Propylene Glycol (PG) is a synthetic organic compound, a clear, odorless, and slightly syrupy liquid widely incorporated into everyday products. Chemically classified as a diol, it contains two alcohol groups and is frequently used as a solvent, humectant, and carrier fluid in various applications, including pharmaceuticals and food items. Yes, PG does evaporate, but it is considered a low-volatility substance. Its chemical properties mean it vaporizes at a far slower rate compared to common liquids like water or alcohol.
The Chemistry of Evaporation
The physical process of evaporation involves molecules escaping from the liquid surface into the surrounding air. Propylene Glycol’s molecular structure is the primary factor governing its slow evaporation rate. The presence of two hydroxyl (-OH) groups allows for strong intermolecular forces known as hydrogen bonds to form between its molecules.
These strong bonds require a significant amount of energy to break, preventing molecules from easily transitioning into a gaseous state. This energy requirement is reflected in its high boiling point, approximately 188.2°C (370.8°F), substantially higher than water’s 100°C. Pure Propylene Glycol also exhibits a very low vapor pressure (about 10.66 Pascals at 20°C), which indicates its low volatility.
The low vapor pressure means only a few molecules have the necessary energy to escape the liquid surface at room temperature. This physical property is the scientific reason for its classification as a low-volatility compound. Consequently, while evaporation is constant, the rate is significantly suppressed, making the process much more gradual than with highly volatile substances.
Practical Evaporation Rate Compared to Water
In real-world conditions, Propylene Glycol’s evaporation speed is dramatically slower than water. Where a spill of water vanishes in minutes or hours, pure PG can take days or even weeks to fully vaporize. This slow rate is often intentionally utilized in industrial and commercial applications.
For instance, its stability and low volatility make it an ideal base for theatrical fog and smoke machines. In these devices, PG is heated to produce a visible vapor that remains suspended in the air for a longer duration, acting as a stable carrier for the visible mist. This practical use demonstrates its resistance to rapid evaporation compared to other liquids.
Hygroscopic Effect
An additional factor influencing its practical evaporation rate is its hygroscopic nature, meaning it readily absorbs and retains water from the atmosphere. In a humid environment, Propylene Glycol on a surface absorbs airborne moisture, creating a mixture. This absorption further slows the gradual process of vaporization by increasing the total amount of liquid present. The hygroscopic property is why PG is used as a humectant in moisturizers and food products to maintain moisture content.
What Happens When Propylene Glycol Evaporates
When pure Propylene Glycol fully evaporates, it does not leave behind any solid residue. Since it is an organic compound that fully transitions into a gas phase, no measurable physical remnant remains on the surface. However, due to its slow evaporation and viscous nature, if the liquid contains dissolved impurities or has absorbed dust and moisture from the air, a sticky film can be left behind.
The resulting vapor state of PG is generally considered safe for human inhalation in the small quantities typically encountered in consumer products like e-cigarettes or asthma inhalers. The vapor is quickly broken down in the atmosphere, with an estimated half-life in the air of between 24 and 50 hours. While prolonged exposure to high concentrations of the vapor can potentially cause mild irritation to the throat or eyes, this is not a concern under normal conditions. Propylene Glycol does not readily decompose into harmful byproducts during the standard evaporation process at ambient temperatures.