CiteULike: dchen's Stanley

Effect of water-wall interaction potential on the properties of nanoconfined water

Pradeep Kumar — 2008-06-11T20:12:46-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 75, No. 1. (2007)

Much of the understanding of bulk liquids has progressed through study of the limiting case in which molecules interact via purely repulsive forces, such as a hard-core or “repulsive ramp” potential. In the same spirit, we report progress on the understanding of confined water by examining the behavior of waterlike molecules interacting with planar walls via purely repulsive forces and compare our results with those obtained for Lennard-Jones (LJ) interactions between the molecules and the walls. Specifically, we perform molecular dynamics simulations of 512 waterlike molecules interacting via the TIP5P potential and confined between two smooth planar walls that are separated by 1.1 nm. At this separation, there are either two or three molecular layers of water, depending on density. We study two different forms of repulsive confinement, when the water-wall interaction potential is either (i) 1/r9 or (ii) a WCA-like repulsive potential. We find that the thermodynamic, dynamic, and structural properties of the liquid in purely repulsive confinements qualitatively match those for a system with a pure LJ attraction to the wall. In previous studies that include attractions, freezing into monolayer or trilayer ice was seen for this wall separation. Using the same separation as these previous studies, we find that the crystal state is not stable with 1/r9 repulsive walls but is stable with WCA-like repulsive confinement. However, by carefully adjusting the separation of the plates with 1/r9 repulsive interactions so that the effective space available to the molecules is the same as that for LJ confinement, we find that the same crystal phases are stable. This result emphasizes the importance of comparing systems only using the same effective confinement, which may differ from the geometric separation of the confining surfaces.

The relationship between liquid, supercooled and glassy water

Osamu Mishima — 2008-04-29T00:31:18-00:00

Nature, Vol. 396, No. 6709. (26 November 1998), pp. 329-335.

Clusters of mobile molecules in supercooled water

Nicolas Giovambattista — 2008-03-01T18:47:31-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 72, No. 1. (2005)

We study the spatially heterogeneous dynamics in water via molecular dynamics simulations using the extended simple point charge potential. We identify clusters formed by mobile molecules and study their properties. We find that these clusters grow in size and become more compact as temperature decreases. We analyze the probability density function of cluster size, and we study the cluster correlation length. We find that clusters appear to be characterized by a fractal dimension consistent with that of lattice animals. We relate the cluster size and correlation length to the configurational entropy, Sconf. We find that these quantities depend weakly on 1/Sconf. In particular, the linearity found between the cluster mass n* and 1/Sconf suggests that n* may be interpreted as the mass of the cooperatively rearranging regions that form the basis of the Adam-Gibbs approach to the dynamics of supercooled liquids. We study the motion of molecules within a cluster, and find that each molecule preferentially follows a neighboring molecule in the same cluster. Based on this finding we hypothesize that stringlike cooperative motion may be a general mechanism for molecular rearrangement of complex, as well as simple liquids. By mapping each equilibrium configuration onto its corresponding local potential energy minimum or inherent structure (IS), we are able to compare the mobile molecule clusters in the equilibrium system with the molecules forming the clusters identified in the transitions between IS. We find that (i) mobile molecule clusters obtained by comparing different system configurations and (ii) clusters obtained by comparing the corresponding IS are completely different for short time scales, but are the same on the longer time scales of diffusive motion.

Connection of translational and rotational dynamical heterogeneities with the breakdown of the Stokes-Einstein and Stokes-Einstein-Debye relations in water

Marco Mazza — 2007-10-16T14:47:04-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 76, No. 3. (2007)

We study the Stokes-Einstein (SE) and the Stokes-Einstein-Debye (SED) relations, Dt=kBT/6R and Dr=kBT/8R3, where Dt and Dr are the translational and rotational diffusivity, respectively, T is the temperature, the viscosity, kB the Boltzmann constant, and R the “molecular” radius. Our results are based on molecular dynamics simulations of the extended simple point charge model of water. We find that both the SE and SED relations break down at low temperature. To explore the relationship between these breakdowns and dynamical heterogeneities (DHs), we also calculate the SE and SED relations for subsets of the 7% “fastest” and 7% “slowest” molecules. We find that the SE and SED relations break down in both subsets, and that the breakdowns occur on all scales of mobility. Thus these breakdowns appear to be generalized phenomena, in contrast with a view where only the most mobile molecules are the origin of the breakdown of the SE and SED relations, embedded in an inactive background where these relations hold. At low temperature, the SE and SED relations in both subsets of molecules are replaced with “fractional” SE and SED relations, Dt~(/T)−t and Dr~(/T)−r, where t0.84(<1) and r0.75(<1). We also find that there is a decoupling between rotational and translational motion, and that this decoupling occurs in both the fastest and slowest subsets of molecules. Further, we find that, the decoupling increases upon cooling, but that the probability of a molecule being classified as both translationally and rotationally fastest also increases. To study the effect of time scale for SE and SED breakdown and decoupling, we introduce a time-dependent version of the SE and SED relations, and a time-dependent function that measures the extent of decoupling. Our results suggest that both the decoupling and SE and SED breakdowns originate at a time scale corresponding to the end of the cage regime, when diffusion starts. This is also the time scale when the DHs are more relevant. Our work also demonstrates that selecting DHs on the basis of translational or rotational motion more strongly biases the calculation of diffusion constants than other dynamical properties such as relaxation times.

Connection between Adam-Gibbs Theory and Spatially Heterogeneous Dynamics

Nicolas Giovambattista — 2008-03-01T18:18:24-00:00

Physical Review Letters, Vol. 90, No. 8. (28 February 2003), 085506.

We investigate the spatially heterogeneous dynamics in the extended simple point charge model of water using molecular dynamics simulations. We relate the average mass n * of mobile particle clusters to the diffusion constant and the configurational entropy. Hence; n * can be interpreted as the mass of the “cooperatively rearranging regions” that form the basis of the Adam-Gibbs theory of the dynamics of supercooled liquids. We also examine the time and temperature dependence of these transient clusters.

Dynamic Heterogeneities in Supercooled Water

N Giovambattista — 2007-06-27T18:37:19-00:00

J. Phys. Chem. B, Vol. 108, No. 21. (27 May 2004), pp. 6655-6662.

Abstract: We investigate dynamic heterogeneities in liquid water by performing molecular dynamics simulations of the SPC/E model. We find clusters of mobile molecules. We study the temperature and time dependence of the cluster size and find that clusters grow as temperature decreases and have a maximum size at the time scale corresponding to the escape of the molecules from the cage formed by neighboring molecules. We relate the average mass n* of mobile particle clusters to the diffusion constant, D, and the configurational entropy, Sconf. We find that n* can be interpreted as the mass of the "cooperatively rearranging regions" hypothesized in the Adam-Gibbs theory of the dynamics of supercooled liquids. In the context of the potential energy landscape (PEL) approach, the diffusion of molecules is related to the change of basins. By studying the dynamics of the system on the PEL, we identify clusters formed by the molecules with large displacements as the system visits consecutive local minima on the PEL. We relate the changing of basins with the restructuring of the hydrogen bond network.