On De Gennes narrowing of fluids confined at the molecular scale in nanoporous materials.

Beyond well-documented confinement and surface effects arising from the large internal surface and severely confining porosity of nanoporous hosts, the transport of nanoconfined fluids remains puzzling in many aspects. With striking examples such as memory, i.e., non-viscous effects, intermittent dynamics, and surface barriers, the dynamics of fluids in nanoconfinement challenge classical formalisms (e.g., random walk, viscous/advective transport)-especially for molecular pore sizes. In this context, while molecular frameworks such as intermittent Brownian motion, free volume theory, and surface diffusion are available to describe the self-diffusion of a molecularly confined fluid, a microscopic theory for collective diffusion (i.e., permeability), which characterizes the flow induced by a thermodynamic gradient, is lacking. Here, to fill this knowledge gap, we invoke the concept of "De Gennes narrowing," which relates the wavevector-dependent collective diffusivity D0(q) to the fluid structure factor S(q). First, using molecular simulation for a simple yet representative fluid confined in a prototypical solid (zeolite), we unravel an essential coupling between the wavevector-dependent collective diffusivity and the structural ordering imposed on the fluid by the crystalline nanoporous host. Second, despite this complex interplay with marked Bragg peaks in the fluid structure, the fluid collective dynamics is shown to be accurately described through De Gennes narrowing. Moreover, in contrast to the bulk fluid, the departure from De Gennes narrowing for the confined fluid in the macroscopic limit remains small as the fluid/solid interactions in severe confinement screen collective effects and, hence, weaken the wavevector dependence of collective transport.

Innovative pressure environment combining hydrostatic pressure gradient and mechanical compression for structural investigations of nanoporous soft films.

The development of a new sample environment enabling X-ray scattering measurements at small and large angles under mechanical compression and hydraulic flow is presented. The cell, which is adapted for moderate pressures, includes beryllium windows, and allows applying simultaneously a compressive pressure up to 2.5 kbar in the perpendicular direction to the flow and either a hydrostatic pressure up to 300 bar or a pressure gradient of the same amplitude. The development of high-pressure devices for synchrotron experiments is relevant for many scientific fields in order to unveil details of a material's structure under relevant conditions of stresses. In particular, mechanical constraints coupled to hydrostatic pressure or flow, leading to complex stress tensor and mechanical response, and therefore unexpected deformations (swelling and pore deformation), are poorly addressed. Here, first the design of the environment is described, and then its performance with measurements carried out on a regenerated cellulose membrane is demonstrated.

The dynamical Matryoshka model: 2. Modeling of local lipid dynamics at the sub-nanosecond timescale in phospholipid membranes.

Biological membranes are generally formed by lipids and proteins. Often, the membrane properties are studied through model membranes formed by phospholipids only. They are molecules composed by a hydrophilic head group and hydrophobic tails, which can present a panoply of various motions, including small localized movements of a few atoms up to the diffusion of the whole lipid or collective motions of many of them. In the past, efforts were made to measure these motions experimentally by incoherent neutron scattering and to quantify them, but with upcoming modern neutron sources and instruments, such models can now be improved. In the present work, we expose a quantitative and exhaustive study of lipid dynamics on DMPC and DMPG membranes, using the Matryoshka model recently developed by our group. The model is confronted here to experimental data collected on two different membrane samples, at three temperatures and two instruments. Despite such complexity, the model describes reliably the data and permits to extract a series of parameters. The results compare also very well to other values found in the literature.

The dynamical Matryoshka model: 3. Diffusive nature of the atomic motions contained in a new dynamical model for deciphering local lipid dynamics.

In accompanying papers [Bicout et al., BioRxiv https://doi.org/10.1101/2021.09.21.461198 (2021); Cissé et al., BioRxiv https://doi.org/10.1101/2022.03.30.486370 (2022)], a new model called Matryoshka model has been proposed to describe the geometry of atomic motions in phospholipid molecules in bilayers and multilamellar vesicles based on their quasielastic neutron scattering (QENS) spectra. Here, in order to characterize the relaxational aspects of this model, the energy widths of the QENS spectra of the samples were analyzed first in a model-free way. The spectra were decomposed into three Lorentzian functions, which are classified as slow, intermediate, and fast motions depending on their widths. The analysis provides the diffusion coefficients, residence times, and geometrical parameters for the three classes of motions. The results corroborate the parameter values such as the amplitudes and the mobile fractions of atomic motions obtained by the application of the Matryoshka model to the same samples. Since the current analysis was carried out independently of the development of the Matryoshka model, the present results enhance the validity of the model. The model will serve as a powerful tool to decipher the dynamics of lipid molecules not only in model systems, but also in more complex systems such as mixtures of different kinds of lipids or natural cell membranes.

