The
Phase Diagram of Water
A phase diagram shows the preferred physical
states of matter at different temperatures and pressure. Within
each phase the material is uniform with respect to its chemical
composition and physical state. At typical temperatures and
pressure (marked by an 'E'
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 represents a phase boundary
and gives the conditions when two phases coexist. Here, 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. Many of the crystalline forms may remain metastable
in much of the low-temperature phase space at lower pressures.
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. The mean surface conditions on Earth, Mars and Venus
are indicated.
All the crystalline 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.j 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°.
Thermodynamic data for the triple points of water
Triple points |
MPa |
°C |
ΔS,
J mol-1 K-1 |
ΔV
cm3 mol-1 |
Ref. |
D2O [711] |
gas |
liquid |
Ih |
0.000611657 |
0.010 |
|
536 |
661 Pa, 3.82°C [70] |
gas liquid |
-132.5 |
-22050 |
|
|
gas Ih
|
-154.5 |
-22048 |
liquid Ih |
-22.0 |
1.634 |
8 |
gas |
Ih |
XI |
0 |
-201.0 |
|
|
711 |
0 MPa, -197°C |
liquid |
Ih |
III |
207.5 |
-22.0 |
|
537 |
220 MPa, -18.8°C |
liquid Ih |
-14.9 |
2.434 |
838 |
|
liquid III |
-13.9 |
-0.839 |
Ih III |
1.0 |
-3.273 |
Ih |
II |
III |
212.9 |
-34.7 |
|
537 |
225 MPa, -31.0°C |
Ih II |
-2.1 |
-3.919 |
838 |
|
Ih III |
1.0 |
-3.532 |
IIIII |
3.2 |
0.387 |
II |
III |
V |
344.3 |
-24.3 |
|
|
537 |
347 MPa, -21.5°C |
IIIII |
3.1 |
0.261 |
838 |
|
IIV |
3.3 |
-0.721 |
IIIV |
0.1 |
-0.982 |
liquid |
III |
V |
346.3 |
-17.0 |
|
537 |
348 MPa. -14.5°C |
liquid III |
-13.2 |
-0.434 |
838 |
|
liquid V |
-13.1 |
-1.419 |
IIIV |
0.1 |
-0.985 |
II |
V |
VI |
~620 |
~-55 |
|
539 |
|
liquid |
V |
VI |
625.9 |
0.16 |
|
537 |
629 MPa, 2.4°C |
liquid V |
-15.7 |
-0.949 |
838 |
|
liquid VI |
-16.2 |
-1.649 |
VVI |
-0.5 |
-0.700 |
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, 894],
where physical properties, such as gas-like or liquid-like
behavior, vary in response to changing density. The critical
isochor (density 322 kg m-3) is shown as the thin
dashed line extension; this may be thought of as dividing
more-liquid-like and more-gas-like properties [540].
The properties of supercritical water are very different from
ambient water. For example, supercritical water is a poor
solvent for electrolytes, which tend to form ion pairs. However,
it is such an excellent solvent for non-polar molecules, due
to its low dielectric constant and poor hydrogen bonding,
that many are completely miscible. Viscosity and dielectric
both decrease substantially whereas auto-ionization increases substantially. The physical properties of water
close to the critical point (near-critical) are particularly
strongly affected [677],
Extreme density fluctuations around the critical point causes
opalescent turbidity. Many properties of cold liquid water
change above about 200 MPa (e.g. viscosity, self-diffusion, compressibility, Raman spectra and molecular
separation), which may be explained by the presence of
a high density liquid phase containing interpenetrating hydrogen
bonds.
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 and supported by diverse experimental
evidence [754a, 861], the existence
of this second, if metastable, critical point is impossible
to prove absolutely at the present time and is disputed
by some [200, 618, 628, 754b]. The transition
between LDA and HDA is due to the increased entropy and attractive
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.
The low-temperature ices, ice-two, ice-eight (VIII), ice-nine (IX), ice-eleven (low pressure
form), ice-thirteen (XIII) [1002] and ice-fourteen (XIV) [1002] all possess (ice-nine and ice-fourteen incompletely) low entropy ordered
hydrogen-bonding whereas in the other ices (except ice-ten [80] and ice-eleven where the hydrogen atoms are symmetrically placed) 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 elsewhere [681].
Structural data on the ice polymorphs
Ice
polymorph |
Density,
g cm-3 a |
Protonsf |
Crystalh |
Symmetry |
Dielectric constant, εSi |
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,
metastable in ice II space |
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-elevenk |
>2.51 |
symmetric |
Hexagonale |
distorted |
|
Found in simulations only |
XII, Ice-twelve |
1.29 |
disordered |
Tetragonal |
one C4 |
|
metastable in ice V phase space |
XIII, Ice-thirteen |
1.23 |
ordered |
Monoclinic |
one C2 |
|
ordered form of ice V phase |
XIV, Ice-fourteen |
1.29 |
mostly ordered |
Orthorhombic |
one C4 |
|
ordered form of ice XII phase |
More structural data on the ice polymorphs
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 (1:4) |
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-elevenk |
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 |
XIII, Ice-thirteen |
7 (all equal) |
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) |
XIV, Ice-fourteen |
2 (1:2) |
7, 8 |
5—fold |
(1) 107,107,107,107,115,115
(2) 67,83,93,106,117,132 |
None |
Other stable or metastable phases of ice have been proposed
(e.g. Ice XIII and ice XIV [958]) and
may exist but their structures have not been established.
Such new phases are thought particularly likely to be found
within the phase space of ice II and ice V.
Several new phases (e.g. ice
1h1c and ice
i) have only been found (so far) in modeling studies.
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. As VHDA is likely to be a a relaxed form of HDA,
this assumption seems unlikely [935].
[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]
j Weaknesses (Bjerrum defects)
in the ice crystal are apparent where the ice
rules are disobeyed. Both O····O
contacts, without an intervening proton (L defect) and O-H····H-O
contacts (D defect) may occur due to molecular rotations where
neighboring water molecules fail to adjust their hydrogen
bonding. Other defects may be caused by the presence of H3O+ and OH- ions. [Back]
k Also known as ice XIII.
[Back]
|