One-Dimensional Confinement Inhibits Water Dissociation in Carbon Nanotubes

The effect of nanoconfinement on the self-dissociation of water constitutes an open problem whose elucidation poses a serious challenge to experiments and simulations alike. In slit pores of width ≈1 nm, recent first-principles calculations have predicted that the dissociation constant of H₂O increases by almost 2 orders of magnitude [Muñoz-Santiburcio and Marx, Phys. Rev. Lett. 2017, 119, 056002]. In the present study, quantum mechanics−molecular mechanics simulations are employed to compute the dissociation free-energy profile of water in a (6,6) carbon nanotube. According to our results, the equilibrium constant K_w drops by 3 orders of magnitude with respect to the bulk phase value, at variance with the trend predicted for confinement in two dimensions. The higher barrier to dissociation can be ascribed to the undercoordination of the hydroxide and hydronium ions in the nanotube and underscores that chemical reactivity does not exhibit a monotonic behavior with respect to pore size but may vary substantially with the characteristic length scale and dimensionality of the confining media.