Aqueous water solid interfaces

Under ambient conditions most surfaces are covered in a thin film of water. At the molecular scale the structure and properties of these thin (often nanometer thick) films of water remain largely a mystery. Largely this is because probing the properties of ambient liquid water films remains a challenge for both experiment and first-principles electronic structure theory alike. The former generally struggle to obtain atomic-level understanding while out of ultra-high vacuum, and the latter are faced with the often computationally prohibitive twin demands of large system sizes and long time scales. However, thanks to recent algorithmic developments and access to some of the world's largest supercomputers it is now possible to explore solid-liquid interfaces from first principles. We are currently examining the structure and dynamics of a number of water-solid interfaces (water/TiO₂; water/MgO, and water/salt). For water on salt (NaCl) interesting progress has already been made:

Density oscillations in a nanoscale water film on salt:The salt-water interface is one of the most important and common on earth, playing a prominent role in disciplines such as atmospheric science and biology. Despite the apparent simplicity of such interfaces, arguably the most fundamental question of what the nature and structure of the liquid water/salt interface is under ambient conditions remains unclear. In our work we have addressed this issue with ab initio molecular dynamics simulations of nanoscale liquid water films on NaCl [1,2]. A pronounced layering is observed in the films, with the density exhibiting a damped oscillatory behavior in the direction of the surface normal. In addition, water molecules in the contact layer are preferentially adsorbed at specific adsorption sites, involved in about 20% fewer hydrogen bonds with each other, and carry considerably reduced dipole moments compared to bulk liquid water.

References:

[1] L. Liu, M. Krack, and A. Michaelides, J. Am. Chem. Soc. 130, 8572 (2008) [pdf]
[2] L. Liu, M. Krack, and A. Michaelides, J. Chem. Phys. 130, 234702 (2009) [pdf]

Molecule exchange: water at TiO2 surface: In this movie we study a collection of water molecules interacting with a perfect TiO2(110) surface using ab initio molecular dynamics simulations. No water dissociation is observed. When we zoom in on the interface zone we see that water molecules can jump away from the first layer and go into the second layer. More details can be found in Liu et al, Phys. Rev. B. 82, 161415(R) (2010).(Credit: Limin Liu, www.chem.ucl.ac.uk/ice).