Pulsed Gradient Stimulated Echo Research Articles

Soft Nanoonions: A Dynamic Overview onto Catanionic Vesicles Temperature-Driven Transition.

Catanionic vesicles are emerging interesting structures for bioapplications. They self-generate by a pairing of oppositely charged ionic surfactants that assemble into hollow structures. Specifically, the anionic-cationic surfactant pair assumes a double-tailed zwitterionic behavior. In this work, the multilamellar-to-unilamellar thermal transition of several mixed aqueous systems, with a slight excess of the anionic one, were investigated. Interestingly, it was found that the anionic counterion underwent a dissociation as a consequence of a temperature increase, leading to the mentioned thermal transition. The present work proposed the spectroscopic techniques, specifically multinuclear NMR and PGSTE (pulsed gradient stimulated echo), as a key tool to study such systems, with high accuracy and effectiveness, while requiring a small amount of the sample. The results presented herein evidence encouraging perspectives, forecasting the application of the studied vesicular nanoreservoirs, for e.g., drug delivery.

Acquisition of spatially-resolved displacement propagators using compressed sensing APGSTE-RARE MRI

A method is presented for accelerating the acquisition of spatially-resolved displacement propagators via under-sampling of an Alternating Pulsed Gradient Stimulated Echo – Rapid Acquisition with Relaxation Enhancement (APGSTE-RARE) data acquisition with compressed sensing image reconstruction. The method was demonstrated with respect to the acquisition of 2D spatially-resolved displacement propagators of water flowing through a packed bed of hollow cylinders. The q,k-space was under-sampled according to variable-density pseudo-random sampling patterns. The quality of compressed sensing reconstructions of spatially-resolved propagators at a range of sampling fractions was assessed using the peak signal-to-noise ratio (PSNR) as a quality metric. Propagators of good quality (PSNR 33.2 dB) were reconstructed from only 6.25% of all data points in q,k-space, resulting in a reduction in the data acquisition time from 4 h to 14 min. The spatially-resolved propagators were reconstructed using both the total variation and nuclear norm sparsifying transforms; use of total variation resulted in a slightly higher quality of the reconstructed image in most cases. To illustrate the power of this method to characterise heterogeneous flow in porous media, the method is applied to the characterisation of flow in a vuggy carbonate rock.

Pulsed gradient stimulated echo (PGStE) NMR shows spatial dependence of fluid diffusion in human stage IV osteoarthritic cartilage.

PurposeHuman osteoarthritic (OA) articular cartilage was investigated with spatially resolved pulsed gradient stimulated echo (PGStE) NMR using strong gradients. In this study, the diffusivity of fluid and biopolymer was characterized as a function of depth within human OA cartilage cores.MethodsOne dimensional (1D) spatially resolved diffusion profiles were measured for human OA cartilage using a standard pulsed gradient stimulated echo (PGStE) sequence with the addition of a read imaging gradient. Low gradient values (0.05 to 1.5 Tm−1) were used to measure fluid motion and high gradients (2.50 to 17.81 Tm−1) enabled analysis of biopolymer motion. Structural damage to cartilage was simulated using either salt or collagenase.ResultsThese data demonstrate that fluid diffusion is sensitive to salt treatment and collagenase digestion, while biopolymer diffusion is sensitive only to collagenase digestion. Biopolymer and fluid diffusion are donor specific for human OA cartilage samples.ConclusionThese data support that fluid diffusion is sensitive to the structure of cartilage while biopolymer diffusion is sensitive to the molecular weight of the biopolymers. The results show that diffusion of fluid and biopolymers in articular cartilage are sensitive to OA and provide complimentary information. Magn Reson Med 80:1170–1177, 2018. © 2018 International Society for Magnetic Resonance in Medicine.

Quantification of non-Newtonian fluid dynamics of a wormlike micelle solution in porous media with magnetic resonance

Nuclear magnetic resonance (NMR) pulsed gradient stimulated echo (PGStE) techniques were used to observe anomalous transport phenomena for flow of a non-Newtonian wormlike micelle solution through a model porous media. Understanding the flow behavior of wormlike micelle solutions in porous media is important due to the growing interest of these solutions in enhanced oil recovery. NMR velocity imaging was unable to discern differences in the flow field between shear-thickening wormlike micelle solution and water due to spatial resolution limitations. However, the probability of displacement, i.e. the propagator, was skewed towards slower velocities and long tails at high displacements for the micelle solution and incorporation of a fractional dynamics approach using the moments of the probability distribution showed a deviation from asymptotic Gaussian statistics.

