Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session K57: Physics of Liquids IFocus
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Sponsoring Units: GSOFT DCP GSNP Chair: Yang Zhang, University of Illinois at Urbana–Champaign Room: LACC 518 |
Wednesday, March 7, 2018 8:00AM - 8:12AM |
K57.00001: Water Hydrogen Bond Dynamics under Electric Fiel Mohammadmahdi Shafiei Alavijeh, Dusan Bratko, Alenka Luzar An external electric field can change the properties of water enabling numerous applications in science and engineering. Using Molecular Dynamics simulations and Reactive Flux Correlation Function approach we studied the hydrogen bond dynamics and it’s coupling to translational diffusion in the presence of external electric fields. Water dipoles tend to be aligned with the field direction. The alignment rapidly increases with the field strengths below 0.05 V/Å, but water molecules continue to exchange bonding partners at an only slightly reduced rate. At fields stronger than 0.05V/Å, the alignment with the field approaches saturation, but the H-bond exchanging process is more affected, showing a slowdown of bond breaking and increased probability of bond reforming in strong fields. We quantify the change in hydrogen bond forming and breaking rates to be close to 15%, the change in self-diffusion coefficient up to 30% in the field direction, and up to 19% when averaged over all three directions. |
Wednesday, March 7, 2018 8:12AM - 8:24AM |
K57.00002: Isotope effects on van-Hove functions for liquid H2O and D2O by inelastic X-ray scattering Takuya Iwashita, Bin Wu, Wei-Ren Chen, Alfred Baron, Takeshi Egami Dynamics of liquid water at the molecular level is crucial to the understanding of transport properties of water, including viscosity. Despite intensive efforts over the past decades the molecular level dynamics has not been fully understood because structural relaxation and its associated hydrogen-bond dynamics have not been clearly identified. To study the structural relaxation the real-space/real-time analysis of inelastic X-ray scattering data on liquid water was carried out. We discovered a clear distinction in structural rearrangements between liquid H2O and D2O and showed that the characteristic time at which the first and second peaks in van-Hove function merged is associated with viscosity or local configurational excitations where an atom loses or gains one neighbor. This result also demonstrated the importance of real-space analysis in studying the local dynamics in the liquid. |
Wednesday, March 7, 2018 8:24AM - 8:36AM |
K57.00003: Molecular Motion of Water in Real Space and Time observed with Inelastic X-ray Scattering Yuya Shinohara, Wojciech Dmowski, Takuya Iwashita, Bin Wu, Daisuke Ishigawa, Alfred Baron, Takeshi Egami We report on real-space and real-time molecular motions of water and its temperature variation at pico-second and sub-nm scales in time and space using van Hove function of water molecules. Measurement of high-resolution inelastic x-ray scattering spectra, S(Q, E), over wide Q (momentum transfer) and E (energy), enables us to determine the van Hove function by Fourier transform. The results show that the dynamics of first and second nearest neighbor molecules are strongly coupled, and that the topological rearrangement of configurations shows slowing-down with decreasing temperature. By comparing the results with molecular dynamics simulation, we find that the time-scale of local topological changes in molecular configuration corresponds to the Maxwell relaxation time, which is determined by macroscopic measurement of viscosity and shear modulus. This supports our earlier findings that the topological changes in molecular connections is the origin of viscosity in liquid. |
Wednesday, March 7, 2018 8:36AM - 8:48AM |
K57.00004: Effects of Strong Electrostatic Correlation on the Solvation Energy of Ions: Comparison of the Modified Born Energy with Experiments Issei Nakamura The Born solvation energy of the ions immersed in liquids is invoked in a vast literature, but it is often inadequate to explain experimental data without adjusting the model parameters, such as the ionic radius and the dielectric constant of solvents. To improve this situation, we have developed a coarse-grained theory of ion solvation in liquids, which draws upon a Ginzburg-Landau type of theory. Our theory accounts simultaneously for the dielectric inhomogeneity and strong electrostatic correlation near the ions. Our result modifies the conventional Born solvation energy using a model parameter that describes the electrostatic correlation length. We show that the outstanding consistency between the theory and experiment--without assuming hypothetical ionic radii in cases of various ions, pure liquids, and even liquid mixtures--is remarkable. Examples include eight different liquids with twelve monovalent, fifteen divalent, and ten trivalent ions. We also mention the application of our theory to the phase instability of a mixture of a polymer and an ionic liquid. |
