194
J. Chem. Eng. Data 1988, 31, 194-197
(4) Bradshaw, A.; Schlelcher, K. Deep-sea Res. 1978. 23, 583. (5) Kell, G. S.; Whalley, E. Philos. Trans. R . SOC.London 1985. A258, 565. (6) Fine, R. A.; Millero, F. J. J. Chem. fhys. 1973, 59,5529. (7) Kell, G. S.:Whalley, E. J . Chem. Phys. 1975, 62,3496. (8) Bradshaw, A.; Schleicher, K. E. Deep-sea Res. 1970, 77, 691. (9) Bradshaw, A.: Schleicher, K. E. J . Mar. Res. 1982, 4 0 , 793. (10) Bett, K. E.; Weale, K. E.: Newltt. D. M. Br. J. Appl. Phys. 1954, 5 , 243. (11) Beattie, J. A.; Blaisdell, B. E.; Kaye, J.; Gerry, H. T.; Johnson, C. A. Proc. Am. Acad. Arts Sci. 1941, 7 4 , 371. (12) Rebel, J.; Slmon, 1.; Walker, J. A. Rev. Scl. Instrum. 1957, 28,828. (13) Kell. G. S.J. Chem. Eng. Data 1975, 2 0 , 97. (14) Bradshaw, A. Deep-sea Res. 1978, 25, 1253. (15) Davis, L. A.; Gordon, R. B. J. Chem. Phys. 1987, 4 6 , 2650. (16) de Haas, W. J. Seances Com. I n ! . Poids Mes. 1950, 22, 85. (17) Del Grosso, V. A. J. Acoust. SOC.Am. 1970, 4 7 , 947. (18) Del Grosso, V. A.; Mader, C. W. J . Acoust. SOC. Am. 1972, 52, 1442.
(19) Bariow, A. J.; Yazgan, E. Br. J. Appl. Phys. 1987, 78, 645. (20) Crease,J., Institute of Oceamgraphlc Sciences, Wormley, Godalmlng, Surrey, England, prhrate communbtlon, 1971. (21) Wang, D.-P.; Mlliero, F. J., J. W p h y s . Res. 1973, 78, 7122. (22) Harlow, F. J. Pfoc. Phys. SOC. [London] 1913, 26, 96. (23) Titton, L. W.; Taylor, J. K. J . Res. Ne#. Bur. Stand. 1937, 78, 205. (24) Chappuls, P. Trav. Mem. Bur. In!. Po& Mes. 1907, 73,~ 1 . (25) Blgg, P. H. Br. J. Appl. Phys. 1987. 78, 521. (26) Thiesen, M.; Scheel. K.: Diesselhorst, H. Wlss. Abh. Phys .-Tech. Reichsanst. 1900, 3, 1. (27) Caldwell, D. R. Deep-Sea Res. 1978, 25, 175. (28) Dorsey, N. E. “Properties of Ordinary Water-Substance”: Reinhold: New York, 1940; p 275-277. (29) Zaworski, R. J.; Keenan, J. H. J. Appl. Mech. 1987, 3 4 , 478. Received for review August 1, 1985. Accepted November 22, 1985. Research supported by the Oceanographic Section of the Natlonal Science Foundation (OCE74-18158).
Room Temperature Fused Salts Based on Copper(1) Chloride-I-Methyl-3-ethylOmidarolSum Chloride Mixtures. 1. Physical Properties Steven A. Bolkan end John T. Yoke” Department of Chemistry, Oregon State University, Cotvallls, Oregon 9733 1
aluminate and chlorocuprate systems in batteries might prove useful.
Mlxtures of CuCl and l-methyC3-ethyhIdazdlum chloride form room temperature lonlc liquids over the compodtlon range 0.33 < mole fractlon CuCl x < 0.67, with glass trans#lon temperatures of about -50 OC. Densltles, vlscoslties, and specific conductlvites are reported In the range 25-50 OC. Density Increases monotonkally with x. Vlscosity 9 goes through a pronounced mlnhnum and speclflc conductlvlty K a pronounced maximum at x = 0.5; these properties have more desirable values than for the previously studled CuCI-trlethylammonlum chloride flow and for fused salt. Actlvatlon energles for VISCOUS spectflc and equivalent conductlvtty are calculated. The Frenkel function ~ ~ n ~ ’=~constant ~ ~ ( 9Is)obeyed.
