THz dielectric relaxation of ionic liquid:water mixtures Mattijs Koeberg a,b , Chien-Ching Wu b , Doseok Kim c , Mischa Bonn a,b, * a Leiden Institute of Chemistry, Leiden University, Einsteinweg 55, P.O. Box 9502, 2300 RA Leiden, The Netherlands b FOM Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands c Department of Physics and Interdisciplinary Program of Integrated Biotechnology, Sogang University, Seoul 121-742, Republic of Korea Received 16 January 2007; in final form 5 March 2007 Available online 28 March 2007 Abstract Using terahertz time-domain spectroscopy (THz-TDS) the complex dielectric response of mixtures of the prototypical ionic liquid [1-butyl-3-methylimidazolium] + :[BF 4 ]  ([bmin]BF 4 ) with water is investigated. The dielectric response can be described well using a two-term Debye model, and these two relaxation processes in the THz region are followed as a function of volume fraction of the con- stituent liquids. For the mixture, slow and fast Debye relaxation times are larger than the values expected from interpolation of pure water and pure ionic liquid values, pointing to increased structural coherence within the mixture. Ó 2007 Elsevier B.V. All rights reserved. 1. Introduction In an ideal liquid mixture, the entropy always increases upon mixing following the rule of entropy of mixing, and the free energy would not change from the interpolated value irrespective of the concentration [1]. In many mix- tures, however, the entropy and the enthalpy of mixing deviate from the ideal behavior, most commonly in aque- ous mixtures. It has been reported, for instance, that the dynamics of an aqueous mixture slows down as compared to the pure constituent liquids [2,3], or a minute amount of water added to the liquid was shown to change the viscos- ity of the liquid drastically [4]. As another example, when non-polar molecules are added to water, it has been believed that the structure of the nearby water molecules is enhanced [5]. This ‘iceberg formation’ hypothesis, how- ever, has been put to question recently at least for the cases of large solute molecules [6,7]. With these issues in mind, there have been several studies on aqueous mixture system using thermodynamics [8,9], vibrational spectroscopy [10– 12], neutron scattering [13], NMR [14], molecular dynamics simulations [15,16], and terahertz spectroscopy [2,3,17]. To study the structure and dynamics towards the under- standing of the above phenomena of aqueous liquid mixture systems, terahertz spectroscopy has many advanta- geous features: the energy range is well-suited to study intermolecular forces and molecular dynamics, as THz time-domain spectroscopy gives direct access to the com- plex dielectric response of the sample. Accordingly, various liquid samples have been studied using this technique [2,3,17,18]. For example, THz spectroscopy has shown that the response of pure water in this frequency range can be described with two main Debye relaxation processes with relaxation times of 8 and 0.2 ps [18]. Subsequent THz studies on various aqueous solutions and mixtures have allowed the understanding of the effect of solute molecules on the dielectric response and the structure of these system [2,3,18,19]. One of the types of system with an interesting dielectric response are ionic liquids (ILs), a salt consisting of specific organic cation and anion in liquid phase at room tempera- ture [20,21]. Unlike ordinary liquids where van der Waals- interaction or hydrogen bonding is the predominant type of intermolecular interaction, electrostatic interactions constitute the main intermolecular interaction for ILs. 0009-2614/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2007.03.075 * Corresponding author. Address: FOM Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands. Fax: +31 20 6684106. E-mail address: bonn@amolf.nl (M. Bonn). www.elsevier.com/locate/cplett Chemical Physics Letters 439 (2007) 60–64