²H-NMR study of molecular reorientation of D₂O confined into the slit-shaped micropores of activated carbon fiber

Herein, the reorientation of heavy water (D₂O) molecules adsorbed in the slit-type micropores of activated carbon fibers is investigated using the ²H-nuclear magnetic resonance technique. The rotational correlation times (τ_c) of D₂O are evaluated from the ²H spin–lattice relaxation time (T₁). The obtained τ_c values are significantly influenced by both the pore-filling ratio (ϕ) and temperature, thus suggesting that the adsorption of D₂O into activated carbon fibers (ACF) effectively influences the reorientation of the D₂O molecules within the ACF. The reorientational motion of D₂O is examined by the extended jump model. According to this model, the nanoconfinement effect, which results from the reduction in free volume around D₂O, is attributed to the transition-state excluded volume effect, whereas the effect of hydrogen bonding between the D₂O and surface functional groups is attributed to the transition-state hydrogen bonding effects. Furthermore, the dependence of τ_c on ϕ is explained by the chemical exchange between the pore surface adsorption sites and the central space of the pore. Thus, the dynamic behavior of adsorbed D₂O molecules reveals the mechanism of D₂O adsorption into the ACF micropores.