Definition
An additive blended into aviation fuels and lubricating oils that prevents water from forming a stable emulsion with the fluid, allowing any water present to separate out so it can be drained or filtered before it reaches the engine.
Plain English
A chemical added to fuel or oil that stops water from mixing with it, so the water settles out and can be removed instead of being carried into the engine.
Context Anchor
Seen in aircraft maintenance discussions about fuel, oil, contamination, and water removal.
Derivation
From 'de-' (to remove or undo) plus 'emulsify' (to mix two liquids that normally don't combine, like oil and water). A demulsifier undoes that mixing, so water and fuel separate cleanly instead of staying blended.
Why Pilots Care
Unresolved water in fuel can freeze at altitude, corrode components, or cause engine flameout; the additive ensures water can be removed before flight.
Analogy
It is like helping a shaken bottle of oil-and-vinegar dressing separate back into layers, instead of staying cloudy and mixed.
Intuition Check
A demulsifier does not make water disappear. It helps water separate from the liquid so it can be removed.
Example Sentence 1
The fuel additive contains a demulsifier so any moisture that condenses inside the tanks settles out and can be drained at the sump.
Example Sentence 2
Preflight procedures sometimes include checking for demulsifier-treated fuel to confirm water has separated into the sump.