1002
Warren T. Ford and Donald J. Hart
increased from methyl (ca. 3.4 A in diameter) to adamantyl (ca. 7 A), the increase in the enthalpy of transfer is about +1 kcal molp1. This shows that the approximate choice for the parameters which gives the curve labeled AH1 is much better than that which gives the curve labeled A H z . More experimental data are needed to further determine the parameters that give the best fit. In any case the experimental findings appear to give some support to the theory used in this'paper. Finally one may comment on the physical interpretation of the calculated thermodynamic quantities of transfer. It has been observed previously1 that when one solute is transferred from one solvent to another with a sufficiently different molecular diameter, the AGt, is mainly a function of the different solvent size (for a given solute hard-sphere diameter), and is relatively independent on the structural features that can be built into the model. On the contrary when two solvents are considered, with the same hard-sphere diameter and very similar molar volumes, as D2O and HzO, the difference in the structure of the two solvents is likely to play an important role in determining the sign of the AG,. Here the interpretation for the AGt, > 0 for solutes with large size is possibly that proposed by Jolicoeur,4 that is that the large solute which cannot fit into the solvent cages is more difficult to insert in the more structured DzO than in H2O.
The interpretation of the sign of the AHtr is much more difficult since it is related to the relative decrease in structure with increasing temperature of H2O and D2O through their G(X). In the present calculation, the sign of AHt, changes with solute size because we have assumed that G(X) decreases sufficiently faster for DzO than for HzO when the temperature is increased.
References and Notes (1) M. Lucas, J. Phys. Chem., 80,359(1976). (2)P. R. Philip and C. Jolicoeur, J. Solution Chem., 4, 105 (1975). (3)M.M. Marciacq-Rousselot and M. Lucas, J. Phys. Chem., 77. 1056 (1973). (4)C. Jolicoeur and G. Lacroix, Can. J. chem., 51, 3051 (1973). (5) D. P. Wilson and W. Y. Wen, J. Phys. Chem., 79, 1527 (1975). (6) R. A. Pierotti, J. Phys. Chem., 69, 281 (1965). (7)F. H. Stillinger, J. Solufion Chem., 2, 141 (1973). (8)A. H. Narten and S. Lindenbaurn, J. Chem. Phys., 51, 1108 (1969). (9) A. H. Narten and H. A. Levy, "Water: a Comprehensive Treatise", Vol. 1, F. Franks, Ed., Plenum Press, New York, N.Y., 1972,Chapter 8. (IO)H. S.Frank, ref 14,Chapter 14. (11) G. C. Kresheck, H. Schneider, and H. A. Shceraga, J. Phys. Chem., 69,3132 (1965). (12)A. Ben Naim, J. Wilf, and M. Yaacobi, J. Phys. Chem., 77, 95 (1973). (13)A. N. Guseva and E. J. Parnov, Sov. Radiocbern., 5, 507 (1963). (14)H. Snell and J. Greyson, J. Phys. Chem., 74,2148 (1970). (15)G. Jancso and W. A. Van Hook, Chem. Rev., 74,689 (1974).
Viscosities and Conductivities of the Liquid Salt Triethyl-n-hexylammonium Triethyl-n- hexylboride and Its Benzene Solutions' Warren T. Ford" and Donald J. Hart Department of Chemistry, University of Illinois, Urbana, lllinois 6 I80 1 (Received October 30, 1975) Publication costs assisted by the University of lllinois
The viscosity of neat liquid triethyl-n-hexylammonium triethyl-n-hexylboride (N2226B2226) decreases nonexponentially with 1/T by a factor of 15 in the range 20-80 "C while its equivalent conductance increases similarly by a factor of 13. Dilution of liquid N2226B2zz6 with an equal volume of benzene increases the specific conductance at 25 "C by a factor of 10.5.
