J. Teixeira

Structure of high‐density amorphous water. II. Neutron scattering study

High‐density amorphous water is studied by neutron scattering in a Q range extending to 16 A−1. The low density form of amorphous water is also analyzed and compared with previous results. There are very important differences in the composite pair correlation functions of the two forms of amorphous ice, in particular beyond the first nearest‐neighbors distance. We conclude that the hydrogen bond network is strongly deformed in a manner analogous to that found in water at high temperature. This is in contrast with the behavior of the pair correlation function of low‐density amorphous water, which is closer to that of supercooled water.

The glass transition and relaxation behavior of bulk water and a possible relation to confined water

Due to the widespread importance of water and the difficulty to study it in the so-called no mans land between 150 and 235 K, deeply supercooled bulk water is currently heavily debated. It speculates about its properties from extrapolations of experimental data on bulk water above 235 K and below 150 K, computer simulations, and experiments on confined water for which the finite size effects may prevent crystallization in the no mans land. However, it is far from obvious how experimental data on bulk water should be extrapolated to the temperature range of the no mans land or how relevant results from computer simulations and studies of confined water are for bulk water. In this paper the structural and dynamical properties of supercooled bulk water are tried to be understood from experimental results on confined water and comparisons with bulk water. We propose that a similar crossover from a high temperature alpha-relaxation to a low temperature beta-relaxation occurs also for bulk water but at a higher temperature than for confined water due to the larger average number of hydrogen bonds between the water molecules in bulk water. In the case of bulk water the crossover is expected to occur around the critical temperature T(s) approximately 228 K when the buildup of an icelike tetrahedral network structure is completed. The proposed interpretation is the simplest one that is able to explain many of the peculiar properties of supercooled water.

Water in confined geometries

Abstract The dynamics of water molecules in the vicinity of a hydrophilic interface is modified as compared to that of bulk water. Recent experiments performed in porous silica show that water dynamics is well described by a stretched-exponential intermediate-scattering function. The behaviour of confined water is similar to that of supercooled water at lower (≈ 30 K) temperature. This temperature shift can be understood within the percolation model of water, assuming that only three hydrogen bonds can be formed between interfacial water molecules instead of four in bulk water.

Small-angle neutron scattering study of D2O–alcohol solutions

Small-angle neutron scattering (SANS) measurements have been carried out on heavy water solutions of ethanol, isopropyl alcohol, n-propyl alcohol, t-butyl alcohol and butoxyethanol between 5 and 37 °C at the concentrations where ultrasonic attenuation exhibits peak values. The wavevector dependence and the absolute intensity of the scattered intensities were analysed according to a microscopic model of the solutions in terms of aggregated complexes. The results indicate that at 25 °C there exist either alcohol ‘micelle-like’ structures or alcohol–heavy water complexes which increase on going from the lower to higher alcohol solutions. As temperature increases from 25 to 37 °C a higher aggregation is observed in butyl alcohol and butoxyethanol solutions. This behaviour is attributed to the demixing tendency of these systems at high temperatures. On going from 25 to 5 °C the aggregation increases again. This trend is associated with the anomalous behaviour of the specific heat and ultrasonic attenuation of these systems. The occurrence of a low-temperature phase transition suggested by these anomalies is supported by our results.

Single-particle dynamics in dimethyl–sulfoxide/water eutectic mixture by neutron scattering

We study water and dimethyl–sulfoxide (DMSO) dynamics in the eutectic DMSO/water mixture, 2H2O:1DMSO, by incoherent quasielastic and inelastic neutron scattering. A temperature range from room down to −27 °C is investigated. Both water and DMSO translational dynamics are significantly slowed in the mixture compared to pure liquids. They exhibit different diffusion dynamics, pointing to the absence of stable hydrogen bonded complexes. Further, the presence of a solute suppresses the dynamic anomalies of water: self-diffusion coefficients for water in the mixture, as well as residence times for jump diffusion reveal an Arrhenius temperature dependence, in contrast to the strong non-Arrhenius behavior in pure water. The density of vibrational states shows a shift of the characteristic librational peak in pure water towards higher energies in the mixture, reflecting hindrance of large amplitude proton motions. These results are compatible with computer simulations which predict longer hydrogen-bond lifetimes ...

