ENHANCED H ION TRANSPORT AND HYDRONIUM ION FORMATION T. S. Mahadevan and S. H. Garofalini * Dept. of Materials Science

1. STRUCTURE AND PROPERTIES OF SIMULATED DISSOCIATIVE WATER MD and experimental Pair Distribution Functions (PDF) for OO, OH, and HH. The O-O PDF is an indicator of the molecular structure of water. Location and intensity of first minimum and second and third OO peak maxima have a strong correlation with the extent of hydrogen bond network. Our MD results are consistent with experimental data from Soper. SiO2 H2O SiO2 Y X Z (A) (B) (C) (D) The potentials were trained to reproduce experimental thermal expansion data for bulk water (A) at 1atm, then used to predict the density/pressure diagram at 348K shown in (B), and the liquid-vapor coexistence curve (C). (D) shows data points from several common water models (TIP4P, SPC, etc), which deviate from the experimental data. 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 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 proton transport 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 expansion of nano-confined water, and predicts enhanced hydronium ion formation at silica surfaces. APPLICATION TO ANOMALOUS EXPANSION OF CONFINED WATER MATCHES EXPERIMENTAL DATA Water shows anomalously high expansion in comparison to bulk water when confined to nanopores 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 simulation results compared to experimental data shown below. WATER ON SILICA SURFACES SHOWS ENHANCED HYDRONIUM ION FORMATION ORDERS OF MAGNITUDE HIGHER THAN BULK WATER H3O+ ions not normally seen in our simulations of bulk water, but are often seen at silica surfaces during reactions between water molecule and surface sites. Silanol formation via formation of hydronium ion, H3O+ (shown as yellow in (c)). (a) Non-dissociative chemisorption of H2O onto 3-coordinated Si at 1, followed by H ion moving to adjacent water molecule (at arrow) in (b), which forms the H3O+ ion shown with yellow oxygen in (c). (d) hydronium ion loses a H ion to the NBO labeled as 2 in (c) to form the second SiOH (arrow in (d)). MULTIPLE HYDRONIUM ION FORMATION AND H ION TRANSFER MD SIMULATIONS (Mahadevan and Garofalini, J Phys Chem C 112(2008) 1507) The MD simulated thermal expansion of 3nm confined water and 7nm confined water results are similar to the experimental data obtained by Scherer et al. a b c d The MD simulated vibration spectrum for bulk water matches experimental peaks reasonably well. CONCLUSIONS A new Dissociative Water Potential has been developed that matches many experimental molecular and bulk properties of water. When confined to nanoscale dimensions, the simulated water shows properties consistent with experimental data, particularly the match with the anomalous expansion of water in small pores. Exposure to the silica surface causes enhanced formation of Hydronium ions and H ion transfer and has potential for increasing proton conduction on wet oxide surfaces, relevant to future energy applications. Ab-initio MD SIMULATIONS (Ma et al, JCP2005) In both our classical MD simulations with the Dissociative Water Potential and ab-initio MD simulations, an H2O molecule non-dissociatively chemisorbs onto a 3-coordinated Si (a), losing the extra H to a nearby free H2O molecule, forming an H3O+ ion (at arrows in b), which transfers its extra H to the 2nd free H2O molecule, forming the second H3O+ ion (c), which finally transfers the extra H to the NBO, forming the second SiOH site. Both MD and ab-initio MD for this reaction took ~150fs. Results show robustness of new potential to reproduce QM simulations in both mechanisms and timeframe of complex reactions. Water-silica interactions available atJ Phys Chem C 112(2008) 1507) * TIP4P, SPC/E and other models DISSOCIATIVE WATER POTENTIAL MATCHES EXPERIMENTAL DATA Available at J Phys Chem B 111(2007)8919