clearly that: (i) in the acid form, the use of a chelating agent does not affect translational diffusion of water, (ii) regardless of sample conditioning, water within Nafion in acid form diffuses more rapidly than in sodium and potassium forms, in particular at low water content (at λ=3.4, D=5.4×10−7cm2s−1 vs. 7.2×10−8cm2s−1), and (iii) for a given water amount, water diffusion is identical in sodium and potassium forms. Point (ii) is due to the presence of oxonium ions within Nafion in acid form, favoring both vehicle and Grotthuss mechanisms. It is also due to a higher number of protons available for diffusion (we find an average value of 1.19 in acid form vs. 0.91 in sodium form). Our results suggest in the case of Nafion in acid form that, at low water content, diffusion occurs by jumps between different sites. Moreover, plotting self-diffusion coefficients vs. relative humidity leads to a clear differentiation between sodium and potassium cations, related to their respective solvation number.

Transport properties in two pyrrolidinium-based protic ionic liquids as determined by conductivity, viscosity and NMR self-diffusion measurements

Conductivity and viscosity measurements of pyrrolidinium hydrogen sulfate, [Pyrr][HSO 4], and pyrrolidinium trifluoroacetate [Pyrr][CF 3COO] were performed at various temperatures over a wide temperature range (i.e., from T = 273 K to 353 K). The results were utilized in the Stokes–Einstein equation to investigate the proton conductivity in both PILs. The self-diffusion coefficients ( D) of the cation and anion species in both studied PILs were independently determined in the same temperature range by observing 1H and 19F nuclei with the pulsed-field gradient spin-echo NMR technique. With regard to the mechanism of self-diffusion, based on the values of the coefficients, a relatively large difference was observed between the two ionic liquids (ILs). Independently of the temperature, the D values indicated that the diffusion of both ions was similar, signifying that they were tightly bound together as ion pairs. Nevertheless, mobile protons attached to nitrogen atoms exhibited D values five times higher than those of the pyrrolidinium cation or hydrogenosulfate anion in [Pyrr][HSO 4], and twofold those in the case of [Pyrr][CF 3CO 2]. In order to comment d.c. conductivities results, the self-diffusion coefficients determined by PGSE NMR were converted into charge diffusivity D by means of the Nernst–Einstein equation. In a similar way, a viscosity-related diffusivity D was calculated with the aid of the Stokes–Einstein equation. The temperature-independent cation transference number and the effective hydrodynamic radius were also deduced from these equations. Such parameters play an important role in charge and mass transports in ILs. Moreover, proton conduction follows a combination of Grotthuss- and vehicle-type mechanisms, which confirms that Brønsted acid–base ionic liquid systems are good candidates as proton conductors in fuel cells or supercapacitor electrolyte devices operating under anhydrous conditions at elevated temperatures.

Hydrodynamic size and scaling relations for linear and 4 arm star PVAc studied using PGSE NMR

In this work the hydrodynamic radius and scaling relations of linear and 4 arm polyvinyl acetate (PVAc) were calculated from self-diffusion measurements. Self-diffusion coefficients at infinite dilution were estimated from measurements of self-diffusion at 25 °C in deuterated chloroform and in the dilute regime using pulsed gradient spin-echo (PGSE) nuclear magnetic resonance (NMR). Scaling laws were observed but unexpectedly these star polymers showed slightly larger hydrodynamic radii than the linear equivalents. Hydrodynamic radii were calculated for different Perrin or shape factors (i.e. spherical, cylindrical and ellipsoidal), and the equivalent hydrodynamic radii for the cylinders and ellipsoids were compared to those obtained for a spherical shaped molecule. These were also compared to an approximate equivalent hydrodynamic radius estimated using approximate dimensions of the polymers at maximum extension for the cylinder/ellipsoid molecular shapes. Results for cylinder and ellipsoid shapes were almost identical for the same aspect ratios as expected.