Gas Adsorption in Zeolite and Thin Zeolite Layers: Molecular Simulation, Experiment, and Adsorption Potential Theory.

Molecular simulations and experiments are used to investigate methane adsorption in bulk and thin layers of MFI zeolite (silicalite-1). After comparing the theoretical adsorption data obtained using Grand Canonical Monte Carlo simulations for bulk MFI at various temperatures against experiments, zeolite layers with different crystalline orientations and levels of surface flexibility are considered. The data obtained for such prototypical systems allow us to rationalize both the qualitative and quantitative impact of external surface in nanoporous solids. In particular, due to strong confinement in zeolite pores, methane is found to adsorb at low pressures in the core of the zeolite while external surface adsorption occurs at pressures where the internal porosity of zeolite is saturated. Using Polanyi's adsorption potential theory, which is derived here from Hill's general scheme for adsorption, we provide a simple thermodynamic formalism to predict consistently adsorption both in the internal porosity and at the external surface of nanoporous solids. While this seminal theory has been already applied for gases in nanoporous solids, its extension to describe both surface and volume adsorption is important to provide a general rational framework for fluid adsorption in finely divided materials. We also discuss the applicability of this formalism for gas adsorption data under supercritical conditions.

How Temperature Rise Can Induce Phase Separation in Aqueous Biphasic Solutions.

Ionic-liquid-based acidic aqueous biphasic solutions (AcABSs) recently offered a breakthrough in the field of metal recycling. The particular mixture of tributyltetradecylphosphonium chloride ([P4,4,4,14]Cl), acid, and water presents the unusual characteristic of a lower solution critical temperature (LCST), leading to phase separation upon a temperature rise of typically a few tens of degrees. We address here the microscopic mechanisms driving the phase separation. Using small-angle neutron scattering, we characterized the spherical micelle formation in a binary ionic liquid/water solution and the micelle aggregation upon the addition of acid due to the screening of electrostatic repulsion. The increase in both the acid concentration and the temperature eventually leads to micelle flocculation and phase separation. This last step is achieved through chloride ion adsorption at the surface of the micelle. This exothermic adsorption compensates for the entropic cost, leading to a counterintuitive behavior, and may be generalized to a number of molecular systems with an LCST.

Oedometric-like setup for the study of water transport in porous media by quasi-elastic neutron scattering.

In comparison to condensed matter, soft matter is subject to several interplaying effects (surface heterogeneities and swelling effect) that influence transport at the nanoscale. In consequence, transport in soft and compliant materials is coupled to adsorption and deformation phenomena. The permeance of the material, i.e., the response of the material to a pressure gradient, is dependent on the temperature, the chemical potential, and the external constraint. Therefore, the characterization of water dynamics in soft porous materials, which we address here, becomes much more complex. In this paper, the development of an original setup for scattering measurements of a radiation in the transmitted geometry in oedometric conditions is described. A specially designed cell enables a uniaxial compression of the investigated material, PIM-1 (Polymers of Intrinsic Microporosity), in the direction perpendicular to the applied hydraulic pressure gradient (up to 120 bars). High pressure boosting of the circulating water is performed with a commercially available high-pressure pump Karcher. This particular setup is adapted to the quasi-elastic neutron scattering technique, which enables us to probe diffusion and relaxation phenomena with characteristic times of 10-9 s-10-12 s. Moreover, it can easily be modified for other scattering techniques.

Time-domain THz spectroscopy of the characteristics of hydroxyapatite provides a signature of heating in bone tissue.

