Murray L. Jansen and Howard L. Yeager
1380
It should be noted that our values for A H for both reactions are greater than those reported by Scott, et ~ l . but , ~ lower than the value of 30 kcal/mol reported by McLaren, et el.^ If the value of McLaren, et al., refers to the 2:l adduct, it and our value may agree to within the large experimental uncertainty. Ack&,wEedgment, ~l~~ authors wish to thank Drs. K . Olszyna and Ihil. h r i a for their help. This work was sup-
ported by the Environmental Protection Agency through Grant No. 800874, for which we are grateful. References and Notes (1) CAES report No. 320-73. ( 2 ) W. D. Scott, D. Lamb, and D. Duffy, J. A m . Sci.. 26, 727 (1969). ( 3 ) E. McLaren, A . J. Yencha, and J. Kushnir, 1.U.G G . Symposium on Trace Gases, Germany, April 2-5, 1973. (4) J. P. Friend, R Leifer. and M .Trichon. J . ~ t m Sci. 30,465 (1973).
Conductance Measurements of Alkali Metal Trifluoroacetates in Propylene Carbonate Mtrrray L. Jansen' and Howard L. Yeager" Deparfmenf of Chemistry, The University of Calgary, Calgary, Alberta, Canada (Received February 6, 7974) Publication costs assisted by the National Research Council of Canada
Precise conductance measurements are reported for the alkali metal trifluoroacetates, tetrapropylammonium chloride, and tetrabutylammonium nitrate in propylene carbonate. Lithium trifluoroacetate is extensively associated, with the formation of ion aggregates at higher concentrations. The degree of association for the trifluoroacetates decreases with increasing size of the alkali metal ion. Single ion mobilities are reported for chloride, nitrate, and trifluoroacetate ions in propylene carbonate.
Introduction Conductance measurements for a variety of alkali metal and tetraalkylammonium halides and perchlorates in propylene carbonate (PC) have been reported.2 All salts were found to be essentially unassociated in this solvent of moderately high dielectric constant (64.92 at 25").3 Single ion mobilities were derived and discussed in terms of the ion solvating ability of PC. We have extended this work to study the conductance behavior of the alkali metal trifluoroacetates. Wu and Friedman4 have measured the heats of solution of these salts in PC. Results indicated that the lithium salt is extensively associated, and the formation of ion pair dimers was suggested. Conductance measurements indicated that the lithium and sodium trifluoroacetates are associated, but no equilibrium constants were calculated .4 Precise conductance measurements are reported here for these salts, in order to accurately evaluate the influence this more strongly basic anion has on association processes in PC. Association trends are discussed in terms of the relative sizes of the alkali metal ions. In addition, conductance measurements are reported for tetrapropylammonium chloride and tetrabutylammonium nitrate in order to estimate single ion mobilities for the respective anions. Trends for anion mobilities in PC are discussed. Experimental Section The purification of PC has been described previously.2 Tetra-n-propylammonium chloride (Eastman Kodak) was precipitated three times from acetone with ether and dried under vacuum at 150" for 40 hr. Analysis by silver nitrate titration: 99.7%. Tetra-n-butylammonium nitrate The Journal of Physical Chemistry, Vol. 78. No. 14, 1974
was prepared by metathesis from tetra-n-butylammonium chloride (Eastman Kodak) and silver nitrate (Engelhard Industries Ltd.) in an ethanol-water mixture. The filtrate remaining after removal of silver chloride was evaporated to yield the crude product. The salt was recrystallized three times from ethyl acetate and dried under vacuum a t 85" for 72 hr. Anal. Calcd: C, 63.12; , 11.92. Found: C, 63.15; H, 11.70. Lithium trifluoroacetate was prepared from recrystallized and dried lithium bromide (Research Organic/Inorganic Chemical Corp.) and dried silver trifluoroacetate (Eastman Kodak) in ether. The filtrate remaining after removal of the silver bromide was reduced in volume, and the precipitated salt was dried under vacuum at 60" for 36 hr, and then at 100" for 72 hr. Analysis by cation exchange and acid titration: 99.2%. Sodium, potassium, rubidium, and cesium trifluoroacetates were prepared by adding equivalent amounts of trifluoroacetic acid (Eastman Kodak) to aqueous solutions of the corresponding carbonates, followed by careful reduction in the volume of the solutions to initiate crystallization. Sodium trifluoroacetate was recrystallized three times from a 6:l dioxanemethanol mixture and then dried under vacuum at 80" for 24 hr and a t 125" for 48 hr. Analysis by cation exchange and acid titration: 99.8%. Potassium trifluoroacetate was recrystallized from dioxane and ethanol and dried under vacuum a t 130" for 72 hr: analysis 100.0%. Rubidium trifluoroacetate was recrystallized twice from a 1:1 dioxaneethanol mixture and dried under vacuum at 70" for 1 week: analysis 100.5%. Cesium trifluoroacetate was recrystallized twice from ethyl acetate and dried under vacuum at 60" for 12 hr: analysis 100.3%. The purification of the alkali metal trifluoroacetates proved to be a formi-
