volumetric characterization of pyridinium-based ionic liquids
TRANSCRIPT
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Fluid Phase Equilibria 317 (2012) 102– 109
Contents lists available at SciVerse ScienceDirect
Fluid Phase Equilibria
j o ur nal homep age: www.elsev ier .com/ locate / f lu id
olumetric characterization of pyridinium-based ionic liquids
ernando Guerrero, Santiago Martín, Victor Pérez-Gregorio, Carlos Lafuente, Isabel Bandrés ∗
epartamento de Química Física, Facultad de Ciencias, Universidad de Zaragoza, 50009 Zaragoza, Spain
r t i c l e i n f o
rticle history:eceived 10 November 2011eceived in revised form8 December 2011ccepted 19 December 2011
a b s t r a c t
Densities for 1-propylpyridinium tetrafluoroborate, 1-butylpyridinium tetrafluoroborate, 1-butylpyridinium triflate, 1-butyl-3-methylpyridinium dicyanamide, and 1-octyl-3-methylpyridiniumtetrafluoroborate are reported in a broad range of temperatures (283.15–333.15 K) and pressures(0.1–65 MPa). The pressure–volume–temperature behaviours of these fluids have been correlated suc-cessfully with the empirical TRIDEN equation; then, relevant derived properties such as the coefficient
vailable online 18 January 2012eywords:onic liquidsensityressure
of thermal expansion, and the isothermal compressibility have been calculated. A comparison of theresults has been carried out to analyse their properties in structural and energetic terms.
© 2012 Elsevier B.V. All rights reserved.
RIDEN equation
. Introduction
One of the most important properties to characterize a newaterial is density. From the practical point of view, density is
elated to the mechanics and engineering of processes, being aey parameter in several design problems including mass andnergy balances [1,2]. For example, density is required to deter-ine rates of liquid–liquid separations or the necessary energy forixing or pumping solutions. Besides, it is particularly important
or energetic compounds, since it is a critical factor that affects det-nation performance [3]. From a theoretical point of view, density islosely related with equations of state and allows the calculation ofther thermodynamic properties such as the coefficient of thermalxpansion or the compressibility. These coefficients are essentialor technical applications in which large temperature and pressureanges are involved [4]. Moreover, density provides useful infor-ation about the structural and energetic factors which explain
he behaviour of materials. For these reasons, systematic studies ofensities are necessary to develop applications in different areas ofnowledge.
Amongst new chemicals, it is worth noting the relevance in thecientific community of ionic liquids (ILs). The unique propertiesf these compounds provide with a possibility to improve several
ndustrial processes in which the classical organic solvents lead toimitations [5–7]. Therefore, an exhaustive investigation of theireatures is essential for developing the true potential of ILs. Volu-∗ Corresponding author. Tel.: +34 976 762330; fax: +34 976 761202.E-mail address: [email protected] (I. Bandrés).
378-3812/$ – see front matter © 2012 Elsevier B.V. All rights reserved.oi:10.1016/j.fluid.2011.12.029
metric data as a function of temperature are very common sincethe importance of density. However, the lack of data at differentpressures is significant for these compounds [8–12].
Taking into account this framework, we report densitiesof a series of pyridinium-based ILs in a broad range oftemperatures and pressures. Specifically, volumetric proper-ties for 1-propylpyridinium tetrafluoroborate ([ppy][BF4]), 1-butylpyridinium tetrafluoroborate ([bpy][BF4]), 1-butylpyridiniumtriflate ([bpy][CF3SO3]), 1-butyl-3-methylpyridinium dicyanamide([b3mpy][N(CN)2]), and 1-octyl-3-methylpyridinium tetrafluorob-orate ([o3mpy][BF4]), have been measured. Temperature andpressure dependence of experimental data have been properly cor-related with the TRIDEN equation [13] and from their parameters,the coefficient of thermal expansion, and the isothermal com-pressibility have been calculated. A comparison of results for theconsidered ILs has been used to understand the effect of the cationalkyl chain length, the dependence of the anionic nature and theinfluence of the substituents on the volumetric behaviour of theseILs. Densities for [bpy][BF4] at two temperatures and different pres-sures were previously reported and good agreement between datahas been found [14].
2. Experimental
Ionic liquids [ppy][BF4] purity 0.98 in mass fraction, [bpy][BF4]0.99 and [bpy][CF3SO3] 0.99 were supplied by IoLiTec whereas
[b3mpy][N(CN)2] 0.98 and [o3mpy][BF4] 0.98 were provided bySolvent Innovation; this information is summarized in Table 1. TheILs were dried for 24 h under a vacuum of ca. 0.05 kPa under stir-ring and stored before use in a desiccator with the aim of decreasing
H. Guerrero et al. / Fluid Phase Equilibria 317 (2012) 102– 109 103
Table 1Sample table.
