Which parameter primarily determines the stability of an emulsion?

• A. Ambient humidity
• B. Type of emulsifier
• C. Color of base
• D. Brand of drug

Answer: (B)

Stability of an emulsion is governed by the barrier that surrounds each droplet. The emulsifier is what creates that barrier by adsorbing at the oil–water interface, reducing interfacial tension and forming a protective film. This film can give droplets a charge (electrostatic repulsion) or create a physical hindrance (steric stabilization), both of which slow or prevent droplets from merging or being pulled together by gravity. Because of this, the type (and amount) of emulsifier largely determines how long an emulsion stays nicely dispersed before separation processes like coalescence or creaming occur.

Ambient humidity, the color of the base, or the brand of drug don’t influence the fundamental interfacial stabilization mechanism. They might affect packaging, appearance, or formulation aspects in indirect ways, but they don’t set the primary droplet-stabilizing interaction in the emulsion.