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The calorimetric glass-transition temperature of water is 136 K, but extrapolation of thermodynamic and relaxation properties of water from ambient temperature to below its homogeneous nucleation temperature TH = 235 K predicts divergence at TS = 228 K. The “no-man’s land” between the TH and glassy water crystallization temperature of 150 K, which is encountered on warming up from the vitrified state, precludes a straightforward reconciliation of the two incompatible temperature dependences of water properties, above 235 K and below 150 K. The addition of lithium chloride to water allows bypassing both TH and TS on cooling, resulting in the dynamics with no features except the calorimetric glass transition, still at 136 K. We show that lithium chloride prevents hydrogen-bonding network completion in water on cooling, as manifested, in particular, in changing microscopic diffusion mechanism of the water molecules. Thus thermodynamic and relaxation peculiarities exhibited by pure water on cooling to its glass transition, such as the existence of the TH and TS, must be associated specifically with the hydrogen-bonding network. [ABSTRACT FROM AUTHOR]

Atsufficiently high temperatures, the center-of-mass microscopicdiffusion dynamics of liquids is characterized by a single component,often with weak temperature dependence. In this regime, the effectivecage made by the neighbor particles cannot be sustained and readilybreaks down, enabling long-range diffusion. As the temperature isdecreased, the cage relaxation becomes impeded, leading to a higherviscosity with more pronounced temperature dependence. On the microscopicscale, the sustained caging effect leads to a separation between afaster in-cage relaxation component and a slower cage-breaking relaxationcomponent. The evidence for the separate dynamic components, as opposedto a single stretched component, is provided by quasielastic neutronscattering experiments. We use a simple method to evaluate the extentof the dynamic components separation as a function of temperaturein a group of related aromatic molecular liquids. We find that, regardlessof the glass-forming capabilities or lack thereof, progressively morepronounced separation between the in-cage and cage-breaking dynamiccomponents develops on cooling down as the ratio of Tb/T, where Tbis the boiling temperature, increases. This reflects the microscopicmechanism behind the empirical rule for the glass forming capabilitybased on the ratio of boiling and melting temperatures, Tb/Tm. When a liquid’s Tb/Tmhappens tobe high, the liquid can readily be supercooled below its Tmbecause the liquid’s microscopic relaxation dynamicsis already impeded at Tm, as evidencedby a sustained caging effect manifested through the separation ofthe in-cage and cage-breaking dynamic components. Our findings suggestcertain universality in the temperature dependence of the microscopicdiffusion dynamics in molecular liquids, regardless of their glass-formingcapabilities. Unless the insufficiently low (with respect to Tb) melting temperature, Tm, intervenes and makes crystallization thermodynamically favorablewhen cage-breaking is still unimpeded and the structural relaxationis fast, the liquid is likely to become supercooled. The propensityto supercooling and eventually forming a glass is thus determinedby a purely thermodynamic factor, Tb/Tm. [ABSTRACT FROM AUTHOR]

Using quasielastic neutron scatteringspectroscopy, we measuredthe averaged translational diffusion of water in solutions of biologicallyrelevant salts, NaCl, a kosmotrope, and KCl, a chaotrope. The analysisrevealed the striking difference in the influence of these ions onwater dynamics. While the averaged water diffusion slows down in thepresence of the structure making (kosmotrope) Na+ion,the diffusion becomes faster in the presence of the structure breaking(chaotrope) K+ion. The latter means that, despite strongCoulombic interactions introduced by the K+ions, theirdisruption of the hydrogen-bonding network is so significant thatit leads to faster diffusion of the water molecules. [ABSTRACT FROM AUTHOR]

For the majority of the water present on earth, the two most important factors influencing its behavior are confinement, in either inorganic or organic matrixes, and the presence of solutes. Here, we investigate the effect of confinement in 3 nm pores on water diffusivity in aqueous solutions with archetypical solutes, a structure making (kosmotrope) NaCl and a structure breaking (chaotrope) KCl, up to 1.0 M in concentration. The water diffusivity in bulk aqueous solutions in such a concentration range is known to decrease very slightly in the presence of NaCl and increase very slightly in the presence of KCl. However, here we observe the water diffusivity in confined H 2 O-KCl increases by a factor of 2 compared to the pure water diffusivity in the same confinement. This unusually strong cumulative effect of confinement and a structure breaking additive may have profound implications for the mobility and transport of aqueous species in nature.

