Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session U42: Focus Session: Supercooled and Nanoconfined Water III |
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Sponsoring Units: DCP Chair: Anders Nilsson, SLAC Room: Hilton Baltimore Holiday Ballroom 3 |
Thursday, March 21, 2013 11:15AM - 11:51AM |
U42.00001: Phase transitions of liquid water at nanoscale Invited Speaker: Christiane Alba-Simionesco The behaviour of fluids confined within nanometric pores significantly differs from that of the bulk. The effect of confinement, surface forces, and reduced dimension is to shift the phase transitions of the confined fluid (condensation, freezing and crystallisation). By postponing or avoiding the inconvenient crystallization process it is often suggested that confinement allows a deeper penetration into the supercooled regime and helps in the understanding the glass formation; in the case of water, confinement might helps to extend the liquid state into the so-called ``no-man's land.'' However below confining conditions of about 10$\sigma$, $\sigma$ being the size of the molecule, water or van der Waals liquids are strongly perturbed by the presence of a surface. Thus a question always remains whether the confined liquid, water or any other fluid, is an extension of the ``bulk'' supercooled regime or refers to specific behavior controlled by external parameters such as the size and the surface interactions imposed to the system. Despite the obvious fundamental interest in understanding bulk water, this situation corresponds to most of the cases in biological and geological systems and deserves particular attention per se. However a prerequisite is to understand and quantify how pores are filled and how much; so we studied the processes of entrance and saturation to a pore (adsorption, imbibition and intrusion) in connection with the structure and the local dynamics of liquid water. Then, we will present new experimental results on the thermodynamic, structural and vibrational properties of water confined within nanometric pores (size of a few molecular diameters). [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U42.00002: How does confinement affect the structure and dynamics of water and other liquids? Anatoli Milischuk, Branka Ladanyi We studied the effects of confinement on static and dynamical properties of liquids including water, acetonitrile, and benzene in amorphous silica nanopores in equilibrium with the bulk liquid at ambient conditions. The model pores are approximately cylindrical, with diameters ranging from 20 to 40 \AA. The filled pores are prepared using grand canonical Monte Carlo simulation and molecular dynamics simulation is used to calculate liquid structure and dynamics. Our studies of dynamics included translational mean squared displacements, orientational time correlations, and survival probabilities in interfacial shells. We also studied polarizability anisotropy time correlations that are related to experimentally observed optical Kerr effect response functions. We found that there is layering and preferential orientational ordering of solvent molecules in the interfacial region. Molecular translational and rotational mobility is reduced in the layers near the interface. Confinement leads to slowdown of the polarizability anisotropy relaxation in agreement with experimental findings. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U42.00003: Strongly Anisotropic Dielectric Response of Confined Water Cui Zhang, Francois Gygi, Giulia Galli We carried out atomistic simulations of water within hydrophobic surfaces, which revealed remarkable modifications of the dynamics and dielectric relaxation of the liquid under confinement. We found that dipolar fluctuations are modified by the presence of surfaces up to strikingly large distances, i.e., tens of nanometers. Fluctuations are suppressed by approximately an order of magnitude in the z direction, perpendicular to the interface, and are enhanced in the x-y plane, giving rise to strong anisotropies in the components of the dielectric response. Our findings are consistent with recent terahertz and ultrafast infrared pump-probe spectroscopy experiments. Work supported by DOE-CMSN DE-SC0005180. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U42.00004: Dynamics of the fast component of nano-confined water under electric field Souleymane Diallo, Eugene Mamontov, Andrey Podlesnyak, Georg Ehlers We have investigated the diffusion of water molecules confined in the pores of folded silica materials (FSM), by means of quasielastic neutron scattering in the time range of 1 picosecond and 65 picoseconds. The measurements were performed on the direct geometry time-of-flight instrument CNCS at the Spallation Neutron Source, for temperatures between 220 K and 245 K, and at two electric field values, 0kV/mm and 2kV/mm. The goal was to investigate the effects of moderate electric field on the previously observed fast component of nano-confined water. In contrast to our earlier observation on the slow dynamics (at longer times) [1], the present results indicate a less drastic effect of applied electric field on the fast dynamics.\\[4pt] [1] S.O. Diallo, E. Mamontov, S. Inagaki, Y. Fukushima, and N. Wada, ``Enhanced Translational Dynamics of Water under Electric Field'' Phys. Rev. E 86, 021506 (2012). [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 1:03PM |
