The
Phase Diagram of Water
A phase diagram shows the preferred physical states
of matter at different temperatures and pressure. At typical room
temperatures and pressure (shown as an 'x'
below) water is a liquid, but it becomes solid (i.e. ice)
if its temperature is lowered below 273 K and gaseous (i.e.
steam) if its temperature is raised above 373 K, at the same pressure.
Each line gives the conditions when two phases coexist but a change
in temperature or pressure may cause the phases to abruptly change
from one to the other. Where three lines join, there is a 'triple
point' when three phases coexist but may abruptly and totally change
into each other given a change in temperature or pressure. Four
lines cannot meet at a single point. A 'critical point' is where
the properties of two phases become indistinguishable from each
other. The phase diagram of water is complex,g
having a number of triple points and one or possibly two critical
points.
The boundaries shown for ice-ten (X)
and the high pressure ice-eleven(XI)
and the boundary between supercritical water and ice-seven
(VII) (see
[691]) are still
to be established.
All the solid phases
of ice involve the water molecules being hydrogen
bonded to four neighboring water molecules. In
all cases the two hydrogen atoms are equivalent, with
the water molecules retaining their symmetry, and
they all obey the 'ice' rules: two hydrogen atoms
near each oxygen, one hydrogen atom on each O····O
bond. The H-O-H angle in the ice phases is expected
to be a little less than the tetrahedral angle (109.47°),
at about 107°.
|
Triple
points |
MPa |
°C |
Ref. |
D2O
[711] |
liquid |
gas |
Ih |
0.000611657 |
0.010 |
536 |
661 Pa, 3.82°C
[70] |
liquid |
gas |
XI |
0 |
-201.0 |
711 |
0 MPa, -197°C |
liquid |
Ih |
III |
207.5 |
-22.0 |
537 |
220 MPa, -18.8°C |
Ih |
II |
III |
212.9 |
-34.7 |
537 |
225 MPa, -31.0°C |
II |
III |
V |
344.3 |
-24.3 |
537 |
347 MPa, -21.5°C |
liquid |
III |
V |
346.3 |
-17.0 |
537 |
348 MPa. -14.5°C |
II |
V |
VI |
~620 |
~-55 |
539 |
|
liquid |
V |
VI |
625.9 |
0.16 |
537 |
629 MPa, 2.4°C |
VI |
VII |
VIII |
2,100 |
~5 |
8 |
1950 MPa, ~0°C |
liquid |
VI |
VII |
2,200 |
81.6 |
8 |
2060 MPa, 78°C |
VII |
VIII |
X |
62,000 |
-173 |
538 |
|
liquid |
VII |
X |
43,000 |
>700 |
612 |
|
|
Both the critical points are shown as red circles in the phase
diagram, above. Beyond the critical
point in the liquid-vapor space (towards the top right, above),
water is supercritical existing as small but liquid-like hydrogen-bonded
clusters dispersed within a gas-like phase [456],
where physical properties, such as gas-like or liquid-like behavior,
vary in response to changing density. The properties of supercritical
water are very different from ambient water. For example, supercritical
water is a very poor solvent for electrolytes, but excellent for
non-polar molecules, due to its low dielectric constant and poor
hydrogen bonding. The physical properties of water close to the
critical point (near-critical) are particularly strongly affected
[677].
The critical point and the orange line in the ice-one phase
space refer to the low-density (LDA) and high-density (HDA)
forms of amorphous water (ice) [16].