The synthesis of heterostructures of different two-dimensional (2D) materials offers an approach to combine advantages of different materials constituting the heterostructure and ultimately enhance their performance for applications such as electrochemical energy storage, achieving high energy, and high-power densities. Understanding the behavior of ions and solvents in confinement between these dissimilar layers is critical to understand their performance and control. Considering aqueous electrolytes, we explore the heterostructure of 2D lepidocrocite-type TiO 2 (2D-TiO 2 ) and hydroxylated or O-terminated Ti 3 C 2 MXene using ReaxFF molecular dynamics simulations and elastic/quasielastic neutron scattering techniques. Simulating a bilayer water intercalation, we find that the extent of interlayer hydration is impacted most by the surface terminations on the MXene and is marginally affected by 2D-TiO 2 . However, the introduction of 2D-TiO 2 decreases the water self-diffusion due to the notch sites (i.e., surface oxygen ridges) entrapping water molecules. Intercalating alkali cations into the heterostructures, we find that Li + is predominantly adsorbed at the 2D-TiO 2 surface instead of the MXenes with the preferential occupation of the notch sites. In contrast, Na + forms a planar solvation with water, while K + is adsorbed both at the O-terminated MXene and 2D-TiO 2 . This behavior is altered when OH-terminated MXene is involved-the repulsion from the protons on the MXene surface forces the K + ions to be adsorbed exclusively to 2D-TiO 2 , while Na + retains some of its solvation in the water layer due to its smaller size. In OH-terminated MXenes, we see a consistent transfer of protons from the MXene surface toward 2D-TiO 2 , implying a greater capacity to store protons in the heterostructures. Of the three cations simulated, Na + hinders the proton migration the least and Li + the most because of its position near the 2D-TiO 2 surface. Therefore, 2D-TiO 2 /MXene heterostructures are likely to exhibit a higher energy density but lower power density, especially with Na + intercalation.

Polymerized ionic liquids (PolyILs) are promising candidates for a broad range of technologies. However, the relatively low conductivity of PolyILs at room temperature has strongly limited their applications. In this work, we provide new insights into the roles of various microscopic parameters controlling ion transport in these polymers, which are crucial for their rational design and practical applications. Using broadband dielectric spectroscopy and neutron and light scattering techniques, we found a clear connection between the activation energy for conductivity, fast dynamics, and high-frequency shear modulus in PolyILs at their glass transition temperature ( T g ). In particular, our analysis reveals a correlation between conductivity and the amplitude of fast picosecond fluctuations at T g , suggesting the possible involvement of fast dynamics in lowering the energy barrier for ion conductivity. We also demonstrate that both the activation energy for ion transport and the amplitude of the fast fluctuations depend on the high-frequency shear moduli of PolyILs, thus identifying a practically important parameter for tuning conductivity. The parameters recognized in this work and their connection to the ionic conductivity of PolyILs set the stage for a deeper understanding of the mechanism of ion transport in PolyILs in the glassy state.

In biological membranes, lipid rafts are now thought to be transient and nanoscopic. However, the mechanism responsible for these nanoscopic assemblies remains poorly understood, even in the case of model membranes. As a result, it has proven extremely challenging to probe the physicochemical properties of lipid rafts at the molecular level. Here, we use all-atom molecular dynamics (MD) simulations and inelastic X-ray scattering (IXS), an intrinsically nanoscale technique, to directly probe the energy transfer and collective short-wavelength dynamics (phonons) of biologically relevant model membranes. We show that the nanoscale propagation of stress in lipid rafts takes place in the form of collective motions made up of longitudinal (compression waves) and transverse (shear waves) molecular vibrations. Importantly, we provide a molecular picture for the so-called van der Waals mediated "force from lipid" [Anishkin, A. et al. Proc. Natl. Acad. Sci. U.S.A. 2014, 111, 7898], a key parameter for the ionic channel mechano-transduction and the mechanism for the lipid transfer of molecular level stress [Aponte-Santamarı́a, C. et al. J. Am. Chem. Soc. 2017, 139, 13588]. Specifically, we describe how lipid rafts are formed and maintained through the propagation of molecular stress, lipid raft rattling dynamics, and a relaxation process. Eventually, the rafts dissipate through the self-diffusion of lipids making up the rafts. We also show that the molecular stress and viscoelastic properties of transient lipid rafts can be modulated through the use of hydrophobic biomolecules such as melatonin and tryptophan. Ultimately, the herein proposed mechanism describing the molecular interactions for the formation and dissolution of lipid rafts may offer insights as to how lipid rafts enable biological function.

