Spin-echo NMR Research Articles

The unseen evidence of Reduced Ionicity: The elephant in (the) room temperature ionic liquids

The unambiguous quantification of the proton transfer in Protic Ionic Liquids (PILs) and its differentiation from the concept of ionicity are still unsolved questions. Albeit researchers awfully quickly treat them as synonyms, the two concepts are intrinsically different and imply a dramatic modification in the expected chemical and physical properties of a PIL. Some attempts have been made to shed light on this discrimination, but single-technique-based approaches fail in giving a clear answer. Aiming at definitively figuring out the differentiation between proton transfer and ionicity, we performed a multi-technique analysis (NMR, Raman, IR, thermal and electrochemical analyses, among others). Indeed, thermal and spectroscopic analyses are employed to determine the acid strength's role in ions' complete formation. To overcome the ambiguity between ionicity and formation degree, we introduce a new paradigm where Reduced Ionicity accounts for both the quantities mentioned above. The reduced ionicity directly affects the thermal stability, the phase behavior, and the spectroscopic observations, resulting in particular features in NMR and vibrational spectra. The combination of physical-chemical analyses and Pulsed-Gradient Spin-Echo (PGSE) NMR allows determining the reduced ionicity (and not the ionicity, as reported so far) of the investigated systems. In this context, being the proton transfer not quantitatively accessible directly, the reduced ionicity of a reference series of triethylamine-based PILs is investigated through transport properties as a function of temperature. Our findings point towards a substantial dependence of the reduced ionicity by the acid strength and the anion's coordination power. Furthermore, some interesting insights about the proton transfer are obtained, combining all the findings collected.

Simulating Resonant Magnetization Reversals in Nanomagnets

Magnetization reversals in magnetic recording media are largely carried out by brute force: a field is applied opposite to the existing magnetization direction of some bit that has sufficient magnitude to nucleate a seed that then grows into an oppositely magnetized bit. The fields used are generally quite large, $\sim 10$ kG, requiring elaborate magnetic circuitry to keep the fields localized so they do not spill over onto neighboring bits. This situation is to be contrasted with the resonant magnetization reversals performed in NMR spin echo experiments in which r.f. fields of a few Gauss coherently reverse the magnetization in the presence of static fields of a few kG, by applying a so-called Pi pulse; two such pulses restores the original alignment.

Dual-Cation Electrolyte System Using Quaternary Ammonium Salts and Ionic Liquid for High Power and High Energy Li4Ti5O12//AC Hybrid Capacitor System