Introduction
Triethyl-n-hexylammonium triethyl-n-hexylboride (N2226B2226) is a liquid salt at room temperature.2 It resembles a charged alkane: the cation and anion are nearly equal in size, and both are isoelectronic with 3,3-diethylnonane. N2226B2226 is miscible in all proportions with most organic solvents, but immiscible with hexane, which it resembles structurally, and immiscible with water, nature's ubiquitous solvent for ions. We now have studied transport properties of NZ226BZZZ6 with two aims. First, we wanted to determine whether N2226B2226 could be a suitable conductor for electrochemical applications. Second, we wanted to compare tempera-
* Address correspondence to this author at Rohm and Haas Co., Research Laboratories, Spring House, Pa. 19477. The Journal of Physical Chemistry, Vol. 80, No. 9, 1976
ture effects on its transport properties, which depend on translational diffusion, to the temperature effects on its carbon-13 spin-lattice relaxation times, which depend on rotational diffusion. The compatibility of N2226B2226 with a wide variety of organic materials makes it intriguing for possible electrochemical uses, either as a supporting electrolyte in high concentration in nonaqueous solvents or as both solvent and supporting electrolyte in neat liquid form. Preliminary conductivity and cyclic voltammetry experiments indicated that neat N2z26B2226 may be suitable for cathodic electrochemi~try.~ Earlier information on viscosities and conductivities of molten salts at low temperature is limited to tetra-n-butylammonium picrate at 91 "C4 and tetra-n-amylammonium thiocyanate at 52-110 "C5. The specific resistance of tetra-
1003
Viscosities and Conductivities of Triethyl-n-hexylammoniumTriethyl-n-hexylboride TABLE I: Viscosity and Conductance of Neat N2226B2226
T, "C O.OOa
10.00 20.00
q,cP 1544 702 346
30.00
188.5
40.00
112.3
50.00
71.1
60.00
47.0
70.00
32.4
80.00a
23.0
T,"C
50-
A, equiv-l ohm-' cm2
d , g cm-3
0.1509 0.204 0.272 0.365 0.447 0.582 0.703 0.875 1.046 1.260 1.477 1.717 1.997
0.8445 0.8405 0.8368 0.8350 0.8330 0.8312 0.8294 0.8273 0.8252 0.8231 0.8213 0.8194 0.8274 0.8156 0.8137
20.02 25.00 30.05 35.01 40.08 45.06 50.05 54.99 60.10 65.08 70.16 74.63 79.73
40-
0 N
5
7 E
30-
c
1 .-
$
2.0-
4
f O . l "C.
TABLE 11: Walden Products and Activation Energies for Viscosity and Conductance of Neat N2226B2226
T, "C \
20.00 30.00 40.00 50.00 60.00 70.00 80.00 A
Aqa
EA,bkcal mol-lEl,b kcal mol-l
52.3 51.1 50.1 49.9 49.0 47.7 46.4
9.86 9.25 8.79 8.15 7.72
10.41 9.71 9.02 8.72 8.39
EVIE" 1.056 1.050 1.026 1.070 1.087
values interpolated from Table I. Calculated from data at
ca. +10 and -10 "C from indicated temperature.
TABLE 111: Viscosity and Conductance of N~z~&zze-Benzene at 25.00 "C
N2226B2226 concn, M
A, equiv-l 9,cp
2.26 2.01 1.617 1.315 1.132 1.004 0.873 0.712
254a 87.2 23.0 9.01 6.00 3.96 3.15 2.33
ohm-l cm2
0.204 0.522 1.769 3.39 4.26 4.59 4.69 4.30
A9
51.7 45.5 40.7 30.5 25.6 18.2 14.8 10.02
Interpolated from 20 and 30 "C values in Table I. n-hexylammonium benzoate hemihydrate at 25 "C also has been reported.6 Viscosities and pVT properties of some tetraalkylammonium tetraalkylborides' and viscosities and conductances of some tetraalkylammonium tetrafluoroborates8 have been explored at >lo0 "C. Walden and coworkers examined viscosities and conductances of molten tetraalkylammonium picrates a t >lo0 "C 50 years ago.g
Results Viscosities and equivalent conductances of neat liquid N2226B2226 are given in Table I. From 0 to 80 "C its viscosity decreases by a factor of 67 and from 20 to 80 "C its conductance increases by a factor of 13. The Walden product decreases somewhat with increased temperature (Table 11). Arrhenius plots of the viscosity and conductance data are not linear; the apparent activation energies decrease with increased temperature (Table 11).
1
I
20
40
$0
lo
Vol % benzene
Figure 1. Specific conductance and equivalent conductance of N2226B2226-benzene solutions at 25 "C.
