The structure of water in the hydration shells of small hydrophobic solutes was investigated through molecular dynamics. The results show that a subset of water molecules in the first hydration shell of a nonpolar solute have a significantly enhanced tetrahedrality and a slightly larger number of hydrogen bonds, relative to the molecules in water at room temperature, consistent with the experimentally observed negative excess entropy and increased heat capacity of hydrophobic solutions at room temperature. This ordering results from the rearrangement of a small number of water molecules near the nonpolar solutes that occupy one to two vertices of the enhanced water tetrahedra. Although this structuring is not nearly like that often associated with a literal interpretation of the term "iceberg" in the Frank and Evans iceberg model, it does support a moderate interpretation of this model. Thus, the tetrahedral orientational order of this ensemble of water molecules is comparable to that of liquid water at ~10 °C, although not accompanied by the small contraction of the O-O distance observed in cold water. Further, we show that the structural changes of water in the vicinity of small nonpolar solutes cannot be inferred from the water radial distribution functions, explaining why this increased ordering is not observed through neutron diffraction experiments. The present results restore a molecular view where the slower translational and reorientational dynamics of water near hydrophobic groups has a structural equivalent resembling water at low temperatures.

Despite being intensively studied, the magnitude of specific structural and dynamic perturbations of water next to hydrophobic surfaces remains a matter of debate. Here we show, from molecular dynamics, that the structure of a subset of water molecules in the first hydration layer, those preserving four nearest water neighbors, resembles that of water at ∼10 °C, and that the origin of the orientational slowdown is mainly a decrease of the hydrogen-bond (HB) acceptor switch frequency, while water structuring plays a minor role, slightly accelerating HB acceptor switches. By portraying the mean HB dynamics of water as a doubly periodic event, we demonstrate that the orientational retardation factor is effectively defined by the ratio of the HB acceptor switch period in the hydration layer and bulk. Excluded volume delays HB acceptor switches, accelerating the orientational relaxation of ∼1/3 of the water molecules on the hydration layer in this time scale, but this is largely exceeded by the decrease of the HB switch frequency, consistent with 2D IR spectroscopy experiments, and at the origin of longer HB lifetimes. The orientational mobility of water populations with long HB lifetimes is also probed, and although a relaxation plateau is observed at ∼10 ps consistent with fs IR spectroscopy experiments, no water molecule is rotationally frozen at any time scale. The proposed molecular picture is consistent with fs IR, 2D IR, and NMR experimental results on the orientational retardation of water and reveals the magnitude of "hidden" enhanced ordered water pentamers formed near hydrophobic solutes.

The structure of water in sodium halide aqueous solutions at different concentrations is studied through molecular dynamics. Emphasis is placed on the extent of ionic-induced changes in the water structure, and the concept of kosmotropes/chaotropes is probed, in terms of perturbations to the tetrahedral H-bond network of water. The results show that at low salt concentrations, the halide anions slightly increase the tetrahedrality of the H-bond network of water in the anionic second hydration shell and I(-) is found to be the strongest kosmotrope, contrary to its structure breaker reputation. The sodium cation in turn induces a significant loss of tetrahedrality in the second cationic hydration shell. At higher concentrations, the dominant disruptive effect of Na(+) cancels the anionic effects, even in the anionic second hydration shell. According to a kosmotropes/chaotropes classification of ions, based on the tetrahedrality of the H-bond network of water, halide anions are therefore weak kosmotropes, while Na(+) is a strong chaotrope. However, if this classification is applied to the salts, rather than to the ions, all of the sodium halides are classified as structure breakers even at low concentrations. Further, the effect of pressure on the tetrahedrality of the H-bond network of water is found to be similar to the average effect of the dissolved salts. The present results indicate that the classification of ions in kosmotropes/chaotropes in terms of long-range perturbations to the tetrahedral H-bond network of water is not correlated to the position of the ions in the respective Hofmeister series.

We study the structural and dynamic transformations of SPC/E water with temperature, through molecular dynamics (MD), and discuss the non-Arrhenius behavior of the transport properties and orientational dynamics, and the magnitude of the breakdown of the Stokes-Einstein (SE) and the Stokes-Einstein-Debye (SED) relations, in the light of these transformations. Our results show that deviations from Arrhenius behavior of the self-diffusion at low temperatures cannot be exclusively explained by the reduction of water defects (interstitial waters) and the increase of the local tetrahedrality, thus, suggesting the importance of the slowdown of collective rearrangements. Interestingly we find that at high temperatures (T  ⩾  340 K) water defects lead to a slight increase of the tetrahedrality and a decrease of the self-diffusion, opposite to water at low temperatures. The relative magnitude of the breakdown of the SE and the SED relations is found to be in accord with recent experiments (Dehaoui et al 2015 Proc. Natl Acad. Sci. USA 112 12020) resolving the discrepancy with previous MD results. Further, we show that SPC/E hydrogen-bond (HB) lifetimes deviate from Arrhenious behaviour at low temperatures in contrast with some previous MD studies. This deviation is nevertheless much smaller than that observed for the orientational dynamics and the transport properties of water, consistent with the relaxation times measured by several experimental methods. The HB acceptor exchange dynamics defined here by the acceptor switch and reform (librational dynamics) frequencies exhibit similar Arrhenius deviations, thus explaining to some extent the non-Arrhenius behavior of the transport properties and of the orientational dynamics of water. Our results also show that the fraction of HB switches through a bifurcated pathway follow a power law with the temperature decrease. Thus, at low temperatures HB acceptor switches are less frequent but occur on a faster time scale consistent with the temperature dependence of the ratio of the rotational relaxation times for the different Legendre polynomial ranks.

