Showing papers by "Peter T. Cummings published in 2004"

Ion adsorption at the rutile-water interface: linking molecular and macroscopic properties.

Abstract: A comprehensive picture of the interface between aqueous solutions and the (110) surface of rutile (α-TiO2) is being developed by combining molecular-scale and macroscopic approaches, including experimental measurements, quantum calculations, molecular simulations, and Gouy−Chapman−Stern models. In situ X-ray reflectivity and X-ray standing-wave measurements are used to define the atomic arrangement of adsorbed ions, the coordination of interfacial water molecules, and substrate surface termination and structure. Ab initio calculations and molecular dynamics simulations, validated through direct comparison with the X-ray results, are used to predict ion distributions not measured experimentally. Potentiometric titration and ion adsorption results for rutile powders having predominant (110) surface expression provide macroscopic constraints of electrical double layer (EDL) properties (e.g., proton release) which are evaluated by comparison with a three-layer EDL model including surface oxygen proton affini...

Electric Double Layer at the Rutile (110) Surface. 1. Structure of Surfaces and Interfacial Water from Molecular Dynamics by Use of ab Initio Potentials

Abstract: A recently developed force field for interactions of water molecules with the (110) surface of rutile (α-TiO2) has been generalized for atomistically detailed molecular dynamics simulations of the interfacial structure of the uncharged mineral surface in contact with liquid SPC/E water at 298 K and 1 atm and for negatively charged surfaces in contact with SPC/E water containing dissolved electrolyte ions (Rb+, Sr2+, Zn2+, Na+, Ca2+, Cl-). Both hydroxylated (dissociative) and nonhydroxylated (associative) surfaces are simulated, since both types of water−surface interactions have been postulated from ab initio calculations and spectroscopic studies under near-vacuum conditions. The positions of water molecules at the interface were found to be very similar for both hydroxylated and nonhydroxylated surfaces, with either terminal hydroxyl groups or associated water molecules occupying the site above each terminal titanium atom. Beyond these surface oxygens, a single additional layer of adsorbed water molecul...

Electric Double Layer at the Rutile (110) Surface. 2. Adsorption of Ions from Molecular Dynamics and X-ray Experiments

Abstract: Molecular dynamics (MD) simulations were conducted to characterize the microstructure of the interface between aqueous solutions and the (110) surface of rutile (R-TiO2) for hydroxylated and nonhydroxylated surfaces, each either neutral or negatively charged. The fully atomistic description of the rutile surface and its interactions with the fluid phase was based on ab initio calculations, while the aqueous phase was described by the SPC/E model and existing parametrizations for Rb + ,N a + ,S r 2+ ,Z n 2+ ,C a 2+ , and Cl - ions. Formation of inner-sphere complexes of cations with surface oxygens was identified for all cations studied. On negatively charged surfaces, Zn 2+ is shown to sorb at two bidentate sites, between a bridging and terminal oxygen, and between two terminal oxygens (hydroxylated surface only), while all other cations occupy a tetradentate site, in contact with two terminal and two bridging oxygens in adjacent rows on the crystal surface, and directly above an additional triply coordinated oxygen in the Ti-O surface plane. These differences in inner-sphere binding configuration appear to be related to the bare ionic radii of the cations. Simulation results agree very well with X-ray standing wave and crystal truncation rod studies of the inner-sphere adsorption sites of the cations Rb + and Sr 2+ . MD and X-ray results for Zn 2+ adsorption are qualitatively consistent, but important differences in adsorption heights are discussed. Both MD simulations and X-ray studies indicate that, on rutile (110), interaction of Cl - with neutral and negatively charged surfaces and with sorbed, multivalent cations is minimal. The hydroxylated surface gives better agreement with experiments than the nonhydroxylated surface and is therefore inferred to be the dominant surface in contact with aqueous solutions at ambient conditions. At the negative, hydroxylated surface, the MD results indicate that Sr 2+ and Ca 2+ also form outersphere species that are laterally ordered with respect to the crystal surface structure, though these are much less abundant than the inner-sphere species. At positively charged hydroxylated surfaces, MD simulations indicate Cl - adsorption in the tetradentate site 4.3 A above the surface, with longer-range ordering of ions and water molecules than was observed on neutral or negatively charged surfaces. The adsorption geometries of ions are not sensitive to an increase of temperature to 448 K.