Because of the importance of bone in the biomedical, forensic and archaeological contexts, new investigation techniques are constantly required to better characterize bone ultrastructure. In the present paper, we provide an extended investigation of the vibrational features of bone tissue in the 0.1-3 THz frequency range by time-domain THz spectroscopy. Their assignment is supported by a combination of X-ray diffraction and DFT-normal modes calculations. We investigate the effect of heating on bone tissue and synthetic calcium-phosphates compounds with close structure and composition to bone mineral, including stoichiometric and non-stoichiometric hydroxyapatite (HA), tricalcium phosphate, calcium pyrophosphate and tetracalcium phosphate. We thus demonstrate that the narrow vibrational mode at 2.1 THz in bone samples exposed to thermal treatment above 750 °C arises from a lattice mode of stoichiometric HA. This feature is also observed in the other synthetic compounds, although weaker or broader, but is completely smeared out in the non-stoichiometric HA, close to natural bone mineral composition, or in synthetic poorly crystalline HA powder. The THz spectral range therefore provides a clear signature of the crystalline state of the investigated bone tissue and could, therefore be used to monitor or identify structural transitions occurring in bone upon heating.

Nanoscale modifications in the early heating stages of bone are heterogeneous at the microstructural scale.

Nanoscale studies of bone provide key indicators to evidence subtle structural changes that may occur in the biomedical, forensic and archaeological contexts. One specific problem encountered in all those disciplines, for which the identification of nanostructural cues could prove useful, is to properly monitor the effect of heating on bone tissue. In particular, the mechanisms at work at the onset of heating are still relatively unclear. Using a multiscale approach combining Raman microspectroscopy, transmission electron microscopy (TEM), synchrotron quantitative scanning small-angle X-ray scattering imaging (qsSAXSI) and polarized light (PL) microscopy, we investigate the ultrastructure of cortical bovine bone heated at temperatures < 300°C, from the molecular to the macroscopic scale. We show that, despite limited changes in crystal structure, the mineral nanoparticles increase in thickness and become strongly disorganized upon heating. Furthermore, while the nanostructure in distinct anatomical quadrants appears to be statistically different, our results demonstrate this stems from the tissue histology, i.e. from the high degree of heterogeneity of the microstructure induced by the complex cellular processes involved in bone tissue formation. From this study, we conclude that the analysis of bone samples based on the structure and organization of the mineral nanocrystals requires performing measurements at the histological level, which is an advantageous feature of qsSAXSI. This is a critical aspect that extends to a much broader range of questions relating to nanoscale investigations of bone, which could also be extended to other classes of nanostructured heterogeneous materials.

Molecular gels in the gas phase? Gelator-gelator and gelator-solvent interactions probed by vibrational spectroscopy.

Benzylidene glucose (BzGlc) is a member of the benzylidene glycoside family. These molecules have the ability to form molecular physical gels. These materials are formed when gelator molecules create a non-covalently bound frame where solvent molecules are trapped. Since the gel formation process and its properties are determined by the subtle balance between non-covalent forces, it is difficult to anticipate them. Quantitative and qualitative understanding of the gelator-gelator and gelator-solvent interactions is needed to better control these materials for important potential applications. We have used gas phase vibrational spectroscopy and theoretical chemistry to study the conformational choices of BzGlc, its dimer and the complexes it forms with water or toluene. To interpret the vibrational spectra we have used the dispersion corrected functional B97D which we have calibrated for the calculation of OH stretching frequencies. Even at the most basic molecular level, it is possible to interrogate a large range of non-covalent interactions ranging from OH → OH hydrogen bonding, to OH → π, and CH → π, all being at the center of gel properties at the macroscopic level.

Investigation into the Relaxation Dynamics of Polymer-Protein Conjugates Reveals Surprising Role of Polymer Solvation on Inherent Protein Flexibility.

Fully biodegradable protein-polymer conjugates, namely, MBP-PMeEP (maltose binding protein-poly methyl-ethylene phosphonate), have been investigated in order to understand the role of polymer solvation on protein flexibility. Using elastic and quasi-elastic incoherent neutron scattering, in combination with partially deuterated conjugate systems, we are able to disentangle the polymer dynamics from the protein dynamics and meaningfully address the coupling between both components. We highlight that, in the dry state, the protein-polymer conjugates lack any dynamical transition in accordance with the generally observed behavior for dry proteins. In addition, we observe a larger flexibility of the conjugated protein, compared to the native protein, as well as a lack of polymer-glass transition. Only upon water hydration does the conjugate recover its dynamical transition, leading to the conclusion that exclusive polymer solvation is insufficient to unfreeze fluctuations on the picosecond-nanosecond time scale in biomolecules. Our results also confirm the established coupling between polymer and protein dynamics in the conjugate.

