Water Dynamics in a Concentrated Poly(<i>N</i>‑isopropylacrylamide) Solution at Variable Pressure

Using quasi-elastic neutron scattering (QENS), we study the dynamics of water in a concentrated poly­(N-isopropyl­acrylamide) solution over a large temperature range across the demixing transition at pressures of 0.1 and 130 MPa. The QENS spectra extending in frequency from 1 to 3 × 103 GHz and in momentum transfer from 0.45 to 1.65 Å–1 reveal the relaxation of hydration water as well as multiple dynamic processes in bulk water. At the cloud point, the fraction of hydration water decreases abruptly at 0.1 MPa, whereas at 130 MPa, it decreases smoothly. The susceptibility spectra of hydration water occur at lower frequencies than those of pure water and the dependence of the relaxation times on momentum transfer can be described by a jump-diffusion model. At a pressure of 0.1 MPa, the hydration water remaining in the two-phase region is more constrained than at 130 MPa. We attribute these findings to the pressure-dependent hydration interactions

Light-Induced Conformational Flexibility of the Orange Carotenoid Protein Studied by Quasielastic Neutron Scattering with <i>In Situ</i> Illumination

The orange carotenoid protein plays a vital role in the photoprotection of cyanobacteria and exhibits a significant structural change upon photoactivation. A rarely considered aspect is the importance of internal protein dynamics in facilitating the structural transition to the active state. In this study, we use quasielastic neutron scattering under (in situ) blue light illumination for the first time to directly probe the protein dynamics of the orange carotenoid protein in the dark-adapted and active states. This shows that the localized internal dynamics of amino acid residues is significantly enhanced upon photoactivation. This is attributed to the photoinduced structural changes exposing larger areas of the protein surface to the solvent, thus resulting in a higher degree of motional freedom. However, the flexibility of the W288A mutant assumed to mimic the active state structure is found to be different, thus highlighting the importance of in situ experiments

Solvent Dynamics in Solutions of PNIPAM in Water/Methanol MixturesA Quasi-Elastic Neutron Scattering Study

The solvent dynamics of concentrated solutions of poly­(<i>N</i>-isopropylacrylamide) (PNIPAM, 25 wt %) in water/methanol mixtures (85:15 v/v) are measured with the aim of shedding light onto the cononsolvency effect. Quasi-elastic neutron scattering (QENS) with contrast variation has been carried out at temperatures below and above the cloud point by using in the first set of experiments the mixture H₂O:<i>d</i>-MeOD (<i>d</i>-MeOD denotes fully deuterated methanol) as a solvent and in the second set of experiments the mixture D₂O:MeOH (MeOH denotes methanol). As a reference, bulk H₂O, bulk MeOH and the mixtures H₂O:<i>d</i>-MeOD and D₂O:MeOH (both 85:15 v/v) have been investigated as well. In the PNIPAM solution in H₂O:<i>d</i>-MeOD, two water populations are identified, namely strongly and less strongly arrested water. At the cloud point, the former is partially released from PNIPAM. The diffusion coefficient of the latter one is similar to the one in the water/methanol mixture, and its residence time decreases at the cloud point. The PNIPAM solution in D₂O:MeOH reveals similar dynamics to the one in H₂O:<i>d</i>-MeOD which may reflect that the dynamics of MeOH near the PNIPAM chain is similar to the one of H₂O. The similarity may, however, partially be due to H/D exchange between D₂O and MeOH. In both PNIPAM solutions, the mean-square displacement of the PNIPAM chain decreases gradually above the cloud point

Hydrogenation Reaction Pathway in Li₂Mg(NH)₂

Results of the first time-resolved in situ neutron diffraction measurements during absorption of deuterium in Li2Mg(ND)2 reveal the occurrence of a two-stage reaction. The first stage involves the reaction to LiND2, LiD, and Li2Mg2(ND)3, with the second stage leading to full deuteration to Mg(ND2)2 and LiD. Since no structural model for Li2Mg2(NH)3 exists in the literature, its structure has been determined from refinement of X-ray and neutron diffraction data. These new data provide key information toward the clarification of the hydrogenation mechanism in this system

From Molecular Dehydration to Excess Volumes of Phase-Separating PNIPAM Solutions