Highly efficient proton conductors, polyoxometalate-poly(ethylene glycol) (POM-PEG) hybrid nanocomposites, have been synthesized by encapsulating a single PEG chain inside the 1D nanochannel defined by the frameworks of POMs. By employing two types of neutron scattering techniques complemented by thermal analysis, we prove that in a nanochannel a single PEG chain stays as a distorted helix. More importantly, we reveal that the PEG segments perform a localized longitudinal random walk and quantitatively show the strong correlation between the local motion of PEG and the macroscopic proton conduction of the material. On the basis of these spatial-temporal characteristics, a microscopic picture for the proton conduction process of POM-PEG hybrid materials is proposed.

Understanding of structural, electrical, and gravimetric peculiarities of water vapor interaction with ion-intercalated MXenes led to design of a multimodal humidity sensor. Neutron scattering coupled to molecular dynamics and ab initio calculations showed that a small amount of hydration results in a significant increase in the spacing between MXene layers in the presence of K and Mg intercalants between the layers. Films of K- and Mg-intercalated MXenes exhibited relative humidity (RH) detection thresholds of ∼0.8% RH and showed monotonic RH response in the 0-85% RH range. We found that MXene gravimetric response to water is 10 times faster than their electrical response, suggesting that H 2 O-induced swelling/contraction of channels between MXene sheets results in trapping of H 2 O molecules that act as charge-depleting dopants. The results demonstrate the use of MXenes as humidity sensors and infer potential impact of water on structural and electrical performance of MXene-based devices.

Selectively labeled GroEL protein was produced in living deuterated bacterial cells to enhance its neutron scattering signal above that of the intracellular milieu. Quasi-elastic neutron scattering shows that the in-cell diffusion coefficient of GroEL was (4.7 ± 0.3) × 10 -12 m 2 /s, a factor of 4 slower than its diffusion coefficient in buffer solution. Internal protein dynamics showed a relaxation time of (65 ± 6) ps, a factor of 2 slower compared to the protein in solution. Comparison to the literature suggests that the effective diffusivity of proteins depends on the length and time scale being probed. Retardation of in-cell diffusion compared to the buffer becomes more significant with the increasing probe length scale, suggesting that intracellular diffusion of biomolecules is nonuniform over the cellular volume. The approach outlined here enables investigation of protein dynamics within living cells to open up new lines of research using "in-cell neutron scattering" to study the dynamics of complex biomolecular systems.

We explore the influence of the solvent dipole moment on cation-anion interactions and transport in 1-butyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl), [BMIM + ][Tf2N - ]. Free energy profiles derived from atomistic molecular dynamics (MD) simulations show a correlation of the cation-anion separation and the equilibrium depth of the potential of mean force with the dipole moment of the solvent. Correlations of the ion diffusivity with the dipole moment and the concentration of the solvent were further demonstrated by classical MD simulations. Quasi-elastic neutron scattering experiments with deuterated solvents reveal a complex picture of nanophase separation into the ionic liquid-rich and solvent-rich phases. The experiment corroborates the trend of concentration- and dipole moment-dependent enhancement of ion mobility by the solvent, as suggested by the simulations. Despite the considerable structural complexity of ionic liquid-solvent mixtures, we can rationalize and generalize the trends governing ionic transport in these complex electrolytes.