Chemical name Source Initial massfraction purity
Purificationmethod
Final massfraction purity
Analysismethod
1-Propylpyridinium tetrafluoroborate IoLiTec 0.98 None 0.98 –1-Butylpyridinium tetrafluoroborate IoLiTec 0.99 None 0.99 –
000
tstl2rMpp[wca
putrSfmawrio[±
seaepcwfh
�
wabDsdoce
3
cu
1-Butylpyridinium triflate IoLiTec
1-Butyl-3-methylpyridinium dicyanamide Solvent innovation
1-Octyl-3-methylpyridinium tetrafluoroborate Solvent innovation
he water content as much as possible. The water contents of theamples were less than 500 ppm as determined by Karl–Fischeritration. The halides contents were less than 0.5% for [ppy][BF4],ess than 100 ppm for [bpy][BF4] and [bpy][CF3SO3], and less than000 ppm and 200 ppm for [b3mpy][N(CN)2] and [o3mpy][BF4],espectively, as the supplier indicates in the certificate of analysis.easurements for [ppy][BF4], [bpy][BF4], and [o3mpy][BF4] were
erformed in the temperature range of 283.15–333.15 K and in theressure range of 0.1–65 MPa. In the case of [bpy][CF3SO3] andb3mpy][N(CN)2], density measurements at lower temperaturesere not possible due to the sample solidification at such work
onditions, the corresponding normal melting points are 301.4 Knd 278 K, respectively [15,16].
Densities, �, were measured using a high pressure, high tem-erature Anton Paar DMA HP cell connected to an evaluationnit Anton Paar DMA 5000. The cell temperature is controlledo ±1 × 10−3 K by means of an integrated Peltier thermostat. Theequired pressure was created by a hand pump 750.1100 fromitec, Switzerland and measured by a pressure transducer US181rom Measuring Specialities, USA. The uncertainty in the pressure
easurement is 5 kPa. A vacuum pump was employed for evacu-ting the whole apparatus. Hexane, water and dichloromethaneere used to calibrate the densimeter in order to cover a wide
ange of densities. Besides, toluene was employed to check the cal-bration over a wide range of temperature and pressure. Detailsf the calibration procedure can be found in a previous paper17]. The estimated uncertainty of our density measurements is1 × 10−5 g cm−3.
It is known that density determination using vibrating tube den-imeters depends on the sample viscosity. Thus, a correction of thexperimental densities of ILs should be performed to obtain reli-ble results due to the high viscosity values of these compounds,specially at high pressures. With this aim, we have followed therocedure proposed by Romani et al. which consists in obtaining aurve of the correction of density, ��, as a function of viscosity, �,ith our densimeter [18,19]. In this way, viscosity-induced errors
or our vibrating tube densimeter, ��, as a function of viscosity, �,ave been adjusted using the following equation:
�(�) = �2
q1 + q2� + q3�2(1)
here q1, q2 and q3 are fitting values. In our case, these parametersre: q1 = 3,516,076, q2 = 129,396 and q3 = 843. Therefore, it is possi-le to correct the density of the ILs obtained from the Anton PaarMA HP cell if experimental viscosities of these chemicals, at the
ame temperature and pressure range at which densities have beenetermined, are available. These measurements have been previ-usly obtained for the studied ILs [20]. More details for the viscosityorrection procedure can be found in a previous paper [21]. Thestimated uncertainty of corrected density is ±1 × 10−4 g cm−3.
. Results
Corrected densities, from now densities, for the studied ILs areollected in Table 2. Previous literature works report density val-es of [bpy][BF4] [14,22] and [b3mpy][N(CN)2] [23] at atmospheric
.99 None 0.99 –
.98 None 0.98 –
.98 None 0.98 –
pressure which agree with our experimental results. However, onlya reference at high pressures and two temperatures have beenfound for [bpy][BF4] [14]. A comparison between experimental andliterature data at different pressures is found in Table 3. Experimen-tal results at T = 298.15 K are similar to our data whereas differencesat T = 323.15 K are slightly greater.
Densities have been fitted with temperature and pressureaccording to the three-dimensional p�T correlating model calledTRIDEN [13]. In this approach, the Tait equation for isothermalcompressed densities has been combined with a modified Rackettequation for the liquid saturation densities:
�0 = AR
BR
[1+(1−T/CR)DR
] (2)
� = �0
1 − CT ln((BT + p)/(BT + p0))(3)
BT = b0 + b1
(T
ET
)+ b2
(T
ET
)2+ b3
(T
ET
)3(4)
Pressure is given in MPa, density in g cm−3 and temperature in Kin this expression. The reference pressure is set to p0 = 0.1 MPa andthe corresponding reference densities, �0, were correlated with Eq.(2). The parameter CT in Eq. (3) was treated as temperature inde-pendent. The relative root-mean-square deviations, RMSDr, havebeen used as statistical values for the equation fit:
RMSDr (%) = 100
(1n
n∑i=1
(�i,exp − �i,cal
�i,exp
)2)1/2
(5)
where n is the number of experimental data. TRIDEN parametersalong with the corresponding deviations for each compound arecollected in Table 4. The overall RMSDr obtained is 0.0081% andthe deviations between experimental and correlated densities areclose to the uncertainty of the experimental densities and randomlydistributed.
The experimental densities of the ionic liquids together withcorrelated densities using the TRIDEN equation are shown inFigs. 1–5. Density decreases with temperature at constant pressuredue to the thermal expansion and weakening of intermolecu-lar interactions between ions [24]. Besides, it undergoes a linearincreasing with pressure.
The analysis of results has been divided in three parts corre-sponding to the structural variations of ions. Thus, we are goingto evaluate in detail the influence of the ionic structure in thisproperty. This study completes a previous thermophysical char-acterization of these ILs that our investigation group has carriedout [15,25,26].
