Thermodynamics of Water Entry in Hydrophobic Channels of Carbon Nanotubes

TL;DR

This study uses the Two Phase Thermodynamics method to analyze the thermodynamics of water entering hydrophobic carbon nanotubes, revealing entropy and energy compensation mechanisms that explain spontaneous filling.

Contribution

It provides a quantitative thermodynamic analysis of water confinement in nanotubes, highlighting entropy contributions and spectral shifts related to tube radius.

Findings

Water molecules inside nanotubes have increased energy balanced by rotational entropy gain.

Confined water's free energy matches bulk water within simulation accuracy.

Spectral shifts in water's vibrational modes depend on nanotube radius.

Abstract

Experiments and computer simulations demonstrate that water spontaneously fills the hydrophobic cavity of a carbon nanotube. To gain a quantitative thermody- namic understanding of this phenomenon, we use the recently developed Two Phase Thermodynamics (2PT) method to compute translational and rotational entropies of confined water molecules inside single-walled carbon nanotubes and show that the increase in energy of a water molecule inside the nanotube is compensated by the gain in its rotational entropy. The confined water is in equilibrium with the bulk wa- ter and the Helmholtz free energy per water molecule of confined water is the same as that in the bulk within the accuracy of the simulation results. A comparison of translational and rotational spectra of water molecules confined in carbon nanotubes with that of bulk water shows significant shifts in the positions of the spectral peaks that are directly related to the tube radius.