[Introduction] The Use of thick electrodes (> 100 µm) is a simple approach to increase the energy density of electrochemical energy storage devices. However, the high ionic resistance in thick electrodes limits the high-power applications of such devices. “Dual-cation” electrolytes for thick-electrode hybrid capacitor systems, typically composed of Li4Ti5O12 (LTO, 200 µm) and activated carbon (AC, 400 µm) electrodes, have been developed[1]. A dual-cation electrolyte is composed of lithium tetrafluoroborate (LiBF4) and spiro-(1,1')-bipyrrolidinium tetrafluoroborate (SBPBF4) dissolved in propylene carbonate (PC). An SBP-based dual-cation electrolyte, comprising 1 M LiBF4 + 2 M SBPBF4 / PC, exhibits a higher total ionic conductivity (7.67 S cm-1) than the single-cation electrolyte (3.40 S cm-1), consisting of 1 M LiBF4 / PC, while Li+ conductivity decreases with an increase of SBPBF4 concentration. Our interesting finding was that the presence of SBP+, despite the low Li+ transport in the electrolyte bulk, reduces overvoltage associated with migration limitation in the thick LTO electrode, and thus enhances power density in LTO//AC hybrid capacitor systems.In this study, we propose another dual-cation electrolyte using a 1-ethyl-3-methylimidazolium (EMI) cation in place of the SBP cation, which further enhances the power performance of LTO//AC hybrid capacitor systems. It has been shown that an EMI-based electrolyte exhibits a higher total ionic conductivity (10.9 S cm-1) than the SBP-based electrolyte. Herein we provide a detailed account of factors that improve the discharge properties of the EMI-based dual-cation electrolyte by using electrochemical and spectroscopic methods.[Experimental] The dual-cation electrolytes were prepared by dissolving 1 M LiBF4 and x M SBPBF4 (0 < x < 3) or x' M EMIBF4 (0 < x' < 4), in PC. The ionic conductivity of the prepared electrolytes was measured using a conductivity meter. The LTO(200 μm)//AC(400 μm) hybrid capacitor system assumed a coin type cell assembly; charge-discharge measurements were conducted at 1 mA cm-2 (at the charge) and 1 to 200 mA cm-2 (at the discharge). Pulsed Gradient Spin Echo (PGSE) NMR analysis was conducted to determine the self-diffusion coefficients of the Li(7Li), SBP (1H), and EMI (1H) cations and the BF4 (19F) anion in the electrolytes.. Electrochemical impedance spectroscopy (EIS) was performed to evaluate the resistance of the LTO electrode using LTO//LTO symmetric cells designed at both the delithiated (SOC of LTO = 0%) and lithiated (SOC of LTO = 25%) states.[Results and discussion]The results of the ionic conductivity for all electrolytes are shown in Figure 1. The EMI-based dual-cation electrolyte exhibited higher ionic conductivity than the SBP-based electrolyte. The results of the charge-discharge test (Figure 2) showed that the EMI-based dual-cation electrolyte exhibits optimal discharge properties among the electrolytes. Moreover, the PGSE NMR analysis results indicated that the self-diffusion coefficient for all ions including Li+ decreased with an increase in EMIBF4 concentration. In contrast, the ionic resistance (R ion) of the LTO negative electrode calculated from the EIS results at the delithiated state (SOC of LTO = 0%) yielded lower values for the EMI-based dual-cation electrolyte (11.1 Ω cm2) than that for the SBP-based (14.0 Ω cm2) and single-cation (27.1 Ω cm2) electrolytes. We also evaluated the charge-transfer resistance (R ct) from the EIS results using LTO//LTO symmetric cells at the lithiated state (SOC of LTO = 25%). The R ct was found to be lower in the EMI-based dual-cation electrolyte (1.71 Ω cm2) than that in the SBP-based (2.28 Ω cm2) and single-cation (3.57 Ω cm2) electrolytes. These results showed that charge-transfer reactions at the LTO electrodes occur more readily in the dual-cation electrolyte.The results indicated that a reduction in R ct occurs because ion potential suppression occurs owing to the accelerated ion transport in the electrode, as well as in our previous study[1]. The suppression of the ionic potential increased the Li+ concentration and reaction current at the collector side, which decreased the R ct. This is because the advantage of an increase in total ionic conductivity is effective even when the transport ofLi+ is reduced in a thick-electrode system. This study confirms that the use of the EMI-based electrolyte, which has a higher total ionic conductivity than the SBP-based electrolyte, exhibits a remarkable suppression of R ion and R ct. The use of thick electrodes, therefore, facilitates an increase in total ionic conductivity, which has a more desirable effect than that of high Li+ conductivity, especially in the EMI-based dual-cation electrolyte, wherein the power density of the thick-electrode LTO//AC hybrid capacitor systems was significantly improved.[1] Y. Chikaoka et al., J. Phys. Chem. C, in press(doi.org/10.1021/acs.jpcc.0c01916). Figure 1

How Does the Solvent Composition Influence the Transport Properties of Electrolyte Solutions?

