Evidence for an anomalous quantum state of protons in nanoconfined water

Abstract

Deep inelastic neutron scattering provides a means of directly and accurately measuring the momentum distribution of protons in water, which is determined primarily by the proton ground-state wave function.We find that in water confined on scales of 20 A, this wave function responds to the details of the confinement, corresponds to a strongly anharmonic local potential, shows evidence in some cases of coherent delocalization in double wells, and involves changes in zero-point kinetic energy of the protons from −40 to +120 meV difference from that of bulk water at room temperature. This behavior appears to be a generic feature of nanoscale confinement. It is exhibited here in 16 A inner diameter carbon nanotubes, two different hydrated proton exchange membranes (PEMs), Nafion 1120 and Dow 858, and has been seen earlier in xerogel and 14 A diameter carbon nanotubes. The proton conductivity in the PEM samples correlates with the degree of coherent delocalization of the proton.