Solvent contribution to the stability of a physical gel characterized by quasi-elastic neutron scattering.

The dynamics of a physical gel, namely, low-molecular-mass organic gelator methyl-4,6-O-benzylidene-α-D-mannopyranoside (α-manno) in water and toluene, are probed by neutron scattering. Using high gelator concentrations, we were able to determine, on a time scale from a few picoseconds to 1 nanosecond, the number of solvent molecules that are immobilized by the rigid network formed by the gelators. We found that only a few toluene molecules per gelator participate in the network which is formed by hydrogen bonding between the gelators' sugar moieties. In water, however, the interactions leading to the gel formations are weaker, involving dipolar, hydrophobic, or π-π interactions, and hydrogen bonds are formed between the gelators and the surrounding water. Therefore, around 10 to 14 water molecules per gelator are immobilized by the presence of the network. This study shows that neutron scattering can give valuable information about the behavior of solvent confined in a molecular gel.

Coupling of the hydration water dynamics and the internal dynamics of actin detected by quasielastic neutron scattering.

In order to characterize dynamics of water molecules around F-actin and G-actin, quasielastic neutron scattering experiments were performed on powder samples of F-actin and G-actin, hydrated either with D(2)O or H(2)O, at hydration ratios of 0.4 and 1.0. By combined analysis of the quasielastic neutron scattering spectra, the parameter values characterizing the dynamics of the water molecules in the first hydration layer and those of the water molecules outside of the first layer were obtained. The translational diffusion coefficients (D(T)) of the hydration water in the first layer were found to be 1.2×10(-5) cm(2)/s and 1.7×10(-5) cm(2)/s for F-actin and G-actin, respectively, while that for bulk water was 2.8×10(-5) cm(2)/s. The residence times were 6.6 ps and 5.0 ps for F-actin and G-actin, respectively, while that for bulk water was 0.62 ps. These differences between F-actin and G-actin, indicating that the hydration water around G-actin is more mobile than that around F-actin, are in concert with the results of the internal dynamics of F-actin and G-actin, showing that G-actin fluctuates more rapidly than F-actin. This implies that the dynamics of the hydration water is coupled to the internal dynamics of the actin molecules. The D(T) values of the water molecules outside of the first hydration layer were found to be similar to that of bulk water though the residence times are strongly affected by the first hydration layer. This supports the recent observation on intracellular water that shows bulk-like behavior.

Communication: crystallite nucleation in supercooled glycerol near the glass transition.

Heterogeneity and solid-like structures found near the glass transition provide a key to a better understanding of supercooled liquids and of the glass transition. However, the formation of solid-like structures and its effect on spatial heterogeneity in supercooled liquids is neither well documented nor well understood. In this work, we reveal the crystalline nature of the solid-like structures in supercooled glycerol by means of neutron scattering. The results indicate that inhomogeneous nucleation happens at temperatures near T(g). Nevertheless, the thermal history of the sample is essential for crystallization. This implies such structures in supercooled liquids strongly depend on thermal history. Our work suggests that different thermal histories may lead to different structures and therefore to different length and time scales of heterogeneity near the glass transition.

Internal motions of actin characterized by quasielastic neutron scattering.

Quasielastic neutron scattering (QENS) experiments were carried out on powders of F-actin and G-actin hydrated with D(2)O to characterize the internal dynamics on the picosecond time scale and the Ångstrom length scale. To investigate the effects of hydration, the measurements were done on samples at hydration ratio (h) of 0.4 (mg D(2)O/mg protein), containing only the first layer of hydration water, and at h = 1.0, containing more layers of water. The QENS spectra, obtained from the measurements at two energy resolutions of 110 and 15 µeV, indicated that the internal motions of both F-actin and G-actin have distributions of motions with distinct correlation times and amplitudes. Increasing hydration changes relative populations of these distinct motions. The effects of hydration were shown to be different between F-actin and G-actin. Elastic incoherent neutron scattering measurements provided the concerted results. The observed effects were interpreted in terms of the dynamical heterogeneity of the actin molecule: in G-actin, more surface loops become flexible and undergo diffusive motions of large amplitudes, whereas in F-actin the molecular interactions that keep the polymerized state suppress the large motions of the surface loops involved with polymerization so that the population of atoms undergoing large motions can increase only to a lesser degree.

Inverse freezing in molecular binary mixtures of alpha-cyclodextrin and 4-methylpyridine.