Firstly, the influence of a substituent within the ring isstudied when [bpy][BF4] is compared with three isomericILs which have been previously characterized by our inves-tigation group, i.e. 1-butyl-2-methylpyridinium tetrafluorobo-
rate ([b2mpy][BF4]), 1-butyl-3-methylpyridinium tetrafluorobo-rate ([b3mpy][BF4]), and 1-butyl-4-methylpyridinium tetraflu-oroborate ([b4mpy][BF4]) [21]. A comparison of the resultsreveals that [bpy][BF4] is denser than the isomeric compounds
104 H. Guerrero et al. / Fluid Phase Equilibria 317 (2012) 102– 109
Table 2Densities, �, of the studied ionic liquids as a function of pressure and temperature.
Tc (K) �a (g cm−3) at pb (MPa)
0.1 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0 55.0 60.0 65.0
[ppy][BF4]283.15 1.2642 1.2661 1.2682 1.2702 1.2722 1.2741 1.2760 1.2779 1.2797 1.2815 1.2832 1.2848 1.2865 1.2881288.15 1.2603 1.2623 1.2644 1.2665 1.2686 1.2704 1.2723 1.2743 1.2761 1.2779 1.2796 1.2814 1.2829 1.2847293.15 1.2566 1.2587 1.2608 1.2628 1.2649 1.2668 1.2689 1.2707 1.2726 1.2743 1.2761 1.2779 1.2795 1.2812298.15 1.2529 1.2551 1.2573 1.2594 1.2615 1.2635 1.2654 1.2674 1.2692 1.2710 1.2729 1.2746 1.2763 1.2780303.15 1.2494 1.2517 1.2538 1.2560 1.2580 1.2600 1.2620 1.2640 1.2659 1.2677 1.2694 1.2713 1.2730 1.2747308.15 1.2459 1.2479 1.2503 1.2523 1.2545 1.2565 1.2585 1.2606 1.2625 1.2643 1.2662 1.2679 1.2697 1.2714313.15 1.2422 1.2445 1.2468 1.2490 1.2511 1.2532 1.2552 1.2572 1.2592 1.2612 1.2631 1.2649 1.2667 1.2683318.15 1.2387 1.2408 1.2432 1.2453 1.2475 1.2496 1.2517 1.2537 1.2557 1.2577 1.2595 1.2612 1.2632 1.2649323.15 1.2352 1.2375 1.2398 1.2421 1.2443 1.2465 1.2485 1.2505 1.2526 1.2545 1.2564 1.2583 1.2601 1.2619328.15 1.2316 1.2339 1.2361 1.2385 1.2407 1.2429 1.2451 1.2472 1.2492 1.2511 1.2531 1.2548 1.2568 1.2586333.15 1.2282 1.2305 1.2329 1.2351 1.2376 1.2398 1.2418 1.2440 1.2460 1.2480 1.2500 1.2519 1.2538 1.2556
[bpy][BF4]283.15 1.2245 1.2266 1.2287 1.2309 1.2329 1.2350 1.2369 1.2388 1.2407 1.2425 1.2443 1.2460 1.2477 1.2494288.15 1.2209 1.2230 1.2251 1.2274 1.2294 1.2314 1.2334 1.2353 1.2373 1.2392 1.2408 1.2427 1.2443 1.2461293.15 1.2173 1.2195 1.2216 1.2238 1.2259 1.2280 1.2300 1.2320 1.2339 1.2357 1.2376 1.2395 1.2411 1.2430298.15 1.2138 1.2160 1.2183 1.2204 1.2227 1.2248 1.2268 1.2287 1.2307 1.2326 1.2346 1.2364 1.2380 1.2397303.15 1.2104 1.2128 1.2151 1.2172 1.2194 1.2215 1.2235 1.2255 1.2275 1.2294 1.2313 1.2332 1.2349 1.2366308.15 1.2069 1.2092 1.2114 1.2137 1.2158 1.2180 1.2201 1.2221 1.2240 1.2260 1.2278 1.2298 1.2316 1.2335313.15 1.2036 1.2059 1.2082 1.2105 1.2128 1.2149 1.2171 1.2191 1.2212 1.2231 1.2251 1.2269 1.2287 1.2305318.15 1.2001 1.2024 1.2049 1.2070 1.2093 1.2114 1.2135 1.2157 1.2177 1.2197 1.2217 1.2235 1.2254 1.2273323.15 1.1968 1.1991 1.2016 1.2039 1.2063 1.2084 1.2106 1.2127 1.2148 1.2167 1.2187 1.2206 1.2225 1.2243328.15 1.1932 1.1956 1.1980 1.2004 1.2027 1.2049 1.2072 1.2094 1.2114 1.2134 1.2154 1.2173 1.2192 1.2213333.15 1.1900 1.1925 1.1949 1.1973 1.1998 1.2019 1.2042 1.2063 1.2085 1.2105 1.2127 1.2146 1.2164 1.2184
[bpy][CF3SO3]303.15 1.3113 1.3142 1.3170 1.3197 1.3223 1.3250 1.3275 1.3299 1.3323 1.3347 1.3370 1.3393 1.3414 1.3434308.15 1.3075 1.3104 1.3133 1.3162 1.3188 1.3214 1.3239 1.3265 1.3290 1.3314 1.3334 1.3358 1.3379 1.3399313.15 1.3036 1.3066 1.3094 1.3123 1.3148 1.3176 1.3203 1.3227 1.3253 1.3276 1.3300 1.3323 1.3345 1.3366318.15 1.2997 1.3028 1.3054 1.3084 1.3111 1.3139 1.3164 1.3192 1.3217 1.3239 1.3263 1.3285 1.3310 1.3331323.15 1.2961 1.2987 1.3020 1.3048 1.3077 1.3105 1.3131 1.3155 1.3182 1.3208 1.3231 1.3254 1.3277 1.3297328.15 1.2920 1.2950 1.2980 1.3010 1.3039 1.3066 1.3093 1.3120 1.3144 1.3170 1.3194 1.3218 1.3240 1.3264333.15 1.2882 1.2913 1.2943 1.2973 1.3003 1.3032 1.3059 1.3086 1.3112 1.3138 1.3162 1.3186 1.3206 1.3232
