for more information on these topics contact: R.Torre ; torre@lens.unfi.it
Vibrational Spectroscopy and Dynamics of Water, Perakis, F.; Marco, L.; Shalit, A.; Tang, F.; Kann, Z. R.; Kuhne, T. D.; Torre, R.; Bonn, M.; Nagata, Y. Chem Rev 116 (2016), 7590–7607.
Optical Kerr Effect of Liquid and Supercooled Water: The Experimental and Data Analysis Perspective. A. Taschin, P. Bartolini, R. Eramo, R. Righini, and R. Torre. Journal of Chemical Physics,141 (2014) 084507.
Evidence of two distinct local structures of water from ambient to supercooled conditions. A.Taschin,P. Bartolini, R. Eramo, R. Righini, and R. Torre, Nat. Commun. 4 (2013) 1–8 ,
Water can remain liquid below its melting point and stays in a metastable phase known as “supercooled”. When water approaches or enters this phase shows a series of anomalous physical behaviors that distinguish water from the other liquids; for example the density shows a maximum for 4 °C and decreases at lower temperatures, the viscosity begins to diverge under the 0 °C. Despite the simplicity of water molecules its liquid phase is very complex to be described by physical models, this is due to the strong network of hydrogen bonds formed by the molecules.
The most accredited model assumed that the water is actually formed by two different liquids: low-density and high- density water. The first is characterized by intermolecular tetrahedral local structures, very similar to those found in ice, the second by less ordered and more compact structures. According to this model, at low temperatures these two water forms are distinct and separate from a phase transition, while at higher temperatures they interpenetrate and continually exchange on very fast time scales, typically 10-12 seconds. The experimental detection of this two water forms is particularly difficult because of the fast and frequent rearrangements of the local liquid structures. To date is still missed a definitive proof of the existence of these two water forms.
Our group, in the laboratories of the European Lab. for Non-Linear Spectroscopy (LENS), measured the dynamics of liquid water at temperatures down to -28 °C using a ultrafast spectroscopy technique based on femtosecond laser sources (10-15 seconds). The measurement of the intermolecular vibrations and the local dynamics of the water molecules have shown the presence of two major molecular organizations: one characterized by a high order of tetrahedral hydrogen bonds, while the other presents strong local distortions of the lattice. These two types of local organization of water molecules can be interpreted as evidence of the existence of water of low and high density.
THz Dynamics of Nanoconfined Water by Ultrafast Optical Spectroscopy, Taschin A., Bartolini P. and Torre R. Meas. Sci. Technol. 28 (2017) 014009,
Confinement, entropic effects and hydrogen bond network fluctuations of water in Nafion membrane. Plazanet, M., Torre, R., Sacchetti, F.
J. of Molecular Liquids, 219 (2016) Pages 1161–1164.
Supercooling and Freezing Processes in Nanoconfined Water by Time-Resolved Optical Kerr Effect Spectroscopy. A.Taschin, P. Bartolini, A. Marcelli, R. Righini, and R. Torre. Journal of Physics: Condensed Matter, 27 (2015) 194107.
A comparative study on bulk and nanoconfined water by time-resolved optical Kerr effect spectroscopy. A. Taschin, P. Bartolini, A. Marcelli, R. Righini, & R. Torre. Faraday Discussion 167 (2013) 293
The study of dynamical and transport processes of liquid matter confined at the nano metric scale is a basic issue for contemporary physics. Its understanding pertains to the technological applications and to fundamental issues.
The question is, at what extend the chemical-physic properties of the nano-confined liquid is overlapping to the bulk features ?
In particular, the investigation and study of nano-confined water is relevant to a broad variety of scientific topics, spanning from biology to geophysics. Nanoconfinement has the advantage of preventing water freezing and of enabling the experimental investigation of the supercooled phase down to very low temperatures. Nevertheless, the water crystallization doesn’t take place only if the confinement is very tight, reaching the nanometric length scale. Already the confinement of water in silica nanopores of 10 nm diameter produces a lowering of the crystallization temperature of about 15 degrees. In order to maintain the water liquid approaching Ts a confinement of about 2.5 nm is required, in pore of diameter ≤ 2 nm water remains in a liquid-like phase.
There is a plethora of materials characterized by nano-heterogeneity that are able to produce a nano-confinement of liquids.
Porous silica glasses have proven to be excellent media for research. A porous glass is a silicate glass presenting a random network of empty pores with a diameter of a few nano-meters. Porous glasses with pores of different diameter, from a few to a hundred nano-meters, are commercially available. Furthermore, the pores can be easily filled with liquids of different nature. Indeed, liquid-filled porous glass represents an interesting material with a partially controllable nano heterogeneity.
The prototype of these materials is Vycor: is an open cell porous glass which is produced by a spinodal demixing in a borate-rich and borate-poor glass and subsequent bleaching of the borate-rich phase. It has nominal values of porosity and mean pore size of 28% and 4 nm respectively. We measure the acoustic propagation, the liquid flow and the the
Moreover, the water maximum density shifts to lower temperatures due to the confinement effect.rmal diffusion by heterodyne detected transient grating measurements on water filled Vycor7930 at variable temperature. The experimental results confirm that transport of elastic energy (i.e. acoustic propagation), heat (i.e. thermal diffusion) and mass (i.e. liquid flow) in a liquid filled porous glass can be described according to hydrodynamic laws in spite of nanometric dimension of the pores.
The optical Kerr effect experiments performed at different hydration levels prove that interfacial water has a different dynamic properties respect to the inner water.
The polymeric electrolyte membranes are extremely interesting materials that adsorb water in nanometric cavities and pores. Despite the relevant applications in Fuel Cells technology, the water sorption/desorption and freezing phenomena taking place in these membranes remain to be explained.
Nafion is the more studied and used polymeric electrolyte membrane showing the presence of water-filled spaces (i.e.cavities and/or pores) inside a polymeric matrix wrapped by sulphonate groups.The characteristic dimensions of these cavities or pores depend on the amount of adsorbed water,typically spanning from 2nm to about 6nm.
Plazanet, M., Sacchetti, F., Petrillo, C., Deme, B., Bartolini, P., & Torre, R. (2013). Water in a polymeric electrolyte membrane: Sorption/desorption and freezing phenomena. Journal of Membrane Science, 453, 419–424. doi:10.1016/j.memsci.2013.11.026
Plazanet, M., Bartolini, P., Sangregorio, C., Taschin, A., Torre, R., & Trommsdorff, H.-P. (2010). Inverse freezing in molecular binary mixtures of alpha-cyclodextrin and 4-methylpyridine. Physical chemistry chemical physics : PCCP, 12(26), 7026–31. doi:10.1039/b923682a