Concentration-dependent switching of the mode of phase separation in ternary self-assembled monolayers of 2-mercaptoethane sulfonic acid, 2-aminoethanethiol and 1-dodecanethiol on Au(1 1 1) Pham Hong Phong, Vladimir V. Sokolov, Naoya Nishi, Masahiro Yamamoto, Takashi Kakiuchi * Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan Received 23 December 2005; received in revised form 13 February 2006; accepted 27 February 2006 Available online 25 April 2006 Abstract Unusually strong apparent surface activity of the pair of 2-mercaptoethane sulfonic acid (MES) and 2-aminoethanethiol (AET) in forming a phase-separated ternary self-assembled monolayer (SAM) composed of MES, AET, and 1-dodecanethiol (DDeT) has been investigated with changing the conditions for preparing the SAM by coadsorption of the thiols from an ethanolic solution. When the total concentration of the thiols, c total , is 1 · 10 3 mol dm 3 or higher, the MES–AET pair forms electrostatically stabilized domains from an early stage of the adsorption after the immersion of a gold substrate in the bathing ethanol solution. The total area of the MES–AET domains does not change with time for longer immersion. The adsorption of DDeT is significantly weaker than the MES–AET pair under such conditions, despite the fact that the intrinsic surface activity of DDeT is greater than those of MES and AET. DDeT-rich islands of typically a few nanometer across spread in the SAM and the size and the distribution of the islands are invariant over a week. In contrast, when c total is lower than 0.1 · 10 3 mol dm 3 , the adsorptivity of DDeT is stronger than that of the MES–AET pair, and MES and AET initially adsorbed are gradually replaced with DDeT at longer immersion time. This switching of the mode of phase separation with the change in c total suggests the crucial roles of the electrostatic interaction of MES and AET in the course of the formation of the SAM and of the hydrophilic barrier formed on the domains to prevent the replacement by hydrophobic DDeT molecules. Ó 2006 Elsevier B.V. All rights reserved. Keywords: 2-Mercaptoethane sulfonic acid; 2-Aminoethanethiol; 1-Dodecanethiol; Ternary self-assembled monolayer; Phase separation; SAM; Au(1 1 1); Reductive desorption; Voltammetry 1. Introduction Phase-separated binary self-assembled monolayers (SAMs) of thiol derivatives on metal surface [1–17] have recently been utilized for a variety of studies in surface elec- trochemistry and surface engineering, for example, the wet- ting of SAM surface in relation to the nanometer-scale phase separation [18], the rate of lateral diffusion of adsorbed alkanethiolates [19], the formation of nanopores in the SAM [20,21], the selective deposition of metals on specific domains [22,23], the attachment of oligonucleotides [24], the immobilization of enzymes on the specific domains of the SAM surface [25–27] and the controlled release of biological cells from the surface [28]. For wider application of SAMs with well defined surface properties in nanometer scale, further elucidation of the phase properties of binary and multi-components SAMs is of decisive importance. We recently reported that the ternary self-assembled monolayer (SAM) of 2-mercaptoethane sulfonic acid (MES), 2-aminoethanethiol (AET), and 1-dodecanethiol (DDeT) formed a phase-separated two types of domains, one consisting of DDeT and the other consisting of 0022-0728/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2006.02.027 * Corresponding author. Tel.: +81 75 753 5528; fax: +81 75 753 3360. E-mail address: kakiuchi@scl.kyoto-u.ac.jp (T. Kakiuchi). www.elsevier.com/locate/jelechem Journal of Electroanalytical Chemistry 600 (2007) 35–44 Journal of Electroanalytical Chemistry