NMR and molecular dynamics study of the size, shape, and composition of reverse micelles in a cetyltrimethylammonium bromide (CTAB)/n-hexane/pentanol/water microemulsion.

The size, shape, and composition of reverse micelles (RMs) in a cetyltrimethylammonium bromide (CTAB)/pentanol/n-hexane/water microemulsion were investigated using pulsed gradient stimulated echo (PGSTE) nuclear magnetic resonance (NMR) measurements and molecular modeling. PGSTE data were collected at observation times (Δ) of 10, 40, and 450 ms. At long observation times, CTAB and pentanol exhibited single diffusion coefficients. However, at short (Δ ≤ 40 ms) observation times both CTAB and pentanol exhibited slow and fast diffusion coefficients. These NMR data indicate that both CTAB and pentanol molecules reside in different environments within the microemulsion and that there is exchange between regions on the millisecond time scale. Molecular dynamic simulations of the CTAB RM, in a solvent box containing n-hexane and pentanol, produced an ellipsoid shaped RM. Using structural parameters from these simulations and the Stokes-Einstein relation, the structure factor and dimensions of the reverse micelle were determined. Analysis of the composition of the interphase also showed that there was a variation in the ratio of surfactant to cosurfactant molecules depending on the curvature of the interphase.

Lipid diffusion in alcoholic environment.

We have studied the effects of a high concentration of butanol and octanol on the phase behavior and on the lateral mobility of 1,2-palmitoyl-sn-glycero-3-phosphocholine (DPPC) by means of differential scanning calorimetry and pulsed-gradient stimulated-echo (PGSTE) NMR spectroscopy. A lowering of the lipid transition from the gel to the liquid-crystalline state for the membrane-alcohol systems has been observed. NMR measurements reveal three distinct diffusions in the DPPC-alcohol systems, characterized by a high, intermediate, and slow diffusivity, ascribed to the water, the alcohol, and the lipid, respectively. The lipid diffusion process is promoted in the liquid phase while it is hindered in the interdigitated phase due to the presence of alcohols. Furthermore, in the interdigitated phase, lipid lateral diffusion coefficients show a slight temperature dependence. To the best of our knowledge, this is the first time that lateral diffusion coefficients on alcohol with so a long chain, and at low temperatures, are reported. By the Arrhenius plots of the temperature dependence of the diffusion coefficients, we have evaluated the apparent activation energy in both the liquid and in the interdigitated phase. The presence of alcohol increases this value in both phases. An explanation in terms of a free volume model that takes into account also for energy factors is proposed.

Size matters! On the way to ionic liquid systems without ion pairing.

Several, partly new, ionic liquids (ILs) containing imidazolium and ammonium cations as well as the medium-sized [NTf2 ](-) (0.230 nm(3) ; Tf=CF3 SO3 (-) ) and the large [Al(hfip)4 ](-) (0.581 nm(3) ; hfip=OC(H)(CF3 )2 ) anions were synthesized and characterized. Their temperature-dependent viscosities and conductivities between 25 and 80 °C showed typical Vogel-Fulcher-Tammann (VFT) behavior. Ion-specific self-diffusion constants were measured at room temperature by pulsed-gradient stimulated-echo (PGSTE) NMR experiments. In general, self-diffusion constants of both cations and anions in [Al(hfip)4 ](-) -based ILs were higher than in [NTf2 ](-) -based ILs. Ionicities were calculated from self-diffusion constants and measured bulk conductivities, and showed that [Al(hfip)4 ](-) -based ILs yield higher ionicities than their [NTf2 ](-) analogues, the former of which reach values of virtually 100 % in some cases.From these observations it was concluded that [Al(hfip)4 ](-) -based ILs come close to systems without any interactions, and this hypothesis is underlined with a Hirshfeld analysis. Additionally, a robust, modified Marcus theory quantitatively accounted for the differences between the two anions and yielded a minimum of the activation energy for ion movement at an anion diameter of slightly greater than 1 nm, which fits almost perfectly the size of [Al(hfip)4 ](-) . Shallow Coulomb potential wells are responsible for the high mobility of ILs with such anions.

How does oil type determine emulsion characteristics in concentrated Na-caseinate emulsions?

