Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Z2: Condensed-Phase Dynamics, Solvation, and Statistical Mechanics |
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Sponsoring Units: DCP Chair: Gilbert Nathanson, University of Wisconsin-Madison Room: 102 |
Friday, March 7, 2014 11:15AM - 11:27AM |
Z2.00001: Solvated Hydroxide and Hydronium in Water Studied by ab initio Molecular Dynamics Based on PBE0 Hybrid Functional with van der Waals Interation Lixin Zheng, Charles Swartz, Xifan Wu The nature of the solvation structures of hydroxide (OH$^{-}$) and hydronium (H$_{3}$O$^{+}$) aqueous solutions is of fundamental interest. It is the prerequisite to understanding the mechanism of proton transfer (PT) through the autoprotolysis process in water. For a long time, ab initio calculations based on local or semi-local approximations have been used to study the solvated OH$^{-}$ and H$_{3}$O$^{+}$. Although successfully giving a qualitative description of the proton mechanism, the semi-local functional suffers from the delocalization error. In addition, the description of dispersion force is missing. To overcome the above errors, we perform ab initio molecular dynamics calculations in both solvated OH$^{-}$ and H$_{3}$O$^{+}$ based on PBE0 hybrid density functional and include the vander Waals interactions. It is found that, compared to the semi-local functional, the proton transfer rate in solvated OH$^{-}$ reduces due to the changes in the solvation structure. [Preview Abstract] |
Friday, March 7, 2014 11:27AM - 11:39AM |
Z2.00002: Computational Modeling of Actinide Ions in Aqueous Solution Raymond Atta-Fynn Unraveling the chemical behavior of actinide species is difficult owing to the complex electronic structure of these species, the fact that many of these species can occur in multiple oxidation states, and the difficulties encountered in their experimental studies. First principles dynamical modeling, although computationally costly, allows us to gain rich insights into the behavior of actinide species. In this talk, we present results of the hydration shell structure and x-ray absorption spectra of aqueous actinides in different oxidation states including U(VI), U(V), U(IV), and Cm(III) using relativistic~\textit{ab initio }molecular dynamics at 300 K. We also probed the thermodynamics of hydrolysis by calculating the first acidity constant for uranium in all three oxidation states (IV, V, and VI). We predicted, for the first time, that UO$_{2}^{+}$ is a weak acid in solution with a pKa value of 8.5. This result is particularly important since no thermodynamic data are available for hydrolyzed species of U(V). In our most recent work on concentrated Cm(III) solutions, we showed that counter-ions can strengthen or weaken the solvent structure itself rather than just the water coordination number. These new results are better explained in terms of the hydrogen bond lifetimes of the solvents. [Preview Abstract] |
Friday, March 7, 2014 11:39AM - 11:51AM |
Z2.00003: A new method for studying nucleation in aqueous environment Raffaela Cabriolu, Tianshu Li Probing nucleation of crystals in aqueous environment at the molecular level poses a major challenge in molecular simulations. The challenges are primarily attributed to two types of slow dynamics ubiquitously present in the nucleation process in aqueous environment: the rare event nature of nucleation and the sluggish dynamics of hydrogen-bond network. While advanced sampling method, such as transition path sampling or forward flux sampling, allows surpassing free energy barrier efficiently, it is unlikely to alleviate the second type of slow dynamics. Here we propose a new approach that combines Monte Carlo and molecular dynamics methods, to overcome the slow natural dynamics of tetrahedral bond network. The new approach exploits the flexibility of Monte Carlo trial moves and the collective motion of molecular dynamics. Recognizing the structural similarities between silicon and water (both belonging to ``tetrahedral materials''), we incorporate the bond-switching Monte Carlo move that was developed for generating random-network models for amorphous silicon, into our approach. The approach will allow modeling nucleation of ice or hydrates based on full atomistic water model. [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:03PM |