[b3mpy][N(CN)2]293.15 1.0524 1.0543 1.0562 1.0580 1.0598 1.0616 1.0633 1.0651 1.0667 1.0685 1.0699 1.0719 1.0731 1.0747298.15 1.0493 1.0512 1.0531 1.0551 1.0569 1.0587 1.0604 1.0622 1.0639 1.0655 1.0672 1.0688 1.0704 1.0719303.15 1.0463 1.0482 1.0501 1.0519 1.0538 1.0557 1.0574 1.0592 1.0609 1.0626 1.0643 1.0660 1.0674 1.0690308.15 1.0431 1.0452 1.0472 1.0491 1.0509 1.0528 1.0547 1.0564 1.0581 1.0598 1.0616 1.0632 1.0646 1.0663313.15 1.0402 1.0422 1.0442 1.0462 1.0481 1.0500 1.0517 1.0536 1.0553 1.0570 1.0587 1.0603 1.0620 1.0635318.15 1.0372 1.0392 1.0411 1.0433 1.0450 1.0469 1.0489 1.0507 1.0523 1.0540 1.0559 1.0575 1.0592 1.0609323.15 1.0342 1.0364 1.0383 1.0404 1.0422 1.0443 1.0460 1.0479 1.0497 1.0514 1.0532 1.0548 1.0564 1.0580328.15 1.0312 1.0331 1.0353 1.0373 1.0393 1.0413 1.0429 1.0449 1.0467 1.0486 1.0505 1.0521 1.0538 1.0554333.15 1.0276 1.0302 1.0323 1.0344 1.0364 1.0383 1.0404 1.0422 1.0440 1.0455 1.0476 1.0492 1.0510 1.0526
[o3mpy][BF4]283.15 1.1052 1.1075 1.1099 1.1122 1.1144 1.1166 1.1187 1.1208 1.1228 1.1248 1.1267 1.1286 1.1304 1.1322288.15 1.1018 1.1042 1.1066 1.1089 1.1113 1.1134 1.1156 1.1176 1.1197 1.1216 1.1236 1.1255 1.1274 1.1292293.15 1.0986 1.1011 1.1035 1.1059 1.1082 1.1104 1.1125 1.1148 1.1168 1.1188 1.1208 1.1227 1.1246 1.1264298.15 1.0952 1.0976 1.1001 1.1025 1.1049 1.1071 1.1094 1.1115 1.1136 1.1156 1.1176 1.1196 1.1215 1.1235303.15 1.0920 1.0945 1.0970 1.0994 1.1018 1.1041 1.1064 1.1086 1.1107 1.1127 1.1148 1.1167 1.1187 1.1206308.15 1.0886 1.0910 1.0936 1.0962 1.0985 1.1008 1.1031 1.1053 1.1075 1.1097 1.1117 1.1137 1.1157 1.1177313.15 1.0853 1.0878 1.0904 1.0930 1.0954 1.0978 1.1001 1.1024 1.1046 1.1067 1.1089 1.1109 1.1129 1.1148318.15 1.0820 1.0846 1.0873 1.0898 1.0923 1.0947 1.0971 1.0994 1.1016 1.1037 1.1058 1.1079 1.1099 1.1119323.15 1.0788 1.0816 1.0842 1.0868 1.0892 1.0917 1.0941 1.0964 1.0987 1.1008 1.1030 1.1051 1.1071 1.1091328.15 1.0756 1.0783 1.0810 1.0835 1.0861 1.0886 1.0910 1.0934 1.0957 1.0979 1.1000 1.1021 1.1042 1.1063333.15 1.0724 1.0751 1.0779 1.0806 1.0831 1.0857 1.0881 1.0905 1.0929 1.0951 1.0973 1.0994 1.1015 1.1036
[tot[
TD
a �(�) = ±1 × 10−4 g cm−3.b �(T) = ±1 × 10−3 K.c �(p) = ±5 × 10−3 MPa.
b2mpy][BF4], [b3mpy][BF4], and [b4mpy][BF4]. Thus, the lack ofhe methyl group in the cation seems to favour a better structural
rganization of the ions inside the fluid. Secondly, experimen-al data for [ppy][BF4] and [bpy][BF4], and for [b3mpy][BF4] ando3mpy][BF4], provide a general idea about how the incrementable 3ensity data reported for [bpy][BF4] at high pressure in the literature and relative root-m
Reference Purity pmin (MPa)
Gu and Brennecke [14] Water content: 0.67 mass% 0.1
Chloride content: 75 ppm 0.1
of the alkyl chain length in the cation affects the compound fea-tures. We have found that density decreases when the alkyl chain
length on the cation increases since [ppy][BF4] and [b3mpy][BF4]show greater values than [bpy][BF4] and [o3mpy][BF4], respec-tively. These data indicate that the presence of a longer alkyl chainean-square deviations, RMSDr, between them and experimental data.
