Anomalies of water and hydrogen bond dynamics in hydrophobic nanoconfinement

Abstract

Using molecular dynamic (MD) simulations of the TIP5P model of water, we investigate theeffect of hydrophobic confinement on the anomalies of liquid water. For confinement lengthLz = 1.1 nm, such that there are 2–3 molecular layers of water, we find the presence of the bulk-like densityand diffusion anomaly in the lateral directions. However, the lines of these anomalies in theP–T plane are shifted tolower temperatures (ΔT≈40 K) and pressures compared to bulk water. Furthermore, we introduce a method to calculatethe effective diffusion constant along the confinement direction and find that the diffusionanomaly is absent. Moreover, we investigate the hydrogen bond dynamics of confinedwater and find that the hydrogen bond dynamics preserves the characteristicsof HB dynamics in bulk water, such as a non-exponential behavior followed byan exponential tail of HB lifetime probability distributions and an Arrheniustemperature dependence of the average HB lifetime. The average number andlifetime of HBs decrease in confined water compared to bulk water at the sametemperature. This reduction may be the origin of the reasons for the different physicalproperties of confined water from bulk water, such as the 40 K temperature shift.