Experimental Sectlon
Matda&. Copper(1) chloride was prepared by the method of Keller and Wycoff (3). l-MethyC3-ethylimidazdiumchloride was prepared and purified as described by Wilkes and coworkers (4); its density as a solid at 21.5 OC was determined by flotation in carbon tetrachlorlde/hexane to be 1.186 g/mL. The dried chlorides were stored and their mixtures were prepared and transferred in a dry nitrogen-filled glovebox which has been described previously (7). Menlng pdnls. Samples were in sealed glass capillaries under nitrogen. Measurements below room temperature were made by a visual method which has been shown (2) to be in agreement with the differential scanning calorimetric technique. The sample capillary and an iron-constantan thermocuple were immersed in ethanol in a small tube which was suspended from the bottom of a rubber stopper in a 400-mL vessel. This vessel was swept with dry nitrogen which first passed through a copper coil immersed in liquid nitrogen: temperature control was achieved by varying the rate of flow of gas. The vessel was mounted in an insulated jacket with a port permitting Mumination and observation with a magnifier. Glass transitions were characterized by a discontinuous increase in viscosity on coding, with fracturing of the melt. The transition temperatures were actually measured as melting points on slow warming after initiil quenching to -100 OC. Measurements above room temperature were made using a conventional melting point apparatus. other PhySka/-. Densities of CuCI/MeEtImCI melts were measured by a dilatometric method as previously descrlbed (I). Viscosities were measured by wing a modified Cannon-Fenske viscosimeter Whose calibration has been described (7). For conductivity measurements, an Industrial Instruments RC16 82 brklge and a cell of constant 11.65 f 0.07 cm-’ were used. Measurements at 60 and 1000 Hz agreed
Introduction Room temperature fused salts containing chlorocuprate(I) anions have been described with various Malkylammonlum and with triethylphosphoniumand triethylchlorophosphnium cations. The density, conductivity, and viscosity of the prototype of these copper-based liquids, triethylammonium dichlorocuprate(I), have been reported over a range of temperatures (7). That compound was used in electrochemical studies of the copper(I1,I) and copper(1,O) couples (7). Practical application, e.g., in batteries, is limited by its excessively high viscosity and by the excessively low exchange current densities of the above couples. An important recent advance in the study of low-melting chloroaluminate electrolytes was the development by Wilkes and co-workers of the 1,3dialkylimidazolIumchloride-aluminum chloride melts. The 1-methyl-3-ethylimidazolium system was found to have favorable conductivity and viscosity behavior and a wide liquid range (2).This has prompted us to investigate the copper(1) chloride-1-methyl-3-ethylimidazolium chloride (MeEtImCI) system to see if low-melting ionic liquids with more desirable physical and electrochemical properties could be obtained for chlorocuprates. I f so, the combination of chloro0021-9568/86/1731-0194$01.50/0
0
1986 American Chemical Society
Journal of Chemical and Engineering Data, Vol. 31,
No. 2, 1986 195
150
125
8
I
e ,*roo
I
u
9 5 75
50
25
A
5OoC ~
0.35
0.40
0.45 0.50 Mole Fraction CuCl
0.55
-~
0.60
0.65
Figure 1. Viscosity isotherms for CuCllMeEtImCl melts.
well and their average was taken. Temperature control was to *O.l OC.