pmax (MPa) T (K) n RMSDr (%)
206.91 298.2 ± 0.1 8 0.03202.81 323.2 ± 0.1 6 0.14
H. Guerrero et al. / Fluid Phase Equilibria 317 (2012) 102– 109 105
Table 4Parameters for the TRIDEN expression along with the corresponding relative root-mean-square deviations, RMSDr, for the studied ILs.
[ppy][BF4] [bpy][BF4] [bpy][CF3SO3] [b3mpy][N(CN)2] [o3mpy][BF4]
AR 0.8929 0.3000 0.3071 0.2759 0.2847BR 0.76843 0.45671 0.44918 0.47547 0.46908CR 443.64 1346.23 749.54 777.83 937.85DR 1.122 0.9743 0.3982 0.4697 0.6508CT 0.0608 0.0634 0.0641 0.0679 0.0684b0 333.733 344.811 103.401 139.117 300.528b1 −19.10 −48.28 21.69 25.95 −17.87b2 −9.624 1.087 4.168 4.365 −9.609b3 1.041 −0.0663 −1.764 −2.107 1.088ET 75.00 75.10 75.00 75.00 75.00RMSDr (%) 0.0069 0.0090 0.0083 0.0086 0.0076
0 10 20 30 40 50 60 70
1.22 5
1.23 0
1.23 5
1.24 0
1.24 5
1.25 0
1.25 5
1.26 0
1.26 5
1.27 0
1.27 5
1.28 0
1.28 5
1.29 0
k
jih
g
f
c
ed
b
a
ρ (
g·c
m-3)
p (MPa)
Fig. 1. Densities, �, for [ppy][BF4] as a function of pressure and temperature: (a)TTT
iflauI
FTTT
0 10 20 30 40 50 60 70
1.28 5
1.29 0
1.29 5
1.30 0
1.30 5
1.31 0
1.31 5
1.32 0
1.32 5
1.33 0
1.33 5
1.34 0
1.34 5
g
h
i
j
k
f
e
ρ (
g·c
m-3)
p (MPa)
Fig. 3. Densities, �, for [bpy][CF3SO3] as a function of pressure and temperature:
= 283.15 K, (b) T = 288.15 K, (c) T = 293.15 K, (d) T = 298.15 K, (e) T = 303.15 K, (f) = 308.15 K, (g) T = 313.15 K, (h) T = 318.15 K, (i) T = 323.15 K, (j) T = 328.15 K, and (k) = 333.15 K.n the cation could avoid a better accommodation of ions in theuid [2,27–29]. Finally, a comparison of results for [b3mpy][BF4]
nd [b3mpy][N(CN)2], and for [bpy][CF3SO3] and [bpy][BF4], allowss to evaluate the dependence of density upon the anion nature.n this case, a direct correlation between the density and the
0 10 20 30 40 50 60 70
1.18 5
1.19 0
1.19 5
1.20 0
1.20 5
1.21 0
1.21 5
1.22 0
1.22 5
1.23 0
1.23 5
1.24 0
1.24 5
1.25 0
1.25 5
kjih
gfedc
b
a
ρ (
g·c
m-3)
p (MPa )
ig. 2. Densities, �, for [bpy][BF4] as a function of pressure and temperature: (a) = 283.15 K, (b) T = 288.15 K, (c) T = 293.15 K, (d) T = 298.15 K, (e) T = 303.15 K, (f) = 308.15 K, (g) T = 313.15 K, (h) T = 318.15 K, (i) T = 323.15 K, (j) T = 328.15 K, and (k) = 333.15 K.
(e) T = 303.15 K, (f) T = 308.15 K, (g) T = 313.15 K, (h) T = 318.15 K, (i) T = 323.15 K, (j)T = 328.15 K, and (k) T = 333.15 K.
formula weight of the anion has been found. Hence, density datafor [bpy][CF3SO3] and [b3mpy][BF4] are considerably greater thanthose for [bpy][BF4] and [b3mpy][N(CN)2] respectively. It is in
agreement with previous works in which the same tendency hasbeen found [27,30].0 10 20 30 40 50 60 70
1.02 5
1.03 0
1.03 5
1.04 0
1.04 5
1.05 0
1.05 5
1.06 0
1.06 5
1.07 0
1.07 5
1.08 0
cd
e
fg
h
ij
k
ρ (
g·c
m-3)
p (MPa)
Fig. 4. Densities, �, for [b3mpy][N(CN)2] as a function of pressure and temperature:(c) T = 293.15 K, (d) T = 298.15 K, (e) T = 303.15 K, (f) T = 308.15 K, (g) T = 313.15 K, (h)T = 318.15 K, (i) T = 323.15 K, (j) T = 328.15 K, and (k) T = 333.15 K.
