Nanostructures and 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.