MD (here)
MD*
EXP
(kcal/mole)
10.5
10.0
10.5
(D)
2.6
2.1- 3.0
D (cm/s) 298K
2.45
1.1- 5.0
2.3
ENHANCED H ION TRANSPORT AND HYDRONIUM ION FORMATION
T. S. Mahadevan and S. H. Garofalini *
Dept. of Materials Science and Engineering, Rutgers University
* shg@rutgers.edu
STRUCTURE AND PROPERTIES OF SIMULATED DISSOCIATIVE WATER
APPLICATION TO ANOMALOUS EXPANSION OF CONFINED WATERMATCHES EXPERIMENTAL DATA
Water shows anomalously high expansion in comparison to bulk water when confined tonanopores in silica. We performed MD simulations with the Dissociative Water Potential on~3nm and 7nm thin films of water confined between amorphous silica. Set-up and simulationresults compared to experimental data shown below.
H2O
SiO2
SiO2
Y
X
Z
The MD simulated thermal expansion of 3nm confined waterand 7nm confined water results are similar to theexperimental data obtained by Scherer et al.
* TIP4P, SPC/E and other models
CONCLUSIONS
A new Dissociative Water Potential has beendeveloped that matches many experimentalmolecular and bulk properties of water.
When confined to nanoscale dimensions, thesimulated water shows properties consistent withexperimental data, particularly the match with theanomalous expansion of water in small pores.
Exposure to the silica surface causes enhancedformation of Hydronium ions and H ion transfer andhas potential for increasing proton conduction onwet oxide surfaces, relevant to future energyapplications.
The potentials were trained to reproduce experimental thermal expansion datafor bulk water (A) at 1atm, then used to predict the density/pressure diagramat 348K shown in (B), and the liquid-vapor coexistence curve (C). (D) showsdata points from several common water models (TIP4P, SPC, etc), whichdeviate from the experimental data.
(C)
(B)
(A)
(D)
MD and experimental Pair DistributionFunctions (PDF) for OO, OH, and HH. TheO-O PDF is an indicator of the molecularstructure of water.
Location and intensity of first minimum andsecond and third OO peak maxima have astrong correlation with the extent ofhydrogen bond network.
Our MD results are consistent withexperimental data from Soper.
Hydronium ion concentration in bulk water is only 10-7. However, water at silica surfaces shows orders of magnitude higher concentration of hydronium ions, allowing for enhanced protontransport and conductivity. This result has potential benefits in fuel cells and H sources.
We have applied Molecular Dynamics Computer Simulations using an accurate dissociative water potential that matches bulk water properties and the anomalously high thermal expansionof nano-confined water, and predicts enhanced hydronium ion formation at silica surfaces.
DISSOCIATIVE WATER POTENTIALMATCHES EXPERIMENTAL DATA
Available at J Phys Chem B 111(2007)8919
The MD simulated vibrationspectrum for bulk water matchesexperimental peaks reasonablywell.
WATER ON SILICA SURFACES SHOWS ENHANCED HYDRONIUM IONFORMATION ORDERS OF MAGNITUDE HIGHER THAN BULK WATER
H3O+ ions not normally seen in our simulations of bulk water, but are often seen at silicasurfaces during reactions between water molecule and surface sites.
Silanol formation via formation ofhydronium ion, H3O+ (shown as yellow in(c)). (a) Non-dissociative chemisorption ofH2O onto 3-coordinated Si at 1, followedby H ion moving to adjacent watermolecule (at arrow) in (b), which forms theH3O+ ion shown with yellow oxygen in (c).(d) hydronium ion loses a H ion to the NBOlabeled as 2 in (c) to form the secondSiOH (arrow in (d)).
MULTIPLE HYDRONIUM ION FORMATION AND H ION TRANSFER
MD SIMULATIONS (Mahadevan and Garofalini, J Phys Chem C 112(2008) 1507)
Ab-initio MD SIMULATIONS (Ma et al, JCP2005)
In both our classical MD simulations with the Dissociative Water Potential and ab-initio MDsimulations, an H2O molecule non-dissociatively chemisorbs onto a 3-coordinated Si (a), losingthe extra H to a nearby free H2O molecule, forming an H3O+ ion (at arrows in b), which transfersits extra H to the 2nd free H2O molecule, forming the second H3O+ ion (c), which finally transfersthe extra H to the NBO, forming the second SiOH site. Both MD and ab-initio MD for this reactiontook ~150fs. Results show robustness of new potential to reproduce QM simulations in bothmechanisms and timeframe of complex reactions.
a
b
c
d
Water-silica interactions available at J Phys Chem C 112(2008) 1507)