106 H. Guerrero et al. / Fluid Phase Equilibria 317 (2012) 102– 109
0 10 20 30 40 50 60 70
1.06 5
1.07 0
1.07 5
1.08 0
1.08 5
1.09 0
1.09 5
1.10 0
1.10 5
1.11 0
1.11 5
1.12 0
1.12 5
1.13 0
1.13 5
kjihgfedcba
ρ (
g·c
m-3)
p (MPa)
Fig. 5. Densities, �, for [o3mpy][BF4] as a function of pressure and temperature:(a) T = 283.15 K, (b) T = 288.15 K, (c) T = 293.15 K, (d) T = 298.15 K, (e) T = 303.15 K, (f)T = 308.15 K, g) T = 313.15 K, (h) T = 318.15 K, (i) T = 323.15 K, (j) T = 328.15 K, and (k)T = 333.15 K.
0 10 20 30 40 50 60 70
0.49
0.51
0.53
0.55
0.57
0.59
T = 333. 15 K
T = 283 .15 K
α
(kK
-1)
p (MPa)
0 10 20 30 40 50 60 70
250
270
290
310
330
350
370
390
410
T = 28 3.15 K
T = 333.15 K
κ T (T
Pa
-1)
p (MPa)
Fig. 6. Coefficient of thermal expansion, ˛, and isothermal compressibility, �T, for[ppy][BF4] as a function of pressure and temperature.
0 10 20 30 40 50 60 70
0.49
0.52
0.55
0.58
T = 3 33. 15 K
T = 283.15 K
α
(kK
-1)
p (MPa)
0 10 20 30 40 50 60 70
260
280
300
320
340
360
380
400
420
440
T = 283 .15 K
T = 333 .15 K
κ T (T
Pa
-1)
p (MPa)
Fig. 7. Coefficient of thermal expansion, ˛, and isothermal compressibility, �T, for[bpy][BF4] as a function of pressure and temperature.
Useful derived properties have been calculated from the depen-dence of temperature and pressure on density; the coefficient ofthermal expansion, ˛, and the isothermal compressibility, �T:
= 1V
(∂V
∂T
)p
= − 1�
(∂�
∂T
)p
(6)
�T = − 1V
(∂V
∂p
)T
= 1�
(∂�
∂p
)T
(7)
Their values for the studied ILs at each temperature and pressureare collected in the supplementary material and they are graphi-cally represented in Figs. 6–10. Data for these derived propertiesat T = 303.15 K and atmospheric pressure are gathered in Table 5.Estimating the accuracy of these coefficients is difficult since theyare affected by the uncertainty associated with the experimentaldetermination of density and the numerical errors related to thefitting equations used [4]. We have found a previous reference ofisothermal compressibility values for [bpy][BF4] [14] and our dataproperly agree with those obtained in this work, with maximumdifferences of 2% corresponding to high pressures.
Coefficients of thermal expansion depend strongly on the
expression used to correlate density values. For this reason, it isessential to choose an equation which could properly describe thevolumetric behaviour of these materials in such work conditions.In spite of the different ionic structures, it is noticeable that these
H. Guerrero et al. / Fluid Phase Equilibria 317 (2012) 102– 109 107
Table 5Coefficient of thermal expansion, and isothermal compressibility for the studied ILs at T = 303.15 K and atmospheric pressure.
[ppy][BF4] [bpy][BF4] [bpy][CF3SO3] [b3mpy][N(CN)2] [o3mpy][BF4]
(kK−1) 0.579 0.571 0.581 0.583 0.6024
cI
slgsBitwbtIhafs
F[
results agree with those obtained in previous studies for other ILs
�T (T Pa−1) 361.8 388.0
oefficients have a similar order of magnitude for all the studiedLs.
The calculated coefficients of thermal expansion for the ILstudied here decrease with pressure at constant temperature fol-owing the general behaviour of liquids [31]. There also exits aeneral agreement that the isotherms of intersect at high pres-ures for many liquids, a characteristic feature first described byridgman [32] and observed for other authors [31,33–36]. Accord-
ng to Figs. 6–10, two different tendencies can be identified. Onhe one hand, this coefficient decreases with rising pressure andith increasing temperature along the isobars for [ppy][BF4], this
ehaviour with temperature, attributed to the high ordering ofhese fluids [37–39], is more evident when pressure increases.t is evident that this coefficient has to present a minimum at
igh temperatures because it is required that diverges to +∞t the critical temperature [39]. On the other hand, the isothermsor [bpy][BF4], [bpy][CF3SO3], [b3mpy][N(CN)2] and [o3mpy][BF4]how an intersection point in which the coefficients are0 10 20 30 40 50 60 70
0.48
0.51
0.54
0.57
0.60
0.63
T = 303 .15 K
T = 333 .15 K
α
(kK
-1)
p (MPa)
0 10 20 30 40 50 60 70
310
330
350
370
390
410
430
450
470
490
510
T = 303 .15 K
T = 333.15 K
κ T (T
Pa
-1)
p (MPa)
ig. 8. Coefficient of thermal expansion, ˛, and isothermal compressibility, �T, forbpy][CF3SO3] as a function of pressure and temperature.
49.0 385.2 477.9
independent of temperature at such pressure. This point is relatedto the minimum of isobaric molar heat capacity values for thesecompounds at this work conditions [40] and it has been observedpreviously for other ionic liquids [41] and molecular solvents [42].The intersection occurs close to atmospheric pressure, 30 MPa and25 MPa for [bpy][BF4] [bpy][CF3SO3] and [b3mpy][N(CN)2], respec-tively, and at higher pressures, 55 MPa, for [o3mpy][BF4].