The electrolyte solutions used in lithium ion batteries have been optimized based on the empirical knowledge and concept. Yet the ion transport phenomena in the electrolyte solution, which composed of mixed solvent with salt concentration of about 1 mol kg−1, are complicated and significantly different from those in dilute solutions. In the present study, the characteristics of the electrolyte solutions are reanalyzed by using physical and spectroscopic data for the electrolyte solutions, which were obtained from our experiment, and by referring to recent theoretical studies 1,2) and the experimentally determined diffusion coefficient in previous studies 3,4).We experimentally measured the viscosity η and ionic conductivity σ of the electrolyte solutions of 1 mol kg−1 of LiPF6 and lithium bis(fluorosulfonyl)imide (LiFSA) dissolved in the binary mixture solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) in the temperature range of 288 ≤ T/K ≤ 328 with varying the EC content from 0 to 60 vol.%, which translates into the molar fraction of EC in 0 ≤ x EC ≤ 0.6 and the solute to solvent ratio constant (total ca. 11 solvent molecules are present per solute in the system). The diffusion coefficients D of each species, Li+, PF6, FSA−, EC and DMC, were determined by pulse gradient spin echo NMR. The state of molecules around Li+ was examined by the Raman spectra of the solvents and anions.With the increase in x EC, DMC solvating Li+ is replaced by EC, resulting in the increase in the total number of solvents around Li+. Reanalysis of Raman spectra based on Borodin's calculations 2) reassessed the solvation numbers and confirmed that the preferential coordination of EC to Li+, but the extent of the preference was not as strong as previously conceived. The addition of EC, more viscous fluid than DMC, into the system leads to the increase in η, which should be responsible for the monotonous decrease in D of all the species with x EC. On the other hand, σ initially increases until marking the maximum at x EC = ~0.3. The opposite trend between D and σ, i.e., dD/dx EC < 0 while dσ/dx EC > 0, at low x EC suggests that the Nernst-Einstein relation does not hold as it ignores the inter-ionic correlations. The analysis of the Stokes radii r St based on η and D obtained in the present study implies that the increase in x EC results in the decrease in r St for all the species, among which the anions are most influenced. Although only an equivocal picture on the cation-anion correlation, such as the ion pair formation, was obtained from the Raman spectroscopy, the present study is consistent with the view that the highly Li+-solvating EC, with its higher dielectric shielding effect than DMC, liberates the anions from the attraction from Li+, enhancing the cation-anion anti-correlation, which positively contributes to the ionic conductivity until the viscosity prevails at high x EC. The Einstein-Stokes formula and van der Waals radius are certainly "classical" concepts; nonetheless, if properly formulated, they can be consistent with the solvation number obtained from Raman spectra not only qualitatively but also semi-quantitatively. References Borodin, G. V. Zhuang, P. N. Ross and K. Xu, Phys. Chem. C, 117, 7433 – 7444 (2013).Borodin, M. Olguin, P. Ganesh, P. R. C. Kent, J. L. Allen and W. A. Henderson, Chem. Chem. Phys., 18, 164 – 175 (2016).Hayamizu, Y. Aihara, S. Arai, C. Garcia-Martinez, J. Phys. Chem. B, 103, 519 – 524 (1999).Hayamizu, J. Chem. Eng. Data, 57, 2012 – 2017 (2012). Figure 1

Interaction of Low Molecular Weight Poly(diallyldimethylammonium chloride) and Sodium Dodecyl Sulfate in Low Surfactant-Polyelectrolyte Ratio, Salt-Free Solutions.

Coacervation is widely used in formulations to induce a beneficial character to the formulation, but nonequilibrium effects are often manifest. Electrophoretic NMR (eNMR), pulsed-gradient spin-echo NMR (PGSE-NMR), and small-angle neutron scattering (SANS) have been used to quantify the interaction between low molecular cationic poly(diallyldimethylammonium chloride) (PDADMAC) and the anionic surfactant sodium dodecyl sulfate (SDS) in aqueous solution as a model for the precursor state to such nonequilibrium processes. The NMR data show that, within the low surfactant concentration one-phase region, an increasing surfactant concentration leads to a reduction in the charge on the polymer and a collapse of its solution conformation, attaining minimum values coincident with the macroscopic phase separation boundary. Interpretation of the scattering data reveals how the rodlike polymer changes over the same surfactant concentration window, with no discernible fingerprint of micellar type aggregates, but rather with the emergence of disklike and lamellar structures. At the highest surfactant concentration, the emergence of a weak Bragg peak in both the polymer and surfactant scattering suggests these precursor disk and lamellar structures evolve into paracrystalline stacks which ultimately phase separate. Addition of the nonionic surfactant hexa(ethylene glycol) monododecyl ether (C12E6) to the system seems to have little effect on the PDADMAC/SDS interaction as determined by NMR, merely displacing the observed behavior to lower SDS concentrations, commensurate with the total SDS present in the system. In other words, PDADMAC causes the disruption of the mixed SDS/C12E6 micelle, leading to SDS-rich PDADAMC/surfactant complexes coexisting with C12E6-rich micelles in solution.

