Water
Lerson Tanasugarn, Ph.D.
Department of Biochemistry
Faculty of Science
Chulalongkorn University
The Water Molecule
Tetrahedral, 2H and 2
lone-pair electron
clouds
Dimensions as in
Figure
[Voet & Voet (1995) p. 30]
Copyright 2003 L. Tanasugarn
Some Physiological
Properties of Water
Melting point
Boiling point
Density of water
Density of ice
Molar heat capacity
Molar heat of fusion
Molar heat of sublimation
Molar heat of vaporization
Dielectric constant
Dipole moment
Viscosity
Serface tension
Diffusion coefficient
273.15 K
373.15 K
0.99987 gml-1 @ 273.15 K
1.00000 gml-1 @ 277.15 K
0.9167 gml-1 @ 273.15 K
75.3 JK-1mol-1
6.01 kJmol-1
46.9 kJmol-1 @273.15 K
40.79 kJmol-1 @ 373.15 K
78.54 @ 298.15 K
1.82 D (6.08 x10-30 mC)
0.01 P @293.15 K
0.07275 Nm-1 at @ 293.15 K
2.4x10-9 m2S-1 @ 298.15 K
[Chang (1981) p. 501]
Copyright 2003 L. Tanasugarn
Structure of Ice
H bonds in H2O, not H2S or NH3
compare the melting points
Extensive 3D meshwork
Each O atom is tetrahedrally bonded to 4 H
atoms in two covalent bonds and two H-bonds
Open lattice, leading to density lower than liquid
water.
Voet & Voet (1995) p. 31.
Water’s heat of sublimation is 46.9 kJmol-1 @ 273.15 K
but only about 6 kJmol-1 can be attributed to the kinetic energy of
gaseous water molecules. The difference, 41 kJmol-1 must
represent the energy required to disrupt H-bonds in the ice
structure.
Copyright 2003 L. Tanasugarn
Structure of Water
H-bonding in dynamic formation and disruption
T^ => more H-bonds broken
free water gets into hole in lattice
density increases
=> kinetic energy increases
density decreases
Maximum density is found at 277.15 K.
Pauling (1952) …
kJmol-1 @
[Chang (1981) p. 500]
Ice’s heat of fusion is 6.0
273.15 K
but from previous slide we have seen that 41 kJmol-1 is required to
disrupt H-bonds in the ice structure. So, liquid water is about 15%
less hydrogen bonded than ice.
Compare with the boiling point of methane (CH4) - similar molecular
mass but no H-bonds
Copyright 2003 L. Tanasugarn
Electrical Conduction
Pure water: negligible conduction
To increase conduction: add salt, acid, or alkali
Conductance depends on ionic mobility, the speed of ionic
movement per unit of electric field strength
Ionic mobility and conductance of H+ and OH- ions are much
greater than other ions since these ions jump along H-bonds:
H+
H
H
O H
O
H
O
H
H
H
H
H
O H
O
H
O
H+
H
The principle of electrical conduction is utilized in
electrophoresis in order to separate biomolecules
under an electric field.
Copyright 2003 L. Tanasugarn
Ionic Mobility
Ion
H+
Li+
Na+
K+
Rb+
Cs+
NH4+
Mg2+
Ca2+
Ba2+
Cu2+
OHFClBrINO3-
Ionic Radius
(Å)
0.60 (33.66)
0.95 (2.80)
1.33 (1.87)
1.48
1.69
0.65
0.99
1.35
0.72
1.36
1.81
1.95
2.16
Ionic Mobility
(cm2s-1V-1)
36.3
4.01
5.19
7.62
8.06
8.01
7.62
5.50
6.17
6.59
5.56
20.06
5.74
7.91
8.10
7.97
7.41
Hydrated radii
Conductance
(Ω-1 equiv-1cm2)
349.81
38.68
50.10
73.50
77.81
77.26
73.5
53.05
59.50
63.63
53.6
198.3
55.4
76.35
78.14
76.88
71.46 [Chang (1981) p. 205]
Copyright 2003 L. Tanasugarn
Ions in
Aqueous Solution
HH O
O
H O O
H
H
_
+
H
H O
H O
H
H
Thermodynamic principles -> hydration
number of ion, which is proportional to


Charge of ion
1/Size of ion
Water in the hydration sphere and bulk water
behave differently. Detectable by
spectroscopy, e.g. NMR.
Dynamic equilibrium between the two types
of water.
Radii of hydrated ionscan be much bigger
than ionic radii or crystal radii
Copyright 2003 L. Tanasugarn
Structure-Making Ions
Structure-Breaking Ions
Small and/or multicharged ions
Li+, Na+, Mg+, Al3+, Er3+, OH-, Fhigh electric field polarizes water & produce additional order
beyond the first hydration layer
slight increase in the viscosity of solution
Large, monovalent ions
K+, Rb+, Cs+, NH4+, Cl-, NO3-, ClO4diffused surface charge -> weak electric field -> polarize water
molecule only the first layer of hydration
viscosity of solution is often lower than that of pure water
Copyright 2003 L. Tanasugarn
Ions vs Ion Pairs
Free ions surrounded by one or more
layers of water molecules
Ion pair = two ions together without
water in between
Determined by potential energy of
attraction vs kinetic or thermal energy
High dielectric constant of water favors
free ions.
Copyright 2003 L. Tanasugarn
Debye-Hückel
Theory of Electrolytes (1923)
Assumptions



Electrolytes are completely dissociated
Dilute solution (<0.01 molal)
On the average, each ion is surrounded by ions of
opposite charges, forming an ionic atmosphere
The theory gives mean activity coefficient in
terms of

Product of charges

Square root of ionic strength, (1/2) Σ miZi2

Inversely proportional to T3/2
Supports the ionic atmosphere picture
Copyright 2003 L. Tanasugarn
The Salting-In and
Salting-Out Effects
Relate solubility of an electrolyte to ionic
strength
From Debye-Hückle Theory
log(s/s0) = 0.509|z+ z-|√I -K’I
where s0 = s where I=0
Copyright 2003 L. Tanasugarn
Salting-In
Increase in
solubility
caused by
increase in
ionic strength
Application in glassware cleaning.
Copyright 2003 L. Tanasugarn
Salting-Out
High ionic
strenghs
precipitate
proteins
Applications in protein precipitation & purification. Ammonium
sulfate is often used owing to its



Multiple ions
High solubility
Acidic nature
Copyright 2003 L. Tanasugarn
The Donnan Effect
Polyelectrolytes cannot get out of the bag
or the matrix
Electrical neutrality keeps the charges in
each compartment.
At equilibrium, the chemical potentials in
both compartments are equal
P-
Many more species exist in the compartment with electrolyte
compared to the other side. The osmotic pressure increases.
Water flows in.
The presence of salt in the aqueous compartment lowers the
osmotic pressure difference inside and outside.
Copyright 2003 L. Tanasugarn