Fluid Phase Equilibria 294 (2010) 131138

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Fluid Phase Equilibria

journa l homepage: www.e lsev ier .co

High-temperature surface tension and density m1-alkyl nic

Mohamm ramJos M.S. loa Instituto de T 0-901b Centro de Qu

a r t i c l

Article history:Received 13 NReceived in reAccepted 16 FAvailable onlin

Keywords:Ionic liquidsDensitySurface tensioThermal expan

of amilyly. Thaturenic lia in toiling

1. Introduction

A way tdetermine tpoints, estaliquidgas e

Ionic liqally low meOn the otheliquid rangThis means most ofuids exhibiliquidvapotemperaturremoved frpoints.

Some timsurface tensposition of tionic liquid

CorresponE-mail add

(L.P.N. Rebelo)

From a pragmatic point of view, on the critical temperature, nor-mal boiling point, and vapor pressure of ionic liquids [1], opened

0378-3812/$ doi:10.1016/j.o dene the pT liquid range of a substance is tohe thermodynamic coordinates of its triple and criticalblishing in thisway theboundaries of the correspondingquilibrium curve.uids are a class of salts characterized by their unusu-lting point temperaturesusually lower than 100 C.r hand, at moderate temperatures (200300 C), their

es are interrupted by the decomposition of their ions.that when compared with traditional inorganic saltsthem less vulnerable to decomposition ionic liq-t just a short segment of their potentially completer saturation line: it starts early starting at their low-e triple point but ends prematurely at a position farom their unreachable critical or even normal boiling

e ago [1], we have used saturation liquid density andion data to estimate by temperature extrapolation thehe critical and normal boiling temperatures of differents, probing in this way the length of the saturation line.

ding authors.resses: jnlopes@ist.utl.pt (J.N. Canongia Lopes), luis.rebelo@itqb.unl.pt.

a window of opportunities as far as the vaporliquid equilibriumproperties of ionic liquids are concerned, originating a wealth ofdifferent studies ranging from the distillation of ionic liquids [24],to the measurement of enthalpies of vaporization [2,58], or thecharacterization of the nature of the vapor phase of ionic liquids[9].

The work reported in Ref. [1] relied heavily on the extensiveuse of empirical correlations, that albeit validated for differentclasses of compounds in fact involved large extrapolations intemperature, from experimental density and surface tension dataending at 343K to hypothetical critical point temperatures around1000K.

In this work, we have decided to measure liquid den-sity and surface tension in temperature ranges up to 473Kand 532K, respectively, for the 1-alkyl-3-methylimidazoliumbis(triuoromethylsulfonyl)amides, [Cnmim][Ntf2] with n=2, 3, 4,5, 6, 7, 8, 9, 10, 12, and 14 series of ionic liquids. The aim is to testthe validity of the above-mentioned extrapolations as well as thesoundness of the approximations previously performed [1] by con-fronting the current results with recently obtained vapor pressureand enthalpy of vaporization data [2] for the same class of com-pounds. Moreover, the compilation of original data in such a vasttemperature range and for so many members of an ionic liquidhomologous family also enables one to draw some important andsystematic conclusions concerning the temperature dependence of

see front matter 2010 Elsevier B.V. All rights reserved.uid.2010.02.020-3-methylimidazolium bistriamide io

ad Tariqa, Ana P. Serrob, Jos L. Matab, Benilde SaS. Esperancaa, Jos N. Canongia Lopesa,b,, Lus Pauecnologia Qumica e Biolgica, ITQB 2, Universidade Nova de Lisboa, Apartado 127, 278mica Estrutural, Instituto Superior Tcnico, 1049-001 Lisboa, Portugal

e i n f o

ovember 2009vised form 2 February 2010ebruary 2010e 6 March 2010

nsion coefcient

a b s t r a c t

The surface tension and densitybis(triuoromethylsulfonyl)amides fatime up to 532K and 473K, respectivethe broad interval of working temperin the thermal expansivity of these ionever been witnessed before. The datthe location of hypothetical normal bm/locate / f lu id

easurements ofliquids

agob,N. Rebeloa,

Oeiras, Portugal

lmost all members of the 1-alkyl-3-methylimidazolium, [Cnmim][Ntf2], with 2n14 were measured for the rste large number of ionic liquids studied within this family andenabled us to show evidence for a thermodynamic anomalyquids (minima in p versus T). The minima themselves havehis extended temperature range also permitted us to discussand critical points.

