Translational diffusion of a fluorescent tracer molecule in nanoconfined water

• Published in: arXiv.org
• Main Authors: Patil, Shivprasad, Ajith, V J
• Format: Paper Journal Article
• Language: English
• Published: Ithaca Cornell University Library, arXiv.org 18.01.2022
• Subjects: Chemical compounds; Confinement; Diffusion rate; Dyes; Fluorescence; Fluorescent dyes; Hydrophilicity; Molecular dynamics; Nanochannels; Neutron scattering; Physics - Fluid Dynamics; Physics - Instrumentation and Detectors; Physics - Mesoscale and Nanoscale Physics; Physics - Soft Condensed Matter; Self diffusion
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2201.06765

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.