Lipid self-assembly into a bilayer membrane — hydrophilic heads outward, hydrophobic tails inward
Amphiphilic lipids have a hydrophilic phosphate head and hydrophobic fatty-acid tails. In water, the hydrophobic effect (entropic, ~ΔG ≈ 3.4 kJ/mol per CH₂) drives self-assembly into bilayers. The two leaflets orient tail-to-tail, creating the ~4 nm hydrophobic core of biological membranes. Lateral diffusion follows Saffman-Delbrück theory (D ~ ln(1/r)).