2010 Elsevier B.V. All rights reserved.

132 M. Tariq et al. / Fluid Phase Equilibria 294 (2010) 131138

Fig. 1. Tempe

the volumerange of thi

2. Materia

2.1. Chemic

Those io4, 6, 8, 12,Universityysis did noof the sampnature of th9 and10we[C9mim][Ntwere driedate temperit graduallyof all sampof less thanevacuationof 250040ride specicthis work [1

2.2. Method

The surfanalysis ofHart chambwere obtainments connaluminiumbox under nthe samples6mmapartstant, withithe positionaluminiumature rangeyielding a v

liquid drop was corrected accordingly. Measurements were madein a dry environment, keeping silica gel inside the chamber, inthe temperature range 313533K. The overall uncertainty in thetemperature measurements (taking into account both the PID con-

, theate

ediadroped oto aalysihapeates,raturas g

the adenedrrepwors meHP,angetermre anmendere

ide atier uthanions,entser, aof t

pandusedorrecthewasprev

omey meacheratine. Thrature-controlled ambient chamber, containing an embedded heater.

tric behavior along a signicant segment of the liquids class of compounds.

ls and methods

als

nic liquids from the [Cnmim][Ntf2] family with n=2, 3,and 14 were synthesized and puried at the QueensIonic Liquid Laboratory (QUILL) in Belfast. NMR anal-t show the presence of any major impurities in anyles except for traces of water due to the hygroscopice compounds. The [Cnmim][Ntf2] samples with n=5, 7,re purchased from Iolitec (all 99% claimedpurity exceptf2] with 98%). Before any measurements, all samplesfor at least 24h under vacuum (0.1 Pa) and moder-

ature (beginning at room temperature and increasingover a 6-h period up to 333K). The water content

les was further checked and found to be in the range70ppm, a value much lower than the original pre-

trolleris estimintermof themountmittedthe anDrop SAssocitempetime wabout

Thefromthviouslyin thissity wamodelature rwas deperaturecomelsewhity insby PelbetterconditsuremHowevbrationthe exliquidswere ction of4 cm3)ter. Towith sdensitwas rethe vibsyringanalysis, which typically showed values in the range00ppm. The chloride content, determined using a chlo-electrode, was less than 20ppm, an adequate value for0].

s

ace tension of each samplewas determined through thethe shape of pendant drops generated inside a Ram-er (model 100-07-00). Drops of approximately 7Led using a micrometric syringe from Gilmont Instru-ected to a Teon tube, which was introduced inside anjacket (see Fig. 1). The syringewaslled inside the gloveitrogen atmosphere due to the hygroscopic nature of. The liquid drops were held between the parallel faces,, of themetallic block,whose temperaturewaskept con-n 0.05K, using a PID controller. The temperature atof the liquid drop (lower than the temperature T in theblock) was calibrated throughout the whole temper-with a differential copperconstantan thermocouple,

alue of T (K) =0.0028T0.77. The temperature of the

effectivene

3. Results

The densented in Tgraphical foexperimentprehensivein elsewher

The volufamily wasnovelties reare their exthe inclusioand [C9mimreported eqis a quasi-approximatpresented ithermocouple and the position gradient between them)d to be around 0.5K. The thermal stabilization at eachte temperature took at least, half an hour. The imagess were acquired using a Video Camera (jAi CV-A50)n a Wild M3Z microscope. The video signal was trans-frame grabber (Data Translation model DT3155), beings performed with the ADSA-P software (AxisymetricAnalysis, Applied Surface Thermodynamics Research

Toronto, Canada). At least 5 drops were formed at eache and 5 images were captured for each drop. Enoughiven to each drop for equilibration. More informationpparatus and method is reported elsewhere [11].sities required for the calculation of the surface tensionop imagedatawere obtainedby interpolationusingpre-orted volumetric results [1214] or new data obtainedk (cf. below). In these new measurements, the den-asured with an Anton-Paar vibrating tube densimeter,operating at atmospheric pressure and in the temper-293473K. The internal calibration of the instrumentined by measuring the densities as a function of tem-d pressure of four standard substances, according to theations of the manufacturer. Further details are given

[13,14]. The Anton-Paar HP cell is embedded in a cav-metallic block, the temperature of which is controllednits. This arrangement allows a temperature stability2mK for periods over 10min. Under these operatingwe found that the repeatability of the density mea-at atmospheric pressure was better than 0.06kgm3.nd taking into account the uncertainties related to cali-he apparatus and the purity of the ionic liquid samples,ed uncertainty was estimated to be 0.4% for the ionicin this work (see Section 3). All reported density data

ted for the effect of viscosity using the internal calibra-densimeter. During the measurements, the liquid (ca.transferred to a syringe and injected into the densime-ent any air bubble, the vibrating tube was rst lledof the contents of the syringe (ca. 3 cm3), and a rstasurement was taken (after the temperature set pointd). Another measurement followed after the liquid ofg tube was replaced with the one that remained in thee agreement between both values is a measure of thess of the method and the absence of air bubbles.

