1,284 research outputs found
The molecular structure of the interface between water and a hydrophobic substrate is liquid-vapor like
With molecular simulation for water and a tunable hydrophobic substrate, we
apply the instantaneous interface construction [A. P. Willard and D. Chandler,
J. Phys. Chem. B, 114, 1954 (2010)] to examine the similarity between a
water-vapor interface and a water-hydrophobic surface interface. The intrinsic
interface refers to molecular structure in terms of distances from the
instantaneous interface. We show that attractive interactions between a
hydrophobic surface and water affect capillary wave fluctuations of the
instantaneous liquid interface, but these attractive interactions have
essentially no effect on the intrinsic interface. Further, the intrinsic
interface of liquid water and a hydrophobic substrate differs little from that
of water and its vapor.The same is not true, we show, for an interface between
water and a hydrophilic substrate. In that case, strong directional
substrate-water interactions disrupt the liquid-vapor-like interfacial hydrogen
bonding network.Comment: 6 pages, 5 figure
Thermodynamics of Coarse Grained Models of Super-Cooled Liquids
In recent papers, we have argued that kinetically constrained coarse grained
models can be applied to understand dynamic properties of glass forming
materials, and we have used this approach in various applications that appear
to validate this view. In one such paper [J.P. Garrahan and D. Chandler, Proc.
Nat. Acad. Sci. USA 100, 9710 (2003)], among other things we argued that this
approach also explains why the heat capacity discontinuity at the glass
transition is generally larger for fragile materials than for strong materials.
In the preceding article, Biroli, Bouchaud and Tarjus (BB&T) [cond-mat/0412024]
have objected to our explanation on this point, arguing that the class of
models we apply is inconsistent with both the absolute size and temperature
dependence of the experimental specific heat. Their argument, however, neglects
parameters associated with the coarse graining. Accounting for these
parameters, we show here that our treatment of dynamics is not inconsistent
with heat capacity discontinuities.Comment: 5 pages, 2 figures. Revised version to appear in J. Chem. Phy
Characterizing heterogeneous dynamics at hydrated electrode surfaces
In models of Pt 111 and Pt 100 surfaces in water, motions of molecules in the
first hydration layer are spatially and temporally correlated. To interpret
these collective motions, we apply quantitative measures of dynamic
heterogeneity that are standard tools for considering glassy systems.
Specifically, we carry out an analysis in terms of mobility fields and
distributions of persistence times and exchange times. In so doing, we show
that dynamics in these systems is facilitated by transient disorder in
frustrated two-dimensional hydrogen bonding networks. The frustration is the
result of unfavorable geometry imposed by strong metal-water bonding. The
geometry depends upon the structure of the underlying metal surface. Dynamic
heterogeneity of water on the Pt 111 surface is therefore qualitatively
different than that for water on the Pt 100 surface. In both cases, statistics
of this adlayer dynamic heterogeneity responds asymmetrically to applied
voltage.Comment: 6 page, 4 figure
Water exchange at a hydrated platinum electrode is rare and collective
We use molecular dynamics simulations to study the exchange kinetics of water
molecules at a model metal electrode surface -- exchange between water
molecules in the bulk liquid and water molecules bound to the metal. This
process is a rare event, with a mean residence time of a bound water of about
40 ns for the model we consider. With analysis borrowed from the techniques of
rare-event sampling, we show how this exchange or desorption is controlled by
(1) reorganization of the hydrogen bond network within the adlayer of bound
water molecules, and by (2) interfacial density fluctuations of the bulk liquid
adjacent to the adlayer. We define collective coordinates that describe the
desorption mechanism. Spatial and temporal correlations associated with a
single event extend over nanometers and tens of picoseconds.Comment: 10 pages, 9 figure
Solvation at Aqueous Metal Electrodes
We present a study of the solvation properties of model aqueous electrode
interfaces. The exposed electrodes we study strongly bind water and have closed
packed crystalline surfaces, which template an ordered water adlayer adjacent
to the interface. We find that these ordered water structures facilitate
collective responses in the presence of solutes that are correlated over large
lengthscales and across long timescales. Specifically, we show that the liquid
water adjacent to the ordered adlayers forms a soft, liquid-vapor-like
interface with concomitant manifestations of hydrophobicity. Temporal defects
in the adlayer configurations create a dynamic heterogeneity in the degree to
which different regions of the interface attract hydrophobic species. The
structure and heterogeneous dynamics of the adlayer defects depend upon the
geometry of the underlying ordered metal surface. For both 100 and 111
surfaces, the dynamical heterogeneity relaxes on times longer than nanoseconds.
Along with analyzing time scales associated with these effects, we highlight
implications for electrolysis and the particular catalytic efficiency of
platinum.Comment: 9 pages, 8 figure
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