Water adsorption in ultrathin silica nanotubes

Silica (SiO$_2$) nanotubes (NTs) are used in a wide range of applications that go from sensors to nanofluidics. Currently, these NTs can be grown with diameters as small as 3 nm, with walls 1.5 nm thick. Recent experimental advances combined with first-principles calculations suggest that silica NTs could be obtained from a single silica sheet. In this work, we explore the water adsorption in such ultrathin silica NTs using molecular simulation and first-principles calculations. Combining molecular dynamics and density functional theory calculations we obtain putative structures for NTs formed by 10, 12, and 15-membered SiO$_2$ rings. Water adsorption isotherms for these NTs are obtained using Grand Canonical Monte Carlo simulations. Computing the accessible cross-section area ($A_\text{free}$) for the NTs, we were able to understand how this property correlates with condensation pressures. We found that $A_\text{free}$ does not necessarily grow with the NT size and that the higher the confinement (smaller $A_\text{free}$), the larger the condensation pressure.