and discussion

sity and surface tension experimental results are pre-ables 1 and 2, respectively. They are also depicted inrm in Figs. 2 and 3, along with comparisons with otheral data sets taken from the literature [1,1214]. A com-analysis of the published density data sets can be founde [12].metric behavior of ionic liquids of the [Cnmim][NTf2]discussed in a very recent publication [12]. The mainlated to the density data reported in the current worktension to a much broader (higher) temperature range,n of two other members of the family ([C7mim][NTf2]][NTf2]), and the conrmation that the previouslyuations that show that the logarithm of the density

linear function of temperature can be, to reasonableion, extended to thepresent range. In fact, the grey linesn Fig. 2 are not t to the new experimental data but the

M. Tariq et al. / Fluid Phase Equilibria 294 (2010) 131138 133

Table 1Experimental density of the [Cnmim][NTf2] ionic liquid series along with the tting equation, ln() (g cm3) = a (T (103 K)) +b.

T (K) (g cm3) T (K) (g cm3) T (K) (g cm3) T (K) (g cm3)

[C2mim][Ntf2] ln( (g cm3)) = (0.691512) (T (103 K)) + (0.626397)293.15 1.5250 343.15 1.4761 393.15 1.4274 443.15 1.3770303.15 1.5151 353.15 1.4664 403.15 1.4175 453.15 1.3666313.15 1.5053 363.15 1.4567 413.15 1.4075 463.15 1.3560323.15 1.4955 373.15 1.4470 423.15 1.3975 473.15 1.3452333.15 1.4858 383.15 1.4372 433.15 1.3873

[C3mim][Ntf2] ln( (g cm3)) = (0.694859) (T (103 K)) + (0.597575)293.15 1.4805 343.15 1.4324 393.15 1.3849 443.15 1.3359303.15 1.4708 353.15 1.4229 403.15 1.3753 453.15 1.3258313.15 1.4611 363.15 1.4135 413.15 1.3656 463.15 1.3156323.15 1.4515 373.15 1.4040 423.15 1.3558 473.15 1.3051333.15 1.4419 383.15 1.3945 433.15 1.3459

[C4mim][Ntf2] ln( (g cm3)) = (0.692099) (T (103 K)) + (0.570547)293.15 1.4423 343.15 1.3956 393.15 1.3494 443.15 1.3019303.15 1.4328 353.15 1.3863 403.15 1.3401 453.15 1.2921313.15 1.4234 363.15 1.3771 413.15 1.3307 463.15 1.2822323.15 1.4141 373.15 1.3679 423.15 1.3212 473.15 1.2720333.15 1.4048 383.15 1.3587 433.15 1.3116

[C5mim][Ntf2] ln( (g cm3)) = (0.695309) (T (103 K)) + (0.547404)293.15 1.4081 343.15 1.3621 393.15 1.3168 443.15 1.2703303.15 1.3987 353.15 1.3531 403.15 1.3076 453.15 1.2608313.15 1.3894 363.15 1.3440 413.15 1.2984 463.15 1.2511323.15 1.3803 373.15 1.3350 423.15 1.2886 473.15 1.2413333.15 1.3712 383.15 1.3259 433.15 1.2797

[C6mim][Ntf2] ln( (g cm3)) = (0.694881) (T (103 K)) + (0.525627)293.15 1.3781 343.15 1.3329 393.15 1.2885 443.15 1.2432303.15 1.3689 353.15 1.3240 403.15 1.2796 453.15 1.2339313.15 1.3598 363.15 1.3152 413.15 1.2706 463.15 1.2245323.15 1.3508 373.15 1.3063 423.15 1.2616 473.15 1.2149333.15 1.3418 383.15 1.2974 433.15 1.2524

[C7mim][Ntf2] ln( (g cm3)) = (0.696503) (T (103 K)) + (0.505060)293.15 1.3495 343.15 1.3050 393.15 1.2614 443.15 1.2170303.15 1.3404 353.15 1.2963 403.15 1.2526 453.15 1.2080313.15 1.3315 363.15 1.2876 413.15 1.2438 463.15 1.1987323.15 1.3226 373.15 1.2789 423.15 1.2350 473.15 1.1894333.15 1.3138 383.15 1.2701 433.15 1.2261

