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

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

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

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 complement our study by a detailed analysis of water hydrogen bond kinetics, which is intimately coupled to diffusion. Despite the inhomogeneity in structure and translational dynamics inside our nanocages, a single set of well-defined rate constants is sufficient to accurately describe the kinetics of hydrogen bond breaking and formation. We find that once system size effects have been eliminated, the residence times of water molecules inside the coordination shell of a hydrogen bond partner are well correlated to average diffusion constants obtained from the procedure above

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