Translational diffusion of a fluorescent tracer molecule in nanoconfined water

TL;DR

This study measures the diffusion of fluorescent dye molecules in nanoconfined water, finding that water's viscosity remains bulk-like down to 5 nm gaps, challenging some recent experimental results.

Contribution

It provides direct experimental evidence that water diffusion remains bulk-like in nanoconfined spaces as small as 5 nm, clarifying conflicting previous findings.

Findings

Diffusion is bulk-like for gaps down to 5 nm.

Slow fluorescence component due to wall adsorption disappears with salt.

Water viscosity under confinement remains unaltered at small scales.

Abstract

Diffusion of tracer dye molecules in water confined to nanoscale is an important subject with a direct bearing on many technological applications. It is not yet clear however, if the dynamics of water in hydrophilic as well as hydrophobic nanochannels remains bulk-like. Here, we present diffusion measurement of a fluorescent dye molecule in water confined to nanoscale between two hydrophilic surfaces whose separation can be controlled with a precision of less than a nm. We observe that the fluorescence intensities correlate over a fast($\sim$ 30 $\mu$s) and slow ($\sim$ 1000 $\mu$s) time components. The slow timescale is due to adsorption of fluorophores to the confining walls and it disappears in presence of 1 M salt. The fast component is attributed to diffusion of dye molecules in the gap and is found to be bulk-like for sub-10 nm separations and indicates that viscosity of water under confinement remains unaltered up to confinement gap as small as $\sim$ 5 nm. Our findings contradict some of the recent measurements of diffusion under nanoconfinement, however they are consistent with many estimates of self-diffusion using molecular dynamics simulations and measurements using neutron scattering experiments.