[C8mim][Ntf2] ln( (g cm3)) = (0.697845) (T (103 K)) + (0.488137)293.15 1.3265 343.15 1.2825 393.15 1.2395 443.15 1.1959303.15 1.3175 353.15 1.2739 403.15 1.2309 453.15 1.1870313.15 1.3086 363.15 1.2653 413.15 1.2222 463.15 1.1780323.15 1.2998 373.15 1.2567 423.15 1.2135 473.15 1.1688333.15 1.2912 383.15 1.2481 433.15 1.2048

[C9mim][Ntf2] ln( (g cm3)) = (0.699866) (T (103 K)) + (0.470928)293.15 1.3031 343.15 1.2597 393.15 1.2174 443.15 1.1745303.15 1.2942 353.15 1.2513 403.15 1.2088 453.15 1.1657313.15 1.2855 363.15 1.2428 413.15 1.2003 463.15 1.1568323.15 1.2768 373.15 1.2343 423.15 1.1918 473.15 1.1478333.15 1.2683 383.15 1.2258 433.15 1.1832

[C10mim][Ntf2] ln( (g cm3)) = (0.701270) (T (103 K)) + (0.457152)293.15 1.2849 343.15 1.2418 393.15 1.2000 443.15 1.1578303.15 1.2760 353.15 1.2335 403.15 1.1917 453.15 1.1489313.15 1.2674 363.15 1.2251 413.15 1.1833 463.15 1.1402323.15 1.2588 373.15 1.2167 423.15 1.1748 473.15 1.1314333.15 1.2503 383.15 1.2084 433.15 1.1664

[C12mim][Ntf2] ln( (g cm3)) = (0.692071) (T (103 K)) + (0.425851)293.15 1.2496 343.15 1.2074 393.15 1.1584 443.15 303.15 1.2409 353.15 1.1992 403.15 1.1502 453.15 313.15 1.2324 363.15 1.1909 413.15 1.1420 463.15 323.15 1.2240 373.15 1.1828 423.15 1.1337 473.15 333.15 1.2156 383.15 1.1747 433.15

[C14mim][Ntf2] ln( (g cm3)) = (0.713555) (T (103 K)) + (0.407854)293.15 343.15 1.1766 393.15 1.1364 443.15 1.0962303.15 353.15 1.1685 403.15 1.1284 453.15 1.0880313.15 363.15 1.1605 413.15 1.1204 463.15 1.0797323.15 1.1931 373.15 1.1524 423.15 1.1124 473.15 1.0714333.15 1.1848 383.15 1.1444 433.15 1.1043

134 M. Tariq et al. / Fluid Phase Equilibria 294 (2010) 131138

Table 2Experimental surface tension data of [Cnmim][NTf2] ionic liquid series along with the tting equation, (mNm1) = a+b (T (103 K)).

T (K) (mNm1) T (K) (mNm1) T (K) (mNm1) T (K) (mNm1)

[C2mim][Ntf ] (mNm1) = (51.13) + (51.39) (T (103 K))313.0 .2332.7 .7353.4 .7372.7 .2392.9 .2

[C3mim][Ntf373.5 .9393.5 .2413.6 .5433.3 .9

[C4mim][Ntf313.3 .6332.6 .6352.6372.7392.7412.5432.5

[C5mim][Ntf312.7333.2353.4373.5393.7

[C6mim][Ntf313.0333.2353.4

[C8mim][Ntf313.0333.3353.8373.6393.6

[C10mim][N314.1333.0353.2373.4393.3413.5433.3453.2

[C12mim][N333.3353.6373.7

[C14mim][N312.8333.2353.3373.5393.6

result of thtions which[C3mim][NTear extrapoat the extrand at a nosible liquidfrom the hyan almost lterms of de6kgm3 (0different so2

35.2 412.8 30.5 53234.2 432.5 29.6 47233.4 453.0 28.7 45232.5 472.5 25.7 31331.4 512.6 24.6 313

2] (mNm1) = (46.34) + (45.07) (T (103 K))28.8 453.2 26.8 35329.4 473.0 24.4 37428.2 312.9 32.5 39427.6 333.6 31.5 414

2] (mNm1) = (46.31) + (47.84) (T (103 K))31.9 452.2 24.2 41330.8 473.0 23.7 432

30.2 313.3 30.4 452.227.9 333.5 29.7 472.026.2 353.6 28.8 312.626.0 373.6 28.1 333.125.6 393.6 25.9 353.7

2] (mNm1) = (44.62) + (48.75) (T (103 K))29.5 413.6 24.6 512.928.3 433.7 23.7 313.227.3 453.6 22.8 353.726.3 473.6 21.7 373.725.6 493.2 20.8 393.7