Simulated water adsorption isotherms in carbon nanopores

Abstract: Water adsorption isotherms are calculated by grand canonical Monte Carlo simulations for the SPC/E water model in carbon nanopores at 298 K. The pores are of slit or cylindrical morphology. Carbon-slit pores are of widths 0.8, 1.0 and 1.6 nm. The simulated single-walled carbon nanotubes are of 1.4 and 2.7 nm diameter ((10:10) and (20:20) respectively). In all cases considered, the adsorption isotherms are characterized by negligible adsorption at low pressures, pore filling by a capillary-condensation-like mechanism and adsorption–desorption hysteresis loops. For both pore morphologies considered, the relative pressures at which pore filling occurs, and the width of the adsorption–desorption hysteresis loop decrease with decreasing pore size. Adsorption isotherms simulated for water in carbon nanotubes show pore filling at lower relative pressures and narrower adsorption–desorption hysteresis loops when compared to adsorption isotherms simulated in carbon-slit pores of similar sizes. By using representati...

Multiscale simulation of the synthesis, assembly and properties of nanostructured organic/inorganic hybrid materials

Abstract: Clare McCabe,1 Sharon C. Glotzer,2 3 John Kieffer,3 Matthew Neurock,4 and Peter Cummings5 6 ∗ 1Department of Chemical Engineering, Colorado School of Mines, Golden C0 80401 2Department of Chemical Engineering, University of Michigan, Ann Arbor, MI 48109-2136 3Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109-2136 4Department of Chemical Engineering, University of Virginia, Charlottesville, VA 22904-4741 5Department of Chemical Engineering, Vanderbilt University, Nashville, TN 37235-1604 6Chemical Sciences Division and Nanomaterials Theory Institute, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6181

Equation of state and liquid-vapor equilibria of one- and two-Yukawa hard-sphere chain fluids: theory and simulation.

Abstract: The accuracy of several theories for the thermodynamic properties of the Yukawa hard-sphere chain fluid are studied. In particular, we consider the polymer mean spherical approximation (PMSA), the dimer version of thermodynamic perturbation theory (TPTD), and the statistical associating fluid theory for potentials of variable attractive range (SAFT-VR). Since the original version of SAFT-VR for Yukawa fluids is restricted to the case of one-Yukawa tail, we have extended SAFT-VR to treat chain fluids with two-Yukawa tails. The predictions of these theories are compared with Monte Carlo (MC) simulation data for the pressure and phase behavior of the chain fluid of different length with one- and two-Yukawa tails. We find that overall the PMSA and TPTD give more accurate predictions than SAFT-VR, and that the PMSA is slightly more accurate than TPTD.

Phase coexistence in polydisperse charged hard-sphere fluids: mean spherical approximation.

Abstract: Taking advantage of the availability of the analytic solution of the mean spherical approximation for a mixture of charged hard spheres with an arbitrary number of components we show that the polydisperse fluid mixture of charged hard spheres belongs to the class of truncatable free energy models, i.e., to those systems where the thermodynamic properties can be represented by a finite number of (generalized) moments of the distribution function that characterizes the mixture. Thus, the formally infinitely many equations that determine the parameters of the two coexisting phases can be mapped onto a system of coupled nonlinear equations in these moments. We present the formalism and demonstrate the power of this approach for two systems; we calculate the full phase diagram in terms of cloud and shadow curves as well as binodals and discuss the distribution functions of the coexisting daughter phases and their charge distributions.

Nanoscience Research for Energy Needs. Report of the National Nanotechnology Initiative Grand Challenge Workshop, March 16-18, 2004

Abstract: This document is the report of a workshop held under NSET auspices in March 2004 aimed at identifying and articulating the relationship of nanoscale science and technology to the Nation's energy future.