Macroscopic properties and ensemble average diffusion of concentrated (dispersed phase 50–60wt%) Na-caseinate-stabilised emulsions for three different oils (soybean oil, palm olein and tetradecane) were explored. On a volume fraction basis, pulsed gradient stimulated echo (PGSTE)-NMR data show that droplet dynamics for all three systems are similar within a region of the emulsion morphology diagram. The exact limits of the emulsion space depend however on which oil is considered. The reduced solubility of tetradecane in water, and Na-caseinate in tetradecane, result in the stabilisation of flocs during formulation. Floc formation is not observed when soybean oil or palm olein is used under identical emulsion formulation conditions. Linear rheology experiments provide indirect evidence that the local structure and the properties of the thin film interfacial domain of tetradecane emulsions vary from those of soybean oil and palm olein emulsions. Collectively these data indicate that protein/oil interactions within a system dominate over specific oil droplet structure and size distribution, which are similar in the three systems.

Morphology and NMR Self-Diffusion in PBA/PEO Miktoarm Star Copolymers

Abstract Morphology and NMR self-diffusion of two miktoarm star copolymers differing in fraction of poly(n-butyl acrylate) and poly(ethylene oxide) (PBA and PEO) arms were under investigation. Structural characteristics of copolymers was obtained on the basis of Small Angle X-Ray Scattering (SAXS) investigations. The phase separated nanoscale morphology of the miktoarm star copolymer with a high fraction of PEO arms was confirmed by Scanning Probe Microscopy (SPM) studies. The modified Avrami approach was used to obtain the information on the non-isothermal crystallization kinetics of the studied systems. It was observed that the crystallization in the system with a higher content of PBA, occurring at higher undercooling, was characterized by a higher crystallization rate. It was also found that increase in PBA arms fraction leads to the reduction in the size of PEO domains. The activation energy of the crystallization process, estimated with Kissinger's method, is lower for miktoarm star copolymer with higher PBA content, which results from facilitation of the transport of PEO chains in the direction of the growing crystal due to the presence of mobile PBA arms. The self-diffusion studies of miktoarm star copolymers melts, carried out with the Pulsed-Gradient STimulated-Echo (PGSTE) Nuclear Magnetic Resonance (NMR) technique, reveals the existence of at least two types of diffusion mechanisms in these systems.

Sizing of Reverse Micelles in Microemulsions using NMR Measurements of Diffusion

This paper reports the size of reverse micelles (RMs) in AOT/octane/H(2)O and CTAB/hexanol/H(2)O microemulsions using magnetic resonance (MR) pulsed field gradient (PFG) measurements of diffusion. Diffusion data were measured using the pulsed gradient stimulated echo (PGSTE) experiment for surfactant molecules residing in the RM interface. Inverse Laplace transformation of these data generated diffusion coefficients for the RMs, which were converted into hydrodynamic radii using the Stokes-Einstein relation. This technique is complementary to those previously used to size RMs, such as dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS), but also offers several advantages, which are discussed. RM sizes, determined using the PGSTE method, in the AOT (sodium bis(2-ethylhexyl) sulfosuccinate) and CTAB (cetyltrimethylammonium bromide) microemulsions were compared with previous DLS and SAXS data, showing good agreement. Methods for determining number distributions from the PGSTE data, through the use of scaling factors, were investigated.

Molecular Weight Distribution Characteristics (of a Polymer) Derived from a Stretched‐Exponential PGSTE NMR Response Function—Simulation

AbstractAssuming the signal response in a pulsed gradient stimulated echo (PGSTE) experiment (on a polymer) to be described by a simple stretched‐exponential function (SEF) and knowing the scaling law between diffusivity and molecular weight, the molecular weight distribution (MWD) characteristics (kurtosis, skewness, moment, and width) are derived and compared to the corresponding distribution characteristics obtained by a log‐normal function fit (to the same MWD). Also, the challenge involved in obtaining a reliable weight average molecular weight from an SEF response function is discussed.

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.

NMR Studies of Cooperative Effects in Adsorption

The conversion of gas adsorption isotherms into pore size distributions generally relies upon the assumption of thermodynamically independent pores. Hence, pore-pore cooperative adsorption effects, which might result in a significantly skewed pore size distribution, are neglected. In this work, cooperative adsorption effects in water adsorption on a real, amorphous, mesoporous silica material have been studied using magnetic resonance imaging (MRI) and pulsed-gradient stimulated-echo (PGSE) NMR techniques. Evidence for advanced adsorption can be seen directly using relaxation time weighted MRI. The number and spatial distributions of pixels containing pores of different sizes filled with condensate have been analyzed. The spatial distribution of filled pores has been found to be highly nonrandom. Pixels containing the largest pores present in the material have been observed to fill in conjunction with pixels containing much smaller pores. PGSE NMR has confirmed the spatially extensive nature of the adsorbed ganglia. Thus, long-range (≥40 μm) cooperative adsorption effects, between larger pores associated with smaller pores, occur within mesoporous materials. The NMR findings have also suggested particular types of pore filling mechanisms occur within the porous solid studied.