The viscosity and conductance data on NzzzsBzz~6-benzene' solutions in Table I11 extend from the molten salt to near the solubility limit of benzene in N2226B2226. (An attempt to prepare a 71.4 vol % benzene solution gave a two phase mixture.) In this range the viscosity decreases by a factor of 109 and the specific conductance and equivalent conductance reach maxima at about 50 and 61 vol % benzene, respectivelypas shown in Figure 1. These maxima are 10.5 times greater than the K,, and 23 times greater than the A of the neat liquid salt.
Discussion Qualitatively the nonexponential dependences of conductance and viscosity of N2226B2226 on 1/T resemble earlier reports of tetraalkylammonium picratesgJOand tetrafluoroborates8 a t >lo0 "C and tetra-n-pentylammonium thiocyanate (N5555SCN) at 52-110 oC.5b The values of EV and E A for N2226B2226 at 20-80 "C are larger than reported for other molten salts, but would not be larger if they were extrapolated to the higher temperatures required to melt the other salts. The small decrease in A7 of N2226B2226 with increasing temperature also resembles the behavior of other molten tetraalkylammonium The ratio EVIE" is greater than unity in a wide variety of molten salts and has been explained in terms of a difference in transport mechanisms. Viscous flow requires movement of both ions a t the same rate in the same direction, but cation and anion must move in opposite directions in electrical conductance and may have unequal transport numbers. When the ions are nearly equal in size, they often are assumed to have equal transport numbers. This assumption is based on limiting equivalent conductances in water of many electrolytes. As their transport numbers approach equality, Eq/EA should approach unity, as it does in N2226B2226. (Actually typical N-C and B-C bond lengths in quaternary ions, 1.52 and 1.65 A, respectively,11J2 imply that the B2226 anion is slightly larger than the N2226 cation.) An alternative explanation for values of Eq/EA close to unity is that the alkyl chains of the N2226 and B2226 ions are entwined, and consequently conductance requires cooperative motion of anion and cation just as viscous flow does.lO The Journal of Physical Chemistry, Vol. 80, No. 9, 1976
Warren T. Ford and Donald J. Hart
1004
The activation energies E? and E A are substantially larger than the activation energies for effective rotational correlation times calculated from carbon-13 NMR spin lattice relaxation times of individual carbon atoms in N22~6B2226.l~ The NMR correlation times depend on both overall reorientation (Brownian motion) and internal rotation about single bonds. At the carbon atoms where the overall reorientation contribution is the greatest, those bound directly to nitrogen or boron, E , = 6.4-7.0 kcal/mol from data at 10-120 "C. Apparently the frictional forces which impede rotational diffusion are less dependent on temperature than those which impede translational diffusion. Molten salts have been described as unassociated because Av of the melt is approximately equal to Aov extrapolated to infinite d i l u t i ~ n . ~As~ ~benzene ,~ is added to N2226B2226 at 25 "C or p-xylene is added to N5555SCN at 52 or 90 0C,5A7 decreases, presumably due to ion association in the nonpolar solvent. The fraction of dissociated ions in concentrated solutions has been defined as F ; = (Av)/(Aq)o, the ratio of the Walden product of the solution to that of the neat liquid salt.5 However, A7 of N5555SCN increases when small amounts of nitrobenzene are added,14 which contradicts the complete dissociation hypothesis of Kenausis, Evers, and Kraus5 for liquid salts. We have rationalized relative rates of some ion-molecule reactions in N2226B2226 on the basis of cooperative anion-cation motion.15 Addition of a low dielectric constant solvent such as benzene or p-xylene may enhance the cooperativity, while a higher dielectric constant solvent such as nitrobenzene may reduce the cooperative motion by separating the ions whose alkyl chains are entwined in the melt. The specific conductance of a 50/50 (v/v) N2226B2226benzene solution is >10 times higher than that of any other benzene solution at 25 "C listed in the massive compilation of Janz and Tomkins.16 Dilution with more polar organic solvents would probably give even higher specific conductances. Such solutions may be highly useful media for cathodic organic electr~chemistry.~ Experimental Section N2226B2226 was prepared as described before and anaM Br, and lyzed to contain