Although generally accepted, the existence of this second,
if metastable, critical point is impossible to prove at the
present time and is disputed
by some [200, 618,
628]. The transition
between LDA and HDA is due to the increased entropy and van
der Waals contacts in HDA compensating for the reduced strength
of its hydrogen bonding. The high-pressure phase lines of
ice-ten (X) and ice-eleven (XI) [81]
are still subject to experimental verification. Two different
forms of ice-eleven have been described by different
research groups: the high-pressure form (also known as ice-thirteen)
involves hydrogen atoms equally-spaced between the oxygen
atoms [84] (like ice-ten)
whereas the lower pressure low
temperature form utilizes the incorporation of hydroxide
defect doping (and interstitial K+ ions) to order
the hydrogen bonding of ice Ih
[207], that otherwise
occurs too slowly. Another ice-ten has been described, being
the proton ordered form of ice-six (VI) occurring
below about 110 K. Only hexagonal ice-one
(Ih), ice-three
(III),
ice-five (V), ice-six
(VI)
and ice-seven (VII) can be
in equilibrium with liquid water, whereas all the others ices,
including ice-two (II, [273]),
are not stable in its presence under any conditions of temperature
and pressure. Ice-two, ice-eight
(VIII),
ice-nine (IX), ice-ten
[80] and ice-eleven
(both) all possess (ice-nine incompletely) ordered hydrogen-bonding
whereas in the other ices the hydrogen-bonding is disordered
even down to 0 K, where reachable. Ice-four (IV) and ice-twelve
(XII)
[82] are both metastable
within the ice-five phase space. Cubic
ice (Ic)
is metastable with respect to hexagonal
ice (Ih).
It is worth emphasizing that liquid water is stable throughout
its phase space above. Kurt Vonnegut's highly entertaining
story concerning an (imaginary) ice-nine, which was capable
of crystallizing all the water in the world [83],
fortunately has no scientific basis (see also
IE) as ice-nine, in reality, is a proton ordered
form of ice-three, only exists at very low temperatures and
high pressures and cannot exist alongside liquid water under
any conditions. Ice Ih
may be metastable with respect to empty clathrate
structures of lower density under negative pressure conditions
(i.e. stretched) at very low temperatures [520].
As pressure increases, the ice phases become
denser. They achieve this by initially bending bonds, forming tighter
ring or helical networks, and finally including greater amounts
of network inter-penetration. This is particularly evident when
comparing ice-five with the metastable ices (ice-four and ice-twelve)
that may exist in its phase space.
The liquid-vapor density data for the graphs above were obtained from
the IAPWS-95
equations [540]. Other phase diagrams
for water are presented elswewhere [681].
Ice
polymorph |
Density,
g cm-3 a |
Protonsf |
Crystalh |
Symmetry |
Dielectric constant,
eSi |
Notes |
Hexagonal
ice, Ih |
0.92 |
disordered |
Hexagonal |
one C6 |
97.5 |
|
Cubic
ice, Ic |
0.92 |
disordered |
Cubic |
four C3 |
|
|
LDA
b |
0.94 |
disordered |
Non-crystalline |
|
|
As prepared, may be mixtures
of several types |
HDA
c |
1.17 |
disordered |
Non-crystalline |
|
|
As prepared, may be mixtures
of several types |
VHDA
d |
1.25 |
disordered |
Non-crystalline |
|
|
|
II,
Ice-two |
1.17 |
ordered |
Rhombohedral |
one C3 |
3.66 |
|
III,
Ice-three |
1.14 |
disordered |
Tetragonal |
one C4 |
117 |
protons may be partially ordered |
IV,