Tube Dilation in Isofrictional Polymer Blends Based on Polyisoprene with Different Topologies: Combination of Dielectric and Rheological Spectroscopy, Pulsed-Field-Gradient NMR, and Neutron Spin Echo (NSE) Techniques

We address the dynamics of isofrictional bimodal polyisoprene (PI) blends and emphasize the effect of concentration and topology of the short component on the dynamics of the long-chain component. ...

Structure Effects on the Ionicity of Protic Ionic Liquids.

We report on the characterisation of 16 protic ionic liquids (PILs) prepared by neutralisation of primary or tertiary amines with a range of simple carboxylic acids, or salicylic acid. The extent of proton transfer was greater for simple primary amine ILs compared to tertiary amines. For the latter case, proton transfer was increased by providing a better solvation environment for the ions through the addition of a hydroxyl group, either on the tertiary amine, or by formation of PIL/molecular solvent mixtures. The library of PILs was characterised by differential scanning calorimetry and a range of transport properties (i. e. viscosity, conductivity and diffusivity) were measured. Using the (fractional) Walden rule, the conductivity and viscosity results were analysed with respect to their deviation from ideal behaviour. The validity of the Walden plot for PILs containing ions of varying sizes was also verified for a number of samples by directly measuring self-diffusion coefficients using pulsed-field gradient spin-echo (PGSE) NMR. Ionicity was found to decrease as the alkyl chain length and degree of branching of both the cations and anions was increased. These results aim to develop a better understanding of the relationship between PIL properties and structure, to help design ILs with optimal properties for applications.

Structural and Li-ion diffusion properties of lithium tantalum phosphate LiTa2PO8

We synthesized a polycrystalline LiTa2PO8 sample at 1323 K by a solid-state reaction. The crystal structure of LiTa2PO8 was examined by selected-area electron diffraction (SAED) and convergent-beam electron diffraction (CBED) techniques. Electrochemical impedance measurements showed that the total conductivity at room temperature was 4.85 × 10−4 S cm−1 and the activation energy was 0.52 eV. The high Li-ion conductive properties were confirmed by 7Li pulse-field gradient (PFG) or pulse-gradient spin-echo (PGSE) NMR spectroscopy. The activation energy of the Li diffusion (0.51 eV) measured by PFG NMR was well consistent with the result of impedance measurement. We believe that the LiTa2PO8 would be as one of the important candidates for the solid electrolyte used in all solid-state lithium batteries.

(Invited) Novel Approaches for the Study of Local Processes in Rocksalt Oxyfluoride Cathodes

