Propylene Glycol (PG)
Propylene glycol or PG is one of the base ingredients used in many e-liquids. Higher PG content is associated increased throat hit and more intense flavor production as many commercially available flavors are PG based flavors. This is balanced by the fact that propylene glycol produces less vapor than vegetable glycerin, making flavor and throat hit a trade off for vapor volume and thickness, this is why many e-liquids on the market today use 70% vegetable glycerin and 30% propylene glycol.
Further Information About Propylene Glycol in General
Propylene glycol (IUPAC name: propane-1,2-diol) is a clear, colorless and hygroscopic synthetic organic compound with the chemical formula C3H8O2. It is a viscous liquid which is nearly odorless but possesses a faintly sweet taste, though not as sweet as vegetable glycerin. Chemically it is classed as a diol and is miscible with a broad range of solvents, including water, acetone, and chloroform which is why it has been used as a flavor carrier.
Propylene glycol contains an asymmetrical carbon atom, so it exists in two enantiomers. The commercial product is a racemic mixture. Pure optical isomers can be obtained by hydration of optically pure propylene oxide. The freezing point of water is depressed when mixed with propylene glycol owing to the effects of dissolution of a solute in a solvent (freezing-point depression). In general, glycols are non-corrosive, have very low volatility and very low toxicity. The closely related ethylene glycol (a key ingredient in antifreeze) is toxic to humans and to many animals and is being slowly replaced by propylene glycol in antifreeze due to it’s lower toxicity.