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Surface pKa of Self-Assembled Monolayers

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J. Justin Gooding

School of Chemical Sciences, The University of New South Wales, Sydney, NSW, 2052 Australia

Penny S. Hale , Leone M. Maddox and Joe G. Shapter

School of Chemistry, Physics and Earth Science (SoCPES), Flinders University of South Australia, Adelaide, SA, 5001, Australia

J. Chem. Educ., 2005, 82 (5), p 779

DOI: 10.1021/ed082p779

Publication Date (Web): May 1, 2005

Abstract

The concept of surface pK_a is introduced and developed for self-assembled monolayers (SAMs). Students use a titration method to determine the pK_a of 3-mercaptopropionic acid (MPA) in solution. At the same time they determine the surface pK_aof a SAM of MPA on a gold electrode by running a series of cyclic voltammograms of ferricyanide in buffers of differing pH. At different pH the extent of ionization of the SAM changes. Therefore, as ferricyanide is anionic, the charge exclusion of the ferricyanide, as ascertained from cyclic voltammetry peak currents, allows the inference of the charge of the MPA SAM. A plot of the peak current versus pH yields surface pK_a values consistent with the literature. The dual aims of elucidating pK_a and gaining experience in cyclic voltammetry are complementary. This practical exercise highlights the differences between surface layers and solution components with applications across a range of disciplines.