Light activation of the visual G-protein-coupled receptor (GPCR) rhodopsin leads to significant structural fluctuations of the protein embedded within the membrane yielding the activation of cognate G-protein (transducin), which initiates biological signaling. Here, we report a quasi-elastic neutron scattering study of the activation of rhodopsin as a GPCR prototype. Our results reveal a broadly distributed relaxation of hydrogen atom dynamics of rhodopsin on a picosecond-nanosecond time scale, crucial for protein function, as only observed for globular proteins previously. Interestingly, the results suggest significant differences in the intrinsic protein dynamics of the dark-state rhodopsin versus the ligand-free apoprotein, opsin. These differences can be attributed to the influence of the covalently bound retinal ligand. Furthermore, an idea of the generic free-energy landscape is used to explain the GPCR dynamics of ligand-binding and ligand-free protein conformations, which can be further applied to other GPCR systems.

MXenes are a recently discovered class of 2D materials with an excellent potential for energy storage applications. Because MXene surfaces are hydrophilic and attractive interaction forces between the layers are relatively weak, water molecules can spontaneously intercalate at ambient humidity and significantly influence the key properties of this 2D material. Using complementary X-ray and neutron scattering techniques, we demonstrate that intercalation with potassium cations significantly improves structural homogeneity and water stability in MXenes. In agreement with molecular dynamics simulations, intercalated potassium ions reduce the water self-diffusion coefficient by 2 orders of magnitude, suggesting greater stability of hydrated MXene against changing environmental conditions.

A typical type of core-shell polyoxometalates can be obtained through the Keggin-type polyoxometalate-templated growth of a layer of spherical shell structure of {Mo72Fe30}. Small-angle X-ray scattering is used to study the structural features and stability of the core-shell structures in aqueous solutions. Time-resolved small-angle X-ray scattering is applied to monitor the synthetic reactions, and a three-stage formation mechanism is proposed to describe the synthesis of the core-shell polyoxometalates based on the monitoring results. New protocols have been developed by fitting the X-ray data with custom physical models, which provide more convincing, objective, and completed data interpretation. Quasi-elastic and inelastic neutron scattering are used to probe the dynamics of water molecules in the core-shell structures, and two different types of water molecules, the confined and structured water, are observed. These water molecules play an important role in bridging core and shell structures and stabilizing the cluster structures.

We report a novel production method for uranium oxyfluoride [(UO2)7F14(H2O)7]·4H2O, referred to as structure D. Structure D is produced as a product of hydrating anhydrous uranyl fluoride, UO2F2, through the gas phase at ambient temperatures followed by desiccation by equilibration with a dry environment. We follow the structure of [(UO2)7F14(H2O)7]·4H2O through an intermediate, liquid-like phase, wherein the coordination number of the uranyl ion is reduced to 5 (from 6 in the anhydrous structure), and a water molecule binds as an equatorial ligand to the uranyl ion. Quasielastic neutron scattering results compare well with previous measurements of mineral hydrates. The two groups of structurally distinct water molecules in D perform restricted motion on a length scale commensurate with the O-H bond (r = 0.92 Å). The more tightly bound equatorial ligand waters rotate slower (Dr = 2.2 ps(-1)) than their hydrogen-bonded partners (Dr = 28.7 ps(-1)).

From macroscopic measurements of deep eutectic solvents such as glyceline (1:2 molar ratio of choline chloride to glycerol), the long-range translational diffusion of the larger cation (choline) is known to be slower compared to that of the smaller hydrogen bond donor (glycerol). However, when the diffusion dynamics are analyzed on the subnanometer length scale, we find that the displacements associated with the localized diffusive motions are actually larger for choline. This counterintuitive diffusive behavior can be understood as follows. The localized diffusive motions confined in the transient cage of neighbor particles, which precede the cage-breaking long-range diffusion jumps, are more spatially constrained for glycerol than for choline because of the stronger hydrogen bonds the former makes with chloride anions. The implications of such differential localized mobility of the constituents should be especially important for applications where deep eutectic solvents are confined on the nanometer length scale and their long-range translational diffusion is strongly inhibited (e.g., within microporous media).