Z2.00004: On the Consequences of Clausius-Duhem Inequality for Electrolyte Solutions Martina Reis, Adalberto Bono Maurizio Sacchi Bassi Based on the fundamentals of thermo-statics, non-equilibrium thermodynamics theories frequently employ an entropy inequality, where the entropy flux is collinear to the heat flux, and the entropy supply is proportional to the energy supply. Although this assumption is suitable for many material bodies, e.g. heat-conducting viscous fluids, there is a class of materials for which these assumptions are not valid. By assuming that the entropy flux and the entropy supply are constitutive quantities, in this work it is demonstrated that the entropy flux for a reacting ionic mixture of non-volatile solutes presents a non-collinear term due to the diffusive fluxes. The consequences of the collinearity between the entropy flux and the heat flux, as well as the proportionality of the entropy supply and the energy supply on the stability of chemical systems are also investigated. Furthermore, by considering an electrolyte solution of non-volatile solutes in phase equilibrium with water vapor, and the constitutive nature of the entropy flux, the stability of a vapor-electrolyte solution interface is studied. Despite this work only deals with electrolyte solutions, the results presented can be easily extended to more complex chemical reacting systems. [Preview Abstract] |
Friday, March 7, 2014 12:03PM - 12:15PM |
Z2.00005: Putting water on a lattice: The importance of long wavelength density fluctuations in theories of hydrophobic and interfacial phenomena. Suriyanarayanan Vaikuntanathan, Phillip Geissler The physics of air-water interfaces plays a central role in modern theories of the hydrophobic effect such as the Lum-Chandler-Weeks (LCW) theory. Implementing these theories, however, has been hampered by the difficulty of addressing fluctuations in the shape of such soft interfaces. We show that this challenge is a fundamental consequence of mapping long wavelength density variations onto discrete degrees of freedom. Specifically, through an analysis of the lattice gas model and related approximations, we identify a narrow parameter regime in which the lattice gas model can optimally be used to describe long wavelength liquid density fluctuations such as the capillary modes at a liquid-vapor interface. Coupling fluctuations in the lattice model to fluctuations on finer molecular scales through the least complicated realization of the LCW perspective, we obtain an effective Hamiltonian for lattice occupation variables in the presence of a hydrophobic solute. We show that this Hamiltonian - with no unknown parameters - in fact suffices to describe quantitatively the the solvation of hydrophobic objects with various shapes and sizes. This model is uniquely well suited for exploring hydrophobic and interfacial phenomena that involve disparate length scales. [Preview Abstract] |
Friday, March 7, 2014 12:15PM - 12:27PM |
Z2.00006: Effetively trapping air or lqiud water for anti-icing applications Jianjun Wang Icing on solid surfaces leads to operational difficulties and high maintenance efforts for power networks, aircrafts, ships, ground transportation vehicles and house-hold refrigerators, to name but a few. In extreme cases, icing on surfaces causes disastrous events such as crash of aircrafts and collapse of power networks, which result in severe economic impact and large loss of life. This talk is focused on the fundamentals of the ice formation and adhesion of ice with solid substrates aiming for fighting against icing on solid surfaces. When the supercooling is low, it would be possible to remove supercooled liquid water from the solid surfaces before freezing occurs. To achieve this, we design and constructed surfaces that can trap the air at the subfreezing temperature thus condensed water microdroplets could be spontaneously removed after the coalescence. When the supercooling is high, icing on surfaces occurs spontaniously. In this case, we constructed coatings on which aqueous lubricating layer could be trapped, thus the ice adhesion on the coating is so low that the ice formed atop could be removed by a wind action or its own gravity. [Preview Abstract] |
Friday, March 7, 2014 12:27PM - 12:39PM |
Z2.00007: Stretched-water equation of state at low temperatures Michael Santiago, Vinay Pagay, David Sessoms, Abraham Stroock Liquids can exist at negative pressure, a metastable state analogous to stretched rubber. This stretched state is ubiquitous. For instance, stretched water exists naturally in many plants, semi dry soils and porous materials, even food. Yet measurements of stretched water's thermodynamic properties are lacking because of experimental challenges. Such measurements could elucidate water's behavior in these settings, unfold the origins of water's anomalies, and perhaps provide conclusive evidence on whether a liquid-liquid critical point exists in highly supercooled water. Here we present the first isothermal measurements of the equation of state (EoS) of stretched water below room temperature, comparing them with predictions of existing theoretical models. For these measurements, we developed a microfabricated sensor that directly measures the pressure in a macroscopic volume of stretched water at known chemical potential and temperature. This sensor uses the metastable vapor liquid equilibrium technique to stretch the water, and is able to reach pressures down to -33 Mpa. Our results agree with the IAPWS EoS at 20 $^{\circ}$C and 15 $^{\circ}$C. We are currently taking further measurements at lower temperatures. [Preview Abstract] |