Wednesday, March 7, 2018 8:48AM - 9:00AM |
K57.00005: Entropy-driven Two-step Nucleation of a Clathrate Colloidal Crystal via a Metastable Fluid-Fluid Phase Transition Sangmin Lee, Michael Engel, Sharon Glotzer Tetrahedral bonding is known to be key to metastable liquid-liquid phase transitions in water, silica, and elements of the carbon family. There, the intermediate liquid phase is believed to be responsible for subsequent nucleation of the crystal. Here we present the first report of an entropy-driven fluid of hard particles following a two-step nucleation pathway mediated by a metastable fluid-fluid phase transition. Monte Carlo simulations reveals that the fluid separates into a low density phase and a high density fluid phase that possesses locally ordered structures mappable to a tetrahedral network (step 1). In turn, a clathrate-like crystal nucleates and grows at the interface of the two fluid phases (step 2). The structural, thermodynamic, and dynamic discontinuities are observed across the fluid-fluid phase transition, and a structural order parameter is developed to clarify each crystallization step. Our observations suggest that the existence of tetrahedral order is the primary reason for the pathway and kinetics of the complex phase behavior, regardless of the origin of the tetrahedral bonding. |
Wednesday, March 7, 2018 9:00AM - 9:12AM |
K57.00006: Molecular dynamics simulations of the thermal conductivity of fluids Marcello Puligheddu, Giulia Galli We present a method to compute the thermal conductivity of fluids using molecular dynamics (MD) simulations in the presence of steady, non-homogenous temperature profiles. The method is a generalization of the approach to equilibrium MD recently applied to solids using empirical potentials and density functional theory (DFT)[1]. Our formulation does not require the definition and calculation of energy densities and has much less stringent requirements, in terms of size and simulation length, than non-Equilibrium MD. We present results for a Lennard-Jones fluid and liquid water using empirical potentials and preliminary results using DFT. |
Wednesday, March 7, 2018 9:12AM - 9:48AM |
K57.00007: The microscopic structural origin of water's anomalies Invited Speaker: Hajime Tanaka Water displays a vast array of unique properties, known as water's anomalies, whose origin remains subject to hot debate. It's now widely believed that the thermodynamic and dynamic anomalies have different origins, criticality associated with the Widom line and glass transition respectively. Contrary to this dual explanation, here we provide the first unified microscopic physical picture of the water's anomalies in terms of locally favored structures. This common structural origin has a hierarchical impact on the anomalies, where thermodynamic and dynamic ones are affected by the underlying structure only at different length scales. Thermodynamic anomalies are described by a microscopic order parameter, which is the translational order of the second shell, while dynamic ones by the same order parameter after spatial coarse-graining up to neighbors, since the dynamics of a molecule is not determined locally but under the influence of its nearest-neighbor environment. To incorporate this, we develop a novel hierarchical two-state model. We show by extensive simulations of two popular water models that both thermodynamic and kinetic anomalies can be almost perfectly explained by the temperature and pressure dependence of these static and dynamic order parameters respectively. |
Wednesday, March 7, 2018 9:48AM - 10:00AM |
K57.00008: Local Electric Fields and Chemical Environment at the Electrode-Ionic Liquid Interface Measured via Vibrational Sum Frequency Generation. Joel Patrow, Yi Wang, Jahan Dawlaty The conductor-electrocyte interface is the main stage for all electrochemical catalysis. At this junction, interfacial electric fields are intimately related to molecular structure and, therefore, influence electrochemistry. To understand this relation, it is necessary to directly probe the local electric field and chemical environment of an interface. We use a well-known vibrational chromophore both as a Stark shift reporter of electric fields and as a probe of the chemical environment present at the electrode-ionic liquid interface. In both cases, we monitor vibrational frequency changes of the chromophore using vibrational sum frequency generation (VSFG). In the first case, we show that electric field strengths of approximately 19 MV/cm are present at the interface. Additionally, we will examine how the ionic liquid anion size affects these fields. In the second case, we will examine how the the addition of water, a common proton source used in ionic liquid electrochemistry, changes the ionic liquid structure at the interface. |
