| 496 |
|
periodic-boundary simulations of the bulk crystal have yielded values |
| 497 |
|
of 175.53 GPa.\cite{QSC2} Using the same force field, we have |
| 498 |
|
performed a series of 1 ns simulations on gold nanoparticles of three |
| 499 |
< |
different radii under the Langevin Hull at a variety of applied |
| 499 |
> |
different radii: 20 \AA~ (1985 atoms), 30 \AA~ (6699 atoms), and 40 |
| 500 |
> |
\AA~ (15707 atoms) utilizing the Langevin Hull at a variety of applied |
| 501 |
|
pressures ranging from 0 -- 10 GPa. For the 40 \AA~ radius |
| 502 |
|
nanoparticle we obtain a value of 177.55 GPa for the bulk modulus of |
| 503 |
|
gold, in close agreement with both previous simulations and the |
| 615 |
|
effects of the empty space due to the vapor phase; for this reason, we |
| 616 |
|
recommend using the number density (Eq. \ref{eq:BMN}) or number |
| 617 |
|
density fluctuations (Eq. \ref{eq:BMNfluct}) for computing |
| 618 |
< |
compressibilities. We achieved the best results using a sampling |
| 619 |
< |
radius approximately 80\% of the cluster radius. This ratio of |
| 620 |
< |
sampling radius to cluster radius excludes the problematic vapor phase |
| 621 |
< |
on the outside of the cluster while including enough of the liquid |
| 622 |
< |
phase to avoid poor statistics due to fluctuating local densities. |
| 618 |
> |
compressibilities. We obtained the results in |
| 619 |
> |
Fig. \ref{fig:compWater} using a sampling radius that was |
| 620 |
> |
approximately 80\% of the mean distance between the center of mass of |
| 621 |
> |
the cluster and the hull atoms. This ratio of sampling radius to |
| 622 |
> |
average hull radius excludes the problematic vapor phase on the |
| 623 |
> |
outside of the cluster while including enough of the liquid phase to |
| 624 |
> |
avoid poor statistics due to fluctuating local densities. |
| 625 |
|
|
| 626 |
|
A comparison of the oxygen-oxygen radial distribution functions for |
| 627 |
< |
SPC/E water simulated using the Langevin Hull and bulk SPC/E using |
| 628 |
< |
periodic boundary conditions -- both at 1 atm and 300K -- reveals an |
| 629 |
< |
understructuring of water in the Langevin Hull that manifests as a |
| 630 |
< |
slight broadening of the solvation shells. This effect may be related |
| 631 |
< |
to the introduction of surface tension around the entire cluster, an |
| 632 |
< |
effect absent in bulk systems. As a result, molecules on the hull may |
| 633 |
< |
experience an increased inward force, slightly compressing the |
| 634 |
< |
solvation shell for these molecules. |
| 627 |
> |
SPC/E water simulated using both the Langevin Hull and more |
| 628 |
> |
traditional periodic boundary methods -- both at 1 atm and 300K -- |
| 629 |
> |
reveals an understructuring of water in the Langevin Hull that |
| 630 |
> |
manifests as a slight broadening of the solvation shells. This effect |
| 631 |
> |
may be due to the introduction of a liquid-vapor interface in the |
| 632 |
> |
Langevin Hull simulations (an interface which is missing in most |
| 633 |
> |
periodic simulations of bulk water). Vapor-phase molecules contribute |
| 634 |
> |
a small but nearly flat portion of the radial distribution function. |
| 635 |
|
|
| 636 |
|
\subsection{Molecular orientation distribution at cluster boundary} |
| 637 |
|
|
| 718 |
|
\subsection{Heterogeneous nanoparticle / water mixtures} |
| 719 |
|
|
| 720 |
|
To further test the method, we simulated gold nanoparticles ($r = 18$ |
| 721 |
< |
\AA) solvated by explicit SPC/E water clusters using a model for the |
| 722 |
< |
gold / water interactions that has been used by Dou {\it et. al.} for |
| 723 |
< |
investigating the separation of water films near hot metal |
| 724 |
< |
surfaces.\cite{ISI:000167766600035} The Langevin Hull was used to |
| 725 |
< |
sample pressures of 1, 2, 5, 10, 20, 50, 100 and 200 atm, while all |
| 726 |
< |
simulations were done at a temperature of 300 K. At these |
| 727 |
< |
temperatures and pressures, there is no observed separation of the |
| 728 |
< |
water film from the surface. |
| 721 |
> |
\AA~, 1433 atoms) solvated by explicit SPC/E water clusters (5000 |
| 722 |
> |
molecules) using a model for the gold / water interactions that has |
| 723 |
> |
been used by Dou {\it et. al.} for investigating the separation of |
| 724 |
> |
water films near hot metal surfaces.\cite{ISI:000167766600035} The |
| 725 |
> |
Langevin Hull was used to sample pressures of 1, 2, 5, 10, 20, 50, 100 |
| 726 |
> |
and 200 atm, while all simulations were done at a temperature of 300 |
| 727 |
> |
K. At these temperatures and pressures, there is no observed |
| 728 |
> |
separation of the water film from the surface. |
| 729 |
|
|
| 730 |
|
In Fig. \ref{fig:RhoR} we show the density of water and gold as a |
| 731 |
|
function of the distance from the center of the nanoparticle. Higher |
