Oil-in-water emulsions coat oil droplets in water-loving surfactants
Oil and water normally separate because oil is hydrophobic and water is hydrophilic. An emulsion is a forced mixture where one liquid is dispersed as tiny droplets in another. In an oil-in-water emulsion (mayo, vinaigrettes), oil droplets are suspended in a water phase. An emulsifier (like egg lecithin in mayo) has a water-loving head and oil-loving tail, forming a protective layer around each oil droplet. This coating prevents droplets from touching, coalescing, and separating back into two layers.
Water-in-oil emulsions (butter, some sauces) disperse water droplets in oil. These are more delicate because even tiny mechanical stress can break the emulsion. Oil-in-water emulsions are more stable because the continuous water phase provides mechanical support and prevents droplets from drifting together.
Temperature and pH affect emulsifier effectiveness
Most emulsions break when heated excessively (the emulsifier denatures) or cooled too far (the oil solidifies). Acidic conditions (low pH) help stabilize many emulsions by protonating the emulsifier's charged head group, improving its anchoring to oil droplets. Salt can either stabilize or destabilize depending on emulsifier type. Egg-based emulsifiers (mayo, Hollandaise) are sensitive to temperature shock: above 160F or below 40F, the emulsifier loses cohesion and the emulsion breaks. Mustard and xanthan gum emulsifiers are more robust to temperature swings.