Free-Base Nicotine Fraction alphafb in Non-Aqueous vs. Aqueous Solutions: Electronic Cigarette Fluids Without vs. With Dilution with Water.

An important design aspect of electronic cigarettes ("e-cigarettes") is the nature of the acid/base chemistry in the e-liquid phase. E-liquids having formulations similar to those of early products are mixes of propylene glycol/glycerol (PG/GL) plus free-base (fb) nicotine and (usually), flavor chemicals that are either non- or rather-weak acid/base actors in PG/GL. The fraction of nicotine in the fb form is denoted (alphafb)e-liquid; the possible range is 0 < (alphafb)e-liquid < 1. For e-liquids of an early design, (alphafb)e-liquid approximately 1. Because e-cigarette aerosols high in fb-nicotine are harsh upon inhalation, many commercial e-liquids now also contain variable levels of an acid additive (e.g., benzoic acid, levulinic acid, etc.) to protonate the nicotine, and form dissolved "nicotine salts": (alphafb)e-liquid values significantly less than 1 are now common. A framework is developed for predicting alphafb values in a given medium based on: 1) acid/nicotine ratios; and 2) overall acid+nicotine protonation constant (Koa) values. This framework is required for understanding: 1) e-liquid design as regards how acid additives affect (alphafb)e-liquid; and 2) demonstrating the inability to measure native (alphafb)e-liquid values by any method using dilution with water.