Electrochimica Acta 55 (2010) 8916–8920 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta Electrochemical double layer at the interfaces of Hg/choline chloride based solvents Renata Costa, Marta Figueiredo, Carlos M. Pereira 1 , Fernando Silva ∗,1 Departamento de Química, Faculdade de Ciências da Universidade do Porto, Centro de Investigac ¸ ão em Química da Universidade do Porto, Rua do Campo Alegre 687, 4169-007 Porto, Portugal article info Article history: Received 24 March 2010 Received in revised form 19 July 2010 Accepted 24 July 2010 Available online 3 August 2010 Keywords: Electric double layer Deep eutectic solvents Room temperature ionic liquids Differential capacitance PZC abstract The electrochemical interfaces of several deep eutectic solvents based on choline chloride mixtures with 1,2-ethanediol, 1,2-propanediol, 1,3-propanediol, urea or thiourea, and mixtures of acetylcholine chloride with urea were studied at a Hg electrode. The cyclic voltammetric results identified the potential domains of electrochemical stability and illustrated their dependence on the deep eutectic solvents composition. The differential capacitance–potentials, C(E), curves for the electrical double layers were obtained from electrochemical impedance data by adjusting the appropriate equivalent circuits. The structure of the interfaces is proposed to be dominated by adsorption of choline cations at large negative polarizations while at less negative or positive polarizations the structure is dominated by the adsorption of the anion. The temperature coefficients of capacitance were found to be nearly zero for Hg. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction Deep eutectic solvents (DES) [1–9] have been proposed as alter- native solvents attractive for electrodeposition of metals (Cr, Mn, Cu, Ag,...) [4,10,11] and alloys (Zn/Cr, Zn/Sn,...) [12,13] in different substrates; in the electropolishing [14,15]; and processes such as electrowinning of metals from complex oxide matrices [5]. Abbott et al. established general formulations for the easy preparation of pure DES using mixtures of substituted quaternary ammonium halide cations such as hydroxyethyltrimethylammonium chloride with metal chlorides [16] and/or suitable hydrogen-bond donors (HBD) such as carboxylic acids, alcohols and amides [1,17]. The interaction between the ammonium salt and the metal chloride or hydrogen-bond donor is the origin for the very large depression of the freezing point up to 200 ◦ C of the mixture [2]. Despite the attention that ionic liquids have received, very little is known about the molecular structure in electrode|ionic liquids electrified interface. Only a few publications on the differ- ential capacitance of imidazolium based ionic liquids are available [18–24]. The data reveals that none of the existing models can be applied to describe the properties of the ionic liquid|electrode inter- face. Similarly to RTILs the structure of DES/electrode interface is not yet understood and a first study of the double layer capaci- ∗ Corresponding author. Tel.: +351 220402613; fax: +351 220402659. E-mail address: afssilva@fc.up.pt (F. Silva). 1 ISE member. tance in a choline chloride based deep eutectic solvent|electrode interface was published [25] recently. The understanding of the structure and the arrangement of the anions and cations of the mixture at the surface of the electrodes can be precious information to the advance and enhancement of their electrochemical properties and consequently industrial appli- cation. In this paper we report the electrochemical characterization and differential capacity–potential data for the interface of Hg and deep eutectic solvents, based on mixtures of choline chloride and differ- ent HBD like ethanediol, propanediol, urea and thiourea. Studies on DES based on acetylcholine and urea mixtures are also reported. 2. Experimental Choline Chloride, [Me 3 NC 2 H 4 OH]Cl, (ChCl) (Sigma–Aldrich, 99,5%) and acetylcholine chloride (Sigma–Aldrich, 99%) were dried at 70 ◦ C overnight. 1,2-propanediol (Fluka, 99.5%), 1,3-propanediol (Fluka, 99.5%) and 1,2-ethanediol (Riedel-de-Haën, 99.8%) were kept dry over 3 ˚ A molecular sieve. Urea (Riedel-de-Haën, 99.0%) and thiourea (Analyticals Carlo Erba, RPE) were dried at 60 ◦ C overnight. The DES were prepared in N 2 filled glove box and further purged with nitrogen for 30 min before each experiment. Concentration of water in the DES solvents was estimated by the Karl Fisher titration method to be between 300 and 400 ppm. The composition and the proportion of the DES liquids prepared and studied are indicated in Table 1. 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.07.070