Wednesday, March 7, 2018 10:00AM - 10:12AM |
K57.00009: Refined potential for LiTFSI-Acetonitrile electrolyte system Zhixia Li, Lily Robertson, Ilya Shkrob, Yu Cao, Joerg Neuefeind, Lu Zhang, Jeffrey Moore, Yang Zhang LiTFSI-Acetonitrile electrolytes have been widely investigated for a variety of applications due to their outstanding properties, such as high conductivity and excellent stability. Molecular Dynamics (MD) simulation is an important technique to elucidate key mechanisms at the molecular level, but depends critically on the priori force field. However, only structure was included in previous development of force field. Herein, we report both structure and dynamics need to be considered when developing a force field according to MD simulation results using parameters from different quantum chemistry calculations. All the force fields parameter yield structures in good agreement with the neutron PDF but dramatically different dynamics. In the extreme case, the simulated self-diffusion coefficient can be one order of magnitude different from the NMR measurement. Furthermore, reducing the charge of oxygen in TFSI and lithium ions yield both structures and dynamics in good agreement experiments. These results suggest that although the force field gives the correct structure, the dynamics may be completely wrong. Therefore, dynamics must be also considered when developing a force field. |
Wednesday, March 7, 2018 10:12AM - 10:24AM |
K57.00010: Slow relaxation and boson peak studies of glass-forming ionic liquids at low temperatures Thamires Lima, Zhixia Li, Mauro Ribeiro, Yang Zhang The relaxation processes and boson peak (BP) behavior of two ionic liquids were investigated by means of QENS and INS at different temperatures (180 K – 300 K) and wave vector transfers (0.3 A-1 – 2.0 A-1), aiming to investigate the spatially heterogeneously dynamic contribution to the glass and melting transitions, and their relationship with the BP. The samples, tributylmethylammonium bis(trifluoromethylsulfonyl)imide and trimethylbutylammonium bis(trifluoromethylsulfonyl) were studied under heating from the glass to the liquid state after an almost instantaneous cooling to 180 K. |
Wednesday, March 7, 2018 10:24AM - 10:36AM |
K57.00011: Activated Relaxation and Transport in Liquids Well Above Tm Marcus Cicerone Microscopic molecular theories of supercooled liquids and glasses often posit the presence of long-lived structures in these systems. Such structures are required to produce observed phenomenology such as spatially heterogeneous dynamics. We find evidence for such structures not only in supercooled liquids, but in simple liquids far above the melting temperature. We present neutron scattering [1, 2], simulation [3], and ultrafast optical experiments showing that these structures and associated dynamic heterogeneities arise at much higher temperature than is typically expected. We also show that they play a very important role in overall transport and relaxation, even in the liquid state. |
Wednesday, March 7, 2018 10:36AM - 10:48AM |
K57.00012: Comparison and Validation of Recent Exchange-Correlation Functionals for First-Principles Simulations of Water Michael LaCount, Francois Gygi We present results of first-principles molecular dynamics simulations of liquid water obtained with the recently proposed SCAN density functional[1]. Results are compared with those obtained with the PBE0 hybrid density functional. Estimates of the variance of pair correlation functions, number of hydrogen bonds and angular correlations are derived in order to determine the duration of simulations needed for statistically significant comparisons. |
Wednesday, March 7, 2018 10:48AM - 11:00AM |
K57.00013: Effects of van der Waals interactions on the hydration of Hofmeister ions (Ca2+, K+, Na+) by ab initio molecular dynamics simulations Liying Zhou, Zhaoru Sun, Limei Xu, Xifan Wu The van der Waals (vdW) interaction is an important physical effect in the description of liquid water. Based on the ab initio molecular dynamics simulations by including vdW interaction through the self-consistent density-dependent dispersion implementation proposed by Tkatchenko-Scheffler, we have systematically studied the influence of vdW interaction on the hydration structure of Ca2+, K+ and Na+ belonging to the Hofmeister series, which are of vital importance to a wide range of biological systems. Similar to liquid water, the H-bond structures of all the ion solutions are found to be significantly softened under the influence of vdW interactions. Embedded in such weakened H-bond network, the hydrophilic propensity of the ions is systematically enhanced as evidenced by the slightly increased coordination number for all three ions, which is facilitated by the increased interstitial water populations by the vdW interactions. |
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