Anisotropic Structure and Dynamics of Water under Static Electric Fields

We study the structure and dynamics of water subject to a range of static external electric fields, using molecular dynamics simulations. In particular, we monitor the changes in hydrogen bond kinetics, reorientation dynamics, and translational motions of water molecules. We find that water molecules translate and rotate slower in elec- tric fields, because the tendency to reinstate the aligned orientation reduces the prob- ability of finding a new hydrogen bond partner and hence increases the probability of reforming already ruptured bonds. Furthermore, dipolar alignment of water mole- cules with the field results in structural and dynamic anisotropies even though the angularly averaged metrics indicate only minor structural changes. Through compar- ison of selected nonpolarizable and polarizable water models, we find that the electric field effects are stronger in polarizable water models, where field-enhanced dipole moments and thus more stable hydrogen bonds lead to slower switching of hydrogen bond partners and reduced translational mobility, compared to a nonpolarizable water model

Dynamics at a Janus interface

Electric field effects on water interfacial properties abound, ranging from electrochemical cells to nanofluidic devices to membrane ion channels. On the nanoscale, spontaneous orientational polarization of water couples with field alignment, resulting in an asymmetric wetting behavior of opposing surfacesa field-induced analogue of a chemically generated Janus interface. Using atomistic simulations, we uncover a new and significant field polarity (sign) dependence of the dipolar- orientation polarization dynamics in the hydration layer. Applying electric fields across a nanoparticle, or a nanopore, can lead to close to 2 orders of magnitude difference in response times of water polarization at opposite surfaces. Typical time scales are within the O(10−1) to O(10) picosecond regime. Temporal response to the field change also reveals strong coupling between local polarization and interfacial density relaxations, leading to a nonexponential and in some cases, nonmonotonic response. This work highlights the surprisingly strong asymmetry between reorientational dynamics at surfaces with incoming and outgoing fields, which is even more pronounced than the asymmetry in static properties of a field-induced Janus interface

Multifaceted Water Dynamics in Spherical Nanocages

We present a new method to study position- dependent, anisotropic diffusion tensors inside spherically confined systems—a geometry that is common to many chemical nanoreactors. We use this method to elucidate the surprisingly rich solvent dynamics of confined water. The spatial variation of the strongly anisotropic diffusion predicted by the model agrees with the results of explicit molecular dynamics simulations. The same approach can be directly transferred to the transport of solutes to and from reaction sites located at nanoreactor interfaces. We com- plement our study by a detailed analysis of wa- ter hydrogen bond kinetics, which is intimately coupled to diffusion. Despite the inhomogene- ity in structure and translational dynamics in- side our nanocages, a single set of well-defined rate constants is sufficient to accurately describe the kinetics of hydrogen bond breaking and for- mation. We find that once system size effects have been eliminated, the residence times of wa- ter molecules inside the coordination shell of a hydrogen bond partner are well correlated to average diffusion constants obtained from the procedure above

Hydrogen Bond Dynamics Near A Micellar Surface: Origin of the Universal Slow Relaxation at Complex Aqueous Interfaces

The dynamics of hydrogen bonds among water molecules themselves and with the polar head groups (PHG) at a micellar surface have been investigated by long molecular dynamics simulations. The lifetime of the hydrogen bond between a PHG and a water molecule is found to be much longer than that between any two water molecules, and is likely to be a general feature of hydrophilic surfaces of organized assemblies. Analyses of individual water trajectories suggest that water molecules can remain bound to the micellar surface for more than a hundred picosecond. The activation energy for such a transition from the bound to a free state for the water molecules is estimated to be about 3.5kcal/mole.Comment: 12 pages. Phys. Rev. Lett. (Accepted) (2002

Network equilibration and first-principles liquid water.

Motivated by the very low diffusivity recently found in ab initio simulations of liquid water, we have studied its dependence with temperature, system size, and duration of the simulations. We use ab initio molecular dynamics (AIMD), following the Born-Oppenheimer forces obtained from density-functional theory (DFT). The linear-scaling capability of our method allows the consideration of larger system sizes (up to 128 molecules in this study), even if the main emphasis of this work is in the time scale. We obtain diffusivities that are substantially lower than the experimental values, in agreement with recent findings using similar methods. A fairly good agreement with D(T) experiments is obtained if the simulation temperature is scaled down by approximately 20%. It is still an open question whether the deviation is due to the limited accuracy of present density functionals or to quantum fluctuations, but neither technical approximations (basis set, localization for linear scaling) nor the system size (down to 32 molecules) deteriorate the DFT description in an appreciable way. We find that the need for long equilibration times is consequence of the slow process of rearranging the H-bond network (at least 20 ps at AIMDs room temperature). The diffusivity is observed to be very directly linked to network imperfection. This link does not appear an artifact of the simulations, but a genuine property of liquid water

Short-Range Structural Transformations in Water at High Pressures

We report results of molecular dynamics simulations of liquid water at the temperature T=277 K for a range of high pressure. One aim of the study was to test the model Amoeba potential for description of equilibrium structural properties and dynamical processes in liquid water. The comparison our numerical results with the Amoeba and TIP5P potentials, our results of \emph{ab initio} molecular dynamics simulations and the experimental data reveals that the Amoeba potential reproduces correctly structural properties of the liquid water. Other aim of our work was related with investigation of the pressure induced structural transformations and their influence on the microscopic collective dynamics. We have found that the structural anomaly at the pressure $p_c\approx 2000$ Atm is related with the changes of the local, short-range order in liquid water within first two coordination shells. This anomaly specifies mainly by deformation of the hydrogen-bond network. We also discuss in detail the anomalous behavior of sound propagation in liquid water at high pressures and compare numerical results with the experimental data.Comment: 1 tex-file and 9 figure