Article pubs.acs.org/jced
Thermophysical Properties of 1‑Butyl-1-methyl-pyrrolidinium Dicyanamide + H2O Mixtures M. C. Cumicheo,† L. C. S. Nobre,‡ A. F. Santos,‡ I. M. S. Lampreia,‡ M. Soledade C. S. Santos,§ F. J. V. Santos,‡ H. Segura,¶,† and C. A. Nieto de Castro*,‡ †
Departamento de Ingeniería Química, Universidad de Concepción, Concepción, Chile Centro de Química Estrutural, and §Centro de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal
‡
S Supporting Information *
ABSTRACT: The properties of mixtures of ionic liquids and water are very important for many applications. One key application is their use as absorbents in absorption refrigeration cycles, replacing water + LIBr and water + ammonia systems. As a possible mixture, the system [Bmpyr][dca] + H2O was studied, as the available data are scarce. Measurements of density, speed of sound, viscosity, and surface tension of [Bmpyr][dca] + H2O were made for seven molar compositions between 288.15 K and 333.15 K. The uncertainty of the density values is 0.07 kg·m−3, while that of the speed of sound was found to be 0.3 m·s−1. Viscosity measurements have an uncertainty of 1.4 %, while the surface tension values have an uncertainty lower than ±0.6 mN·m−1. All the measurement were measured at atmospheric pressure and are reported to 0.1 MPa ± 1 kPa. From density data, isobaric molar expansions were calculated and from combined density and speed of sound data, isentropic molar compressions were also obtained. Finally, excess molar volumes, excess molar isobaric expansions and excess molar isentropic compressions were derived and fitted to Redlich−Kister polynomial equations. Estimated surface entropy and enthalpy were calculated. Regarding the viscosity measurements, the [Bmpyr][dca] was found to be a Newtonian fluid and the deviations of the viscosity of the mixtures from ideality were found to be very dependent on the molar fraction of water. Observed changes of the several properties with composition and temperature were analyzed in terms of intermolecular interactions and changes in structure patterns. From the results obtained, it can be concluded that this mixture has adequate thermophysical properties for absorption refrigeration applications. and 150) μmol/L, showed that [Emim][dca] and [Bmpyr][dca] were very well behaved, with low toxicity, while [Bmim][dca] was orders of magnitude more toxic.4 The structure of IL + water solutions is far from being completely understood. Interactions present between the ions themselves and the water molecules comprise several intermolecular forces that contribute to an overall complex result, namely Coulombic, van der Waals and hydrogen bonding. If the structure near infinite dilution for water is clear, the structure of the pure ionic liquid is not totally clear, although there is a current acceptance of polar and nonpolar domains in its structure. However, when we start to add water molecules to the pure ionic liquid, the structure of the mixture is highly dominated by an “IL-type” structure, that accepts water to a given extent, before adapting to a “water-type”
1. INTRODUCTION The need for new absorption refrigeration systems pointed to the study of mixtures of ionic liquids with water because of their thermophysical properties and ease of preparation. Ionic liquid systems have now been applied to many areas, the areas of energy and environment being remarkably successful. However, not all these liquids can be considered “green solvents” as their toxicological properties are strongly dependent on chemical composition, and care has to be taken with the chosen ionic liquid. A preliminary screening of the possible ionic liquids to be used led us to study liquids based on dicyanamide anion (dca) with alkylmethylimidazolium or alkylmethylpyrrolidinium cations, in the search for heat transfer fluids applications with IoNanofluids.1,2 The use of the dicyanamide anion was chosen for the low viscosity and high thermal stability of the ILs containing it.3 However, in ecotoxicity tests in vitro to determine LC50, the average lethal concentration (50 % of deaths), after 24 h of contact of IL molecules with Artemia Salina eggs (brine shrimp larva), one organism of the aquatic ecosystems (salted water), with a concentration of (1, 50, 100, © XXXX American Chemical Society
Special Issue: Memorial Issue in Honor of Anthony R. H. Goodwin Received: August 19, 2015 Accepted: November 12, 2015
A
DOI: 10.1021/acs.jced.5b00944 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
Article
made thermostated micro-PTFE sample vessel (≅ 2 mL) and the sample temperature was monitored with an independent external Pt100 thermometer, inserted next to the sample vessel. Measurements were performed after long equilibrium periods (≥ 1 h), in a closed sample vessel, and temperature fluctuations were below 0.2 K. Equipment calibration and experimental procedure were identical to the one previously described.7 The IL mixture was transferred to the sample cell under a dry nitrogen flux. The surface tension values are the average of over more than 30 surface tension values obtained over at least two independent sets of immersion detachment cycles. The surface tension uncertainty (at 95 %, k = 2.04) is lower than ± 0.6 mN· m−1(1.9 %). Viscosity was measured using capillary viscometers. Glass capillary viscometers from Cannon (Ostwald type, C size, #71) were modified (arm length adjustment) to be properly fitted to a ViscoClock unit from Schott-Gerätte to measure flow times to ± 0.01 s and were calibrated by a step-up procedure based on water satisfying ISO36968 requisites for grade 1, using kinematic viscosity values from ISO/TR3666.9 The measuring procedure was previously described in ref 10. The viscosity relative uncertainty was found to be better than 1.4 % (k = 2). The test for a Newtonian liquid behavior of pure ionic liquid was performed with a TA Instruments rheometer (TA AR1500ex). All property measurements were made at atmospheric pressure. Taking into account its variation in the laboratory during the measurement period, the values are reported to 0.1 MPa ± 1 kPa, the uncertainty being estimated by the maximum daily variation in the barometric pressure in the laboratory. 2.3. Methodology. The water content in the ionic liquid was monitored by Karl Fischer coulometry just before each series of measurements, and the mole fractions of the binary mixtures were always corrected for these values. In no case was it greater than 3000 ppm, after all the manipulations and property measurements. The the IL + water mixtures were prepared by weight with an expanded uncertainty (k = 2) associated with the mole fraction of u(x) = 0.0002.
structure. The measurements herein presented were also designed to contribute to a better understanding of the structure of the [Bmpyr][dca] + water binary mixture, for which existing experimental data are still limited, with more accurate data and new properties, like speed of sound at a wider temperature range and surface tension. We report in this paper density, speed of sound, surface tension, and viscosity for seven molar compositions of 1-butyl1-methyl-pyrrolidinium dicyanamide ([Bmpyr][dca]) + water (H2O), at temperatures between 288.15 K and 333.15 K and 0.1 MPa. The data obtained was used to derive thermodynamic excess properties (including surface tension), and was interpreted using current solution chemistry model analysis.
2. MATERIALS AND METHODS 2.1. Materials. [Bmpyr][dca] was purchased from IoLiTec, Ionic Liquids Technologies, Germany, and the product specifications of the certificate analysis can be found in Table 1. The estimated mass fraction purity claimed by the supplier is Table 1. Ionic Liquids Product Specifications (In Mass Percentage, When Applicable)
a
property
[Bmpyr][dca]
identity (NMR. IR) assay (NMR) cation (IC) anion (IC) halides (IC) water (KF)a appearance date
pass > 98 % 99.9 % 99.5 %
