Francesco Mallamace
Istituto Sistemi Complessi del CNR, Roma, Italia
The bulk liquid water density data (ρ) are considered in a very large temperature-pressure range including also the glass phases. A care analysis of their isobars, together with the suggestions of recent thermodynamical studies, give evidence of two crossovers at T* and P* above which the hydrogen bond interaction (HB) is unable to arrange the tetrahedral network that is at the basis of the liquid polymorphism giving rise to the low density liquid (LDL). The curvature of these isobars, as a function of T, are completely different: concave below P* (where maxima are) and convex above.
In both the cases a continuity between liquid and glass is observed with P* as the border of the density evolution towards the two different polymorphic glasses (low and high density amorphous, LDA and HDA). The experimental data of the densities of these two glasses also show a marked different pressure dependence. Here, on the basis of these observations in bulk water and by considering recent studies on the growth of the LDL phase, by decreasing temperature we discuss the water liquid-liquid transition and evaluate the isothermal compressibility inside the deep supercooled regime. Such a quantity shows an additional maximum that is pressure dependent that under ambient conditions agrees with a recent X-ray experiment. In particular, the present analysis suggests the presence of a liquid-liquid critical point located at about 180 MPa and 197 K.
A result, this latter fully confirmed by NMR dynamical data studied in the frame of the mode-coupling theory.