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Water structure and behavior

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H2O

 

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.

[Phase Diagram] melting line of ice VII, Ln(P/2216)=1.73683x(1-355/T)-0.0544606x(1-(T/355)^5)+0.806106x(10^-7)x(1-(T/355)^22); P in MPa, T in Kelvin melting line of ice VI, P/632.4=1-1.07476x(1-T/273.31)^4.6; P in MPa, T in Kelvin 2nd critical point, ~182 K, ~195 MPa, ~1.1 g cm-3  [580] room temperature and pressure critical point, 647.096 K, 22.064 MPa, 322 Kg m-3 ice Ih-vapor line; Ln(P, Pa)=-5866.6426/T +22.32870244+0.0139387003T-3.4262402x10^(-5)xT^2+2.7040955x10^(-8)xT^3+0.67063522LnT; T in Kelvin liquid-vapor line; Ln(P, Pa)=-2836.5744/T^2 -6028.076559/T+19.54263612-0.02737830188T+1.6261698x10^(-5)xT^2+7.0229056x10^(-10)xT^3-1.8680009x10^(-13)xT^4+2.7150305LnT; T in Kelvin Vapor pressure Ice Ih; density (P=0) 0.9167 g/cm^3 Ice Ic; density (P=0) 0.92 g/cm^3 Ice-ten Ice-nine; density (P=0) 1.16 g/cm^3 Ice-eight; density (P=0) 1.49 g/cm^3 Ice-two; density (P=0) 1.17 g/cm^3 Phase boundaries [ref 273]; I-II P=176.0+0.918(T-198.15) MPa; II-III P=213+[(T/238)^19.676-1] MPa; II-V P=412.0-7.01(T-239.15) MPa Ice-five; density (P=0) 1.23 g/cm^3; melting point P=346+410*((T/256.15)^8.1-1) MPa; ice V/VI P=625.9+0.06086*(T-273.15)-0.0008571*(T-273.15)^2 MPa Ice-six; density (P=0) 1.31 g/cm^3; melting point P=625+707*((T/273.31)^4.46-1) MPa Ice-seven; density (P=0) 1.50 g/cm^3; melting point P=2210+534.2*((T/355)^5.22-1) MPa Ice Ih; melting point, P=-395.2*((T/273.16)^9.0 -1) MPa Ice XI, ice XI/ice Ih P=(T-72)*67 MPa supercritical water Triple point, liquid 18.019 cm^3 mol^-1, gas 3.355 m^3 mol^-1, ice Ih 19.66 cm^3 mol^-1; 0.01°C, 612 Pa Triple point, liquid,15.15 cm^3 mol^-1, ice-five 14.5 cm^3 mol^-1, ice-six 13.8 cm^3 mol^-1; 0.16°C, 625.9 MPa Triple point, ice-six, ice-seven, ice-eight; ~5°C, 2.1 GPa Triple point, ice-two, ice-five, ice-six; ~-55°C, ~620 MPa Ice-three; density (P=0) 1.14 g/cm^3; melting point  P=207+62*((T/251.15)^60-1)MPa; Triple points, (liquid 16.52 cm^3 mol^-1, iceIh 19.4  cm^3 mol^-1, ice-three 15.7 cm^3 mol^-1; -22.0°C, 207.5 MPa) (liquid 15.90 cm^3 mol^-1, ice-three 15.7 cm^3 mol^-1, ice-five 14.5 cm^3 mol^-1; -17.0°C, 346.3 MPa) (iceIh, ice-two, ice-three; -34.7°C, 212.9 MPa) (ice-two, ice-three, ice-five; -24.3°C, 344.3 MPa); ice 1/III P=186.1-1.335*(T-273.15)-0.1028*(T-273.15)^2 MPa; ice III/V P=344.3-0.275*(T-273.15)-0.01099*(T-273.15)^2 MPa Triple point, liquid 13.36 cm^3 mol^-1, ice-six 13.8 cm^3 mol^-1, ice-seven 11.5 cm^3 mol^-1; 81.6°C, 2.2 GPa Triple point, ice-seven, ice-eight, ice-ten; 100 K, 62 GPa

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.

 

[2-D Pressure-Temperature-Density graph, liquid-gas data derived from ref 540]

[3-D Pressure-Temperature-Density graph, liquid-gas data derived from ref 540]
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
None [747]
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]

[Phase diagram of water, if it behaved like a more typical  material of its molecular weight] room temperature and pressure

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]