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In the present work, published in Physical Review Letters, the combination of two spectroscopic techniques reveals the microscopic mechanisms that control the behavior of water`s dielectric response.
Water dynamics is involved in practically all biological, chemical and geological processes and, therefore, is of utmost importance in many areas of research and industrial applications. Due to the separation of electrical charges, water molecules have a permanent dipole moment that can be used as a marker for molecular orientation upon application of an electric field. Exploiting this idea, dielectric spectroscopy has been one of the main techniques used from the early times to investigate how water molecules move . However, in spite of a 100-years effort, the microscopic nature of the dielectric relaxation of water, in particular, its main component (the so-called Debye peak) and other weaker contributions phenomenologically introduced to give account for the high-frequency data (see Fig. 1(a), ) still remain elusive. The problem is that, since water molecules participate in a dynamic hydrogen-bond network, the dielectric relaxation is collective in nature and cannot be associated to any single-molecule (dipole) reorientation as in the case of non-associative liquids.
In the present study, published in Physical Review Letters, as well as highlighted in `Physics´, coherent and incoherent neutron scattering has allowed identifying water’s nuclei motions, directly monitoring the structural relaxation, and clarifying the origin of the dielectric response. This work also opens a new way of approaching water dynamics under different conditions (supercooled, confined, etc.) and that of other hydrogen-bonded liquids.
Our time-of-flight experiments on protonated and heavy water covered the interesting frequency range for the dynamics at room temperature, revealing single-nuclei (H) motions and collective dynamics respectively with space-time resolution around the main structure factor peak (Qmax ≈ 2 Å-1 for water) and at intermediate length scales. Proper renormalization with the Bose factor delivered the corresponding susceptibilities. In these functions, we can directly distinguish three processes [see Figs. 1(b) and (c)]: (i) The one dominating at low frequencies shows dispersion in Q, indicating diffusive behaviour. (ii) In the other extreme of the spectra, the relevant process is Q-independent and with a rather high characteristic frequency (≈ THz), suggesting an inelastic vibrational origin. (iii) A third intermediate process –more evident in the low-Q coherent data– shows, if any, a very weak Q-dependence. Assuming for the relaxational component, as in the model proposed in Refs. [3,4], simultaneous occurrence of a local process and diffusion, the intermediate contribution would reflect such local process affected by diffusive motions (i. e., an `effective local´ process). This scenario provides an excellent description of all the experimental data [see Fig. 1(b) and (c)].
Figure 2: Mean displacement experimentally obtained for Hs (different symbols for different Q-values in the range 0.19 ≤ Q ≤ 2.0 Å-1) and calculated from the simulations for Hs (solid line) and Os (dashed-dotted line). Dotted lines mark the coordinates corresponding to τD, τl, τv (inverse frequencies of the susceptibility components’ maxima).
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Dielectric Susceptibility of Liquid Water: Microscopic Insights from Coherent and Incoherent Neutron Scattering.
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Physical Review Letters (2016).
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