Three‐dimensional water diffusion in impermeable cylindrical tubes: theory versus experiments

Characterizing diffusion of gases and liquids within pores is important in understanding numerous transport processes and affects a wide range of practical applications. Previous measurements of the pulsed gradient stimulated echo (PGSTE) signal attenuation, E(q), of water within nerves and impermeable cylindrical microcapillary tubes showed it to be exquisitely sensitive to the orientation of the applied wave vector, q, with respect to the tube axis in the high-q regime. Here, we provide a simple three-dimensional model to explain this angular dependence by decomposing the average propagator, which describes the net displacement of water molecules, into components parallel and perpendicular to the tube wall, in which axial diffusion is free and radial diffusion is restricted. The model faithfully predicts the experimental data, not only the observed diffraction peaks in E(q) when the diffusion gradients are approximately normal to the tube wall, but their sudden disappearance when the gradient orientation possesses a small axial component. The model also successfully predicts the dependence of E(q) on gradient pulse duration and on gradient strength as well as tube inner diameter. To account for the deviation from the narrow pulse approximation in the PGSTE sequence, we use Callaghan's matrix operator framework, which this study validates experimentally for the first time. We also show how to combine average propagators derived for classical one-dimensional and two-dimensional models of restricted diffusion (e.g. between plates, within cylinders) to construct composite three-dimensional models of diffusion in complex media containing pores (e.g. rectangular prisms and/or capped cylinders) having a distribution of orientations, sizes, and aspect ratios. This three-dimensional modeling framework should aid in describing diffusion in numerous biological systems and in a myriad of materials sciences applications.

Application of PGSTE-NMR technique to characterize the porous structure of pharmaceutical tablets

Direct compaction of pharmaceutical tablets is a complex process that results in a heterogeneous density distribution inside the compact. In the present study, we have used a non-invasive and non-destructive technique: the pulsed-gradient stimulated-echo (PGSTE) NMR method to access to topological information (connectivity, tortuosity) about the porous structure of the tablets obtained with three different pharmaceutical excipients: the microcrystalline cellulose, the lactose and the anhydrous calcium phosphate. These materials were chosen since their mechanical properties under pressure are highly differentiated. To probe the pore space with the PGSTE-NMR technique, the tablets were initially impregnated with silicone oil that is NMR sensitive ( 1H NMR). The time-dependent apparent self-diffusion coefficient was measured over a suitable range of diffusion time in the directions perpendicular and parallel to the compression axis, from which the tortuosity factor and the anisotropy of the porous structure can be studied. These results show that the porous structure varies with pressure and depends on the excipient behaviour under pressure. Then, this work demonstrates that PGSTE-NMR could be an alternative and a very interesting technique to obtain useful information on the structural properties of such compacted materials.

Transport Properties in Ionic Liquids and Ionic Liquid Mixtures: The Challenges of NMR Pulsed Field Gradient Diffusion Measurements

Pulsed field gradient NMR is a powerful method for the measurement of diffusion coefficients in liquids and solids and has begun to attract much attention in the ionic liquids field. However, aspects of the methodology as traditionally applied to solutions may not be uniformly applicable in these more viscous and chemically complex systems. In this paper we present data which shows that the Pulsed Gradient Spin Echo (PGSE) method in particular suffers from intrinsic internal gradients and can produce apparent diffusion coefficients which vary by as much as 20% for different 1H nuclei within a given molecule--an obvious anomaly. In contrast, we show that the Pulsed Gradient Stimulated Echo method does not suffer from this problem to the same extent and produces self-consistent data to a high degree of accuracy (better than 1%). This level of significance has allowed the detection, in this work, of subtle mixing effects in [C(3)mpyr][NTf(2)] and [C(4)mpyr][NTf(2)] mixtures.

A novel improved method for analysis of 2D diffusion–relaxation data—2D PARAFAC-Laplace decomposition

