13C Single Pulse Research Articles

Molecular-Level Understanding of Phase Stability in Phase-Change Nanoemulsions for Thermal Energy Storage by NMR Spectroscopy.

Phase change materials (PCMs) are latent heat storage materials that can store or release thermal energy while undergoing thermodynamic phase transitions. Organic PCMs can be emulsified in water in the presence of surfactants to enhance thermal conductivity and enable applications as heat transfer fluids. However, PCM nanoemulsions often become unstable during thermal cycling. To better understand the molecular origins of phase stability in PCM nanoemulsions, we designed a model PCM nanoemulsion system and studied how the molecular-level environments and dynamics of the surfactants and oil phase changed upon thermal cycling using liquid-state nuclear magnetic resonance (NMR) spectroscopy. The model system used octadecane as the oil phase, stearic acid as the surfactant, and aqueous NaOH as the continuous phase. The liquid fraction of octadecane within the nanoemulsions was quantified noninvasively during thermal cycling by liquid-state 1H single-pulse NMR measurements, revealing the extent of octadecane supercooling as a function of temperature. The mean droplet size of the PCM nanoemulsions, measured by dynamic light scattering (DLS), was correlated with the liquid content of octadecane to explain phase instability in the solid-liquid coexistence region. Quantitative 13C single-pulse NMR experiments established that the carbonyl surfactant head groups were present in multiple distinct environments during thermal cycling. After repeated thermal cycling, the 13C signal intensity of the carbonyl surfactant head groups decreased, indicating that the surfactant head groups lost molecular mobility. The results explain, in part, the origin of phase instability of PCM nanoemulsions upon thermal cycling.

Adsorption and Thermal Decomposition of Electrolytes on Nanometer Magnesium Oxide: An in Situ 13C MAS NMR Study

Mg batteries have been proposed as potential alternatives to lithium-ion batteries because of their lower cost, higher safety, and enhanced charge density. However, the Mg metal readily oxidizes when exposed to an oxidizer to form a thin MgO passivation surface layer that blocks the transport of Mg2+ across the solid electrode-electrolyte interface (SEI). In this work, the adsorption and thermal decomposition of diglyme (G2) and electrolytes containing Mg(TFSI)2 in G2 on 10 nm-sized MgO particles are evaluated by a combination of in situ 13C single-pulse, surface-sensitive 1H-13C cross-polarization (CP) magic-angle spinning (MAS) nuclear magnetic resonance, and quantum chemistry calculations. At 180 °C, neat G2 decomposes on MgO to form surface-adsorbed -OCH3 groups that are captured as a distinctive peak located at about 50 ppm in the CP/MAS spectrum. At low Mg(TFSI)2 salt concentration, the main solvation structure in this electrolyte is solvent-separated ion pairs without extensive Mg-TFSI contact ion pairs. G2, likely including a small amount of G2-solvated Mg2+, adsorbs onto the MgO surface. At high Mg(TFSI)2 salt concentrations, contact ion pairs between Mg and TFSI are formed extensively in the solution with the first solvation shell containing one pair of Mg-TFSI and two G2 molecules and the second solvation shell containing up to six G2 molecules, namely, MgTFSI(G2)2(G2)6+. In the presence of MgO, MgTFSI(G2)2(G2)6+ adsorbs onto the MgO surface. At 180 °C, the MgO surface stimulates a desolvation process converting MgTFSI(G2)2(G2)6+ to MgTFSI(G2)2+ and releasing G2 molecules from the second solvation shell of the MgTFSI(G2)2(G2)6+ cluster into the solution. MgTFSI(G2)2+ and MgTFSI(G2)2(G2)6+ tightly adsorb onto the MgO surface and are observed by 1H-13C CP/MAS experiments. The results contained herein show that electrolyte composition has a directing role in the species present on the electrode surface, which has implications on the structures and constituents of the solid-electrolyte interface on working electrodes and can be used to better understand its formation and the failure modes of batteries.

