van der Waals Forces

ESS5855 LectureFall 2010

Introduction

• There are 4 principal types of physicalforces acting between atoms and molecules at the molecular and macro-scopic levels

• The first, coulombic interaction involving at least one formally charged species

• The remaining 3 forces make up what are commonly termed van der Waals forces

van der Waals Forces

• Intermolecular forces• The attractive forces between molecules

other than those due to covalent bonds or to the electrostatic interaction of ions with one another or with neutral molecules

• Relatively weak compared to normal chemical bonds, but play a fundamental role in diverse fields

van der Waals Forces

• Almost all interfacial phenomena are influenced to various extents by forces that have their origin in atomic- and molecular-level interactions due to the permanent orinduced polarities created in molecules by the electric fields of neighboring molecules or due to the instantaneous dipoles caused by the positions of the electrons around the nuclei

van der Waals Forces

• Permanent dipole - induced dipole interaction (Debye equation)

• Permanent dipole - permanent dipole interaction (Keesom equation)

• Induced dipole - induced dipole interaction (London equation)

London Force

• Of the three forces mentioned above, the London force is always present (like the gravitational force) because it does not require the existence of permanent polarity or charge-induced polarity in the molecules

• Even neutral atoms or molecules such as helium or hydrocarbons give rise to the London interaction

• Intermolecular forces play very important parts in determining the strengths of solids, properties of gases and liquids, heat of melting and vaporization of solids, and the like

Remarks

Lennard-Jones Potential

Potential energy versus distance of separation for two methane molecules

Dipole–Dipole Interactions

+Q -Q

μ1

Permanent dipole to Induced dipole

Permanent dipole to Permanent dipole

Induced dipole to Induced dipole

• All energies considered so far can be calculated using classical physics, which fail to explain the attraction between non-polar molecules

• To calculate the dispersion force, quantum mechanical perturbation theory is required

• Consider an atom with its positively charged nucleus around which electrons circulate with a high frequency of typically 1015–1016 Hz

• At every instant, the atom is therefore polar and only the direction of the polarity changes with this high frequency

• When two such oscillators approach, they start to influence each other

• Attractive orientations have higher probabilities than repulsive ones, which leads to an attractiveforce

Induced dipole to Induced dipole

van der Waals Forces between Large Particles and over Large Distances

Homework