Dynamics of water confined in ∼5 Å diameter channels of beryl and cordierite single crystals were studied by using inelastic (INS) and quasielastic (QENS) neutron scattering. The INS spectra for both samples were similar and showed that there are no hydrogen bonds acting on water molecule, which experiences strong anisotropic potential, steep along the channels and very soft perpendicular to it. The high-resolution (3.4 μeV) QENS data revealed gradual freezing out of the water molecule dynamics for both minerals at temperatures below about 80 K when the scattering momentum transfer was parallel to the channels, but not when it was perpendicular to the channels. The QENS study with medium energy resolution (0.25 meV) of the beryl with the scattering momentum transfer along the channels showed gradual freezing out of water molecule dynamics at temperatures below about 200 K, whereas at higher temperatures the data could be described as 2-fold rotational jumps about the axis coinciding with the direction of the dipole moment (that is, perpendicular to the channels), with a residence time of 5.5 ps at 225 K. The energy resolution dependence of the apparent dynamics freezing temperature suggests gradual slowing down of the rotational jumps as the temperature is decreased, until the associated QENS broadening can no longer be detected, rather than actual freezing.

The mechanical and dynamical properties of cellulose, the most abundant biomolecule on earth, are essential for its function in plant cell walls and advanced biomaterials. Cellulose is almost always found in a hydrated state, and it is therefore important to understand how hydration influences its dynamics and mechanics. Here, the nanosecond-time scale dynamics of cellulose is characterized using dynamic neutron scattering experiments and molecular dynamics (MD) simulation. The experiments reveal that hydrated samples exhibit a higher average mean-square displacement above ∼240 K. The MD simulation reveals that the fluctuations of the surface hydroxymethyl atoms determine the experimental temperature and hydration dependence. The increase in the conformational disorder of the surface hydroxymethyl groups with temperature follows the cellulose persistence length, suggesting a coupling between structural and mechanical properties of the biopolymer. In the MD simulation, 20% hydrated cellulose is more rigid than the dry form, due to more closely packed cellulose chains and water molecules bridging cellulose monomers with hydrogen bonds. This finding may have implications for understanding the origin of strength and rigidity of secondary plant cell walls. The detailed characterization obtained here describes how hydration-dependent increased fluctuations and hydroxymethyl disorder at the cellulose surface lead to enhancement of the rigidity of this important biomolecule.

The emergence of intrinsically disordered proteins (IDPs) as a recognized structural class has forced the community to confront a new paradigm of structure, dynamics, and mechanical properties for proteins. We present novel data on the similarities and differences in the dynamics and nanomechanical properties of IDPs and other biomacromolecules on the picosecond time scale. An IDP, β-casein (CAS), has been studied in a calcium bound and unbound state using neutron and light scattering techniques. We show that CAS partially folds and stiffens upon calcium binding, but in the unfolded state, it is softer than folded proteins such as green fluorescence protein (GFP). We also see that some localized diffusive motions in CAS have a larger amplitude than in GFP at this time scale but are still smaller than those observed in tRNA. In spite of these differences, CAS dynamics are consistent with the classes of motions seen in folded protein on this time scale.

Atomistic molecular dynamics (MD) simulations were carried out to investigate the local dynamics of polyelectrolyte dendrimers dissolved in deuterium oxide (D2O) and its dependence on molecular charge. Enhanced segmental dynamics upon increase in molecular charge is observed, consistent with quasielastic neutron scattering (QENS) measurements. A strong coupling between the intradendrimer local hydration level and segmental dynamics is also revealed. Compelling evidence shows these findings originate from the electrostatic interaction between the hydrocarbon components of a dendrimer and the invasive water. This combined study provides fundamental insight into the dynamics of charged polyelectrolytes and the solvating water molecules.

The dynamics of RNA within the β-relaxation region of 10 ps to 1 ns is crucial to its biological function. Because of its simpler chemical building blocks and the lack of the side methyl groups, faster relaxational dynamics of RNA compared to proteins can be expected. However, the situation is actually opposite. In this work, the relaxational dynamics of tRNA is measured by quasielastic neutron scattering and analyzed using the mode coupling theory, originally developed for glass-forming liquids. Our results reveal that the dynamics of tRNA follows a log-decay within the β-relaxation region, which is an important trait demonstrated by the dynamics of proteins. The dynamics of hydrated tRNA and lysozyme compared in the time domain further demonstrate that the slower dynamics of tRNA relative to proteins originates from the difference in the folded states of tRNA and proteins, as well as the influence of their hydration water.

