The Boson peak in confined water: An experimental investigation of the liquid-liquid phase transition hypothesis

Abstract

The Boson peak (BP) of deeply cooled confined water is studied by using inelastic neutron scattering (INS) in a large interval of the (P, T ) phase plane. By taking into account the different behavior of such a collective vibrational mode in both strong and fragile glasses as well as in glass-forming materials, we were able to determine the Widom line that characterizes supercooled bulk water within the frame of the liquid-liquid phase transition (LLPT) hypothesis. The peak frequency and width of the BP correlated with the water polymorphism of the LLPT scenario, allowing us to distinguish the "low-density liquid" (LDL) and "high-density liquid" (HDL) phases in deeply cooled bulk water. Moreover, the BP properties afford a further confirmation of the Widom line temperature T W as the (P, T ) locus in which the local structure of water transforms from a predominately LDL form to a predominately HDL form.

Experimental tests for a liquid-liquid critical point in water

“…In the second case we are inside a region in which the neutron scattering data indicate that the dynamical crossover disappears [32]. The consistency of these results are fully supported by the studies of the Boson peak in which the Widom line is detectable in the P-T plane due to changes in its frequency and width at the crossover from the fragile (HDL dominated phase) to the strong region (dominated instead by the LDL phase) [212].…”

“…The link between the BP and the water polymorphism is verified via the observed BP behaviors in the "LDL" and "HDL" phases of cooled water. In conclusion, the Widom line can be detected from the changes in the frequency and width of the Boson Peak as one approaches a phase region dominated by the LDL phase [212,220]. Figure 37 shows that the pressure-temperature behavior of the self diffusion of water, D S , is also anomalous.…”

“…They have been used to study the BP at the water FSDC and to verify the links between the BP (observed in confined water [210]) and the Widom line hypothesized for liquid water [211]. This has been tested experimentally using inelastic neutron scattering (INS) to verify whether the BP in deeply cooled confined water can confirm the LLCP existence [212]. Figure 35 shows the INS S (Q, ω) of water confined in nanotubes at the ambient pressure in the 290-190 K range.…”

Two structural relaxations in protein hydration water and their dynamic crossovers

“…[69][70][71][72] Recently the onset of the Boson peak in water has been also connected to the presence of a liquid-liquid WL both in bulk and confined water. 69,[73][74][75][76] The oscillating features above 1 ps are not present for hydration water since they are finite size effects that disappear for larger box sizes, as discussed in detail by Kumar et al 69 In summary, the performed MCT test on the τ α of protein hydration water in our system establishes that the correspondent process is an α-relaxation typical of glass formers and it is analogous to that of bulk water. As stated in the Introduction, the α-relaxation and the FSC in hydration water was also found on a slightly different system: a lysozyme protein with a monolayer coverage of water both in experiments and in simulations.…”

Low frequency Raman study of the Boson peak in a Tellurite-tungstate glass over temperature

“…Even though glasses have been so extensively used and studied, some properties remains a puzzle. One of the most interesting example is the observed population of states at Terahertz frequencies found in glasses and any amorphous material [4,5,6]. This excess of vibrational density of states at such frequencies is known as "Boson Peak" (BP) and it has been observed since the 70's [7] in Raman spectra, however, its origin is still subject of debate in the literature [8,9,10].…”