Pulsed Gradient Spin-echo NMR Research Articles

Zwitter-ionic amphiphilic calixresorcinarenes: Micellization in the bulk solution and non-surface activity

Here the amphiphilic calix[4]resorcinarenes, which form self-associates in an aqueous solution but do not exhibit surface activity, are described. Aqueous solutions of calix[4]resorcinarenes with different zwitter-ionic groups on the upper rim: carboxybetaine (C5-CB, C11-CB, C12OPh-CB), sulfobetaine (C5-SB, C11-SB), hydroxysulfobetaine (C5-hSB, C11-hSB), - and different hydrophobic substituents on the lower rim (pentyl, undecyl, dodecyloxyphenyl) were studied by fluorimetry, Fourier transform pulsed-gradient spin-echo (FT-PGSE) NMR spectroscopy, UV–vis spectroscopy, dynamic light scattering (DLS), and tensiometry. It was found that all macrocycles spontaneously form self-associates in an aqueous solution, but only sulfobetaine and hydroxysulfobetaine calixresorcinarenes with pentyl substituents on the lower rim are surface active. The critical association concentration (CAC) values, which were obtained by different methods, as well as values of averaged hydrodynamic diameter, averaged self-association numbers, solubilizing power, and other parameters of self-association are presented and discussed.

Water diffusion in complex systems measured by PGSENMR using chemical shift selective stimulated echo: Elimination of magnetization exchange effects.

The interpretation of molecular translational diffusion as measured by pulsed gradient spin-echo NMR (PGSENMR) can be complicated by the presence of chemical exchange and/or dipolar cross-relaxation (including relayed cross-relaxation via spin diffusion). The magnitude of influence depends on the kinetics of exchange and/or dipolar cross-relaxation present within the system as well as the PGSENMR sequences chosen for measurements. First, we present an exchange induced zero-crossing phenomenon for signal attenuation of water in lipidic cubic phases (formed by a mixture of monoolein and water) in the presence of pulsed gradients observed using a standard STimulated Echo (STE) sequence. This magnetization exchange induced zero-crossing phenomenon, a pseudo-negative diffraction-like feature, resembles that reported previously for restricted diffusion when locally anisotropic pores are polydisperse or randomly oriented. We then demonstrate the elimination of these exchange and/or dipolar cross-relaxation induced effects with the use of a chemical shift selective STE (CHESS-STE) sequence, adapted from the previously reported band-selective short transient STE sequence, along with results obtained from the bipolar pulse pair STE sequence for comparison. The CHESS-STE sequence introduced here represents a generic form of PGSENMR sequences for obtaining water diffusion coefficients free from the influence of exchange and/or dipolar cross-relaxation in complex systems. It has potential applications in measuring translational diffusion of water in biopolymer mixtures as well as probing the microscopic structure in materials via water restricted diffusion measured by PGSENMR, particularly when the potential presence of exchange/cross-relaxation is of concern.

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.

Diffusion Spectrum of Polymer Melt Measured by Varying Magnetic Field Gradient Pulse Width in PGSE NMR.

The translational motion of polymers is a complex process and has a big impact on polymer structure and chemical reactivity. The process can be described by the segment velocity autocorrelation function or its diffusion spectrum, which exhibit several characteristic features depending on the observational time scale—from the Brownian delta function on a large time scale, to complex details in a very short range. Several stepwise, more-complex models of translational dynamics thus exist—from the Rouse regime over reptation motion to a combination of reptation and tube-Rouse motion. Accordingly, different methods of measurement are applicable, from neutron scattering for very short times to optical methods for very long times. In the intermediate regime, nuclear magnetic resonance (NMR) is applicable—for microseconds, relaxometry, and for milliseconds, diffusometry. We used a variation of the established diffusometric method of pulsed gradient spin-echo NMR to measure the diffusion spectrum of a linear polyethylene melt by varying the gradient pulse width. We were able to determine the characteristic relaxation time of the first mode of the tube-Rouse motion. This result is a deviation from a Rouse model of polymer chain displacement at the crossover from a square-root to linear time dependence, indicating a new long-term diffusion regime in which the dynamics of the tube are also described by the Rouse model.

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

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

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.

Unravelling the role of a green rejuvenator agent in contrasting the aging effect on bitumen: A dynamics rheology, nuclear magnetic relaxometry and self-diffusion study

This paper evaluated the potentialities of a green and biocompatible rejuvenator agent (HR) in conferring an appreciable resistance against the effects caused by artificial aging on a given bitumen. Both neat and aged bitumens were analyzed and compared to analogous samples modified with HR. Control samples containing a vegetable oil as softening agent were also tested for comparison. The tested samples were subjected to a second aging cycle. Structural differences between the samples were carried out through an inverse Laplace transform of the NMR spin-echo decay (T2) and self-diffusion measurements by pulsed gradient spin echo nuclear magnetic resonance (PGSE-NMR) spectroscopy. In addition, dynamic rheological analyses were conducted to determine the dependence of the gel-sol transition temperature on both the type of additive and ageing process. The present study clearly highlighted the fact that artificial ageing, realized here by the rolling thin film oven test (RTFOT) and the pressure ageing vessel (PAV) test, induced important structural modifications. The analysis of relaxation times and self-diffusion coefficients indicated that ageing promoted the formation of molecular populations characterized by a shift of the distribution toward higher molecular weights compared to unaged bitumen. Diffusion data showed also an Arrhenius-like temperature dependence. A correlation between all the data was attempted to understand the role of the investigated additives. The eco-friendly biocompatible rejuvenator helped not only to restore the structure of the aged bitumen, but even slowed down the processes of a second aging (aiming at the first aged sample).

