Temperature-variation of capacitance of alkanethiol monolayer films Takahiro Nakamura, Koichi Aoki *, Jingyuan Chen Department of Applied Physics, Faculty of Engineering, Fukui University, 3-9-1 Bunkyo 3-chrome, Fukui-shi 910-8507, Japan Received 26 November 2001; received in revised form 28 January 2002 Abstract Capacitances, C , in KCl solution and Faradaic currents, I , of a ferrocene derivative were obtained at the polycrystalline gold electrode coated with self-assembled hexadecanethiol monolayer when temperature, T , was continuously varied. Both capacitances and currents increased with temperature (from 0 to 70 8C). Curves of DC /DT vs. T were composed of two different lines, exhibiting large variations at T c (ca. 45 8C). A similar variation was also found at curves of DI /DT vs. T for the oxidation current, I , of ferrocene derivative. The critical variation suggests phase transition of the SAM. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Self-assembled monolayer; Phase transition; Temperature control; Capacitance; Faradaic current 1. Introduction Self-assembled monolayer (SAM) films formed by irreversible adsorption of alkanethiols on a gold surface are stable as well as reproducible in electrochemical responses [1,2]. This stability is ascribed not only to the strong adsorption of the alkanethiols on gold atoms [3,4] but also to the strong interaction of alkane chains [5,6].A role of the interaction between alkane chains has been demonstrated by dependence of the chain length on the reductive desorption potential [7  /10]. SAMs including chemically functional groups exhibit more complicated interactions such as electro-static repulsion [11  /15], the steric interaction owing to closely dense packing [16], and the hydrogen-bonding stabilization [17,18]. These effects have been well investigated for carboxylate-terminated alkanethiols [14,16,18 /24]. Intensity of these interactions can be expressed by a function of pair interaction energy divided by temperature [10,14,24]. Thus, the intensity should be altered by changing not only functional groups but also temperature. Work on the temperature-dependence of SAMs has aimed at evaluating activation energies of the desorption from metal surfaces [25  /28], chemical reactions at SAMs [29  /31], and heterogeneous reactions through SAMs [32,33]. These investigations have provided kinetic data of chemical properties of SAMs. In contrast, temperature-dependence on physical structure is also interesting; of which examples are thermal effects on interactions [34,35], and phase separation [36  /38]. It is not easy to obtain data on the phase transition because temperature-variation is too small to be de- tected in comparison with errors of reproducibility and durability. We obtained long-term variations of differential capacitance of the hexadecanethiol film adsorbed on polycrystalline gold surface [39]. Then, the capacitance data were sensitive to temperature. This work reports continuous temperature-variation of capacitance of hexadecanethiol films on polycrystalline gold electrodes. 2. Experimental All chemicals were of analytical grade. A polycrystal- line Au disk electrode, 1.6 mm in diameter, was used as a substrate of the SAM. After polished with a suspen- sion of alumina powder (#2000), ultrasonicated in an ion-exchanged water, and rinsed with C 3 H 6 O, the electrode was immersed into 10 mM 1-hexadecanethiol (Wako) ethanol solution for 12 h to generate the thiol SAM. The SAM electrode was immersed in EtOH for 5 min and then in ion-exchange water for 5 min. It was transferred to an electrochemical cell. * Corresponding author. Tel.:  /81-776-27-8665; fax:  /81-776-27- 8494. E-mail address: d930099@icpc00.icpc.fukui-u.ac.jp (K. Aoki). Electrochimica Acta 47 (2002) 2407  /2411 www.elsevier.com/locate/electacta 0013-4686/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII:S0013-4686(02)00045-2