Correlation lengths in nanoconfined water and transport properties

Abstract

We report the existence of disparate static and dynamic correlation lengths that could describe the influence of confinement on nanoconfined water (NCW). Various aspects of viscous properties, such as anisotropy and viscoelasticity, of NCW are studied by varying the separation distance “ d” between two confining hydrophobic plates. The transverse component of the mean square stress exhibits slow spatial decay (measured from the surface) beyond ∼1.8 nm, which was not reported before. The static correlation length obtained from fitting the exponential decay of the transverse mean-square stress with d is 0.75 nm, while the decay time of the stress–stress time correlation function gives a dynamic correlation length of only 0.35 nm. The shortness of the dynamic correlation length seems to arise from the low sensitivity of orientational relaxation to confinement. In the frequency-dependent viscosity, we observe a new peak at about 50 cm−1 that is not present in the bulk. This new peak is prominent even at 3 nm separations. The peak is absent in the bulk, although it is close to the intermolecular –O–O–O– bending mode well known in liquid water. We further explore the relationship between diffusion and viscosity in NCW by varying d.