Session A04: Water Dynamics in Different Environments: Experiment and Theory I. Interfacial Water

Electric Fields, Reorientation, and Water at the Air-water Interface of Iron and other Salt Solutions

Water at the air-water interface organizes and facilitates the accommodation of ion complexes and that of surface active species. The hydration shells of these surface active components are evident by their spectroscopic signatures mostly in the OH stretching regions using both vibrational sum frequency generation and infrared reflection absorption spectroscopies. In addition, surface potential measurements reveal information on the interfacial electric field strength and thus ordering of molecular and ion complexation dipoles in the interfacial region. We report evidence of interfacial iron (III) speciation and other ions and their interfacial hydration effects. We also report on the resistance to dipole reorientation of ion and molecular species at the air-aqueous interface of iron(III) sulfate and nitrate versus iron(III) chloride.
  

The structure and polarization of the water-graphene interface from molecular dynamics simulations and X-ray reflectivity experiments

Nanconfined water is found in numerous applications across natural and technological systems where water mediates chemical reactions, adsorption, diffusion, ion transport, among other processes. Water affects the interactions among ions, molecules, and surfaces. The electrostatic forces in the systems and processes are modified through polarization which is the collective orientation of the molecules' dipole moment. The polarization of water is different in bulk, in confinement, and at interfaces. Here we study the water structure and polarization next to uncharged graphene surfaces by means of molecular dynamics simulations and X-ray reflectivity experiments. Despite graphene being hydrophobic, our simulations and experiments show an adsorbed water layer on graphene. Even in the absence of an external electric field this adsorbed water layer has a persistent polarization and an induced electrostatic potential. In addition, the adsorbed water molecules form a hydrogen bond network and make the graphene surface repulsive to ions.   

Polarizable Potentials For Metals: The Density Readjusting Embedded Atom Method (DR-EAM)

In simulations of metallic interfaces, a critical aspect of metallic behavior is missing from the some of the most widely used classical molecular dynamics force fields. We present a modification of the embedded atom method (EAM) which allows for electronic polarization of the metal by treating the valence density around each atom as a fluctuating dynamical quantity. The densities are represented by a set of additional fluctuating variables (and their conjugate momenta) which are propagated along with the nuclear coordinates. This “density readjusting EAM” (DR-EAM) preserves nearly all of the useful qualities of traditional EAM. We show that DR-EAM can successfully model polarization in response to external charges, capturing the image charge effect in atomistic simulations. DR-EAM also captures behaviors of metals in the presence of uniform electric fields, predicting surface charging and shielding internal to the metal. Our additional studies of DR-EAM used to model a metal-water interface show local surface ordering of water dipoles due to the polarizability of the metal. We also discuss the interfacial thermal conductivity of the metal-water interface calculated using both polarizable and non-polarizable models for the metal and for water.   

Orientational Dynamics of the Hydrogen-bonded OH of Interfacial Water

We report on the orientational dynamics of the bonded OH (bOH) of interfacial water, i.e., the OH groups in which the H atom participates in a hydrogen bond (HB). A vibrationally resonant infrared-visible sum-frequency signal provides a surface sensitive probe of the dynamics following excitation by a vibrationally resonant infrared pump. Using p-polarized probe beams, we measure the evolution of the pump-induced orientational anisotropy following p- and s-polarized pump excitation.
Previous work showed that the dangling OH (dOH) groups (i.e., OH groups in which the H atom does not participate in a HB) reorient much faster than do bulk bOH[1,2]. Measuring the dynamics of interfacial bOH is challenging, though, on account of strong intermolecular resonant vibrational energy transfer (RVET). Using isotopic dilutions down to 1 part H₂O in 19 parts D₂O, we minimize RVET and show that interfacial bOH reorient on the same timescale as do bulk bOH. These dynamics are robust to interfacial ion concentrations and suggest that the interfacial hydrogen bond network becomes bulk-like within a single molecular distance from the surface.
References:
[1] S.H. Xiao et al, JACS 138, 5551 (2016).
[2] C.S. Hsieh et al, PNAS 110, 18780 (2013).   

Why are Water-Hydrophobe Interfaces Electrically Charged?