2] (mNm1) = (44.53) + (48.17) (T (103 K))29.4 373.4 26.5 433.628.1 393.5 25.6 453.427.2 413.3 24.8 473.4

2] (mNm1) = (44.51) + (50.33) (T (103 K))29.2 413.4 24.3 512.628.2 433.3 23.2 313.227.3 453.4 22.1 332.626.3 473.6 20.7 352.825.3 493.0 20.1 372.8

tf2] (mNm1) = (44.57) + (51.89) (T (103 K))27.7 473.3 19.3 392.426.7 493.1 18.5 412.525.4 513.8 17.5 432.224.5 533.2 16.9 452.823.5 313.3 29.1 472.722.4 333.6 28.1 492.521.5 352.7 27.0 313.220.4 372.6 26.3 333.3

tf2] (mNm1) = (42.39) + (48.03) (T (103 K))26.4 393.6 23.6 453.825.4 413.7 22.5 473.724.5 433.6 21.4 493.3

tf2] (mNm1) = (42.08) + (52.92) (T (103 K))26.3 413.5 20.9 512.923.5 433.6 18.7 323.323.5 453.6 17.5 333.922.1 473.5 16.6 354.021.5 493.4 15.9 374.0

e application of the corresponding ln() =AT+B equa-were reported in Table 1 of Ref. [12]. In the case off2] and [C5mim][NTf2], the lines result from the lin-lation of two points taken from Ref. [13], consideredemes of the temperature range studied in that workminal pressure 0.1MPa. This proves that in the acces-range of this family of ionic liquids quite removedpothetical critical point density (or its logarithm) is

inear function of temperature. Fig. 2 also shows that innsity one should expect expanded uncertainties below.4%) when comparing reliable volumetric data fromurces. It must be stressed that the apparatus used to

measure thpressure de200 C butsensitive elintrinsicallyin other atmsions [12].pressure, alanother remthis family otemperaturbut they ar23.7 333.3 33.626.4 352.4 33.427.3 373.2 31.533.8 393.2 31.134.7 413.1 30.1

30.2 435.2 26.329.1 455.5 26.128.0 475.7 24.627.1

25.6 374.1 29.224.9 394.2 28.1

24.4 414.4 27.223.3 434.5 26.631.9 455.0 25.831.2 475.2 24.630.1

20.1 413.8 23.530.5 433.5 22.627.2 453.5 21.726.6 473.3 21.124.7 493.0 21.6

24.2 493.2 20.723.6 512.7 19.321.8 532.4 18.5

19.2 392.9 24.028.5 412.9 23.127.4 432.8 22.126.1 452.5 21.525.2 472.4 19.7

24.9 353.3 25.323.6 373.3 24.322.9 392.4 24.622.2 412.3 23.521.1 433.0 22.220.1 452.9 21.228.7 472.8 19.127.0

20.4 512.9 17.819.5 532.6 16.918.8

15.5 394.7 21.324.9 414.9 19.724.1 434.8 19.623.7 454.5 17.922.6 474.5 17.0

e densities in the extended temperature range is a high-nsimeter, capable of operating at temperatures up to

also to withstand pressures up to 70MPa. The density-ement of the densimeter (a vibrating tube) is thereforeless accurate than the corresponding elements usedospheric-pressure densimeters used in previous occa-

Its calibration, as a function of both temperature andso entails further loss of accuracy. Fig. 2 also shows twoarkable properties of the volumetric behavior withinf ionic liquids: not only theplots of ln() as a functionofe for the studied ionic liquids are almost straight lines,e also quite parallel to each other. This means that the

M. Tariq et al. / Fluid Phase Equilibria 294 (2010) 131138 135

Fig. 2. Logarithmof density as a functionof temperature for the [Cnmim][NTf2] ionicliquid series with n=2, 3, 4, 5, 6, 7, 8, 9, 10, 12, and 14. The symbols represent theexperimental data points. The thin lines result from the linear t of the experimentalresults obtained in this work. The thick grey lines show the range of the alreadyavailable literature data [12,13].

Fig. 3. Surface tension as a function of temperature for the [Cnmim][NTf2] ionic liq-uid series. The solid lineswere calculated using thetting equations given in Table 2.Experimental points are plotted only for the [C2mim][NTf2] and [C14mim][NTf2]series. The scatter for the other series is similar. The error bar corresponds to theestimated overall uncertainty of the surface tension measurements. The short greysegments correspond to previously obtained data [1].

