To understand nicotine pharmacology, it’s important to know that absorption of nicotine is pH dependent. Because nicotine is a weak base (pKa = 8.0):

In acidic media…
nicotine is ionized and poorly absorbed across membranes.

In alkaline media…
nicotine is nonionized and well absorbed across membranes.

Under physiologic conditions (pH = 7.3–7.5), approximately 31% of nicotine is nonionized and readily crosses cell membranes (Benowitz, 1992; Hukkanen et al., 2005).

Benowitz NL. (1992). Cigarette smoking and nicotine addiction. Med Clin N Am 76:415–437.
Hukkanen J, Jacob P III, Benowitz NL. (2005). Metabolism and disposition kinetics of nicotine. Pharmacol Rev 57:79–115.

Slide is used with permission, Rx for Change: Clinician-Assisted Tobacco Cessation. Copyright © 1999-2007 The Regents of the University of California, University of Southern California, and Western University of Health Sciences. All rights reserved.