The nicotine molecule contains two nitrogen atoms with basic properties. The nicotine molecule can thus add one proton to form a monoprotonated species or two protons to form the diprotonated species. Nicotine in tobacco smoke exists in either a protonated or unprotonated form, depending on the presence of natural acids and bases, the tobacco blend, tip ventilation, and the use of additives. Whereas protonated nicotine is positively charged and not volatile, unprotonated nicotine ("free-base nicotine") from tobacco smoke is volatile and readily passes into lipid cell membranes. More unprotonated nicotine in a puff delivers more nicotine to the brain. The same holds true for other (tobacco and non-tobacco) alkaloids. The approximate dividing line between dominance by protonated forms or by the unprotonated form is a pH of 8. At pH 8, the two fractions are present in equal parts. At any lower pH, the fraction of protonated nicotine is greater.