Ice-four |
1.27 |
disordered |
Rhombohedral |
one C3 |
|
metastable in ice V
phase space |
V,
Ice-five |
1.23 |
disordered |
Monoclinic |
one C2 |
144 |
protons may be partially ordered |
VI,
Ice-six |
1.31 |
disordered |
Tetragonale |
one C4 |
193 |
protons can be partly ordered |
VII,
Ice-seven |
1.50 |
disordered |
Cubice |
four C3 |
150 |
two interpenetrating ice Ic
frameworks |
VIII,
Ice-eight |
1.46 |
ordered |
Tetragonale |
one C4 |
4 |
low temperature form of ice
VII |
IX,
Ice-nine |
1.16 |
ordered |
Tetragonal |
one C4 |
3.74 |
low temperature form of ice
III |
X,
Ice-ten |
2.51 |
symmetric |
Cubice |
four C3 |
|
symmetric proton form of ice
VII |
XI,
Ice-eleven |
0.92 |
ordered |
Orthorhombic |
three C2 |
|
low temperature form of ice
Ih |
XI,
Ice-eleven |
>2.51 |
symmetric |
Hexagonale |
distorted |
|
Found in simulations only |
XII,
Ice-twelve |
1.29 |
disordered |
Tetragonal |
one C4 |
|
metastable in ice V phase space |
Ice
polymorph |
Molecular
environments |
Small
ring size(s) |
Helix |
Approximate
O-O-O angles, ° |
Ring
penetration hole size |
Hexagonal
ice, Ih |
1 |
6 |
None |
All 109.47±0.16 |
None |
Cubic
ice, Ic |
1 |
6 |
None |
109.47 |
None |
LDA
b |
3+ |
5, 6 |
None |
mainly
108, 109 and 111 |
None |
HDA
c |
6+ |
5, 6 |
None |
broad
range |
None |
VHDA
d |
6+ |
5, 6 |
None |
broad
range |
|
II,
Ice-two |
2 (1:1) |
6 |
None |
80,100,107,118,124,128;
86,87,114,116,128,130 |
None |
III,
Ice-three |
2 (1:2) |
5, 7 |
4—fold |
(1) 91,95,112,112,125,125
(2) 98,98,102,106,114,135 |
None |
IV,
Ice-four |
2 (1:3) |
6 |
None |
(1) 92,92,92,124,124,124
(3) 88,90,113,119,123,128 |
some 6 |
V,
Ice-five |
4 (1:2:2:2) |
4, 5,
6, 8 |
None |
(1) 82,82,102,131,131,131
(2) 88,91,109,114,118,128
(3) 85,91,101,103,130,135
(4) 84,93,95,123,125,126 |
8 (1 bond) |
VI,
Ice-six |
2 (1:2) |
4, 8 |
None |
(1) 77,77,128,128,128,128
(2) 78,89,89,128,128,128 |
8 (2 bond) |
VII,
Ice-seven |
1 |
6 |
None |
109.47 |
every
6 |
VIII,
Ice-eight |
1 |
6 |
None |
109.47 |
every
6 |
IX,
Ice-nine |
2 (1:2) |
5, 7 |
4—fold |
(1) 91,95,112,112,125,125
(2) 98,98,102,106,114,135 |
None |
X,
Ice-ten |
1 |
6 |
None |
109.47 |
every
6 |
XI,
Ice-eleven |
1 |
6 |
None |
109.47 |
None |
XI,
Ice-eleven |
undetermined |
6 |
None |
undetermined |
every
6 |
XII,
Ice-twelve |
2 (1:2) |
7, 8 |
5—fold |
(1) 107,107,107,107,115,115
(2) 67,83,93,106,117,132 |
None |
a density at atmospheric pressure.
[Back]
b Low-density amorphous ice (LDA).
The structural data in the Table is given assuming LDA has
the structure of ES.
[Back]
c High-density amorphous ice (HDA).
The structural data in the Table is given assuming HDA has
the structure of crushed CS.
[Back]
d Very high-density amorphous
ice (VHDA). The structural data
in the Table assumes no hydrogen bond rearrangements from
LDA or HDA. [Back]
e Structure consists of two interpenetrating
frameworks. [Back]
f Although primarily ordered or
disordered, ordered arrangements of hydrogen bonding may not
be perfect and disordered arrangements of hydrogen bonding
are not totally random as there are correlated and non-bonded
preferential effects. [Back]
g If water behaved more
typically as a low molecular weight material, its phase
diagram may have looked rather like this:
[Back] |
|
h Crystal cell parameters have
been collated [711]. [Back]
i Dielectric constants fall into
two categories dependent on whether the hydrogen bonds are
ordered (low values) or disordered (high values). [Back]
|