This paper demonstrates how the multi-linear PARAFAC model can with advantage be used to decompose 2D diffusion–relaxation correlation NMR spectra prior to 2D-Laplace inversion to the T 2– D domain. The decomposition is advantageous for better interpretation of the complex correlation maps as well as for the quantification of extracted T 2– D components. To demonstrate the new method seventeen mixtures of wheat flour, starch, gluten, oil and water were prepared and measured with a 300 MHz nuclear magnetic resonance (NMR) spectrometer using a pulsed gradient stimulated echo (PGSTE) pulse sequence followed by a Carr–Purcell–Meiboom–Gill (CPMG) pulse echo train. By varying the gradient strength, 2D diffusion–relaxation data were recorded for each sample. From these double exponentially decaying relaxation data the PARAFAC algorithm extracted two unique diffusion–relaxation components, explaining 99.8% of the variation in the data set. These two components were subsequently transformed to the T 2– D domain using 2D-inverse Laplace transformation and quantitatively assigned to the oil and water components of the samples. The oil component was one distinct distribution with peak intensity at D = 3 × 10 −12 m 2 s −1 and T 2 = 180 ms. The water component consisted of two broad populations of water molecules with diffusion coefficients and relaxation times centered around correlation pairs: D = 10 −9 m 2 s −1, T 2 = 10 ms and D = 3 × 10 −13 m 2 s −1, T 2 = 13 ms. Small spurious peaks observed in the inverse Laplace transformation of original complex data were effectively filtered by the PARAFAC decomposition and thus considered artefacts from the complex Laplace transformation. The oil-to-water ratio determined by PARAFAC followed by 2D-Laplace inversion was perfectly correlated with known oil-to-water ratio of the samples. The new method of using PARAFAC prior to the 2D-Laplace inversion proved to have superior potential in analysis of diffusion–relaxation spectra, as it improves not only the interpretation, but also the quantification.

NMR relaxation and pulsed field gradient study of alginate bead porous media

Experiments are presented, which correlate molecular displacement with the multi-exponential T 2 relaxation times of water flowing and diffusing through an alginate bead pack. Three systems were studied comprising beads of 3, 1 or <1 mm in diameter. T 2-resolved propagators were obtained through a combined pulsed gradient stimulated echo (PGSTE) and Carr–Purcell–Meiboom–Gill (CPMG) experiment. Fourier transformation with respect to q produces a propagator for each echo in the CPMG train. Inverse Laplace transformation of the CPMG decays for each point ( Z) in the propagator produced a two-dimensional propagator. Analysis of these two-dimensional propagators provided insight into the transport and exchange behaviour of water flowing through this system. This experiment has been simulated in a model bead structure and the resulting T 2 relaxation time behaviour and T 2-resolved propagators were found to be in good agreement with the experimental data. We also present a theoretical analysis of the response to the combined PGSTE/CPMG sequence in the simple model case of Pouseille flow in a cylindrical capillary, where diffusion to a surface sink is assumed to be the dominant relaxation mechanism.

Competitive/Cooperative Electrostatic Interactions in Macromolecular Complexes: Multinuclear NMR Study of PDADMAC−PMANa Complexes in the Presence of Al3+ Ions

Interaction of Al3+ ions with electrostatic complexes of poly(N-diallyldimethylammonium chloride) (PDADMAC) and sodium polymethacrylate (PMANa) in D2O were studied by using 1H, 27Al, 23Na, and 35Cl NMR, relaxations, and pulsed-gradient stimulated echo (PGSTE) self-diffusion experiments. Al3+ ions are shown to enter the complex structure, substituting up to about 20% of the DADMAC groups in their electrostatic links. The structure of the complex somewhat changes, the free PDADMAC chains being pushed to the outer area of the complex, but remains otherwise intact. This process has an equilibrium nature and larger excess of Al3+ ions does not perceptibly influence its product. It is shown to be an interesting example of a combination of competitive and cooperative interaction: Al3+ ions compete with DADMAC groups in their electrostatic coupling with the polyanion and, at the same time, cooperate with them in the stabilization of the complex.

Cooperative Interactions of Unlike Macromolecules: 3. NMR and Theoretical Study of the Electrostatic Coupling of Sodium Polyphosphates with Diallyl(dimethyl)ammonium Chloride−Acrylamide Copolymers

Electrostatic coupling of sodium polyphosphates (polymerization degrees 15 and 65) with statistical copolymers of diallyl(dimethyl)ammonium chloride (DADMAC) and acrylamide (AA) of variable charge densities was studied using theoretical models, quantum chemical calculations, 1H and 23Na NMR spectra, relaxations, and pulsed-gradient-stimulated-echo (PGSE) experimental methods. Sodium polyphosphates with polymerization degrees 15−65 are readily coupled with DADMAC−AA statistical copolymers, containing 8−73% mol of the DADMAC ionic groups. The quantum-chemical prediction that even polycations with low density of ionic groups can be linked to densely charged polyphosphates in a ladder-like fashion is in accordance with the experimental results. Despite the predicted decrease in stabilization energy with increasing coupled sequence, the systems show a definitely cooperative behavior with the polyphosphate sequence of about 15 monomeric units, already sufficient for the full effect. Longer polyphosphate chains ...