Lecture 14:
Special interactions
What did we cover in the last
lecture?
Restricted motion of molecules near
a surface results in a repulsive force
which acts to push surfaces apart
and increase the entropy
This method is used to stabilise
colloids and surfaces when charges
cannot be used effectively (i.e. in
non polar organic solvents)
For rod like molecules with a
separation, d, the pressure between
the surfaces at separation, D, is
given by
k BT
P 2
d D
In this lecture…
1) The properties of water as a solvent
2) Hydrogen bonding
3) Hydrophobic interactions
Further Reading
Intermolecular and surface forces, J.
Israelachvili, Chapter 8, p122
Water is odd stuff
Water is arguably the most important
liquid on earth
All biological organisms have a high
water content
Nanoscience also exploits the dielectric
properties of water to help stabilise
particles and surfaces in solution
(electrostatic colloidal interactions)
It has high melting and boiling points for a low molecular
weight liquid
It also has a high latent heat of vapourisation and surface
energy when compared to other liquids
What gives water these
properties?
These properties are due to
the special nature of the
interactions between water
molecules
Atoms such as O, N, F and
Cl are highly electronegative
(they tend to pull electrons
towards themselves)
Covalent bonds with these atoms can be quite polar. Bonds
between these atoms and hydrogen tend to have a greater
polarity.
Dipolar interactions between these bonds are referred as
hydrogen bonds
How strong are hydrogen bonds?
The energies associated with individual H-bonds are
typically
10-40 kJmol-1 (~ 4-16 kBTroom)
Comparing this with the energies associated with other
bonds we have
Metallic and Covalent Bonds ~ 500 kJmol-1
(~200 kBTroom)
Van der Waals (dispersion forces) bonds ~ 1 kJmol-1 (~0.4
kBTroom)
They are easier to overcome than simple dispersion
interactions but also relatively stable at room temperature
The structure of ice
The directional nature of hydrogen
bonds causes water molecules to pack
into an open tetrahedral structure
In doing so, the average
number of nearest
neighbour molecules
decreases from ~5 in
liquid to 4 in ice (resulting
in a lower density)
This is unusual as crystalline solids are usually more
dense than the liquid because atoms/molecules become
more closely spaced
The structure of water
At low (~ room or
body) temperature,
the polar nature of the
interactions between
water molecules
causes liquid water to
retain some of this
structure
The hydrogen bonds are still present, but the molecules
are more mobile. The interactions between H-bonds are
dipole-dipole interactions (see lecture 3)
Hydrogen bonding
1) Dispersion forces act between all molecules and have no
preferred direction
2) H bonds are dipole-dipole interactions between specific
functional groups. As such they have some directionality
3) They also have energies of ~ 4-16 kBTroom (dispersion
energies ~ 1 kBTroom )
4) This makes them ‘sticky’ and more stable than simple
dispersion forces but they can also be disrupted by thermal
motion of the molecules
Hydrogen bonding in
nanoscience
Molecules which form
many H bonds with
neighbours have higher
dissociation energies
and are thus more
sticky
If the molecules are
designed carefully, the
directionality of Hbonds can be used to
form supramolecular
aggregates which self
assemble in solution or
on a surface
PTCDI +
melamine
network
Courtesy
of Peter
Beton
Hydrogen bonding in nature
Biological molecules also exploit the directionality of H-bond formation to
produce rigid structural elements that give these large molecules a well
defined shape
DNA
Proteins (a-helices and b-sheets)
(Base pair bonding)
Problem
Calculate an estimate of the electrostatic energy of
interaction associated with a single hydrogen bond in
water if the intramolecular O-H separation is 0.1 nm, the
intermolecular O-H separation is 0.176nm and the amount
of charge separation along each bond is +/- 0.1e.
Give your answer in units of kBTroom
The interaction of water with non
polar molecules
When a nonpolar molecule comes
into contact with polar liquids such
as water it can only interact via
dispersion forces (~0.4 kBTroom)
These are much weaker than the
hydrogen bonds (~4-16 kBTroom)
As a result water molecules tend
to reorient themselves near non
polar molecules to form a more
ordered structure which
maximises H-bonding (called a
Clathrate cage)
These
structures
greatly
reduce
the
entropy of the water
molecules surrounding
the non polar molecule
The hydrophobic effect
There is a large entropic penalty
(~10-30kJmol-1) associated with
increasing the local order
(decreasing disorder) of water
molecules around a nonpolar
molecule
If the interfacial area between water
and the nonpolar groups is reduced
the system can offset this
unfavourable entropy
This is physical origin of the
hydrophobic effect
Area, S2
Area, S1
+
V2
V1
Area < S1 + S2
V1 + V2
Note: The hydrophobic effect is not the result of a direct
interaction between nonpolar molecules.
Hydrophobic interaction energy
There is no satisfactory
theory to explain the
details of the hydrophobic
interaction.
This is because the
interaction between two
surfaces involves many
layers
of
intervening
molecules
Measurements show that
the interaction energy, U,
decays exponentially with
the separation between
surfaces
 D
U  2 1S exp   
 o 
where 1 is the interfacial
energy with water, S, is the
interfacial area and o is the
range of the interaction
o ~1-2nm
The hydrophobic force
The hydrophobic force between two flat surfaces is given
by
 D
dU
2 1S
F 

exp   
dD
o
 o 
where 1 is the interfacial energy with water, S, is the
interfacial area and o is the range of the interaction
o
o ~1-2 nm
Area, S
Problem
Calculate the magnitude of
a) The Van der Waals (dispersion) pressure
and
b) The pressure due to hydrophobic forces
acting between two non polar interfaces at a distance of 0.5
nm in water if the Hamaker constant for the system is
A=0.5x10-20 J and the interfacial energy between dodecane
and water is 40 mJm-2 . Assume that the range of the
hydrophobic interaction is 1 nm
Hydrophobic interactions in
nature
The hydrophobic interaction is also
used by protein molecules to ensure
that different sections of the molecule
fold up in a specific way. This is
important because structure controls
function
As we will see in future lectures,
the hydrophobic force is also
important in the formation of
biological (cell) membranes
Summary of key concepts
Hydrogen bonds and hydrophobic
interactions are attractive interactions.
They are stronger than simple
dispersion interactions (~4-30 kJmol-1
vs ~1kJmol-1)
They have more specificity than
dispersion forces and are used in both
nature and nanoscience to build
structures
The hydrophobic effect arises due to
changes in the entropy of water
surrounding nonpolar molecules.
F 
2 1S
o
 D
exp   
 o 