Cation-disordered rocksalt (DRX) lithium transition metal oxides have recently emerged as a new class of high energy density lithium-ion cathodes, but in most cases suffer from rapid performance degradation upon electrochemical cycling. Unlike layered lithium transition metal oxides, these DRXs are amenable to bulk fluorination (as demonstrated by 19F NMR spectroscopy) and significant improvements in their electrochemical properties, especially cyclability, have been observed upon partial oxygen substitution by fluorine.1 We present here our findings on the impact of fluorine doping on the local cation order and structural and electrochemical properties of several DRX cathodes, using a combination of solid-state NMR experiments and theoretical calculations. Specifically, the impact of F insertion on the distribution of Li and F species in the as-synthesized cathodes is evaluated, and changes in the local structure upon electrochemical cycling are monitored. Ex situ 19F NMR spectra collected on cathode samples stopped at different stages of (dis)charge are particularly difficult to interpret, due to the presence of paramagnetic transition metals leading to very large shifts and broad resonances. In addition, the intrinsic disorder on the cation sublattice leads to a large number of F environments with similar shifts, resulting in broad and overlapping signals. To assist the interpretation of the NMR data, we use Monte Carlo simulations and ab initio calculations of the paramagnetic NMR parameters to determine the distribution of F environments in the materials and predict the chemical shift for these various F sites.Monte Carlo simulations clearly indicate short-range order in the as-synthesized materials, with the incorporation of F in octahedral anion sites with at least five Li nearest-neighbors, in good agreement with 19F NMR results (see Fig. 1). Yet, first principles calculations of NMR parameters reveal that 19F NMR signals from F nuclei directly bonded to the redox-active (and paramagnetic) transition metal species are too broad to be observed.2 Hence, all experimentally-observed 19F NMR signals arise from F sites that are not directly bonded to the paramagnetic metal. These initial insights clearly reveal the strength of our combined experimental and theoretical approach but also suggest that further insight into short-range order in these complex materials requires new experimental (NMR) approaches and complementary techniques. Figure 1. a) Distribution of F environments in the model Li1.166Ni0.333Ti0.5O1.833F0.166 DRX structure obtained from Monte Carlo simulations at an equilibration temperature of 973 K. This distribution is used as a proxy for the distribution of F environments in the Li1.15Ni0.45Ti0.3Mo0.1O1.85F0.15material synthesized at 700°C. b) 19F spin echo NMR spectrum obtained at 11.7 T and at a magic angle spinning (MAS) speed of 60 kHz. The observed 19F signal arises from F nuclei with no Ni nearest-neighbor. Figures adapted from Clément et al.2

Incorporating a Tetrapeptide into Lyotropic Direct Hexagonal Mesophase.

An approach to incorporate a bioactive hydrophobic substance, C22H32N4O7 tetrapeptide (TP), into the structure of the hexagonal mesophases C12EO10/H2O and C12EO10/La(III)/H2O was proposed. Concentration and temperature ranges of mesophases in the C12EO10/H2O/TP and C12EO10/La(III)/H2O/TP systems were established. The analysis of the X-ray diffraction data revealed a change in the structural characteristics of mesophases in the presence of tetrapeptide. Formation of a denser packing of molecules in the mesophases with TP was detected. Based on the FTIR spectroscopy data, intermolecular changes in the systems were examined. Pulsed-gradient spin-echo NMR self-diffusion experiments were performed to characterize the structure of lyomesophases depending on system composition and temperature. The degree of hydration of water molecules in lyomesophases was analyzed. The data confirmed successful incorporation of tetrapeptide into the structure of lyomesophase and, therefore, the possibility of using hexagonal mesophases for both incapsulation and delivery of biomolecules.

Electrophoretic NMR Characterization of Charged Side Chain Cationic Polyelectrolytes and Their Interaction with the Anionic Surfactant, Sodium Dodecyl Sulfate.

Oppositely charged polymers and surfactants show a complex phase behavior with large regions of solubility and insolubility dependent on the concentrations of the species present. Here, a series of quaternized hydroxyethyl cellulose (cationic) polymers have been characterized by pulsed-gradient spin-echo NMR (PGSE-NMR) and electrophoretic NMR (eNMR) in simple aqueous (D2O) solutions and in combination with the oppositely charged (anionic) surfactant, sodium dodecyl sulfate (SDS). Analysis of the effective charge on the polymer derived from both the eNMR and PGSE-NMR results yields a readily interpretable insight into the polymer behavior; the effective charge on the polymer at infinite dilution shows a linear relationship with the degree of modification. On addition of low concentrations of SDS, typically Csurf < 5 mM, the surfactant interacts with the charged polymers, leading to substantial changes in the dynamics of the system (polymer diffusion, viscosity). At these levels of surfactant addition, there is no macroscopic phase separation. Further, with the absence of an interaction with the parent, the uncharged polymer strongly suggests that the SDS only interacts with the charged moieties present on the functionalized side groups and not the polymer backbone. Ultimately, the charge on the soluble polymer/surfactant complex was found to depend linearly on the level of surfactant binding across a series of polymers with differing levels of modification with the charge becoming effectively zero at the macroscopic phase separation boundary.

