2.1. E-Liquid Formulation

E-liquid formulations with freebase (FB) nicotine, used for the metal coil material study, were prepared with 6 mg/mL nicotine (0.6% FB) in 30% propylene glycol (99%, from Sigma-Aldrich) and 70% vegetable glycerin (≥99.5% from Sigma-Aldrich) by volume (i.e., 30:70 PG/VG), with relevance to commercially available e-liquids.^45,46 Solutions were stored at 2–8 °C prior to use. E-liquid formulations with nicotine salts were prepared at 2% (w/w) or 20 mg/mL, with relevance to commercially available e-liquids (20–50 mg/mL concentration range).⁴⁶⁻⁴⁸ Benzoic acid (≥99.5%, from Sigma-Aldrich), was chosen as the acid for the nicotine salt (1:1 molar ratio with nicotine)^7,8 due to its popularity in commercial formulations.^9,10,46−48