Fig. 4. Averaglength, nC, forp values werlinear ts (preaverage slope

thermal expof temperasponding tothe alkyl-ch

Furthermextensivebsity withinsubtler detaversus T dap show achain lengt0.75610at473K, resresults [15of the presing temperinverse probehavior. Mbetween thof short-chumetric beliquids havmodynamicsufcientlyperature indiverge toature depepresent datand thepreperature intting the pvalidity of sthe linear ranges (293e thermal expansion coefcient,p , as a functionof the alkyl side chaineach member of the [Cnmim][NTf2] family. In this case the average

e obtained directly from the slope of the corresponding ln() versus Tsented for each ionic liquid in Table 1). The error bar represents theerror of the ts.

ansion coefcient, p, is almost constant, both in termsture and along the whole family of ionic liquids, corre-an average value of 0.689103 K1. Fig. 4 illustratesain effect for a mean temperature of 373K.ore, thepresent volumetric data set, probably themost

oth in termsof temperature rangeand ionic liquiddiver-

a given homologous series, enables one to explore evenils of its thermal variation. If for each system the ln()ta are tted to a quadratic function, then the values ofslight increase with increasing temperature and alkyl-h, being all comprised between 0.615103 K1 and3 K1, for [C2mim][NTf2] at 293K and [C14mim][NTf2]pectively. Thismayseemincontradictionwithprevious19] obtained for different ionic liquids (including someent family) that exhibit a decrease in p with increas-ature in the 270350K range. It must be noted that thisportionality betweenp and T constitutes an anomalousost traditional liquids exhibit direct proportionality

e two properties. Another anomalous exception is thatain n-alcohols [20] and comparisons between the vol-havior of this class of compounds and that of ionice been drawn in the past [15,21]. Moreover, the ther-s of liquidvapor equilibrium dictates that at a givenhigh-temperature p ought to start rising with tem-

crease because at the liquidvapor critical point it must+. This apparent contradiction between the temper-ndence of p obtained from the quadratic ts of thea (increase with temperature in the 293473K range)viously obtained volumetric results (decreasewith tem-the 270350K range) can in fact be lifted simply byresent data to a series of cubic equations (Fig. 5). Theuch option can be veried by checking the residues ofts of the present data in the low- and high-temperature343K and 353473K, respectively): the existence of

136 M. Tariq et al. / Fluid Phase Equilibria 294 (2010) 131138

Fig. 5. Thermal expansion coefcient, p , as a function of temperature for eachmember of the [Cnmim][NTf2] family (C14C2 from top to bottom). In this case thep values were obtained from the temperature derivatives of the correspondingln() data tted to cubic functions of T. The error bar corresponds to an averageuncertainty for all ts.

two regions with opposite curvatures, shown in Fig. 6, means thata correct dapolynomialwhich meanin the low-tincreasingseem to coof such a prin the tempof ionic liqudue care asthan the untematic errU-tube den

If one noof the molathe lengthothe previouture each mmolar volumis true alonexhibiting stribution ofreaching va473K).

Fig. 6. Residu[C10mim][NTfhigh- (35346

Fig. 7. Molar vf the c473.1isothe

surf(T),eciseis s

he mionicaintyantl

. In thnt mof thssedo inion

durining frature.3 shows that the surface tension decreases with increasingrature and with the increase of the alkyl side chain lengththe [Cnmim][NTf2] family. The latter trend (in agreementarvalho et al. [23]) is not as regular as it is in the case ofta t can only be achieved using at least a third degreefunction. Moreover, and by denition, p = ln()/T,s that the positive curvature displayed by the residualsemperature region corresponds to a decrease ofp withtemperature. Thus, the data determined in this worknstitute the rst experimental proof for the existenceeviously predicted crossover, from negative to positive,erature derivative of the thermal expansion coefcientids. Nonetheless, this conclusion should be taken withthe residuals shown, e.g. in Fig. 6 are not much greatercertainty of the experimental data, including the sys-ors associated with the calibration of the high-pressuresimeter used in this work (cf. Section 2).w analyses the volumetric data in terms of incrementsr volume along the ionic liquid family (as a function off thealkyl side chainof the cation), one is able to conrmsly reported fact [22] that at around room tempera-ethylene group added to the side chain increases thee of the ionic liquid always by about 17 cm3/mol. This

g the entire family, with the plots represented in Fig. 7traight lines for each represented temperature (the con-eachmethylene group is larger at higher temperatures,lues slightly above 19 cm3/mol at temperatures around

length obottom,for each

Theature,and prpicturewith tof theuncertsignicmentsdifferetermsbe streleast twstudiedtakenincreastempe