Inverse Laplace Transform (ILT) NMR: A powerful tool to differentiate a real rejuvenator and a softener of aged bitumen

ILT is particularly useful when the signal is characterized by multi-exponential decay, for example in spin relaxation or in the dephasing of the NMR spin echo signal associated with sopra molecular aggregation under the influence of pulsed magnetic or internal field gradients. As novel approach to observe the real rejuvenating effect of the potential additive, an Inverse Laplace Transform of the NMR spin-echo decay (T2) was applied. The potentialities of a new, non-toxic and eco-friendly biocompatible additive on aged bitumen are explored for the first time as bitumen rejuvenator, by means of advanced rheological and Relaxometry-NMR measurements. Pristine, aged, and doped aged bitumen morphology have been investigated by SEM. The new rejuvenator helps to rearrange the structure of the aged bitumen (aiming at the original one), and this mechanism can be observed by ILTNMR analysis.

Diffusion Behavior of Water Molecules in Hydrogels with Controlled Network Structure

Diffusion behavior of particles in hydrogels is important both for the fundamental understanding of mass transport and for practical applications and has been investigated for a long time. There are three major theories describing the diffusion behavior of small particles in a polymer network: obstruction, hydrodynamic, and free volume theories. Although many researchers have examined these three theories, their applicability is still unclear due to ambiguity stemming from the heterogeneity of conventional hydrogels. Recently, we have developed a near-ideal hydrogel called Tetra-PEG gel that provides a unique possibility to correlate gel structure with properties. In this study, we measured the diffusion coefficient of water molecules (D) in Tetra-PEG gels by pulsed field gradient spin-echo 1H NMR. By comparing D and the correlation length of a polymer network (ξ) measured by small-angle neutron scattering, we observed an identity formula similar to the hydrodynamic theory (D/D0 = exp(−d/ξ), where D0 is t...

Comprehensive Structural Assessment of Linear Block Polymers by NMR and SEC

Block polymers are an important class of materials offering unique and useful properties different from random polymers. Because of their tailored structure, these materials present synthetic challenges. For this reason, it is necessary to provide a thorough structural analysis to drive the synthetic effort. We describe an analytical methodology that includes size exclusion chromatography (SEC), quantitative 13C NMR, and 1H pulsed gradient spin echo (PGSE) NMR coupled with direct exponential curve resolution algorithm (DECRA) data analysis. The combination of these complementary techniques provides a comprehensive assessment of the chemical structure of the final polymeric material, including the detection and quantification of homopolymer formation, compositional heterogeneity with molecular weight, and monomer sequencing. We present work on two different linear block polymers: poly(styrene-b-allyl methacrylate) and poly[styrene-b-(styrene-alt-maleic anhydride)].

Evaluating the Ion Transport of 1-Ethyl-3-Methylimidazolium Acetate Solutions Containing Carbohydrate Solutes

