Vape juice, or e-liquid, is a complex mixture of ingredients that delivers nicotine and flavor when heated in an electronic cigarette. Its primary components include Propylene Glycol (PG), Vegetable Glycerin (VG), flavorings, and often nicotine. Since it is a liquid, it is subject to evaporation over time. The volume of e-liquid can decrease, and the rate at which this occurs is governed by the physical properties of its constituent parts.
The Science of Evaporation in E-Liquids
Evaporation is the process where a liquid turns into a gas without reaching its boiling point. This occurs when molecules at the liquid’s surface gain enough kinetic energy to escape into the surrounding air. This phenomenon is directly related to a substance’s vapor pressure, which measures its tendency to transition into a gaseous state. E-liquids are a blend of PG and VG, which have very different vapor pressures.
Propylene Glycol has a significantly higher vapor pressure and a lower boiling point (around 188°C) compared to Vegetable Glycerin. This difference in volatility means PG molecules require less energy to escape the liquid mixture. PG is the primary component responsible for volume loss through evaporation, so e-liquids with a higher ratio of PG tend to reduce in volume more quickly.
Vegetable Glycerin (VG) is a thicker, more viscous liquid with a much lower vapor pressure and a higher boiling point (around 290°C). This makes VG less volatile, evaporating at a much slower rate than PG. Since flavorings and nicotine are dissolved within these carrier liquids, the preferential evaporation of PG can slowly alter the balance of the mixture. Trace amounts of water, which has a far higher vapor pressure than either glycol, will evaporate the fastest.
Environmental Factors That Speed Up Evaporation
The rate at which e-liquid evaporates is highly dependent on the external environment where it is stored. Heat is a significant accelerator of evaporation, as higher temperatures provide more thermal energy to the liquid molecules. This increased kinetic energy allows volatile molecules, particularly PG and water, to overcome intermolecular forces and escape as vapor.
Air exposure is another major factor; constant airflow carries away vaporized molecules from the liquid surface. This continuous removal prevents the air immediately above the liquid from becoming saturated, maintaining a gradient that drives further evaporation. E-liquid sitting in an open tank is far more susceptible to this effect than a tightly capped bottle.
Direct exposure to light, especially ultraviolet (UV) light, also contributes to degradation and volume loss. UV radiation introduces energy into the system, accelerating the chemical breakdown and oxidation of components like nicotine and flavorings. This rapidly destabilizes the e-liquid, increasing the speed at which volatile components are lost.
Preventing Volume Loss Through Proper Storage
Minimizing volume loss relies on controlling the external factors that accelerate evaporation. The most straightforward action is ensuring that all containers, including storage bottles and tank caps, are tightly sealed when not in use. A secure seal limits air exposure and prevents the escape of vaporized molecules, effectively slowing the evaporation rate.
The ideal storage environment should be cool, dark, and stable, with temperature recommendations generally falling between 10°C and 21°C (50°F and 70°F). Storing e-liquid in a cupboard or drawer keeps it away from direct sunlight and heat sources. This reduces the kinetic energy of the molecules, keeps the vapor pressure low, and preserves the overall integrity of the mixture.
The long-term consequence of evaporation is a change in the concentration of the remaining, non-volatile components. As the PG and water evaporate, the percentage of VG, flavorings, and nicotine left in the reduced volume increases. This can lead to a harsher, more concentrated flavor and a potentially stronger nicotine delivery than intended.