Mettlng Pdnts. The xCuCI/( 1 - X)MeEtImCI mixtures are room temperature liquids over the composition range 0.33 < mole fraction x < 0.67. On cooling, sudden glass transitions were observed. The melting points of these glasses fell between -40 and -64 O C over the composition range 0.33 < x < 0.67. At compositions just above and below this range, the mixtures separated into two phases, liquids which formed glasses at a low temperature plus normal solids. I n the composition range 0 < x < 0.20 the melting points dropped from 82 to 52 OC, while for the range 0.85 < x < 1.00 the melting points rose from 296 to 430 OC. Densttles and Molar Volumes. I n the temperature range 25-50 O C , the data are well represented by linear functions. The results are given in Table I. At a given temperature, density increases monotonically with mole fraction of copper(1) chloride, and densities of melts of compositions other than those in Table I were obtained by curvilinear interpolation. Also shown in Table I are values of the coefflcients of thermal expansion, evaluated at 40 OC by using (Y (l/V)(dV/dT) = (d4oe)(l/dsoo - l/d300)/20.0 (1) The volumes of the xCuCI/( 1 - X)MeEtImCI melts exceed the
Table I. Densities in the Range 25-50 "C and Coefficients of Expansion (alo.)" linear coeff of CuCl mole correln expansion, fraction a b coeff deg-I 0.332 1.3835 7.59 X 0.9968 5.92 X lo4 0.400 0.498 0.604 0.666
1.4058 1.5423 1.7162 1.9041
8.44 1.08 1.07 1.24
X 1O-l X X X
0.9961 0.9999 0.9988 0.9998
6.57 7.34 6.61 5.65
X X X 10"' X lo4
" p (g/mL) = a - b(t "C).
sum of the molar volumes of their components, e.g., by 9.0% for x = 0.5. This behavior closely resembles that of the CuCI/Et,NHCI system and is attributed to the existence of holes in the liquid structure (5). VlscosHes and Conductlv~les Isotherms showing the variation in viscosity 71 and specific conductivity K with composition are given in Figures 1 and 2. Pronounced extrema are seen at mole fraction 0.5 corresponding to the nominal MeEtImCuCI, composition. Quantitatively, the decrease in viscosity at the composition midpoint is relatively greater than the increase in conductivity, so that the Walden product PK passes through a minimum at mole fraction 0.5 at each temperature studied. Although at 25 OC the viscosity of the equimolar CuCI/MeEtImCI melt, 45.9 cP, is significantly less than that of
.
106 Journal of Chemical and Engineering Data, Voi. 3 1, No. 2, 1986
3 .O
2.5
c(
2
2,o
Y
-Q
A
c
.->
I
1.5
0
U
.Y
* ._ Y
t
1.0
an
0.5
0
0.35
0.40
0.45
0.50
0.60
0.55
0.65
Mole Froctian CuCl
Flgure 2. Specific conductivity isotherms for CuCI/MeEtImC1 melts.
Table 11. Activation Energies (kcal/mol), Viscosities ( T ) , Specific Conductivities ( K ) , and Values of the Frenkel Function FF = (n)(K)Ent/E"t
~
25.0
30.0
O C
CuCl mole fraction 0.334 0.400 0.425 0.450 0.475 0.500 0.525 0.550 0.575 0.600 0.666
E,,+, E:, kcal/mol kcal/mol 13.8 10.7 9.68 8.79 8.40 8.29 8.81 9.02 10.4 11.1 13.2
12.1 9.42 8.54 7.67 7.10 7.15 7.51 8.09 8.62 10.1 10.8
7 , CP
ohm-' cm-'
539 173 112 72.0 51.6 45.9 55.9 72.5 101 138 357
1.64 4.10 5.58 8.28 10.8 11.6 9.95 8.19 6.85 4.82 3.45
FF
T , CP
0.352 361 0.329 124 0.314 82.3 0.296 54.6 0.242 39.7 0.262 35.3 0.250 42.5 0.342 53.4 0.253 71.8 0.377 93.5 0.360 235
ohm-' cm-' 2.45 5.50 7.38 10.7 13.6 14.5 12.6 10.7 8.91 7.04 4.86
