A joint seminar on Wed.

[Cond.Matt. & Stat.Mech.Section]

JOINT ICTP/SISSA CONDENSED MATTER SEMINAR

 

 

Wednesday, 7 March    -    4:00 p.m.

 

Luigi Stasi Seminar Room,  ICTP Leonardo Building (1st floor)

 

Ali A. HASSANALI   ( ETH Zürich )

 

" Ions in water: The amorphous silica interface and the recombination of H30+ and OH- "


Abstract

 

In this talk I will begin by sharing results on the structure and dynamics of ions at the water-amorphous silica interface. For 80 years, scientists have employed models in which ions and water near the silica surface form a stagnant layer called the Stern layer.  To account for all experimental features, these models invoke puzzling properties such as the transport of ions through immobile water.  In this talk I will present a realistic theoretical description of the water-amorphous silica interface. We have successfully constructed and validated a model for the water-amorphous silica interface and have begun to examine the fate of biomolecules near this important interface. Our simulations challenge the classical textbook Stern layer model.  Both ions and water exhibit a substantial degree of mobility, yet the phenomena the Stern layer was originally invoked to explain, are reproduced by our calculations.

In the second part of my talk I will discuss some very recent results on the mechanism of the recombination of hydronium and hydroxide ions in water. This process following water ionization is one of the most fundamental processes determining the pH of water. The neutralization step once the solvated ions are in close proximity, is phenomenologically understood to be fast but the molecular mechanism has not been directly probed by experiments.   We elucidate the mechanism of recombination in liquid water with ab initio molecular dynamics simulations and it emerges as quite different from the conventional view of the Grotthuss mechanism. The neutralization event involves a collective compression of the water wire bridging the ions which occurs in 0.5 ps triggering a concerted triple-jump of the protons. This process leaves the neutralized hydroxide in a hypercoordinated state, with the implications that enhanced collective compressions of several water molecules around similarly hypercoordinated states, are likely to serve as nucleation events for the autoionization of liquid water.