According to Figs. 6–10, isothermal compressibility for theseILs increases with temperature and it decreases when pressurerises. Then, these ILs show a regular liquids behaviour. No sig-nificant differences have been found when the isomers and theIL missing a methyl group are compared. Otherwise, compress-ibility increases with the alkyl chain length of the cation. These
[42]. These results also seem to support the conclusions obtainedfrom density and other thermophysical properties which suggest agreater molar free volume for those compounds with bulky groups
0 10 20 30 40 50 60 70
0.49
0.52
0.55
0.58
0.61
T = 293 .15 K
T = 333 .15 K
α
(kK
-1)
p (MPa)
0 10 20 30 40 50 60 70
260
280
300
320
340
360
380
400
420
440
T = 293 .15 K
T = 333 .15 K
κ T (T
Pa
-1)
p (MPa)
Fig. 9. Coefficient of thermal expansion, ˛, and isothermal compressibility, �T, for[b3mpy][N(CN)2] as a function of pressure and temperature.
108 H. Guerrero et al. / Fluid Phase Equ
0 10 20 30 40 50 60 70
0.51
0.54
0.57
0.60
0.63
T = 283 .15 K
T = 333 .15 K
α
(kK
-1)
p (MPa)
0 10 20 30 40 50 60 70
310
330
350
370
390
410
430
450
470
490
510
530
550
T = 283.15 K
T = 333.15 K
κ T (T
Pa
-1)
p (MPa)
F[
[tiwp
4
caaoniosti
A
dt
[
[[
[[[
[
[
[
[
[
[
[
ig. 10. Coefficient of thermal expansion, ˛, and isothermal compressibility, �T, foro3mpy][BF4] as a function of pressure and temperature.
25,26]. Thus, [o3mpy][BF4] and [bpy][BF4] are more compressiblehan [b3mpy][BF4] and [ppy][BF4], respectively. Finally, compress-bility values increase with increasing anion formula weight. Hence,
e have found that [b3mpy][BF4] and [bpy][CF3SO3] are more com-ressible than [b3mpy][N(CN)2] and [bpy][BF4], respectively.
. Conclusions
Densities, coefficients of thermal expansion, and isothermalompressibilities of a series of ILs in a broad range of temperaturesnd pressures have been reported. Results have been evaluatedttending to structural effects. Density data indicate that the lackf the methyl group in the cation favours a better structural orga-ization of the ions inside the fluid. Besides, density decreases and
sothermal compressibility increases when the alkyl chain lengthf the cation increases. Finally, it has been found a direct relation-hip between the formula weigh of the anion and properties forhese ILs. In this case, coefficients of thermal expansion seem to bendependent of the ionic structures.
cknowledgements
We are grateful for financial assistance from Diputación Generale Aragón, Universidad de Zaragoza (ref-23301). I. Bandrés thankshe predoctoral fellowship from DGA.
[
[[
ilibria 317 (2012) 102– 109
List of symbolsAR, . . ., DR parameters of the modified Rackett equationBT, bx parameters of the Tait equation (MPa)CT parameter of the Tait equationE parameter in the Tait equation for reducing the tempera-
ture (K)n number of experimental pointsp pressure (MPa)p0 pressure of reference (=0.1 MPa)qi parameters of Eq. (1)RMSDr relative root-mean-square deviations (%)T temperature (K)V molar volume (cm3 mol−1)
Greek letters˛ coefficient of thermal expansion (kK−1)�T isothermal compressibility (T Pa−1)�� density correction (g cm−3)� viscosity (mPa s)� density (g cm−3)�0 reference density (g cm−3)
Subscriptscal calculatedexp experimental
Appendix A. Supplementary data
Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.fluid.2011.12.029.
References
[1] J.O. Valderrama, K. Zarricueta, Fluid Phase Equilibr. 275 (2009) 145–151.[2] J.G. Huddleston, A.E. Visser, W.M. Reichert, H.D. Willauer, G.A. Broker, R.D.
Rogers, Green Chem. 3 (2001) 156–164.[3] C.F. Ye, J.M. Shreeve, J. Phys. Chem. A 111 (2007) 1456–1461.[4] J. Jacquemin, P. Nancarrow, D.W. Rooney, M.F.C. Gomes, P. Husson, V. Majer,
A.A.H. Padua, C. Hardacre, J. Chem. Eng. Data 53 (2008) 2133–2143.[5] J.F. Brennecke, E.J. Maginn, AlChE J. 47 (2001) 2384–2389.[6] T.L. Greaves, C.J. Drummond, Chem. Rev. 108 (2008) 206–237.[7] M. Armand, F. Endres, D.R. MacFarlane, H. Ohno, B. Scrosati, Nat. Mater. 8 (2009)
621–629.[8] L.I.N. Tome, R.L. Gardas, P.J. Carvalho, M.J. Pastoriza-Gallego, M.M. Pineiro, J.A.P.