Depolarized light scattering of heavy water, and hydrogen bond dynamics

We measured the depolarized Rayleigh scattering of heavy water from 70°C down to -13°C. The spectra were analysed and show a low frequency component. Both the intensity and the width of this line are temperature dependent. The characteristic time associated with the width has an Arrhenius behaviour with an activation energy of 2·5 kcal mol-1. The intensity has a quadratic shape with a maximum around 50°C. We interpret these results as a manifestation of the hydrogen-bond dynamics and propose a mechanism for the breaking of H-bonds through the librational movements. Comparisons are made with theoretical models and molecular dynamics simulations.

The inelasticity correction for liquid water in neutron scattering

The authors show that an empirical method using a polynomial expansion allows one to perform a reliable inelasticity correction for liquid water studied by neutron diffraction, at three different wavelengths. A unique liquid structure factor has been deduced, even in the case of the mixture D2O/H2O (10% wt), and the parameters of the water molecule determined.

Dynamics of water studied by coherent and incoherent inelastic neutron scattering

Abstract This paper reviews the more recent results obtained on the dynamics of water by neutron scattering and shows that some information can be obtained by this technique at the microscopic level of the hydrogen bond. It also accounts for some very recent results obtained with the hydrated protein C-phycocyanin. Incoherent quasi-elastic and inelastic neutron scattering by water has been performed in a temperature range extending to the supercooled state. The analysis of the quasi-elastic spectrum separates two main components and gives two characteristic times, one of them being related to the hydrogen-bond lifetime. The inelastic spectra extend until 600 meV, i.e. covering the intramolecular vibration region, showing for the first time the stretching band. Collective excitations propagating at 3310 m s −1 have been observed by coherent inelastic neutron scattering. This result was predicted by previous computer molecular dynamics simulations of water. The data are interpreted as a manifestation of short wavelength collective modes propagating within patches of highly bonded water molecules, and distinct from the ordinary sound wave.

Structural changes in dipalmitoylphosphatidylcholine bilayer promoted by Ca2+ ions: a small-angle neutron scattering study.

Small-angle neutron scattering (SANS) curves of unilamellar dipalmitoylphosphatidylcholine (DPPC) vesicles in 1-60mM CaCl2 were analyzed using a strip-function model of the phospholipid bilayer. The fraction of Ca2+ ions bound in the DPPC polar head group region was determined using Langmuir adsorption isotherm. In the gel phase, at 20 degrees C, the lipid bilayer thickness, dL, goes through a maximum as a function of CaCl2 concentration (dL=54.4A at approximately 2.5mM of CaCl2). Simultaneously, both the area per DPPC molecule AL, and the number of water molecules nW located in the polar head group region decrease (DeltaAL=AL(DPPC))-AL(DPPC+Ca)=2.3A2 and Deltan=n(W(DPPC))-n(W(DPPC+Ca))=0.8mol/mol at approximately 2.5mM of CaCl2). In the fluid phase, at 60 degrees C, the structural parameters d(L), A(L), and n(W) show evident changes with increasing Ca2+ up to a concentration C(Ca)(2+) < or = 10mM. DPPC bilayers affected by the calcium binding are compared to unilamellar vesicles prepared by extrusion. The structural parameters of DPPC vesicles prepared in 60mM CaCl2 (at 20 and 60 degrees C) are nearly the same as those for unilamellar vesicles without Ca2+.

X-ray and neutron scattering studies of the structure of water in a hydrogel

Abstract The structure of water absorbed in polyHEMA is studied by X-ray diffraction in the vitreous state at 77 K as well as in the liquid state at room temperature. Molecular packing at the level of the first neighbors is found similar to that in bulk water but a distortion is observed at the level of the second neighbors which is attributed to the bending of the H bonds. From the widths of the Bragg peaks of the crystalline ices from the small-angle neutron scattering results, we deduce that the pore size and the interpore distance in the swollen polyHEMA are smaller than 30 A.