Study of ion dynamics of LiI·6H2O in the supercooled liquid state using NMR spectroscopy and ionic conductivity measurements

Ion dynamics in the liquid and supercooled liquid state of LiI·6H2O were studied by nuclear magnetic resonance (NMR) spectroscopy and ionic conductivity measurements. The self-diffusion coefficients of the water molecules and lithium ions were measured by 1H and 7Li pulsed gradient spin echo NMR (PGSE-NMR). The temperature dependences of the DC ionic conductivity and diffusion coefficients of lithium ions and water molecules were well-fitted by the Vogel-Tamman-Fulcher (VTF) law, similar to those of the LiCl·RH2O system. The conductivity and lithium diffusion coefficients of the LiI·6H2O system were more than twice those of LiCl·7H2O in the supercooled liquid state below 200 K, which could be attributed to the larger size of the iodide anions than that of chloride anions. The temperature dependence of the correlation times calculated from NMR T1 relaxation deviated from the VTF law at low temperatures because of the local orientational fluctuation of the water molecules.

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.

Auto-fluorescent PAMAM-based dendritic molecules and their potential application in pharmaceutical sciences.

The epithelial permeation of water-soluble fluorescent PAMAM dendrons based on 7H-benz[de] benzimidazo [2,1-a] isoquinoline-7-one as a fluorescent core across epithelial cell models MDCK I and MDCK II has been quantified. Hydrodynamic radii have been derived from self-diffusion coefficients obtained via pulsed-gradient spin-echo Nuclear Magnetic Resonance (PGSE-NMR). Results indicate that these dendritic molecules are molecularly disperse, non-aggregating, and only slightly larger than their parent homologues. MDCK I permeability studies across epithelial barriers show that these dendritic molecules are biocompatible with the chosen epithelial in-vitro model and can permeate across MDCK cell monolayers. Permeability is demonstrated to be a property of dendritic size and cell barrier restrictiveness indicating that paracellular mechanisms play the predominant role in the transport of these molecules.

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.

Toward understanding the anomalous Li diffusion in inorganic solid electrolytes by studying a single-crystal garnet of LLZO-Ta by pulsed-gradient spin-echo nuclear magnetic resonance spectroscopy.

Li diffusion was observed by 7Li nuclear magnetic resonance (NMR) spectroscopy in three single-crystal samples of LLZO-Ta (Li6.5La3Zr1.5Ta0.5O12) grown by the floating zone melting method as well as a crushed sample in this study. Previously, the pulsed-gradient spin-echo 7Li NMR method was applied to Li+ diffusion measurements in inorganic solid electrolyte powder samples. Anomalous Li+ diffusion behaviors were observed such as dependence of the observing time (Δ) and pulsed-field-gradient strength (g), and the diffusive-diffraction patterns in short Δ in the echo-attenuation plots. In the powder samples, it is uncertain that the Li ions diffuse in the bulk within grain, across grains, or both. To date, the origins of the anomalous Li+ diffusion have not yet been clearly understood. From models of atomic-level lithium pathways, the micrometer-space diffusion channels are assumed to be narrow with curvatures. In contrast to the powder samples, a single crystal is supposed to be uniform without grain boundaries and the Li ions in single-crystal samples can diffuse in the bulk with negligible effects from the surface. The single-crystal samples are expected to give us proper answers. We found that the 7Li echo-attenuation plots of the single-crystal samples showed anomalous phenomena in dependence on Δ and g with much reduced manners. We found that the phenomena are inherent characteristics of Li+ diffusion in inorganic solid electrolytes. From the aspects of Li+ carrier numbers, the fast divergent Li+ diffusion constants, observed at short Δ with small g, contribute importantly to the electrochemical high ionic conduction measured by impedance spectroscopy.

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)].

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.

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.

7Li NMR diffusion studies in micrometre-space for perovskite-type Li0.33La0.55TiO3 (LLTO) influenced by grain boundaries

Perovskite-type oxide solid electrolyte Li0.33La0.55TiO3 (LLTO) samples are known to have high ionic conductivities and grain-boundary resistances from AC impedance spectroscopy. In addition, the existence of domain structures has been shown by transmission electron microscopy and other techniques. In this study Li+ migration in tetragonal and cubic samples (t- and c-LLTO) was studied in micrometre space by pulsed-gradient spin-echo (PGSE) NMR from the room temperature up to 100 °C. The grain-boundary effects appeared clearly in the 7Li echo attenuation plots, which were not linear and included at least two components. Previously, we reported that the Li+ diffusion in inorganic solid electrolytes (garnets, NASICON, sulfides) distributes widely in time and space, illustrated by the dependences of the measuring parameters such as observation time (Δ) and pulsed-field gradient (PFG) strength (g). In addition to the complexities of parameter dependent Li+ diffusion, grain-boundary disturbance effects were observed for Li+ diffusion phenomena in LLTO samples. We indicated that a unique DLi value could be estimated from the linear echo attenuation plot at convergent measuring conditions with long Δ and large g for garnets and NASICON. The LLTO samples showed curved echo attenuation plots at the convergent measuring conditions, which were analysed by two components to give two Li+ diffusion constants (DLi-fast and DLi-slow). These values are consistent with the tracer diffusion constants measured at higher temperatures (>150 °C). For c-LLTO, DLi-fast and DLi-slow showed good correspondences with the bulk ionic conductivity (σbulk) and grain-boundary ionic conductivity (σgb), respectively. The Li carrier numbers estimated from DLi-fast and σbulk for c- and t-LLTO showed similar values, whereas those estimated from DLi-slow and σgb for c-LLTO were smaller by approximately one-order of magnitude.