Diffusion of Water Molecules on the Surface of Silica Nanoparticles -- Insights from Nuclear Magnetic Resonance Relaxometry

TL;DR

This study uses NMR relaxometry to investigate water molecule diffusion on silica nanoparticle surfaces, revealing temperature-independent residence times and detailed dynamics of bound and free water populations.

Contribution

It provides new quantitative insights into water diffusion and residence times on silica nanoparticle surfaces using broad-frequency NMR relaxometry.

Findings

Water diffusion occurs in a surface layer close to nanoparticles.

Residence lifetime of water molecules is temperature-independent.

Correlation times vary with nanoparticle size and temperature.

Abstract

$^{1}$H spin-lattice nuclear magnetic resonance (NMR) relaxation experiments have been performed for water dispersions of functionalized silica nanoparticles of diameters of 25 and 45 nm. The experiments have been performed in a broad frequency range spanning 3 orders of magnitude, from 10 kHz to 10 MHz, versus temperature, from 313 to 263 K. On the basis of the data, two-dimensional translation diffusion (diffusion close to the nanoparticle surface within a layer of the order of a few diameters of water molecules) has been revealed. The translational correlation times as well as the residence life times on the nanoparticle surface have been determined. It has turned out that the residence lifetime is temperature-independent and is on the order of 5 x 10$^{-6}$ s for the smaller nanoparticles and by about a factor of 3 longer for the larger ones. The translational correlation time for the case of 25 nm nanoparticles is also temperature-independent and yields about 6 x 10$^{-7}$ s, while for the dispersion of the larger nanoparticles, the correlation times changed from about 8 x 10$^{-7}$ s at 313 K to about 1.2 x 10$^{-6}$ s at 263 K. In addition to the quantitative characterization of the two-dimensional translation diffusion, correlation times associated with bound water molecules have been determined. The studies have also given insights into the population of the bound and diffusing water on the surface water fractions.