Emulsion Chemistry: Thermodynamics & Interface
An emulsion is a dispersion of one immiscible liquid (the dispersed phase) within another (the continuous phase), stabilised by an emulsifier. Food emulsions include: oil-in-water (O/W) emulsions like milk, cream, mayonnaise, and salad dressing (oil droplets in an aqueous continuous phase), and water-in-oil (W/O) emulsions like butter and margarine (water droplets in an oily continuous phase). All emulsions are thermodynamically unstable—they form spontaneously only when the input of energy is greater than the energy gained by phase separation. The Gibbs free energy of emulsion formation is: ΔG = ΔH - TΔS. Mixing oil and water is entropically unfavourable (immiscible liquids have a very strong tendency to separate to reduce interfacial area), making ΔG positive—meaning all emulsions will eventually separate without stabilisation. Creating an emulsion requires: (1) Energy input (homogenisation, blending) to reduce droplet size and increase interfacial area; (2) Reduction of interfacial tension (γ) through the addition of an emulsifier, which adsorbs at the interface and reduces the energy cost of creating new surface area (ΔG ≈ γ × ΔA).