U42.00005: The structure of water in bulk and in confinement by total neutron and x-ray scattering Invited Speaker: Alan Soper In the past decade or so there has been a significant worldwide effort to try to obtain a consistent set of radial distribution functions for water. Exactly how those distribution functions should be interpreted in terms of the local order in water remains a somewhat open question -- whether for instance they imply water has a degree of heterogeneity in its local structure or whether it is in fact a uniform fluid with normal statistical fluctuations in density and structure. However combining a number of different x-ray and neutron data sets together is now indicating a rather consistent view of the local distribution functions in water. This consistency is achieved partly as a result of different researchers applying state-of-the-art data analysis methods to their data, both neutron and x-ray, but partly also by the application of computer simulation methods of structure refinement which help to eliminate some of the artifacts that can be introduced by uncertainties in that data analysis. The situation as regards confined water is much less clear. However it is possible to investigate water near a surface using radiation total scattering methods in the case where the pores which contain the water, whether sheet-like, cylindrical, or spherical, have a regular arrangement in the material. This is because the Bragg peaks arising from that regular arrangement are strongly affected depending on how the fluid is distributed within the pore. This talk will focus on the MCM41 silicas which have cylindrical pores on a hexagonal lattice. Combining the scattering data with computer structure refinement in the same way that is done for the bulk liquid is leading to unprecedented insight into how water is organized near the silicate surface. This work is aimed at clarifying the underlying processes that may have lead to recent observations of fragile to strong transitions in these materials. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U42.00006: Fluctuating confinement of water in aqueous organic nanodroplets Gerald Wilemski, Fawaz Hrahsheh Supercooled and nano-confined water occurs frequently as nanometer-sized aqueous-organic aerosol droplets that are ubiquitous in the atmosphere and in many industrial processes. Nanodroplet structure is important because it influences droplet growth and evaporation rates, heterogeneous reaction rates, and radiative properties. We use classical molecular dynamic simulations to study the structure of ternary water-butanol-nonane nanodroplets for several temperatures and droplet sizes. We study the effects of butanol on the wetting of the water/butanol core-shell droplet by the nonane lens. At low concentrations, butanol acts as a surfactant to significantly enhance the wetability of the water droplet by nonane. At 250 K, with sufficient butanol and nonane, perfect wetting (thin film formation by nonane) occurs. Perfect wetting also occurs at higher temperatures, 270 K to 300 K, but this wetting state is progressively destabilized at higher temperature. All of the nanodroplets studied undergo distinct transitions between partial dewetting and perfect wetting states due to isothermal fluctuations in the local distribution of butanol on the surface of the water core. These fluctuations favor the wetted state at lower temperatures and the dewetted state at higher temperatures. [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U42.00007: The Effect of Contact Angle on the Depletion Layer when Water Meets a Hydrophobic Surface Adele Poynor By definition hydrophobic substances hate water. Water placed on a hydrophobic surface will form a drop in order to minimize its contact area. What happens when water is forced into contact with a hydrophobic surface? One theory is that an ultra-thin low-density depletion layer forms near the surface. We investigate the effect of contact angle on depletion layer formation using the surface sensitive technique of Surface Plasmon Resonance. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U42.00008: ABSTRACT WITHDRAWN |
Thursday, March 21, 2013 1:39PM - 2:15PM |
U42.00009: The dynamical relaxation: a key to understand Water Anomalies. Results from bulk and confined water Invited Speaker: Francesco Mallamace The anomalous behavior of thermodynamic response functions is an unsolved problem in the physics of water. The mechanism that causes the apparently indefinite increase in the heat capacity, the compressibility, and the coefficient of thermal expansion, inside the supercooled regime, is unknown. We explore this problem by analyzing both new and old experimental data coming out from the power spectrum S(Q,), on bulk and confined water at ambient pressure. On decreasing the temperature, we find that the liquid undergoes a structural transformation with the onset of an extended hydrogen bond network.Such a structure is at the basis of the marked viscoelastic behavior observed as a well defined frequency and wave vector dependence of the water sound velocity, and thus of the water response functions. All these observed properties appear consistent with the water polymorphism. We stress that, under these conditions, the thermal response functions and their corresponding fluctuations remain finite at ambient pressure. From the observation that the water density maximum dominating the system thermodynamics under ambient conditions is strongly P-dependent and disappears at a crossover pressure (P$_{cross}$ 1.8kbar) we have studied such a variable in a wide area of the T-P phase diagram. On these basis we have considered new and old data of both the isothermal compressibility $_{T}$(T,P) and the coefficient of thermal expansion $_{P}$(T,P). In the first case the main observation is that $_{T}$(T) shows a minimum located at the same temperature (T$_{MC}$ 315$\pm$5K) for all the studied pressures. As in the $_{T}$(T) case, also the behavior of $_{P}$ is surprising: all the $_{P}$(T) curves measured at different P cross at T$_{MC}$; specifically, the experimental data show a ``singular and universal expansivity point'' at T$_{MC}$ 315K and $_{P}$(T$_{MC}$) 0.44 10$^{-3}$K$^{-1}$. Moreover, on the contrary of other water singularities we stress that such temperature has a precise thermodynamical consistence lying in the relationship connecting the two studied response functions. [Preview Abstract] |
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