Intermolecular forces and the liquid state

The Lennard-Jones potential, London dispersion, the bulk modulus from molecular stiffness, hydrogen bonds, the van der Waals equation, and the tensile spinodal.

A liquid or solid is a continuum at macroscopic scales, but its bulk modulus, tensile strength, and surface tension all descend from the forces between individual molecules. This chapter builds those forces from the pair potential up: the repulsion and attraction that shape it, the quantum origin of the attraction, and the macroscopic mechanics — stiffness, phase behaviour, and the limit of tension — that follow. It supplies the molecular groundwork the thermodynamics, free-energy, and surface-tension chapters build on.

• 8.1 The Lennard-Jones pair potential — repulsion versus attraction, the well depth, and the equilibrium and inflection radii.
• 8.2 London dispersion: the origin of the attraction — fluctuating dipoles, polarisability, and why the attraction scales as $1/r^6$.
• 8.3 From pair stiffness to the bulk modulus — the well curvature as a spring, the lattice sum, and $K \sim \varepsilon/\sigma^3$.
• 8.4 Hydrogen bonds and the anomalies of water — a directional bond, the tetrahedral network, and where water’s anomalies come from.
• 8.5 The van der Waals equation of state — excluded volume and cohesion, the critical point, and phase separation.
• 8.6 Tensile strength and the spinodal — negative pressure, the spinodal limit, and why real liquids fail far earlier.