Local order in aqueous NaCl solutions and pure water: X-ray scattering and molecular dynamics simulations study

The microstructures of pure water and aqueous NaCl solutions over a wide range of salt concentrations (0-4 m) under ambient conditions are characterized by X-ray scattering and molecular dynamics (MD) simulations. MD simulations are performed with the rigid SPC water model as a solvent, while the ions are treated as charged Lennard-Jones particles. Simulated data show that the first peaks in the O...O and O...H pair correlation functions clearly decrease in height with increasing salt concentration. Simultaneously, the location of the second O...O peak, the signature of the so-called tetrahedral structure of water, gradually disappears. Consequently, the degree of hydrogen bonding in liquid water decreases when compared to pure fluid. MD results also show that the hydration number around the cation decreases as the salt concentration increases, which is most likely because some water molecules in the first hydration shell are occasionally substituted by chlorine. In addition, the fraction of contact ion pairs increases and that of solvent-separated ion pairs decreases. Experimental data are analyzed to deduce the structure factors and the pair correlation functions of each system. X-ray results clearly show a perturbation of the association structure of the solvent and highlight the appearance of new interactions between ions and water. A model of intermolecular arrangement via MD results is then proposed to describe the local order in each system, as deduced from X-ray scattering data.     

Structural studies of water in hydrophilic and hydrophobic mesoporous silicas: an x-ray and neutron diffraction study at 297 K

Water confined in a sol-gel network has been characterized by x-ray and neutron diffraction for two samples of mesoporous silica: one with a hydrophilic character (a nonmodified one) and another with a hydrophobic character (a modified one with a methylated internal pore surface). The pore size has been previously characterized [J. Jelassi et al., Phys. Chem. Chem. Phys. 134, 1039 (2010)] to have a mean pore diameter of approximately 55 A?. The diffraction measurements presented in this paper have been made at room temperature [293 K] for a filling factor of 0.45, giving a mean thickness of 8-9 A? for the water layer. The results show that the local order of the confined water molecules in the intermediate region of 3-6 A? is significantly different from that of the bulk water and also for the two different environments. For the hydrophilic sample, the siloxyl groups at the surface modify the water structure through the effects of interfacial hydrogen-bonding, which influences the orientational configuration of local water molecules and creates a modified spatial arrangement in the pore. In the case of the hydrophobic sample, there is no specific interaction with the pore wall, which is primarily van der Waals type, and the water molecules at the interface are differently oriented to create a hydrogen-bonded network linked more directly to the rest of the water volume. In the present circumstances, the thickness of the water layer has a relatively small dimension so that the interpretation of the measured diffraction pattern is not as straightforward as for the bulk liquids, and it is necessary to consider the effects of diffraction-broadening from a distributed sample volume and also the contribution from cross-terms that remain after conducting a "wet-minus-dry" analysis procedure. These analytic difficulties are discussed in the context of the present measurements and compared with the work of other groups engaged in the study of water confined in different environments. The present results, again, emphasize the complexity influencing the properties of water in a confined geometry and the strong influence of surface interactions on its behavior.     

Interlayer water molecules in vanadium pentoxide hydrate. IX. Anisotropic translational diffusion leading to anisotropic ac conductivity

The anisotropy of the dynamic properties of interlayer water molecules along the a and b axes of vanadium pentoxide hydrate, orthorhombic V2O5.nH2O, was studied using quasielastic neutron scattering (QENS) in relation to the anisotropy of the ac conductivity. The QENS spectra were analyzed using a stretched exponential function and a Lorentzian function. Both methods showed that the double-layer water molecules along the b axis are more mobile than those along the a axis. The difference in mobility between the two axes is more pronounced using a Lorentzian function analysis. These facts suggest that the diffusion coefficient of water molecules along the b axis is larger than that along the a axis, which is closely related to the ac conductivity originating from proton hopping. The anisotropy of the dynamic motion of water molecules can be attributed to the shorter b-axis length (b=3.60 A), with respect to the longer and less regular repetition of the atomic arrangements along the a axis (42.34 A).     

Studies of cavitation and ice nucleation in 'doubly-metastable' water: time-lapse photography and neutron diffraction

High-speed photographic studies and neutron diffraction measurements have been made of water under tension in a Berthelot tube. Liquid water was cooled below the normal ice-nucleation temperature and was in a doubly-metastable state prior to a collapse of the liquid state. This transition was accompanied by an exothermic heat release corresponding with the rapid production of a solid phase nucleated by cavitation. Photographic techniques have been used to observe the phase transition over short time scales in which a solidification front is observed to propagate through the sample. Significantly, other images at a shorter time interval reveal the prior formation of cavitation bubbles at the beginning of the process. The ice-nucleation process is explained in terms of a mechanism involving hydrodynamically-induced changes in tension in supercooled water in the near vicinity of an expanding cavitation bubble. Previous explanations have attributed the nucleation of the solid phase to the production of high positive pressures. Corresponding results are presented which show the initial neutron diffraction pattern after ice-nucleation. The observed pattern does not exhibit the usual crystalline pattern of hexagonal ice [I(h)] that is formed under ambient conditions, but indicates the presence of other ice forms. The composite features can be attributed to a mixture of amorphous ice, ice-I(h)/I(c) and the high-pressure form, ice-III, and the diffraction pattern continues to evolve over a time period of about an hour.     

Hydration water rotational motion as a source of configurational entropy driving protein dynamics. Crossovers at 150 and 220 K

The existence of a protein dynamic transition around 220 K is widely known and the central role of the protein hydration shell is now largely recognized as the driving force for this transition. In this paper, we propose a mechanism, at the molecular level, for the contribution of hydration water. In particular, we identify the key importance of rotational motion of the hydration water as a source of configurational entropy triggering (i) the 220 K protein dynamic crossover (the so-called dynamic transition) but also (ii) a much less intense and scarcely reported protein dynamic crossover, associated to a calorimetric glass transition, at 150 K.     

Thermodynamic, structural, and dynamic properties of supercooled water confined in mesoporous MCM-41 studied with calorimetric, neutron diffraction, and neutron spin echo measurements

Thermodynamic, structural, and dynamic properties of heavy water (D(2)O) confined in mesoporous silica glass MCM-41 C10, C12, and C14 were investigated by differential scanning calorimetry, neutron diffraction, and neutron spin echo (NSE) measurements, respectively. The DSC data showed that no crystallization of D(2)O confined in C10 occurs in a temperature range between 298 and 180 K, and that crystalline ice is formed at 204 and 221 K for C12 and C14, respectively. For C10, the neutron radial distribution functions of confined D(2)O suggested a structural change in the supercooled state between 223 and 173 K. For C10 sample, it has been found that the tetrahedral-like water structure is partially enhanced in the central part of pores at 173 K. For all the samples, the intermediate scattering functions from the NSE measurements are fitted by the Kohlrausch-Williams-Watts stretched exponential function which implies that confined supercooled D(2)O exhibits a wide distribution of relaxation times. For C10, C12, and C14 samples, between 298 and 240 K, the relaxation times of supercooled D(2)O follow remarkably well the Vogel-Fulcher-Tamman equation; for C10 sample, below 240 K, the relaxation times of nonfreezing D(2)O show an Arrhenius type behavior. From the present experimental results on calorimetric, structural, and dynamic properties, it has been concluded that supercooled D(2)O confined in MCM-41 C10 experiences a transition from high-density to low-density hydrogen-bonded structure at around 229 K.