nanostructures and liquids
Nanoconfined Liquids
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 thermal 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.Moreover, the water maximum density shifts to lower temperatures due to the confinement effect.The optical Kerr effect experiments performed at different hydration levels prove that interfacial water has a different dynamic properties respect to the inner water.
... more in:
THz Dynamics of Nanoconfined Water by Ultrafast Optical Spectroscopy,
Taschin A., Bartolini P. and Torre R.
Meas. Sci. Technol. 28 (2017) 014009, doi:10.1088/1361-6501/28/1/014009
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
Cucini, R., Taschin, A, Bartolini, P., & Torre, R. (2010). Acoustic phenomena and hydrodynamic flow in a water filled nano-porous glass studied by transient grating spectroscopy. Journal of Physics: Conference Series, 214, 012032. doi:10.1088/1742-6596/214/1/012032
Cucini, R., Taschin, A., Bartolini, P., & Torre, R. (2010). Acoustic, thermal and flow processes in a water filled nanoporous glass by time-resolved optical spectroscopy. Journal of the Mechanics and Physics of Solids, 58(9), 1302–1317. doi:10.1016/j.jmps.2010.06.002
Taschin, A., Cucini, R., Bartolini, P., & Torre, R. (2010). Temperature of maximum density of water in hydrophilic confinement measured by transient grating spectroscopy. EPL (Europhysics Letters), 92(2), 26005. doi:10.1209/0295-5075/92/26005
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.
Performing neutron diffraction and Transient grating experiments, we monitored the quantity of ice formed during the sorption/desorption as a function of temperature down to 180 K. Upon cooling, we observe that ice forms outside of the membrane .and crystallizes. Simultaneously, the membrane shrinks and dehydrate, leading to an increase of the hydronium ions concentration inside the matrix. Reversibly, the ice melts and the membrane re-hydrate upon heating. We proposed a model that explain the main observed processes.
... more in:
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.
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
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