Fig.tempealongwith Ces ln()res = ln()exp ln()t of the ln() experimental data of2] relative to linear ts of the same data in the low- (293343K) and3K) temperature ranges.

density: th[C5mim][Nrange, and[C14mim][Nto the oversurface tenamounts ofpart of the oarrangemenmoietiesofalkyl side chis happenintion studiesfamily, theruids aroundomains). Iatoms of caan isotropiolume of the series [Cnmim][NTf2] as a function of the alkyl side chaination for three distinct temperatures. The isotherms are, from top to5K, 393.15K and 303.15K. The lines represent a linear t of the resultsrm.

ace tensiondata, plotted inFig. 3 as a functionof temper-display a quite different scenario from the fairly regulartrends exhibited by the volumetric data. Basically, the

omehow blurred by the overall uncertainty associatedethod that was used to measure the surface tensionliquids in such an extended temperature range; that(estimated to be in the order of 1mNm1 or 3%) is

y larger than that associated with the density measure-e surface tension case, we can expect errors between

ethods (even if the ionic liquid samples are reliable ineir purity) in the order of a few percent points. It mustthat the data reported in this work are the result of atdependent surface tension measurement runs for eachic liquid. Moreover, each run comprised measurementsg heating and cooling regimes, with the temperaturerom around 313K to 513K and then back to the initiale decrease is more signicant from [C2mim][NTf2] toTf2], less so in the [C6mim][NTf2] to [C12mim][NTf2]

with a noticeable drop from [C12mim][NTf2] toTf2]. Part of this irregular behavior can be attributedall uncertainty of the data, including the fact that thesion is a property sensitive to the presence of smallimpurities. Nevertheless one can always speculate thatbserved trends are a manifestation of different types ofts at the surface as the ratio of the non-polar to polar

the ionic liquids is alteredby the increase in lengthof theains. Interestingly this could also be a reection ofwhatg in the bulk phase: MD simulations and X-ray diffrac-[24] have shown that in the case of the [Cnmim][NTf2]e is a transition in the bulk structure of the ionic liq-d [C6mim][NTf2] (formation of non-polar continuoust is also known that with alkyl side chains above 12rbon long there is the possibility of a transition fromc (albeit nano-structured) uid to a partially layered

M. Tariq et al. / Fluid Phase Equilibria 294 (2010) 131138 137

Fig. 8. Estimauids as a funcwith results frfrom this workTb using the Cfrom Refs. [5,2

(more smec(n=1218)ever found

SeveralFig. 3: (i) tnation of thpartially unviously repsolid bold liable in termto their shotions exhib[Cnmim][NTlarge uncer

The EtvV(2/3) at a ginterval betture, Tc. Onlinear corre (9/11) =bface tensionto yield newatures for aresults wertemperaturthe prelimi(over manyare now conboiling poinexperimentilarities to tabove [21].

The resupublished Ttemperaturdata [5,27,

ClausiusClapeyron relation. The rather large uncertainty associ-ated with the estimated Tb values reect the lack of precision of thesurface tension data, the large extrapolations involved in the use ofthe Etvos and Guggenheim empirical correlations, and the (also

cal) cer have ttailserthres w, whmentdatailinged onetes thof t

l. Tha calfactlow

dictiodicatmixte comshorabovnce9]. Hcounpressust bn tepresshe heprespy ofapidnested normal boiling point temperatures, Tb , of [Cnmim][Ntf2] ionic liq-tion of alkyl side chain length, n. () Tb using the Etvos equationom this work; () Tb using the Guggenheim equation with results; () Tb using the Guggenheim equation with data from Ref. [1]; ()

lausiusClapeyron relation with experimental vapor pressure values7,28].

tic-like)uid [25], although in thecaseof [Cnmim][Ntf2]ionic liquids no evidence of liquid crystal formationwas[26].other conclusions can be drawn from the analysis ofhere is an inherent lack of precision in the determi-e slope of the (T) functions, a fact that will alwaysdermine attempts to extrapolate such data; (ii) pre-orted literature values [1], which are also plotted asnes in Fig. 3 for some selected cases, are even less reli-s of dening the temperature dependence of due

empiritheothpointsalso en

Nevperatuliquidsexperisets ofmal boincreas[1] none usinsteadintervathe dat

Theexhibitcontrathat inbinaryvolatilwith avaporresonatrend [into acvapor[5]) mpositiovaportions, tvaporenthalmore rintervert measuring ranges; (iii) nevertheless, the (T) func-it very similar slopes for the different members of thef2] family, specially if one takes into consideration the

