Coherent neutron scattering study of confined water in nafion

The vibrational dynamics of water molecules confined inside the cavities of a nafion membrane has been investigated exploiting the experimental technique of Brillouin neutron scattering to cover two different kinematic regions. Despite the complexity of the experiments, the inelastic data show unambiguously that confined water still sustains collective modes, although the mode lifetime is affected by a larger damping than in bulk water. The collective-mode velocity (3040 m/s) is found to be much higher than the hydrodynamic value (1320 m/s), such as in the case of bulk water. The structure of the inelastic peaks is consistent with a complex picture where normal vibrational modes of a water droplet coexist with the collective excitation. In addition, no evidence of the second nondispersive excitation at similar or equal to 6 meV observed in bulk water is seen in the present experiment. Finally, the analysis of the inelastic integrated intensities suggests that the anomalous trend observed in bulk water at low-momentum transfers disappears under geometrical confinement.