For aqueous poly­(<i>N</i>-isopropyl acrylamide) (PNIPAM) solutions, a structural instability leads to the collapse and aggregation of the macromolecules at the temperature-induced demixing transition. The accompanying cooperative dehydration of the PNIPAM chains is known to play a crucial role in this phase separation. We elucidate the impact of partial dehydration of PNIPAM on the volume changes related to the phase separation of dilute to concentrated PNIPAM solutions. Quasi-elastic neutron scattering enables us to directly follow the isotropic jump diffusion behavior of the hydration water and the almost freely diffusing water. As the hydration number decreases from 8 to 2 for the demixing 25 mass % PNIPAM solution, only a partial dehydration of the PNIPAM chains occurs. Dilatation studies reveal that the transition-induced volume changes depend in a remarkable manner on the PNIPAM concentration of the solutions. The excess volume per mole of H₂O molecules expelled from the solvation layers of PNIPAM during phase separation probably strongly increases from dilute to concentrated PNIPAM solutions. This finding is qualitatively related to the immense strain-softening previously observed for demixing PNIPAM solutions

Magnesium Imide: Synthesis and Structure Determination of an Unconventional Alkaline Earth Imide from Decomposition of Magnesium Amide

Magnesium imide (MgNH) was produced by monitoring the decomposition process of magnesium amide with in situ neutron diffraction. Significant changes in the structure of magnesium amide are detected during heat treatment and eventually result in the formation of crystalline MgNH. A model for the crystal structure of magnesium imide (MgNH) is presented for the first time. Remarkably, magnesium imide offers unique structural features similar to the cyclosilicate class and can be described as a porous solid formed by a sequence of linked chains of face sharing Mg6N6 hexagonal prism clusters

Magnesium Imide: Synthesis and Structure Determination of an Unconventional Alkaline Earth Imide from Decomposition of Magnesium Amide

Magnesium imide (MgNH) was produced by monitoring the decomposition process of magnesium amide with in situ neutron diffraction. Significant changes in the structure of magnesium amide are detected during heat treatment and eventually result in the formation of crystalline MgNH. A model for the crystal structure of magnesium imide (MgNH) is presented for the first time. Remarkably, magnesium imide offers unique structural features similar to the cyclosilicate class and can be described as a porous solid formed by a sequence of linked chains of face sharing Mg6N6 hexagonal prism clusters

LiBH₄−Mg(BH₄)₂: A Physical Mixture of Metal Borohydrides as Hydrogen Storage Material

The LiBH4−Mg(BH4)2 system has been investigated as a possible hydrogen storage material. Several composites were synthesized by ball milling, namely, xLiBH4−(1−x)Mg(BH4)2 with x = 0, 0.10, 0.25, 0.33, 0.40, 0.50, 0.60, 0.66, 0.75, 0.80, 0.90, 1. The physical mixture was investigated by using X- ray powder diffraction and thermal analysis. Interestingly, already a small amount of LiBH4 makes the α to β transition of Mg(BH4)2 reversible, which has not been reported before. The eutectic composition was found to exist at 0.50 x x = 0.50 composite releases about 7.0 wt % of hydrogen

Structure and Thermodynamic Properties of the NaMgH₃ Perovskite: A Comprehensive Study

One of the bottlenecks in the implementation of a hydrogen economy is the development of storage materials that can uptake high content of H2 and release it within a suitable temperature and pressure range. Among the proposed hydride systems, the perovskite NaMgH3 is receiving increasing attention, not only as the Mg ternary based hydride with the highest hydrogen gravimetric (6 wt %) and volumetric density (88 g L−1) but also as a stable hydride likely to be formed in the transformation reactions of mixed hydrides. However, there is a large scatter in the literature for both the structure of the NaMgH3 compound and the thermodynamics of the hydrogenation/dehydrogenation processes. In this paper a critical review of the literature data, supported by a new set of experimental (in situ synchrotron X-ray diffraction, infrared spectroscopy, high-pressure differential scanning calorimetry, pressure composition isotherms) and theoretical data is presented. The influence of ball milling on the microstructure is studied in the NaMgH3 in comparison to NaH and MgH2. The infrared spectrum of NaMgH3 compound, assigned by calculated and experimental results, is characterized by vibrational regions around 1100 and 600 cm−1. In situ synchrotron X-ray diffraction measurements show the desorption reaction of NaMgH3 into NaH and Mg at about 673 K under 0.2 MPa H2, and the successive reabsorption of NaH and Mg back to NaMgH3 at 623 K under 0.5 MPa H2. From high-pressure differential calorimetry, it was measured a formation enthalpy of 141 kJ/mol f.u for NaMgH3 compound. It was confirmed the possible reaction of NaH with Mg with observation of NaMgH3 formation in 1.0 MPa H2. Finally, this work provides a thermodynamic description of the NaMgH3 phase by a critical assessment of the available information using the CALPHAD approach and the equilibrium pressure−temperature phase diagram is presented