The peculiar size and temperature dependence of water diffusion in carbon nanotubes studied with 2D NMR diffusion–relaxation <i>D</i> –<i>T</i>2<i>eff</i> spectroscopy

Abstract

It is well known that water inside hydrophobic nano-channels diffuses faster than bulk water. Recent theoretical studies have shown that this enhancement depends on the size of the hydrophobic nanochannels. However, experimental evidence of this dependence is lacking. Here, by combining two-dimensional nuclear magnetic resonance diffusion-relaxation ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>D</mml:mi></mml:math> - <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>T</mml:mi> <mml:mrow><mml:mn>2</mml:mn> <mml:mi>e</mml:mi> <mml:mi>f</mml:mi> <mml:mi>f</mml:mi></mml:mrow> </mml:msub> </mml:math> ) spectroscopy in the stray field of a superconducting magnet and molecular dynamics simulations, we analyze the size dependence of water dynamics inside Carbon Nanotubes (CNTs) of different diameters ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mn>1.1</mml:mn></mml:math> - <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mn>6.0</mml:mn></mml:math> nm), in the temperature range of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mn>265</mml:mn></mml:math> - <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mn>305</mml:mn></mml:math> K. Depending on the CNT diameter, the nanotube water is shown to resolve in two or more tubular components acquiring different self-diffusion coefficients. Most notably, a favorable CNT diameter range ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mn>3.0</mml:mn></mml:math> - <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mn>4.5</mml:mn></mml:math> nm) is experimentally verified for the first time, in which water molecule dynamics at the center of the CNTs exhibits distinctly non-Arrhenius behavior, characterized by ultrafast diffusion and extraordinary fragility, a result of significant importance in the efforts to understand water behavior in hydrophobic nanochannels.