tainty of the data.os equation, V(2/3) =m(T Tc), states that the productiven temperature shouldbedirectly proportional to theween that temperature and the critical point tempera-the other hand, the Guggenheim equation establishes alationbetween the function (9/11) and the temperature,(b/Tc)T. Both equationswere usedwith density and sur-data in the extended temperature range (300500K)estimates for the (hypothetical) critical point temper-

ll studied members of the [Cnmim][NTf2] family. Thee then converted to estimates of normal boiling pointes using the empirical relation Tb =0.7Tc. In contrast tonary estimates performed in [1], where an averagedtypes of substances) value of 0.6Tc was employed, wevinced that 0.7Tc should better describe the critical-to-t scaling of many ionic liquids because this is the factorally known for n-alkanols, substances with many sim-hese commonly used aprotic liquid salts as discussed

lts are presented in Fig. 8 along with the previouslyb values calculated using data in a much more limitede range [1] and values obtained using vapor pressure28] extrapolated to atmospheric pressure using the

indeed exhmore volatrange, as prsented in th

4. Conclus

The preencompassof numero[Cnmim][Ntematic unproperties ocan be meatheir trendside chaina situationconsidered

Ionic liqids, compocontinuousthe [Cnmimchanges inresults showthe volumeonversion from Tc to Tb values using the 0.7 factor. Onnd, the extrapolationof vaporpressuredata (froma fewo six orders of magnitude below atmospheric pressure)a signicant loss of precision.eless, all predictions indicate normal boiling point tem-ell above the decomposition temperatures of the ionicich means that the results may never be corroboratedally by direct measurements. Moreover, the differentshow that there is a tendency for a decrease in the nor-point temperature as the length of the alkyl side chain isthus qualitatively corroborating preliminary estimatesheless showing that that tendency is not so strong ifese new and V data in an extended temperature rangehe old data taken from a much narrower temperatureis subdued tendency is also in better agreement withculated from the vapor pressure measurements.that cations with larger alkyl side chains seem to

er normal boiling point temperatures may seem inn with experimental fractional distillation results [3,4]e that in a ([Cnmim][NTf2] + [Cmmim][NTf2], 4n

138 M. Tariq et al. / Fluid Phase Equilibria 294 (2010) 131138

chain length dependence. This suggests that those reorganizationsoccurwithout the creationor eliminationof cavities inside the ionicliquid, a fact that supports the idea that ionic liquids are not onlynano-structured but also extremely efcient in terms of using allavailable space. On the other hand, the surface tension data showthat such rearrangements when occurring at the surface may leadto a somewhat less regular behavior, although this last idea is stillspeculative and needs further experimental conrmation.

When combined together, the density and surface tension datacan also be used to estimate hypothetical normal boiling point tem-peratures within this family of ionic liquids (the original rationalebehind the present work). The results corroborated previous con-clusions regarding several vaporliquid properties of ionic liquids,which constituted the subject of a recent review [2].

Acknowledgements

Financiacia e TecnPCDT/QUI/6postdoctora2007, respe

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393Mohammad Tariq obtained his Ph.D. (2007) from JamiaMillia University, New Delhi, India. Since September 2007he is a FCT postdoctoral fellow at Instituto de TecnologiaQumica e Biolgica (ITQB), Portugal. His research inter-ests are centered on the measurement of thermophysicaland transport properties of liquids and liquidmixtures. Heis also interested in the self-assembly of conventional andionic liquid surfactants.

Jos Nuno Canongia Lopes was born in 1965 in Lisbon,Portugal. He obtained his Ph.D. in chemical thermody-namics from the Instituto Superior Tcnico (IST), Lisbon,in 1993. He is currently Assistant Professor at IST, andalso (since 2005) at ITQB, where he helds a post of InvitedAssistant Professor. His main research interests lie in theareas ofmolecularmodeling of complexuids andmateri-als, namely the thermodynamics of ionic liquids and theirmixtures.

Lus Paulo N. Rebelo was born in 1960 in Lisbon, Por-tugal. In 1989 he received his Ph.D. in physical chemistryfromtheUniversidadeNovadeLisboa (UNL).He joined theInstituto de Tecnologia Qumica e Biolgica (ITQB) in 2000where he is currently Full Professor and Vice-Director.His research interests are centered on molecular ther-modynamics of liquids and liquid solutions, in particular,isotope effects, ionic liquids, and metastability.

High-temperature surface tension and density measurements of 1-alkyl-3-methylimidazolium bistriflamide ionic liquidsIntroductionMaterials and methodsChemicalsMethods

Results and discussionConclusionsAcknowledgementsReferences