Lecture 3: Polar interactions

1. Lecture 3: Polar interactions

2. 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 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 kT Ionic U ~ kT Metallic U ~100-300 kT

3. 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

4. Many molecules are electrically neutral and carry no net charge However, they can still possess an electric dipole moment Consider 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 What are polar molecules?

5. 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 Zwitterions and dipolar ions

6. How do we define an electric dipole? 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

7. Electric field due to a dipole Consider two charges separated by a small distance, d in a medium of dielectric constant,e We can calculate the field at the point, x, by summing up the fields due to the two point charges (see OHP) When x >> d, this gives the result that;

8. 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) Potential energy of a dipole in an electric field

9. Polarisation using external fields Atom and molecules are polarised by external electrical fields. Opposite charges move in different directions when an E field is applied The dipole moment of an atom or molecule can be related to the external applied electric field by Units of a = C2m2J-1 Where a is the polarisability of the atom/molecule

10. Polarisability of atoms and molecules We can determine the polarisability of an atom/molecule in an applied external field (see OHP) Atom or Molecule Key result d is typically smaller than, but comparable to radius of atom/molecule R. So a scales with volume of atom/molecule (V~R3)

11. p69 ‘Surface and Intermolecular Forces’ by J. Israelachvili Some examples of Polarisabilities a(CH4) = 4x a(C-H) a(H2O) ~ 2x a(O-H)

12. Summary of key results Dipole moment Electric field due to a dipole Potential energy of dipole in an E field Atoms and molecules can be polarised by external fields