SUPERCOOLED WATER: TWO LIQUIDS? · Francesco Sciortino Dipartimento di Fisica, Sapienza,...
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SUPERCOOLED WATER: TWO LIQUIDS? Francesco Sciortino Dipartimento di Fisica, Sapienza, Universita' di Roma, Piazzale Aldo Moro 2, 00165 Roma In 1992, the results of a computer simulation study of the ST2 model of water were used to propose that a liquid-liquid phase transition (LLPT) occurs in supercooled water[1]. According to this hypothesis, supercooled water could coexist in two different liquid forms, differing in their density and in their local order. Below the critical temperature T c for the proposed LLPT, two distinct phases of water, the low density liquid (LDL) and high density liquid (HDL) phases are separated by a first-order phase transition. Along the liquid-liquid coexistence line, LDL floats on HDL. An appealing feature of the LLPT proposal is that it simultaneously accounts for (a) the unusual thermodynamic behavior of liquid water in the supercooled region, and (b) the occurrence of two distinct forms of amorphous solid water in the glassy regime. Fig.1. The phase diagram of ST2-water in the pressure-temperature and in the temperature-density planes. The liquid-liquid critical point is marked C. The other lines indicate the line of extrema in the density, compressibility and specific heat. Triangles indicate the liquid-liquid spinodal curves (from Ref. [2]). Experimentally, a LLPT has yet to be decisively confirmed in supercooled water, and efforts to resolve this question in the laboratory continue. The predicted location of the critical point in the supercooled regime is challenging to study in experiments because of rapid ice crystallization. In simulations, this problem is avoided when the liquid can be studied on a time scale that is long relative to the liquid-state relaxation time, but short compared to crystal nucleation times. Evidence for a LLPT has been reported in a number of simulation studies of water and water-like systems, when crystallization does not interfere with the possibility of equilibrating the liquid in supercooled states. Other studies have fiercely contrasted the hypothesis of the LLPT in water. The idea of a LLPT has been also found useful to interpret the behavior of other model systems, including Silicon, SiO 2 , Carbon and other tetrahedral network forming liquids. Interestingly, it has also been found relevant in the interpretation of soft-matter systems where the possibility of overcoming nucleation is significantly higher. One case is offered by four-arm DNA constructs, where theoretical calculations[3] predicts
SUPERCOOLED WATER: TWO LIQUIDS? · Francesco Sciortino Dipartimento di Fisica, Sapienza, Universita' di Roma, Piazzale Aldo Moro 2, 00165 Roma In 1992, the results of a computer simulation
Dipartimento di Fisica, Sapienza, Universita' di Roma, Piazzale
Aldo Moro 2, 00165 Roma
In 1992, the results of a computer simulation study of the ST2
model of water were used to propose that a liquid-liquid phase
transition (LLPT) occurs in supercooled water[1]. According to this
hypothesis, supercooled water could coexist in two different liquid
forms, differing in their density and in their local order. Below
the critical temperature Tc for the proposed LLPT, two distinct
phases of water, the low density liquid (LDL) and high density
liquid (HDL) phases are separated by a first-order phase
transition. Along the liquid-liquid coexistence line, LDL floats on
HDL. An appealing feature of the LLPT proposal is that it
simultaneously accounts for (a) the unusual thermodynamic behavior
of liquid water in the supercooled region, and (b) the occurrence
of two distinct forms of amorphous solid water in the glassy
regime.
Fig.1. The phase diagram of ST2-water in the
pressure-temperature and in the temperature-density planes. The
liquid-liquid critical point is marked C. The other lines indicate
the line of extrema in the density, compressibility and specific
heat. Triangles indicate the liquid-liquid spinodal curves (from
Ref. [2]). Experimentally, a LLPT has yet to be decisively
confirmed in supercooled water, and efforts to resolve this
question in the laboratory continue. The predicted location of the
critical point in the supercooled regime is challenging to study in
experiments because of rapid ice crystallization. In simulations,
this problem is avoided when the liquid can be studied on a time
scale that is long relative to the liquid-state relaxation time,
but short compared to crystal nucleation times. Evidence for a LLPT
has been reported in a number of simulation studies of water and
water-like systems, when crystallization does not interfere with
the possibility of equilibrating the liquid in supercooled states.
Other studies have fiercely contrasted the hypothesis of the LLPT
in water. The idea of a LLPT has been also found useful to
interpret the behavior of other model systems, including Silicon,
SiO2, Carbon and other tetrahedral network forming liquids.
Interestingly, it has also been found relevant in the
interpretation of soft-matter systems where the possibility of
overcoming nucleation is significantly higher. One case is offered
by four-arm DNA constructs, where theoretical calculations[3]
predicts
the possibility of a cascade of liquid-liquid phase
separations.
Fig.2. Graphic representation of a simple model for DNA
constructs with four single-strand self-complementary arms in a
bonded and in a un-bonded configuration. Under the appropriate
conditions of temperature and density the system organizes itself
into interpenetrating tetra-coordinated networks. (from Ref. [3]).
In the talk, I will review recent numerical investigations[4,5]
confirming the LLCP scenario and discuss the possibilities of
observing it experimentally using DNA-made particles. References.
[1] P. H. Poole, F. Sciortino, U. Essmann, H. E. Stanley Phase
behavior of metastable water Nature 360, 324-328, 1992 [2]P.H.
Poole, I. Saika-Voivod, F. Sciortino Density minimum and
liquid-liquid phase transition J. Phys. Cond. Matt.17, L431-L437,
2005 [3] C.W. Hsu, J. Largo, F. Sciortino, and F. W. Starr
Hierarchies of networked phases induced by multiple liquid--liquid
critical points PNAS, 105, 13711-13715 (2008). [4] F. Sciortino, I.
Saika-Voivod and P. H. Poole Study of the ST2 model of water close
to the liquid-liquid critical point Phys. Chem. Chem. Phys. 13,
19759-19764 (2011) [5] P. H. Poole, R. K. Bowles, Ivan
Saika-Voivod, and F. Sciortino Free energy surface of ST2 water
near the liquid-liquid phase transition J. Chem. Phys. 138, 034505
(2013); doi: 10.1063/1.4775738 (see
http://glass.phys.uniroma1.it/sciortino/publications.htm for
article download) Corresponding Author: F. Sciortino, Dipartimento
di Fisica, Sapienza Universita' di Roma, Piazzale Aldo Moro 2,
00165 Roma, Italy – Phone: +390649913799 – Email:
[email protected]
