Water is certainly the most important liquid for life on earth. Moreover, it is also perhaps the liquid with the most puzzling physical properties. One of waters best known anomalies is the decrease of density upon cooling below 4 8C at atmospheric pressure. Other anomalous properties include the sharp increase of specific heat and compressibility upon cooling and supercooling. In addition to these thermodynamic features, the kinetic properties are also unusual, as the increase of diffusivity and decrease of viscosity upon compression indicate. [1,2] Adding solutes considerably broadens the spectrum of observed effects. For this reason the structure and dynamics of water in the vicinity of solutes have been studied for decades. The structure-making (kosmotrope) and structure-breaking (chaotrope) influence of ions on the hydration water has been understood as emerging from a balance between the water-water and ion-water interactions, which varies considerably with the charge density on the solute surface. [3][4][5][6][7][8][9] Lebermann and Soper used neutron diffraction to compare the effects of applied pressure and high salt concentrations on the hydrogen-bonding network of water. They found that the ions induce a change in structure equivalent to the application of high pressures, and that the size of the effect is ion-specific.[10] Similar effects have been reported by Botti et al., [11,12] who studied the solvation shells of H + and OH À ions in water. Mancinelli et al. could show that the structural perturbation due to monovalent ions (in aqueous solutions of NaCl and KCl) exists outside the first hydration shell of the ions.[13] Their study emphasized longer-ranged ion-induced perturbation and related shrinkages of the second and third coordination shells of water molecules, while the first hydration shell is largely unchanged. The O-O pair correlation function of water was modified by the ions in a manner closely analogous to what happens in pure water under pressure. In contrast, recent molecular dynamics (MD) simulations of aqueous CaCl 2 solutions indicate unequivocally that the changes of the water structure caused by the presence of ions in solution cannot be emulated as a pressure effect owing to the local nature of such a structure perturbation. [14] Growing evidence is emerging that the anomalous behavior of water and aqueous solutions is closely related to the existence of at least two major distinct local structural forms of water. [15,16] At low temperatures and at low to moderate pressures, water approaches a low-density liquid (LDL) state, which exhibits an almost perfectly interconnected random tetrahedral network. In the LDL state water has an average of four nearest neighbors, similar to the situation in ice I h (see Figure 1 a). Close to the low-density structure the mobility of water strongly slows down. [17][18][19][20][21][22] MD simulations suggest that defects in the random tetrahedral hydrogen-bonding network show fivefold water coordination. [18,22] With increasing pressure, these d...

Mercury is a priority pollutant as its mobility between the hydrosphere and the atmosphere threatens the biosphere globally. The air-water gas transfer of elemental mercury (Hg0) is controlled by its diffusion through the water-side boundary layer and thus by its diffusion coefficient, D(Hg), the value of which, however, has not been established. Here, the diffusion of Hg0 in water was modeled by molecular dynamics (MD) simulation and the diffusion coefficient subsequently determined. Therefore the movement of either Hg(0) or xenon and 1000 model water molecules (TIP4P-Ew) were traced for time spans of 50 ns. The modeled D(Xe) of the monatomic noble gas agreed well with measured data; thus, MD simulation was assumed to be a reliable approach to determine D(Hg) for monatomic Hg(0) as well. Accordingly, Hg(0) diffusion was then simulated for freshwater and seawater, and the data were well-described by the equation of Eyring. The activation energies for the diffusion of Hg0 in freshwater was 17.0 kJ mol(-1) and in seawater 17.8 kJ mol(-1). The newly determined D(Hg) is clearly lower than the one previously used for an oceanic mercury budget. Thus, its incorporation into the model should lead to lower estimates of global ocean mercury emissions.

We study aqueous solutions of alkaline chlorides (NaCl, KCl, RbCl and CsCl) with a combination of attenuated total reflection infrared (ATR-IR) spectroscopy and molecular dynamics (MD) simulations using the TIP4P-Ew water model (Horn et al., J. Chem. Phys. 2004, 120, 9665), covering concentration ranges between 0.1 and 6 M. Spectral modifications in the OH stretch region are evaluated and correlated to the various salts and salt concentrations. By taking the difference spectra between the spectral line shapes of aqueous salt solutions and those of pure water, we specifically focus on the small quasi-"free OH" band appearing at the highest wavenumbers in the spectra. This free-OH feature is found constantly at 3650 cm(-1) for all salts and salt concentrations, but it shows a characteristic intensity depending on the chosen cation. In the order from Na(+) to Cs(+), the free OH intensity decreases compared to that of pure water. To interpret the experimental results, we performed MD simulations for similar salt solutions. The experimentally observed effects can be correlated with structural alterations indicated by differences between the site-site pair correlation functions of water in aqueous salt solutions and those of pure water.

We perform molecular dynamics (MD) simulations of aqueous salt (NaCl) solutions using the TIP4P-Ew water model (Horn et al., J. Chem. Phys. 2004, 120, 9665) covering broad temperature and concentration ranges extending deeply into the supercooled region. In particular we study the effect of temperature and salt concentration on the solvation of methane at infinite dilution. The salt effect on methane's solvation free energy, solvation enthalpy and entropy, as well as their temperature dependence is found to be semi-quantitatively in accordance with the data of Ben-Naim and Yaacobi (J. Phys. Chem. 1974, 78, 170). To distinguish the influence of local (in close proximity to ions) and global effects, we partition the salt solutions into ion influenced hydration shell regions and bulk water. The chemical potential of methane is systematically affected by the presence of salt in both sub volumes, emphasizing the importance of the global volume contraction due to electrostriction effects. This observation is correlated with systematic structural alterations similar to water under pressure. The observed electrostriction effects are found to become increasingly pronounced under cold (supercooled) conditions. We find that the influence of temperature and salt induced global density changes on the solvation properties of methane is well recovered by simple scaling relation based on predictions of the information theory model of Garde et al. (Phys. Rev. Let. 1999, 77, 4966).

Die „freie Wasserphase“ in einer wässrigen Salzlösung verhält sich bezüglich thermischer Ausdehnung, Dynamik und lokaler Struktur ähnlich wie reines Wasser unter Druck. Moleküldynamiksimulationen geben das anomale Verhalten von Wasser und wässriger Salzlösungen nahezu quantitativ wieder. Im unterkühlten Bereich stabilisieren Ionen die „hochdichte“ Konfiguration des Wassers (siehe Bild; H weiß, O rot, blau: H‐Brücken).

The unusual increase of diffusivity of supercooled water upon compression seems to be related to the existence of at least two major distinct local structural forms: a "low-density" structure, exhibiting an almost perfect tetrahedral hydrogen bonding network, and a "high-density" structure, characterized by defects in this network. The structural changes can be measured by the "tetrahedricity parameter", describing the deviation from the ideal tetrahedron. In this paper we show that the anomalous diffusion behavior upon compression cannot only be related to the structural heterogeneities but also to dynamical heterogeneities occuring in supercooled liquid water. This is shown for translational heterogeneities which decrease with temperature and pressure. Both, static and dynamic heterogeneities can be correlated. They are substantial at low temperatures and moderate pressures and diminish with increasing temperature and pressure, respectively. Our results are based on molecular dynamics simulations of the TIP4P-Ew water model [1].

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The cover picture shows water molecules in different hydration shells at the surface of the model protein ubiquitin. The properties of proteins and the role of solvent in conformational dynamics is a central topic to the DFG-Foschergruppe (FOR 436) which forms the basis of this special issue. The polymorphism, dynamics and function of water at molecular interfaces is discussed with contributions from R.