This study evaluated the ion transport properties of 1-ethyl-3-methylimidazolium acetate (EMIAc)-based solutions containing additional acetonitrile (AN), H2O, and up to 3.0 wt-% carbohydrate solutes. Solution ion conductivity was directly measured via frequency response, and ion self-diffusivity coefficients determined via pulsed-field gradient spin-echo (PGSE) NMR. These data combined to determine solution ionicity. Multiple linear regression analysis shows ionicity is primarily impacted by the added neutral solvent, and weakly impacted by temperature. Addition of equimolar AN or H2O increases both cation and anion self-diffusivity (D+ and D−, respectively), and each of these solvent systems decreases the D+/D− quotient. Addition of carbohydrate solutes decreases cation and anion self-diffusivity, but solute additions slightly increase the D+/D− quotient. Our observation that carbohydrate solutes reduce Ac self-diffusivity by a greater fraction than EMI supports the general assertion that acetate anions are more heavily involved in solvating cellulose than EMI cations. Finally, EMIAc solutions containing microcrystalline cellulose (MCC) were evaluated by NMR spin-lattice relaxation (T1) measurements. These T1 data reveal the microviscosity of EMIAc:MCC solutions is virtually unchanged with addition of up to 3.0 wt-% solute, in agreement with the ion transport properties measured at the same scale.

Understanding the behavior of mixtures of protic-aprotic and protic-protic ionic liquids: Conductivity, viscosity, diffusion coefficient and ionicity

We have investigated the physicochemical properties such as electrical conductivity, viscosity and diffusion coefficient for the binary mixtures of protic ionic liquids with aprotic ionic liquids and of protic with protic ionic liquids at 298.15 K. A significant enhancement in the electrical conductivity is observed for the binary mixtures of ionic liquids, as compared to those of the constituent pure ionic liquids and varied with the composition of the mixtures. The viscosity of binary mixtures of protic with aprotic ionic liquids, 1‑butyl‑3‑methylimidazoliumbis(trifluoromethylsulfonyl)imide [bmIm][NTf2], 1‑butyl‑1‑methylpyrrolidiumbis(trifluoromethylsulfonyl)imide [bmPyrr][NTf2] and 1,3-dimethylimidazolium methyl sulfate, [mmIm][CH3SO4] decreases with an increase in the composition of the [HmIm][CH3COO]. On the contrary, the viscosity for binary mixtures of protic with protic ionic liquid, 1‑methylpyrrolidium acetate [HmPyrr][CH3COO] and 4‑methylmorpholine acetate [HmMorph][CH3COO] increases upon the addition of 1‑methylimidazolium acetate [HmIm][CH3COO]. The self diffusion coefficients were determined for all the binary mixtures of ionic liquids by using Pulsed Gradient Spin Echo (PGSE) NMR method. Self diffusion coefficients of [bmIm][NTf2]-[HmIm][CH3COO], [bmPyrr][NTf2]-[HmIm][CH3COO], [mmIm][CH3SO4]-[HmIm][CH3COO] are enhanced, while those of [HmPyrr][CH3COO]-[HmIm][CH3COO] and [HmMorph][CH3COO]-[HmIm][CH3COO] decreases on addition of [HmIm][CH3COO]. This is converse in the case of viscosity. Furthermore, the above correlations were interpreted with the help of NMR spectroscopy on the basis of interactions of ions in the binary mixtures of ionic liquids. Finally, we have quantified the ionicity through the Nernst–Einstein equation and have confirmed the validity of the Walden rule for the binary mixture of ionic liquids.

Diffusion of Rodlike Polymers: Pulsed Gradient Spin Echo NMR of Poly(γ-stearyl-α,l-glutamate) Solutions and the Importance of Helix Stability.

