Abstract
The problem of large-density variations in supercooled and ambient water has been widely discussed in the past years. Recent studies have indicated the possibility of nanometer-sized density variations on the subpicosecond and picosecond time scales. The nature of fluctuating density heterogeneities remains a highly debated issue. In the present work, we address the problem of possible association of such density variations with the dynamics of terahertz longitudinal acoustic-like modes in liquid water. Our study is based on the fact that the subpicosecond dynamics of liquid water are essentially governed by the structural relaxation. Using a mode coupling theory approach, we found that for typical values of parameters of liquid water, the dynamic mechanism coming from the combination of the structural relaxation process and the finiteness of the amplitude of terahertz longitudinal acoustic-like mode gives rise to a soliton-like collective mode on a temperature-dependent nanometer length scale. The characteristics of this mode are consistent with the estimates of the amplitudes and temperature-dependent correlation lengths of density fluctuations in liquid water obtained in experiments and simulations. Thus, the fully dynamic mechanism could contribute to the formation and dynamics of fluctuating density heterogeneities. The soliton-like collective excitations suggested by our analysis may be relevant to different phenomena connected with supercooled water and can be expected to be associated with some ultrafast biological processes.
Highlights
The problem of large-density variations in supercooled and ambient water has been widely discussed in the past years
Molecular dynamics (MD) simulations based on the SPC/E model showed the presence of transient low- and high-density (< 0.9 or > 1.1 kg/m3) regions in water for both supercooled and ambient conditions, interpreted within a continuum model
Large-density fluctuations in liquid water can be explained in the framework of heterogeneous models based on fluctuations between two classes of different structures[12], known as low-density liquid (LDL) and high-density liquid (HDL) s tructures[1], locally favoured and normal s tructures[32], as well as symmetrical and
Summary
The problem of large-density variations in supercooled and ambient water has been widely discussed in the past years. Coexisting low- and high-density regions in ambient water have been identified using Voronoi voids on a larger length scale of 1–2 nm than those predicted by local order parameters and those reported previously[19]. An expected density difference between LDL-like and HDL-like species is 20–30%21,34–36, while an average length scale of density fluctuations was estimated as ~ 1.5 nm for ambient w ater[1,16].
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