Understanding why e-liquid goes bad helps you understand why storage rules matter. Three chemical processes drive degradation:
1. Nicotine Oxidation
Nicotine is an alkaloid that reacts with oxygen. When exposed to air, it undergoes oxidation, converting to nicotine oxide and eventually to cotinine — a compound with different pharmacological properties. The visible sign is browning: fresh nicotine e-liquid is pale yellow or clear; oxidized nicotine turns dark amber or brown. This is the same process that turns a cut apple brown.
The oxidation rate depends on:
- Temperature: Every 18°F (10°C) increase doubles the oxidation rate (Arrhenius equation)
- Oxygen exposure: More headspace in the bottle = faster oxidation
- UV light: Ultraviolet light catalyzes oxidation — amber glass blocks 90%+ of UV
- Nicotine type: Salt nicotine (lower pH 5-6) oxidizes faster than freebase (pH 7-8)
- PG/VG ratio: Higher PG ratios oxidize slightly faster due to lower viscosity and higher oxygen solubility
2. Flavor Compound Degradation
E-liquid flavoring compounds — esters, aldehydes, terpenes, and lactones — are volatile organic compounds. Over time, they:
- Evaporate (especially top notes like citrus and mint)
- React with nicotine (some flavors accelerate nicotine oxidation)
- Break down into simpler compounds (cinnamaldehyde → cinnamic acid + aldehyde)
- Polymerize (individual molecules link together, changing viscosity and taste)
3. VG/PG Separation
Vegetable glycerin (VG) and propylene glycol (PG) have different densities (VG: 1.26 g/mL, PG: 1.04 g/mL). In extreme cold (below 32°F/0°C), VG can partially crystallize and separate from PG. This is reversible with gentle warming, but repeated freeze-thaw cycles degrade the emulsion and alter the vaping experience. For the full chemistry of PG/VG thermal behavior, see our ingredient science guide linked above.
