Definition: An emulsion is a colloidal dispersion where both dispersed phase and dispersion medium are immiscible liquids.

Two Types:

Oil-in-Water $\frac{O}{W}$ Emulsions:

• Oil droplets dispersed in water (water is continuous phase)
• Characteristics: Feel non-greasy; dilutable with water; electrically conducting; colored dye test — water-soluble dye colors whole system, oil-soluble dye only colors droplets
• Examples: Milk, vanishing cream, cod liver oil emulsion, mayonnaise, salad cream

Water-in-Oil $\frac{W}{O}$ Emulsions:

• Water droplets dispersed in oil (oil is continuous phase)
• Characteristics: Greasy feel; dilutable with oil; poorly conducting; water-soluble dye only colors droplets (not continuous phase)
• Examples: Butter, cold cream, cold liver oil (original), petroleum emulsions, margarine

Identification Tests:

1. Dilution test: Add water → if disperses = $\frac{O}{W}$; if droplets form = $\frac{W}{O}$
2. Conductance test: O/W conducts (ionic water medium); W/O does not
3. Dye test (Sudan III, fat-soluble dye): O/W = only droplets colored; W/O = whole system colored uniformly
4. Filter paper test: O/W spreads quickly; W/O leaves oil stain on filter paper

Role of Emulsifiers: Emulsifiers are amphiphilic (dual-natured) molecules: hydrophilic head + hydrophobic tail.

• At oil-water interface, tails go into oil, heads point to water
• Form an interfacial film around oil droplets
• Reduce interfacial tension
• Prevent coalescence of droplets
• Examples: Soap $\frac{anionic surfactant for O}{W}$, lecithin, proteins, gum arabic

Demulsification (Breaking Emulsions):

• Heating (disrupts interfacial film)
• Centrifugation (physical separation)
• Adding electrolyte (neutralizes emulsifier charge)
• Adding demulsifier (competitive adsorption at interface)

Industrial Significance of Emulsion Type: The butter-making process converts O/W milk (oil droplets in water) → W/O butter (water droplets in fat). This emulsion inversion (O/W → W/O) during churning is a commercially important application of emulsion chemistry.