Molecular Mechanisms Causing Anomalously High Thermal Expansion of Nanoconfined Water

Anomalously high thermal expansion is measured in water confined in nanoscale pores in amorphous silica and the molecular mechanisms are identified by molecular dynamics (MD) simulations using an accurate dissociative water potential. The experimentally measured coefficient of thermal expansion (CTE) of nanoconfined water increases as pore dimension decreases. The simulations match this behavior for water confined in 30 Å and 70 Å pores in silica. The cause of the high expansion is associated with the structure and increased CTE of a region of water ～6 Å thick adjacent to the silica. The structure of water in the first 3 Å of this interface is templated by the atomically rough silica surface, while the water in the second 3 Å just beyond the atomically rough silica surface sits in an asymmetric potential well and displays a high density, with a structure comparable to bulk water at higher pressure.