Friday, March 7, 2014 12:39PM - 12:51PM |
Z2.00008: Charge-Controlled Colloids on Liquid-Liquid Interfaces Daniel A. Kunz, Bernd Reck, Vinothan N. Manoharan The tendency of colloidal particles to stabilize interfaces has been exploited for many years to generate Pickering emulsions with a variety of industrial applications. However, the exact stabilization mechanism and its dependence on the surface properties of the colloidal particles are not yet fully understood. We provide new interfacial studies on the nonequilibrium dynamics of a colloidal system with tunable surface charge density. We push individual sub-micron colloidal particles towards an oil-water interface and track their motion in three-dimensions using holographic microscopy to examine the influence of zeta potential on the dynamics of the system. [Preview Abstract] |
Friday, March 7, 2014 12:51PM - 1:03PM |
Z2.00009: Toward an Accurate Model for the Line Shape Analysis of 2D Correlation Spectra Mohammadhasan Dinpajooh, Dmitry Matyushov Bilinear (linear plus quadratic) coupling of a dipolar polarizable chromophore to a Gaussian thermal bath (Q-model\footnote{D. V. Matyushov and G. A. Voth, J. Chem. Phys., \textbf{113}, 5413 (2000)}) is applied to study the effect of non-Gaussian statistics of the transition frequency on time-resolved linear and nonlinear correlation (2D) spectra. Exact lineshape functions of time-resolved fluorescence spectra and broadening functions of 2D correlation spectra (2DCS) are derived based on the summation of an infinite cumulant series of the transition frequency, in contrast to the two-cumulant approximation of the Gaussian models. The formal theory is supported by molecular dynamics simulations of a dipolar polarizable chromophore dissolved in a modified TIP4P water. We show that the Q-model, unlike the Gaussian model, can capture the asymmetry of 2DCS and bending of the center line in 2DCS reported experimentally. The theory provides a consistent formalism of the line shape analysis in cases when Gaussian models do not apply. [Preview Abstract] |
Friday, March 7, 2014 1:03PM - 1:15PM |
Z2.00010: Structure and Dynamics of Octamethyl-POSS Nanoparticles Michael Crawford, Kerwin Dobbs, Robert Smalley, William Guise, Niina Jalarvo, Olivier Gourdon, Georg Ehlers, Anibal Ramirez-Cuesta, Christoph Wildgruber, Madhusudan Tiyagi, Sanat Kumar Polyoligosilsesquioxanes (POSS) are a large family of Si-O cage molecules that can be viewed as 1-2 nm diameter, monodisperse silica nanoparticles. Here we report the results of a study of the crystal structure and ligand dynamics of one of the simplest POSS nanoparticles, octamethyl-POSS or Si$_{\mathrm{8}}$O$_{\mathrm{12}}$(CH$_{\mathrm{3}})_{\mathrm{8}}$, where the central Si$_{\mathrm{8}}$O$_{\mathrm{12}}$ cage is surrounded by eight methyl ligands. Neutron powder diffraction, inelastic neutron scattering, Raman spectroscopy, and quasielastic neutron scattering were used to characterize the structural, vibrational and dynamical properties of the polycrystalline form of this material. The structural data clearly show the presence of strongly temperature dependent methyl group torsional vibrations. The torsional vibration energy, the magnitude of the torsional energy barrier, and the activation energy for methyl rotations over the barrier were determined from the neutron measurements. These results provide a detailed picture of the structure and ligand dynamics of this POSS molecule. [Preview Abstract] |
Friday, March 7, 2014 1:15PM - 1:27PM |
Z2.00011: IR Spectroscopy of Gasses Evolved During Roasting Coffee Beans Alexander Clain, Xavier Capaldi, Samuel Amanuel We measured the IR spectra of the gasses that evolve during roasting of coffee beans. The spectra recorded at different temperature revealed that the intensity of certain IR bands increase as the temperature increases. For instance, the intensity of the CO$_{2}$ band increased by a factor of four and reached a plateau as the roasting temperature approached 200$^{\circ}$C. The intensity further increased as the temperature increased above 200$^{\circ}$C, however, in two steps. Similarly the intensity of the OH bands monotonically increased until 200$^{\circ}$C and then increased further in two rapid steps above 200$^{\circ}$C. The temperature ranges where IR intensities change in two steps coincides with the temperature ranges where typically commercial roasting is done and where the first and second ``cracks'' are heard during roasting. [Preview Abstract] |