Coutinho, J. Chem. Eng. Data 56 (2011) 2205–2217.[9] H. Machida, Y. Sato, R.L. Smith, Fluid Phase Equilibr. 264 (2008) 147–155.10] J. Esperanca, Z.P. Visak, N.V. Plechkova, K.R. Seddon, H.J.R. Guedes, L.P.N. Rebelo,
J. Chem. Eng. Data 51 (2006) 2009–2015.11] J. Klomfar, M. Souckova, J. Patek, J. Chem. Eng. Data 56 (2011) 426–436.12] E. Rilo, A.G.M. Ferreira, I.M.A. Fonseca, O. Cabeza, Fluid Phase Equilibr. 296
(2010) 53–59.13] E.C. Ihmels, J. Gmehling, Ind. Eng. Chem. Res. 40 (2001) 4470–4477.14] Z.Y. Gu, J.F. Brennecke, J. Chem. Eng. Data 47 (2002) 339–345.15] I. Bandres, F.M. Royo, I. Gascon, M. Castro, C. Lafuente, J. Phys. Chem. B 114
(2010) 3601–3607.16] I. Bandres, B. Giner, I. Gascon, M. Castro, C. Lafuente, J. Phys. Chem. B 112 (2008)
12461–12467.17] H. Guerrero, C. Lafuente, F. Royo, L. Lomba, B. Giner, Energy Fuels 25 (2011)
3009–3013.18] Y.A. Sanmamed, D. Gonzalez-Salgado, J. Troncoso, C.A. Cerdeirina, L. Romani,
Fluid Phase Equilibr. 252 (2007) 96–102.19] Y.A. Sanmamed, D. Gonzalez-Salgado, J. Troncoso, L. Romani, A. Baylaucq, C.
Boned, J. Chem. Thermodyn. 42 (2010) 553–563.20] I. Bandres, R. Alcalde, C. Lafuente, M. Atilhan, S. Aparicio, J. Phys. Chem. B 115
(2011) 12499–12513.21] H. Guerrero, M. García-Mardones, P. Cea, C. Lafuente, I. Bandres, Thermochim.
Acta, doi:10.1016/j.tca.2011.12.020, in press.22] B. Mokhtarani, A. Sharifi, H.R. Mortaheb, M. Mirzaei, M. Mafi, F. Sadeghian, J.
Chem. Thermodyn. 41 (2009) 323–329.
23] L.G. Sanchez, J.R. Espel, F. Onink, G.W. Meindersma, A.B. de Haan, J. Chem. Eng.Data 54 (2009) 2803–2812.24] J.F. Wang, C.X. Li, C. Shen, Z.H. Wang, Fluid Phase Equilibr. 279 (2009) 87–91.25] I. Bandres, B. Giner, H. Artigas, C. Lafuente, F.M. Royo, J. Chem. Eng. Data 54
(2009) 236–240.
e Equ
[
[
[
[
[
[
[[[
[
[
[
[
[
H. Guerrero et al. / Fluid Phas
26] I. Bandres, M.C. Lopez, M. Castro, J. Barbera, C. Lafuente, J. Chem. Thermodyn.44 (2012) 148–153.
27] C.P. Fredlake, J.M. Crosthwaite, D.G. Hert, S. Aki, J.F. Brennecke, J. Chem. Eng.Data 49 (2004) 954–964.
28] H. Tokuda, K. Hayamizu, K. Ishii, M. Susan, M. Watanabe, J. Phys. Chem. B 109(2005) 6103–6110.
29] A.P. Froba, H. Kremer, A. Leipertz, J. Phys. Chem. B 112 (2008)12420–12430.
30] H. Tokuda, K. Hayamizu, K. Ishii, M. Abu Bin Hasan Susan, M. Watanabe, J. Phys.Chem. B 108 (2004) 16593–16600.
31] M. Taravillo, V.G. Baonza, M. Cáceres, J. Núnez, J. Phys.: Condens. Matter 15(2003) 2979–2988.
32] P.W. Bridgman, The Physics of High Pressure, Dover, New York, 1970.33] G. Jenner, M. Millet, High Temp.—High Press. 2 (1970) 205–213.34] G. Jenner, M. Millet, High Temp.—High Pressure 5 (1973) 145.
[
[[
ilibria 317 (2012) 102– 109 109
35] W.G. Cutler, R.H. McMickle, W. Webb, R.W. Schiessler, J. Phys. Chem. 29 (1958)727–740.
36] O. Fandino, A.S. Pensado, L. Lugo, E.R. López, J. Fernández, Green Chem. 7 (2005)775–783.
37] M.J. Davila, S. Aparicio, R. Alcalde, B. Garcia, J.M. Leal, Green Chem. 9 (2007)221–232.
38] J. Troncoso, C.A. Cerdeirina, P. Navia, Y.A. Sanmamed, D. Gonzalez-Salgado, L.Romani, J. Phys. Chem. Lett. 1 (2010) 211–214.
39] L.P.N. Rebelo, V. Najdanovic-Visak, Z.P. Visak, M.N. da Ponte, J. Szydlowski, C.A.Cerdeirina, J. Troncoso, L. Romani, J. Esperanca, H.J.R. Guedes, H.C. de Sousa,
Green Chem. 6 (2004) 369–381.40] M.J.P. Comunas, A. Baylaucq, C. Boned, J. Fernandez, Int. J. Thermophys. 22(2001) 749–768.
41] E.K. Goharshadi, H. Azizi-Toupkanloo, J. Mol. Liq. 151 (2010) 117–121.42] A. Kumar, J. Solution Chem. 37 (2008) 203–214.