Dielectric response of light, heavy and heavy-oxygen water: isotope effects on the hydrogen-bonding network's collective relaxation dynamics

Abstract

Isotopic substitutions largely affect the dielectric relaxation dynamics of hydrogen-bonded liquid water; yet, the role of the altered molecular masses and nuclear quantum effects has not been fully established. To disentangle these two effects we study the dielectric relaxation of light (H₂¹⁶O), heavy (D₂¹⁶O) and heavy-oxygen (H₂¹⁸O) water at temperatures ranging from 278 to 338 K. Upon ¹⁶O/¹⁸O exchange, we find that the relaxation time of the collective orientational relaxation mode of water increases by 4–5%, in quantitative agreement with the enhancement of viscosity. Despite the rotational character of dielectric relaxation, the increase is consistent with a translational mass factor. For H/D substitution, the slow-down of the relaxation time is more pronounced and also shows a strong temperature dependence. In addition to the classical mass factor, the enhancement of the relaxation time for D₂¹⁶O can be described by an apparent temperature shift of 7.2 K relative to H₂¹⁶O, which is higher than the 6.5 K shift reported for viscosity. As this shift accounts for altered zero-point energies, the comparison suggests that the underlying thermally populated states relevant to the activation of viscous flow and dielectric relaxation differ.