Friday, March 7, 2014 1:27PM - 1:39PM |
Z2.00012: A First-Principles Polarized Raman Method for Determining Whether a Sample is Crystalline or Isotropic Andrew Weisman, Kateri DuBay, Katherine Willets, Richard Friesner We have discovered a simple way to apply basic vibrational Raman spectroscopy to unambiguously determine whether a region of a sample is crystalline or isotropic. Unlike previous methods for determining crystallinity, ours is completely general and rigorously founded in the theory of Raman scattering; it applies independently to any mode of any material and requires no previously established relationships between peak parameters and degree of crystallinity. By applying this technique while scanning an unknown, heterogeneous sample, the borders of the crystalline and isotropic regions can be delineated clearly, after which more developed methods can be applied to determine the orientations of the crystalline regions, thereby completely characterizing the molecular structure of the sample. [Preview Abstract] |
Friday, March 7, 2014 1:39PM - 1:51PM |
Z2.00013: Rotation-induced grain growth and stagnation in phase-field crystal models Jens Tarp, Mathias Bjerre, Luiza Angheluta, Joachim Mathiesen Polycrystalline microstructures are typically formed by thermal processes such as quenching or annealing of melts, through the nucleation and growth of grains of different crystallographic orientation. Since these microstructures have a controlling role on the large scale material properties, it is crucial to understand their formation and late stage evolution. Here we consider the grain growth and stagnation in polycrystalline microstructures using a phase-field crystal model. We identify a transition from a grain growth stagnation upon deep quenching below the melting temperature $T_{m}$ to a continuous coarsening at shallower quenching near $T_{m}$. We find that the grain evolution is mediated by local grain rotations. In the deep quenching regime, the grain assembly typically reaches a metastable state where the kinetic barrier for recrystallization across boundaries is too large and grain rotation with subsequent coalescence or boundary motion is infeasible. For quenching near $T_{m}$ the grain growth depends on the average rate of grain rotation, and follows a power-law behavior with time, with a scaling exponent that depends on the quenching depth. [Preview Abstract] |
Friday, March 7, 2014 1:51PM - 2:03PM |
Z2.00014: Nested Sampling as a blind search method in condensed matter systems Robert Baldock, Gabor Csanyi, Livia Bartok-Partay, Albert Bartok-Partay Nested Sampling is a Bayesian blind search method for calculating the density of states. Nested Sampling samples almost entirely in the region of first order phase transitions, thereby efficiently delivering entire phase diagrams without specific system knowledge. Here we present the latest methods for applying nested sampling to condensed matter systems. [Preview Abstract] |
Friday, March 7, 2014 2:03PM - 2:15PM |
Z2.00015: Pulling at the fabric of the exotic phase diagram for a simple 2D model Ahmad Almudallal, Ivan Saika-Voivod, Sergey Buldyrev We use computer simulation to study a simple, two-dimensional off-lattice model that was originally devised to understand the anomalous properties of water. The model comprises core-softened disks interacting through a repulsive square shoulder located inside a longer attractive square well. In calculating the phase diagram for the model we discover that the system exhibits the truly remarkable phenomenon of inverse melting, for which the system crystallizes upon isobaric heating, over a small range in pressure. Despite occurring in two dimensions, the melting transition is first order and to a liquid, rather than to a hexatic or quasicrystal phase. We find that by increasing the extent of the shoulder, we increase the pressure range over which inverse melting occurs. But as this range increases, the stability fields of other crystal phases must bend to accommodate the changing inverse melting line. This continues until the phase diagram breaks, with a triple point disappearing, new phases appearing, and a channel of liquid stability to low temperatures forming. [Preview Abstract] |
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