The purpose of this investigation is to determine whether a large oligomeric protein, inorganic pyrophosphatase (IPPase) from Thermococcus thioreducens with quaternary structural complexity, would have distinguishable dynamic characteristics compared to those of the small simple monomeric model protein, lysozyme. In this study, the β-relaxational dynamics of the two proteins, IPPase and lysozyme, are compared in the 10 ps to 0.5 ns time interval using quasi-elastic neutron scattering (QENS). Both of the protein dynamics show a characteristic logarithmic-like decay in the intermediate scattering function (ISF) of the hydrogen atoms. Distinguishable dynamical behavior found between two proteins reveals local flexibility and conformational substates unique to oligomeric structures. Moreover, the temperature dependence of the mean square displacement (MSD) of the hydrogen atoms in protein molecules, which is a traditional way to determine the "softness" of the protein molecule, is measured and shows no difference for the two proteins.

We used quasielastic neutron scattering (QENS) to study the dynamics of phenanthrenequinone (PQ) on the surface of onion-like carbon (OLC), or so-called carbon onions, as a function of surface coverage and temperature. For both the high- and low-coverage samples, we observed two diffusion processes; a faster process and nearly an order of magnitude slower process. On the high-coverage surface, the slow diffusion process is of long-range translational character, whereas the fast diffusion process is spatially localized on the length scale of ∼4.7 Å. On the low-coverage surface, both diffusion processes are spatially localized; on the same length scale of ∼4.7 Å for the fast diffusion and a somewhat larger length scale for the slow diffusion. Arrhenius temperature dependence is observed except for the long-range diffusion on the high-coverage surface. We attribute the fast diffusion process to the generic localized in-cage dynamics of PQ molecules, and the slow diffusion process to the long-range translational dynamics of PQ molecules, which, depending on the coverage, may be either spatially restricted or long-range. On the low-coverage surface, uniform surface coverage is not attained, and the PQ molecules experience the effect of spatial constraints on their long-range translational dynamics. Unexpectedly, the dynamics of PQ molecules on OLC as a function of temperature and surface coverage bears qualitative resemblance to the dynamics of water molecules on oxide surfaces, including practically temperature-independent residence times for the low-coverage surface. The dynamics features that we observed may be universal across different classes of surface adsorbates.

Studies of the low-temperature dynamics of proteins in aqueous solutions are limited by the crystallization of water. In this work, we use a solution of LiCl in D2O as a solvent for a protein to prevent crystallization and study the dynamics of both the protein and its aqueous solvent by quasielastic neutron scattering (QENS) in the temperature range of 210 to 290 K. Our results reveal that, while the dynamics of the aqueous solvent undergoes a crossover at about 220 K, the dynamics of the protein itself shows no transition at this temperature. The prevailing view is that the β-fluctuations of the protein are governed by the α-fluctuations of the solvent; therefore, observation of the apparent decoupling between the dynamics of the protein and its solvent below the crossover temperature is remarkable.

Using quasielastic neutron scattering, we have investigated diffusion dynamics of protons in the protic ionic liquid, N,N,N',N'-tetramethylguanidinium bis(perfluoroethylsulfonyl)imide, a promising new compound for application as an electrolyte in proton-conducting fuel cells. A temperature range of 30-360 K has been studied. The melting temperature of N,N,N',N'-tetramethylguanidinium bis(perfluoroethylsulfonyl)imide is about 290 K. We have found four distinct dynamic processes. First, the methyl group rotations exhibit broadly distributed dynamics which, on the nanosecond time scale, become visible above approximately 100 K. Second, there is a localized process with a characteristic confinement radius of about 1.6 A, which likely involves protons of the -NH(2) groups. These two processes take place in both solid and liquid phases, even though the methyl group rotations in the liquid phase are likely too fast to be detected in our experiment. Above the melting temperature, there are two new diffusion processes contributing to the dynamics of the liquid phase. Both of them appear to be of translational character. However, only the slower process represents unrestricted translation diffusion. The faster process is better described as spatially restricted translational diffusion with a characteristic confinement radius of about 8 A. It is likely that the long-range proton transfer in N,N,N',N'-tetramethylguanidinium bis(perfluoroethylsulfonyl)imide is associated primarily with the unrestricted translational diffusion process, which is characterized by a diffusion coefficient varying from 0.4 x 10(-10) to 1.4 x 10(-10) m(2)/s in the temperature range of 320-360 K.