The structure and dynamics of phospholipid reverse micelles are studied by molecular dynamics. We report all-atom unconstrained simulations of 1,2-dioleoyl-sn-phosphatidylcholine (DOPC) reverse micelles in benzene of increasing sizes, with water-to-surfactant number ratios ranging from W = 1 to 16. The aggregation number, i.e., the number of DOPC molecules per reverse micelle, is determined to fit experimental light-scattering measurements of the reverse micelle diameter. The simulated reverse micelles are found to be approximately spherical. Larger reverse micelles (W > 4) exhibit a layered structure with a water core and the hydration structure of DOPC phosphate head groups is similar to that found in phospholipid membranes. In contrast, the structure of smaller reverse micelles (W ≤ 4) cannot be described as a series of concentric layers successively containing water, surfactant head groups, and surfactant tails, and the head groups are only partly hydrated and frequently present in the core. The dynamics of water molecules within the phospholipid reverse micelles slow down as the reverse micelle size decreases, in agreement with prior studies on AOT and Igepal reverse micelles. However, the average water reorientation dynamics in DOPC reverse micelles is found to be much slower than in AOT and Igepal reverse micelles with the same W ratio. This is explained by the smaller water pool and by the stronger interactions between water and the charged head groups, as confirmed by the red-shift of the computed infrared line shape with decreasing W.

We study the structure of water through molecular dynamics, specifically the compression/expansion of the hydrogen-bond (H-bond) network, with temperature and pressure, and in salt solutions of alkali chlorides and sodium halides, and relate the observed local spatial perturbations with the tetrahedrality and the average number and lifetime of water H-bonds. The effect of transient H-bonds and transient broken H-bonds on H-bond lifetimes is further investigated, and results are compared with depolarized Rayleigh scattering lifetimes for neat water. A significant electrostriction is observed in the first hydration shell of Li(+) and F(-), while larger ions cause a small expansion of the H-bond network of water instead. However, both alkali cations and halide anions induce a minor contraction of the H-bond network in the second hydration shell. Further, water in the second hydration shell of Li(+), Na(+), and K(+) is less tetrahedral than neat water, resembling water at high pressures, while the H-bond network in the respective hydration shell of halide anions resembles water at low temperatures. Nevertheless, neither ion induced H-bond contraction nor expansion can be exactly mapped onto P or T effects on the local structure of water. Even though the average number and lifetime of H-bonds in the ionic hydration shells of most ions are not very different from those found in neat water, Li(+) and F(-) significantly increase the lifetime of water donor and acceptor H-bonds, respectively, in the first hydration shell. The non-monotonic increase of cation and anion mobility, with ionic size, observed experimentally, is interpreted in terms of the water local tetrahedrality around cations and anions.

The thermal conductivity of molten NaCl and KCl was calculated through the Evans-Gillan nonequilibrium molecular dynamics (NEMD) algorithm and Green-Kubo equilibrium molecular dynamics (EMD) simulations. The EMD simulations were performed for a "binary" ionic mixture and the NEMD simulations assumed a pure system for reasons discussed in this work. The cross thermoelectric coefficient obtained from Green-Kubo EMD simulations is discussed in terms of the homogeneous thermoelectric power or Seebeck coefficient of these materials. The thermal conductivity obtained from NEMD simulations is found to be in very good agreement with that obtained through Green-Kubo EMD simulations for a binary ionic mixture. This result points to a possible cancellation between the neglected "partial enthalpy" contribution to the heat flux associated with the interdiffusion of one species through the other and that part of the thermal conductivity related to the coupled fluxes of charge and heat in "binary" ionic mixtures.

We study the temperature dependence of the lifetime of geometric and geometric/energetic water hydrogen-bonds (H-bonds), down to supercooled water, through molecular dynamics. The probability and lifetime of H-bonds that break either by translational or librational motions and those of energetic broken H-bonds, along with the effects of transient broken H-bonds and transient H-bonds, are considered. We show that the fraction of transiently broken energetic H-bonds increases at low temperatures and that this energetic breakdown is caused by oxygen-oxygen electrostatic repulsions upon too small amplitude librations to disrupt geometric H-bonds. Hence, differences between geometric and energetic continuous H-bond lifetimes are associated with large H-bond energy fluctuations, in opposition to moderate geometric fluctuations, within common energetic and geometric H-bond definition thresholds. Exclusion of transient broken H-bonds and transient H-bonds leads to H-bond definition-independent mean lifetimes and activation energies, ~11 kJ/mol, consistent with the reactive flux method and experimental scattering results. Further, we show that power law decay of specific temporal H-bond lifetime probability distributions is associated with librational and translational motions that occur on the time scale (~0.1 ps) of H-bond breaking /re-forming dynamics. While our analysis is diffusion-free, the effect of diffusion on H-bond probability distributions where H-bonds are allowed to break and re-form, switching acceptors in between, is shown to result in neither exponential nor power law decay, similar to the reactive flux correlation function.