Water-ice coexistence under nanoscale confinement revealed by magnetization-transfer NMR

Low-temperature ice-liquid phase coexistence and nuclear magnetization exchange dynamics of water confined in the ordered mesoporous silica material KIT-5 were investigated by nuclear magnetic resonance (NMR). The temporal evolution of the magnetization following the preparation of non-equilibrium spin states was measured over a range of temperatures to quantify magnetization exchange between the coexisting ice and liquid phases. The analysis of the NMR spectra yielded the phase fractions, spectral linewidths, and magnetization dynamics of both phases. The equilibrium phase fractions were used to determine the temperature dependence of the non-frozen layer thickness separating the silica surface from the ice core, which was found to be in good agreement with theoretical predictions. The magnetization exchange kinetics were quantitatively described by a diffusion model, yielding an effective diffusion coefficient in the order of 10-16 m2/s for magnetization transport within the confined ice phase. Spin diffusion rates were estimated independently using several complementary approaches to assess the relative contributions of spin diffusion and molecular diffusion. While these estimates constrain the possible mechanisms of magnetization exchange, they do not permit an unambiguous separation of the spin diffusion and molecular diffusion contributions, suggesting that confinement-induced structural disorder may play an important role in magnetization transport in nanoconfined ice.