High-resolution magic angle spinning (1) H NMR measurement of ligand concentration in solvent-saturated chromatographic beads.

A method based on 1H high‐resolution magic angle spinning NMR has been developed for measuring concentration accurately in heterogeneous materials like that of ligands in chromatography media. Ligand concentration is obtained by relating the peak integrals for a butyl ligand in the spectrum of a water‐saturated chromatography medium to the integral of the added internal reference. The method is fast, with capacity of 10 min total sample preparation and analysis time per sample; precise, with a reproducibility expressed as 1.7% relative standard deviation; and accurate, as indicated by the excellent agreement of derived concentration with that obtained previously by 13C single‐pulse excitation MAS NMR. The effects of radiofrequency field inhomogeneity, spin rate, temperature increase due to spinning, and distribution and re‐distribution of medium and reference solvent both inside the rotor during spinning and between bulk solvent and pore space are discussed in detail. © 2016 The Authors Magnetic Resonance in Chemistry published by John Wiley & Sons Ltd.

Silica capsules enclosing P123 triblock copolymer micelles for flurbiprofen storage and release.

Flurbiprofen was incorporated in 200-400 nm silica capsules filled with Pluronic P123 polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer. The assembly process of the capsules and the molecular organization of drug molecule, surfactant and silica were investigated using SAXS, TGA, SEM, DLS, DSC, 13C single-pulse, CPMAS and 1H-1H two-dimensional NMR. Flurbiprofen molecules are molecularly dispersed inside polypropylene cores of P123 surfactant micelles occluded in a 20-30 nm thick silica shell. Flurbiprofen molecules in polypropylene cores of P123 micelles exhibit high mobility and are easily released after introduction in simulated gastrointestinal fluid and the solubility limit is reached within minutes. Release rates are favored at high pH due to acid dissociation of the carboxylic acid group of the flurbiprofen molecule. The molecular environment of flurbiprofen in these P123 filled silica capsules is different from ordered mesoporous silica materials synthesized using P123 as sacrificial template having the drug molecules adsorbed on the silica surface of pore walls. These findings uncover a new concept of storage and release of hydrophobic bioactive molecules.

Fingerprints of Phalaenopsis Tissues in Growth and Spike Induction Periods—A Solid‐state 13C NMR Approach

AbstractSolid‐state Nuclear Magnetic Resonance (ss‐NMR) 13C single‐pulse excitation spectroscopy in combination with the magic‐angle spinning (MAS) technique was applied to a series of Phalaenopsis tissues, including the leaf, sheath, stem, and root, at different growth and spiking periods. Compared with{1H}/13C cross‐polarization MAS spectra, the 13C single‐pulse excitation MAS spectra displayed very distinct spectral patterns, recognizable as fingerprints of the tissues studied. 1Here, we demonstrate that solid‐state 13C single‐pulse excitation NMR spectroscopy provides a direct and robust analytical tool for studying the various tissues of Phalaenopsis in different growth and spiking induction periods.

Hydration properties of regioselectively etherified celluloses monitored by 2H and 13C solid-state MAS NMR spectroscopy

The hydration properties of 2,3-O-hydroxypropylcellulose (HPC) and 2,3-O-hydroxyethylcellulose (HEC) were analyzed by multi-nuclear solid-state MAS NMR spectroscopy. By 13C single-pulse (SP) MAS and cross-polarization (CP) MAS NMR, differences between the immobile regions and all parts of the polysaccharides were detected as a function of hydration. Complementary information about the water environments was observed by 2H MAS NMR. By this approach it was demonstrated that side chains in 2,3-O-HPC and 2,3-O-HEC were easier to hydrate than the cellulose backbone. Furthermore the motion of water was more restricted (slower) in 2,3-O-HPC than in 2,3-O-HEC. For both polysaccharides the hydration could be explained by a two-step process: in step one increased ordering of the immobile regions occurs after which the entire polymer is hydrated in step two.