Mechanisms underlying the electrification of water-hydrophobe interfaces are of much interest in chemical science, but are not entirely clear. In response, a systematic investigation of excess electrical charges carried by water droplets dispensed from capillaries was performed by: (i) studying the deflections of pendant droplets under uniform electric fields, and (ii) the direct measurement of electrical charges of the dispensed droplets using an ultrasensitive electrometer coupled with a Faraday cup. Thus, the effects of the following crucial factors were unraveled: hydrophobicity/hydrophilicity of the capillary, the presence/absence of a water reservoir inside the capillary, water pH, ionic strength, dielectric constant, the dissolved CO₂ content, and the relative humidity. The emerging picture is that the electrification at interfaces of common hydrophobic materials, e.g., polytetrafluoroethylene and polypropylene, is not limited to interfaces with water alone; nor is this phenomenon entirely dependent on the specific adsorption of OH^- or H₃O⁺ ions, as commonly believed. Our exhaustive study also draws together an extensive body of literature on this subject   

Free Energy of Water Dissociation at the Water - TiO2 Interface from Ab Initio Deep Potential Molecular Dynamics

TiO2 is a widely used photocatalyst in science and technology and its interface with water is important in fields ranging from geochemistry to biomedicine. Yet, it is still unclear whether water adsorbs in molecular or dissociated form on TiO2 even for the case of well-defined crystalline surfaces. To address this issue, we simulated the TiO2-water interface using molecular dynamics with an ab initio-based deep neural network potential. Our simulations show a 6% equilibrium fraction of water dissociation at room temperature, in agreement with enhanced sampling estimates of the dissociation free energy [1]. Due to the relevance of surface hydroxyl groups to the surface chemistry of TiO2, our model might be key to understanding phenomena ranging from surface functionalization to photocatalytic mechanisms.
Refs: [1] Marcos F. Calegari Andrade, Hsin-Yu Ko, Linfeng Zhang, Roberto Car and Annabella Selloni, submitted.   

Laser-Assisted Dissolution of Geological Samples Submerged in Water: Evidence of Hydrothermal Processing

We investigate the use of a 240 ns pulse Nd:YLF (3 kHz, 0.33 mJ/pulse) laser to rapidly break down and dissolve an obsidian sample submerged in water. We propose a mechanism, a hydrothermal surface interaction, by which this laser-assisted dissolution process occurs. The laser fluence is below the ablation thresholds, 0.4-7 J/cm²vs 10 J/cm²and is directed onto the submerged sample causing an increase in temperature and pressure at the water-sample interface. Our results indicate that this process is highly efficient at removing, and potentially dissolving, amorphous glassy substrates (i.e. an obsidian sample) at the water-sample interface. This is further shown by the formation of purified SiO₂structures that reform on the surface post-pulse. We additionally show how various parameters (e.g. dwell time, power, repetition rate, and focal length) affect the efficiency of this process.   

Intrinsic pH of water/vapor interface revealed by ion-induced water alignment

Protons at the water/vapor interface are relevant for atmospheric and environmental processes, yet to characterize their surface affinity on the quantitative level is still challenging. Here we utilize phase-sensitive sum-frequency vibrational spectroscopy to quantify the surface density of protons (or their hydronium form) at the intrinsic water/vapor interface, through inspecting the surface-field-induced alignment of water molecules in the electrical double layer of ions. With hydrogen halides in water, the surface adsorption of protons is found to be independent of specific proton-halide anion interactions and to follow a constant adsorption free energy, ΔG ~ –3.74 (±0.56) kJ/mol, corresponding to a reduction of the surface pH with respect to the bulk value by 0.66 (±0.10), for bulk ion concentrations up to 0.3 M. Our spectroscopic study is not only of importance in atmospheric chemistry, but also offers a microscopic-level basis to develop advanced quantum-mechanical models for molecular simulations.   

The behavior of water confined between two hydrophobic surfaces with grafted polymeric segments studied using molecular simulations

We have performed Indirect Umbrella Sampling (INDUS) molecular simulations to calculate the free energy barrier of evaporation of water confined between two hydrophobic surfaces grafted with polymeric segments. Dependence of the free energy barrier on grafting density and segment flexibility will be discussed. We have calculated the potential of mean force between the two surfaces as a function of the distance between them. The implications of the presence of flexible grafted segments on the hydrophobic collapse of the surfaces will be illustrated.