Thermochimica Acta 572 (2013) 39–44 Contents lists available at ScienceDirect Thermochimica Acta jo ur nal ho me page: www.elsevier.com/locate/tca Refractive properties of binary mixtures containing pyridinium-based ionic liquids and alkanols Mónica García-Mardones, Pilar Cea, M. Carmen López, Carlos Lafuente ∗ Departamento de Química Física, Facultad de Ciencias, Ciudad Universitaria, 50009 Zaragoza, Spain a r t i c l e i n f o Article history: Received 2 May 2013 Received in revised form 18 August 2013 Accepted 19 August 2013 Available online 1 October 2013 Keywords: Alkanol Excess property Ionic liquid Mixing rule Refractive index a b s t r a c t Refractive indices for six binary systems formed by three pyridinium-based ionic liquids: 1-butylpyridinium tetrafluoroborate, 1-butyl-3-methylpyridinium tetrafluoroborate or 1-butyl-4- methylpyridinium tetrafluoroborate and two alkanols: methanol or ethanol were measured at four temperatures, T = 293.15, 303.15, 313.15 and 323.15 K. From these data excess refractive indices were calculated and fitted. The excess refractive indices of the mixtures, which were found positive, were related to their corresponding excess volumes. Furthermore, several mixing rules were used to predict both refractive indices and densities of the binary mixtures. © 2013 Elsevier B.V. All rights reserved. 1. Introduction The thermophysical characterization of binary mixtures of ionic liquids is fundamental in order to obtain information about molec- ular interactions and structural changes involved in the mixing process. This information is relevant for the design of processes and products involving ionic liquids; the interest in these kinds of compounds that can be used as “tunable solvents” has increased in the last years [1–7]. The refractive index is a optical property of the matter very sensitive to changes in the molecular organization of the sample, polarity of the medium, [8,9] and it is also related through the molar refraction with the free volume, so its study is adequate to obtain the above mentioned information. There are some publications about the refractive behaviour of binary mixtures involving imidazolium-based ionic liquids. Zafarani-Moattar et al. [10] have determined the viscosity, density, speed of sound and refractive index of 1-butyl- 3-methylimidazolium hexafluorophosphate in tetrahydrofuran, dimethylsulfoxide, methanol, and acetonitrile at T = 298.15 K. Xu et al. [11] reported densities, viscosities, and refractive index of 1- butyl-3-methylimidazolium alanine acid salt with methanol and benzylalcohol at temperatures from T = 298.15 to T = 313.15 K at intervals of 5 K. Taib et al. [12] measured density, refractive index of 1-butyl-3-methylimidazolium tetrafluoroborate with water and monoethanolamine in the temperature range 293.15–353.15 K. ∗ Corresponding author. Tel.: +34 976762295; fax: +34 976761202. E-mail address: celadi@unizar.es (C. Lafuente). They also predicted the refractive index of binary mixtures using two mixing rules: the Gladstone-Dale and the Lorentz- Lorentz equations, the first mixing rule showed the best results. Singh et al. [13] presented data of density, speeds of sound, and refractive index for binary mixtures of [bmim][SO 4 ] with methanol, 1-propanol, 2-propanol or 1-butanol at the tempera- tures from T = 298.15 K to T = 313.15 K. Moreover, they used the Lorentz–Lorentz mixing rule to predict the refractive index. Gómez et al. [14] studied the thermophysical properties binary mixtures of 1-propyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide with methanol, ethanol, 1-propanol, or 2-propanol; they deter- mined density, speed of sound, and refractive index at T = 298.15 K. However, there are not studies on refractive properties of mixtures containing pyridinium-based ionic liquid, with the aim to enlarge the information about these kind of mixtures, we present now the refractive indices for the binary sys- tems: 1-butylpyridinium tetrafluoroborate, [bpy][BF 4 ], 1-butyl-3- methylpyridinium tetrafluoroborate, [b3mpy][BF 4 ], or 1-butyl-4- methylpyridinium tetrafluoroborate, [b4mpy][BF 4 ], with methanol or ethanol over the whole range of compositions at the tem- peratures 293.15 K, 303.15 K, 313.15 K, and 323.15 K; the system [bpy][BF 4 ] + ethanol shows a miscibility gap at T = 293.15 K so, the measurements were not performed at this temperature. From the experimental data, the excess refractive indices were calculated according to Reis et al. suggestions. In order to establish a rela- tionship between the excess refractive and volumetric properties of the studied mixtures, we represented excess refractive indices versus excess volumes and analyzed the obtained plots. Finally, we used several mixing rules: Lorentz–Lorenz [15,16], Gladstone-Dale 0040-6031/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2013.08.016