Many natural and synthetic polymers and particles have a rodlike shape, leading to important and intriguing solution behavior, such as high intrinsic viscosities and liquid crystalline phases. Much of what is known about suspensions of rods has been learned by studying helical polypeptides, even though such molecules are not perfectly rigid, smooth cylinders. Previous optical tracer self-diffusion studies of poly(γ-benzyl-α,l-glutamate) (PBLG) revealed that the molecule initially resists topological constraints imposed by neighboring molecules, but diffusion strongly decreases as concentration rises beyond a certain number density. In contrast, the tracer self-diffusion coefficient of truly rigid tobacco mosaic virus begins decreasing immediately with concentration. We used pulsed gradient spin echo NMR to measure another polypeptide, poly(γ-stearyl-α,l-glutamate) (PSLG), to gain physical insight into the question of polypeptide diffusion in crowded isotropic solutions. The PSLG molecule, with long alkyl sidechains, is semiflexible like PBLG but does not exhibit the same ability to evade topological constraints. Instead, PSLG follows a simple exponential decay, D/ DKR = A e(-κν/ν*) + B, where DKR is the Kirkwood-Riseman expectation for rod diffusion, ν is the number density of rods, ν* is the Onsager expectation for the number density at the onset of liquid crystal formation, A = 1 ± 0.1, B = 0.1 ± 0.01, and κ = 4.5 ± 0.5. The results emphasize the importance of helix stability when choosing rodlike polypeptides as model systems, particularly with regard to the chain ends.

Application of the LASSO algorithm for fitting the multiexponential data of the NMR relaxometry

The problem of time series fitting by the sum of several exponentials with different decay parameters often arises analyzing the data of a physical experiment. In particular, such a problem arises determining the nuclear transverse relaxation times from the spin-echo decay NMR data. Mathematically, the problem can be formulated as a solution of the Fredholm integral equation of the first kind with an exponential kernel, or an equivalent system of linear equations. To solve this equation, various regularization methods are used, based on the L2-norm in most cases. We report the application of the L1-regularization for the problem, using the LASSO algorithm. A comparison was made between the results of L1- and L2-regularization on model time series, which were the sum of several exponential decays with noise. Also the L1- and L2-regularization were applied to analyze the 1H NMR experimental data of the spin-echo decays of n-hexane adsorbed in the porous volume of aluminium poor MFI-type zeolites known as ZSM-5. It was found out that the L1-regularization is effective method for determination of the transverse relaxation times from NMR data.

Influence of Diffusion Time on the Diffusion Coefficients of Gases in Polymers Determined by Pulsed Gradient Spin Echo NMR

The diffusive behavior of pure and mixed 13CO2, methane, and ethylene in poly(4,4′-hexafluoroisopropylidene diphthalic anhydride-2,3,5,6-tetramethyl-1,4-phenylenediamine) membranes is investigated using a pulsed gradient spin echo NMR method. Also, proton and 13C spin–lattice and spin–spin relaxation times are measured. In all cases, the relaxation times of sorbed gases are well described assuming a single component, indicating that gas molecules in the membrane recognize one environment. If there is more than one, they are in fast exchange between them, and average values of relaxation times are observed. The diffusion coefficient of 13CO2 is independent of the diffusion time, exhibiting a Fickian behavior in all cases. Meanwhile, those of methane and ethylene vary with the diffusion time, and data analysis was performed considering their non-Fickian behavior. The diffusion coefficient of methane increases in the presence of 13CO2 and ethylene, suggesting that the sieving properties of the glassy polyimi...

Synthesis and Characterization of Isosorbide-Based Polyurethanes Exhibiting Low Cytotoxicity Towards HaCaT Human Skin Cells.

The synthesis of four samples of new polyurethanes was evaluated by changing the ratio of the diol monomers used, poly(propylene glycol) (PPG) and D-isosorbide, in the presence of aliphatic isocyanates such as the isophorone diisocyanate (IPDI) and 4,4′-methylenebis(cyclohexyl isocyanate) (HMDI). The thermal properties of the four polymers obtained were determined by DSC, exhibiting Tg values in the range 55–70 °C, and their molecular structure characterized by FTIR, 1H, and 13C NMR spectroscopies. The diffusion coefficients of these polymers in solution were measured by the Pulse Gradient Spin Echo (PGSE) NMR method, enabling the calculation of the corresponding hydrodynamic radii in diluted solution (1.62–2.65 nm). The molecular weights were determined by GPC/SEC and compared with the values determined by a quantitative 13C NMR analysis. Finally, the biocompatibility of the polyurethanes was assessed using the HaCaT keratinocyte cell line by the MTT reduction assay method showing values superior to 70% cell viability.