the CuCI/Et,NHCI melt, 123 cP, and the specific conductivity higher, 1.16 X vs. 4.30 X lo3 ohm-' cm-', the values of their Waklen products are the same within experimental error, 0.532 vs. 0.529 CP ohm-' cm-'. Equivalent conductivites, ohm-' cm2equiv-l, were calculated by using
h =~
40.0
O C
103K,
103~,
w / p
(2)
50.0
FF
7 , CP
0.374 167 0.331 68.7 0.316 48.4 0.301 33.8 0.246 25.1 0.260 22.5 0.250 26.3 0.340 33.3 0.248 41.0 0.387 53.3 0.360 115
"C
103~,
ohm-' cm-'
FF
ohm-'
4.64 9.32 11.7 16.1 19.9 21.3 18.8 16.7 14.3 11.9 8.98
0.360 0.334 0.313 0.297 0.244 0.260 0.249 0.346 0.250 0.394 0.370
7, CP cm-' 89.8 42.7 31.7 22.8 17.2 15.5 17.7 22.1 26.0 32.5 64.1
7.98 13.9 17.1 22.7 27.3 29.5 26.6 23.8 21.1 18.2 14.0
FF 0.359 0.328 0.315 0.298 0.248 0.262 0.251 0.342 0.252 0.385 0.357
(3)
and p is the density at the temperature at which K was measured. Over the range 25-50 OC, llnear plots vs. 1/T were obtained of in K (correlation coefficients -0.9963 to -0.9994),
mean FF and std dev 0.361 f 0.009 0.331 f 0,002 0.315 f 0.001 0.298 f 0.002 0.245 f 0.003 0.261 f 0.001 0.250 f 0.001 0.343 f 0.003 0.251 f 0.002 0.386 f 0.007 0.362 f 0.006
in A (correlation coefficients -0.9963 to -0.9995), and In q (correlation coefficients 0.9987 to 0.9994), permitting calculation of energies of activation for specific conductivity and equivalent conductivity EAT,and for viscous flow E:. Although Walden products are not constant for a given molten salt composltiin as viscosity and conductivity vary over a temperature range, the empkical Frenkel function (6, 7)
€2
FF = ( 8 ~ ~ ' ~ s ~=X constant q )
where the equivalent weight is given by
w = ~(98.99)-I-(1 - xX146.63)
"C
103~,
(4)
has been found to describe the behavior of CUCi/Et$lHCI metts (1)as well as molten alkali haHdes (8) and cadmium ha#des (9). The data in Table I 1 show that this function is remarkably constant for each composition studied in the CuCI/MeEtImCI system.
Journal of Chemical and Engineering Data, Vol. 3 1, No. 2, 1986 197
Table 111. Comparison of Activation Energies EA' (kcal/mol) Calculated from (I) EAt= E > aRT2 and (11) from the Arrhenius Equation CuCl mole fraction EAt(I) EAt(II)
+
0.334 0.400 0.500
12.2
0.600 0.666
10.2
10.2
10.9
10.9
12.2 9.53 7.29
9.55 7.19
The two conductivity activation energies are theoretically related by (70) EAt = EKt4- aRT2
(5)
and values of EAt calculated by this equation using the 40 OC coefflclent of expansion (Y values from Table I are compared in Table 111 with values of EAtobtained from Arrhenius plots. Dlscusslon Just as I-methyl-3-ethylimidazolium chloride was found by Wilkes and co-workers to be a desirable component in aluminum chloride room temperature melts from the standpoint of the wide liquid range available, low viscosity, and high conductivity, so it is now found superior to triethylammonium chloride as a component of low melting copper(1) chloridebased fused salts. Two reasons have been proposed for the liquid nature of these copper(1) electrolytes: first, low lattice energies related to the bulk and low symmetry of the cations and consequent poor packing of these with the chlorocuprate(1) anions, and, second, the existence of a mixture of species depressing each other's melting points, arlsing from such chloride ion transfer equilibria as CUc1,CuCI,-
+ nC1- + cucI,+,'-" + yCuCl + Cu,+flI,+; 2cUcI2-
Cu,CI,-
( n = 1, 2)
(6)
(y = 1, etc.)
