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Lecture 3: Polar interactions. What did we cover in the last lecture?. The force on an object is the gradient in potential energy The range of an interaction is the distance at which thermal effects start to dominate. - PowerPoint PPT Presentation
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Lecture 3: Polar interactions
What did we cover in the last lecture?The force on an object is the gradient in potential energy
The range of an interaction is the distance at which thermal effects start to dominate
dx
dUxF )(
kTxU range )(
Electrostatics can be used to explain the interactions in ionic solids. The cohesive energy and Born energy of ions/particles can be used to explain why they are more soluble in water than other solvents
The energies associated with the formation/breaking of strong covalent, ionic and metallic bonds are
Covalent U ~100-300 kTIonic U ~ kT
Metallic U ~100-300 kT
In this lecture…
• Polar interactions
• What is a dipole?
• E field due to a dipole
• Energy of a dipole in an external field
• Polarisability
P48-69 in ‘Surface and Intermolecular Forces’ by J. Israelachvili
What are polar molecules?
Many molecules are electrically neutral and carry no net charge
However, they can still possess an electric dipole momentConsider the molecule HCl (hydrogen chloride)
More electronegative atoms such as chlorine have a higher affinity for electrons. In the case of HCl, the electrons are pulled towards the chlorine atom giving rise to a permanent dipole
Zwitterions and dipolar ions
In some cases, the polar properties of the molecules depend upon the local environment (e.g. presence of solvents). Water soluble polar molecules of this type are called Zwitterions (Zwitter- German meaning ‘hybrid’ )
Some molecules contain both a net charge and a dipole moment. These molecules are referred to as dipolar ions
Recall from electromagnetism that a dipole consists of two charges (+q and -q) separated by a distance d
The dipole moment, p, is then defined as
Units of Cm
How do we define an electric dipole?
~~r̂qdp
Consider two charges separated by a small distance, d in a medium of dielectric constant,
We can calculate the field at the point, x, by summing up the fields due to the two point charges (see OHP)
Electric field due to a dipole
When x >> d, this gives the result that;
~3~
1
2i
x
qdE
o
Energy of a dipole in an external electric field
In later lectures we will need to determine the energy of interaction between dipoles
We will therefore need to calculate the energy of a dipole in an external electric field (see OHP)
extE~
~~.EpU dipole
Potential energy of a dipole in an electric field
The dipole moment of an atom or molecule can be related to the external applied electric field by
Units of = C2m2J-1
Polarisation using external fieldsAtom and molecules are polarised by external electrical fields.
Opposite charges move in different directions when an E field is applied
extEp~~
Where is the polarisability of the atom/molecule
Polarisability of atoms and molecules
Key result
d is typically smaller than, but comparable to radius of atom/molecule R. So scales with volume of atom/molecule (V~R3)
34 do
Atom or Molecule
We can determine the polarisability of an atom/molecule in an applied external field(see OHP)
Some examples of Polarisabilities
p69 ‘Surface and Intermolecular
Forces’ by J. Israelachvili
(CH4) = 4x (C-H)
(H2O) ~ 2x (O-H)
Summary of key results
~3~
1
2i
x
qdE
o
~~.EpU dipole
~~r̂qdp Dipole moment
Electric field due to a dipole
Potential energy of dipole in an E field
extEp~~
Atoms and molecules can be polarised by external fields