Ternary solutions of alpha-cyclodextrin (alphaCD) in 4-methylpyridine (4MP)/water mixtures solidify when heated and melt when cooled, and the crystalline solid phase exhibits a rich phase behavior as a function of temperature. In this work, we extend these earlier investigations to pure binary mixtures of alphaCD in water free 4MP, characterized via temperature and time dependent measurements of viscosity, X-ray diffraction, and infrared spectroscopy, complemented by observations of acoustic properties and small angle neutron diffraction. At high concentrations (>500 g l(-1)), these solutions enter an amorphous solid phase not only with decreasing but also with increasing temperature, before crystallizing at higher temperatures. This inverse solidification is attributed to the growth of hydrogen bonded clusters, leading to a steep increase of the viscosity with temperature.

trans-Decahydronaphthalene (decalin) from powder diffraction data.

The title compound, C(10)H(18), a decalin stereoisomer, crystallizes with Z' = 0.5 in the space group P2(1)/n. The trans-decalin molecule is located on an inversion centre with both rings in a chair conformation, making for a quasi-flat overall shape. Despite the absence of hydrogen bonds, it crystallizes easily. In this work the unknown crystal structure of trans-decalin has been solved and refined using X-ray powder diffraction data.

Lattice dynamics to trigger low temperature oxygen mobility in solid oxide ion conductors.

SrFeO(2.5) and SrCoO(2.5) are able to intercalate oxygen in a reversible topotactic redox reaction already at room temperature to form the cubic perovskites Sr(Fe,Co)O(3), while CaFeO(2.5) can only be oxidized under extreme conditions. To explain this significant difference in low temperature oxygen mobility, we investigated the homologous SrFeO(2.5) and CaFeO(2.5) by temperature dependent oxygen isotope exchange as well as by inelastic neutron scattering (INS) studies, combined with ab initio (DFT) molecular dynamical calculations. From (18)O/(16)O isotope exchange experiments we proved free oxygen mobility to be realized in SrFeO(x) already below 600 K. We have also evidence that low temperature oxygen mobility relies on the existence of specific, low energy lattice modes, which trigger and amplify oxygen mobility in solids. We interpret the INS data together with the DFT-based molecular dynamical simulation results on SrFeO(2.5) and CaFeO(2.5) in terms of an enhanced, phonon-assisted, low temperature oxygen diffusion for SrFeO(3-x) as a result of the strongly reduced Fe-O-Fe bond strength of the apical oxygen atoms in the FeO(6) octahedra along the stacking axis. This dynamically triggered phenomenon leads to an easy migration of the oxide ions into the open vacancy channels and vice versa. The decisive impact of lattice dynamics, giving rise to structural instabilities in oxygen deficient perovskites, especially with brownmillerite-type structure, is demonstrated, opening new concepts for the design and tailoring of low temperature oxygen ion conductors.

Differences in internal dynamics of actin under different structural states detected by neutron scattering.

F-actin, a helical polymer formed by polymerization of the monomers (G-actin), plays crucial roles in various aspects of cell motility. Flexibility of F-actin has been suggested to be important for such a variety of functions. Understanding the flexibility of F-actin requires characterization of a hierarchy of dynamical properties, from internal dynamics of the actin monomers through domain motions within the monomers and relative motions between the monomers within F-actin to large-scale motions of F-actin as a whole. As a first step toward this ultimate purpose, we carried out elastic incoherent neutron scattering experiments on powders of F-actin and G-actin hydrated with D(2)O and characterized the internal dynamics of F-actin and G-actin. Well established techniques and analysis enabled the extraction of mean-square displacements and their temperature dependence in F-actin and in G-actin. An effective force constant analysis with a model consisting of three energy states showed that two dynamical transitions occur at approximately 150 K and approximately 245 K, the former of which corresponds to the onset of anharmonic motions and the latter of which couples with the transition of hydration water. It is shown that behavior of the mean-square displacements is different between G-actin and F-actin, such that G-actin is "softer" than F-actin. The differences in the internal dynamics are detected for the first time between the different structural states (the monomeric state and the polymerized state). The different behavior observed is ascribed to the differences in dynamical heterogeneity between F-actin and G-actin. Based on structural data, the assignment of the differences observed in the two samples to dynamics of specific loop regions involved in the polymerization of G-actin into F-actin is proposed.