The potential of zero charge (pzc) of the thiol-modified Au(1 1 1) electrode has been determined for self-assembled monolayers (SAMs) of octadecanethiolate (ODT), undecanethiolate (UDT), propanethiolate (PT), and 1H, 1H, 2H,2H-perfluorodecanethiol ate (PFDT) by measuring the contact angle of a drop of an aqueous 0.1 mol dm(-3) NaClO4 solution on the SAM-covered electrode as a function of the electrode potential. The values of the pzc for ODT, UDT, PT, and PFDT SAMs were -0.52, -0.49. -0.3, and 1.04 V (vs. Ag\AgCl\satd. KCl), respectively. The variation of the values is ascribed to the change in the surface potential on the solution side of the electrode\solution interface due to the difference in the dipole moments of alkanethiolates on Au(111), which is supported by the ab initio calculation of these thiolates adsorbed on a four-atom cluster of An atoms. (C) 2003 Elsevier B.V. All rights reserved.