(7)
+ c1-
(8)
Spectroscopic evidence has been presented ( 7 7 ) for species such as CuC12- in the chloride-rich (basic) and Cu2CI,- in the chloride-poor (acidic) CuCl room temperature fused salts. The CUCI,~- and CUcl4& species were found in CuCl solutions in the basic AICI,/MeEtImCI melts ( 72), and various polynuclear complexes Cu,,,Cl,,-("-"') have been reported (73)in aqueous CuCl CI- systems. Clearly, a much wider variety of anions is present than in the aluminum chloride ionic liquids where the anionic species present are completely described (74) by the equation
+
2AICI,- + AI,CI,-
-ICI-
(9)
A solid compound MeEtIm'CuC1,has not been observed, but in the liquid in the temperature range 25-50 O C and at the
mole fraction O.SCuC1/0.5MeEtImCI composition, the predominant anionic species is certainly CuCI,-. The isotherms of Figures 1 and 2 show a pronounced minimum in viscosity and maximum in conductivlty at this composition, corresponding to the predominance of this simplest of the chlorocuprate(1) anions. I n both the MeEtImCI-enriched basic melts and the CuCI-enriched acidic melts, the lower mobilities of the CuCl2and CUCI,~anions or the polynuclear Cu,Cln-("-"') anions are clearly reflected in the increased viscosity and decreased specific conductivity. The coefficients of thermal expansion also pass through at maximum at the equimolar composition. All past experience with molten salts shows that the energy of activation for viscous flow is greater than that for electric conductivity, since flow is limited by the ion with lower mobility while conductivity is influenced more by the ion with higher mobility. The €:/E: ratlo ranges from 3 to 7 for molten alkali halides, and Is about 2 for bivalent metal halides (9). The present data on CuCI/MeEtImCI meits provide a contrast with E J l E ; ratios 1.1 to 1.2. These values suggest that the substituted imidazollum cation and the chbrocuprate(1) anions have similar mobilities. Acknowledgment We thank Jeffry S. Thompson for technical assistance in the phase transition studies. Registry No. MeEtImCI, 65039-09-0; CuCi, 7758-89-6.
Llterature CHed Siikey, J. R.; Yoke, J. T. J. Electrochem. SOC. 1980, 127, 1091. Fanin, A. A., Jr.; Fioreani, D. A.; King, L. A.; Landers, J. S.; Piersma, 8. J.; Stech, D. J.; Vaughn, R. L.; Wilkes, J. S.;Williams, J. L. J. Phys. Chem. 1984, 88, 2614. Keller. R. N., Wycoff, H. D. Inwg. Synth. 1946. 2 , 1. Wiikes, J. S.;Levisky, J. A.; Wilson, R. A.; Hussey, C. L. Inorg. Chem. 1982. 2 1 , 1263. Bloom, H.; Bockris, J. O'M. I n "Modern Aspects of Electrochemistry"; Bockris, J. O'M., Ed.; Butterworths: London, 1959; Voi. 2, Chapter 3. Frenkei, J. "Kinetic Theory of Liquids"; Oxford University Press: London, 1946; p 441. Bloom, H.; Heymann. E. Proc. R. Soc., Ser. A 1947, 188, 392. Yaffe, I . S.;Van Artsdalen, E. R. J. Phys. Chem. 1958, 6 0 , 1125. Bloom, H.; Harrap, B. S.; Heymann, E. Proc. R. SOC.,Ser. A 1948, 194, 237. Bloom, H. "The Chemistry of Molten Salts"; W. A. Benjamin: New York. 1967: D 81. Axteii, D. D.: Good, B. w.; Porterfield, W. w.; Yoke, J. T. J . Am. Chem. Soc. 1973. 95. 4555. Laher, T. M.; Hussey, C.L. Inorg. Chem. 1983, 22, 3247. Sukhova, T. G.; Temkln, 0. N.; Fiid, R. M. Russ. J . Inorg. Chem. 1970, 15. 949; Zh. Neofg. Khim. 1970, 15, 1849. Cheek, G. T.; Osteryoung, R. A. Inorg. Chem. 1982, 21, 3581. Received for review June 24, 1985. Accepted November 14, 1985. This work was supported in part by the Oregon State University Research Councli, and Is based on part of the work to be submitted by S.A.B. to the Graduate School, Oregon State University, in partial fulfillment of the requirements for the Ph.D. degree.