Alkali Metal Trifluoroacetetes Conductance Measurements
1381
TABLE I: Conductance Parameters f o r 1:l Electrolytes in PC at 25" An
KA
27.2 zt 1 . 4 27.2 i 1 . 2 29.04 f 0.009 29.04 i 0.008 29.57 f 0.003 30.21 f 0.002 28.74 & 0.004 28.70 f 0.007 29.42 f 0.003 29.39 & 0.002
189 i 58 189 $7 50 42.3 i: 0 . 2 42.5 i 0 . 2 26.7 i 0 . 1 1 8 . 1 f 0.04 2.0 zt 0 . 1 2 . 1 Ifr: 0 . 2 1 . 9 zt 0 . 1 1 . 9 Ifr: 0 . 1
Salt
NaCF3COz KCF3COz RbCFaC02 CsCFzCOz Pr4NCl ]BU,NNO~
A
CJl
9318 9373 912 897 419 278 163 151 257 262
f 5340 f 4581 =k 37 i. 32 & 10 i: 7 & 33 f 54 f 20 i 13
OA
-~ 0.5 0.6 0.006 O r 006 0,001
0,001 0,002 0.004 0.002 0,001
15-
IO -
5t
* . 4.0
2.0
8.0
6.0
cbx lo2 Figure 1. Equivalent conductance vs. the square root of molar concentration for lithium trifluoroacetate in PC at 25". The line shows calculated values assuming A0 = 26.4 and K A = 1900.
dable task; temperatures of drying were found to be a critical factor 435 The apparatus and procedure used to make the conductance measurements and to measure electrolyte solution densities and viscosities have been described previously.2 All salt transfers were performed in a glovebox under N2 atmosphere. Results The measured equivalent conductances and corresponding electrolyte concentrations together with solution density and salt viscosity coefficients appear in the microfilm version of this volume of the journal.s For all salts except lithium trifluoroacetate, the conductance data were analyzed by the Fuoss-Wsia equation728
A
+
+
A, - S(CY)"' E c log ~ CY ( J , - BAJcy - J,(cY)"' - K,cyAf*'
(1)
where the ion pair association constant is given by
K?
= (1
- Y)/Y2cf*L
(2)
In eq 1, S = 0.30464Ao -t 23.597, and E = 0.93398A0 10.595. Values of 64.923 and 2.513 cP2 for the dielectric constant and viscosity of PC at 25" were used to calculate the coefficients. In the calculation of JI and f*, the ion size parameter was set equal to the sum of estimated ionic radii. A computer least-squares program was used to evaluate the adjustable parameters Ao, K A , and Jz. These values with their standard deviations are listed in Table I, along with the standard deviations of fit, a A .
Figure 2. Single ion conductivities in PC vs. the reciprocal of estimated crystallographic radii. Lines representing cation and anion trends have no theoretical significance.
Attempts to analyze the conductance data for lithium trifluoroacetate by eq 1 were unsuccessful. Results indicated that the association behavior of this salt is more complex than ion pairing. A plot of A us. c1I2 is shown in Figure 1. Also shown is a plot calculated using eq 1, assuming that A0 = 26.4 and K A = 1900. These values were obtained by a graphical application of the Fuoss-Onsager conductance e q ~ a t i o n using ,~ only the data for the more dilute solutions. Although the two lines are coincident to a concentration of about 4 x M , the experimental plot shows marked negative deviation at higher concentrations. We attribute this to the formation of higher ionic aggregates. Two conductance equations which treat the formation of ion triples in addition to ion pairs were applied to the data. The Kraus equationlo assumes that both cationic and anionic triple species form, and the Wooster equationll assumes that only one kind of triple ion is present. Of the two treatments, the Wooster equation was found to provide a better fit of the data, although even with this equation deviations became large a t higher concentrations. A conductance function which assumes the formation of only ion pairs and ion pair dimers was derived and applied to the data, but the fit was poor. We conclude that lithium trifluoroacetate forms a variety of ion aggregate species in PC. Although it was not possible to evaluate all equilibrium constants for these processes, the value of 1900 for the ion pair formation constant is estimated to have an uncertainty of about 30%. The large standard deviation in the least-squares analysis of sodium trifluoroacetate data may indicate a slight amount of ion aggreThe Journal of Physfcal Chemfstry. Vol 78. No. 14. 1974
Murray L. Jansen and Howard L. Yeager
gation as well. Kohlrausch differences calculated from A0 values for alkali metal perchlorates2 and the corresponding trifluoroacetates differ by about 0.3 units in the worst case.
Discussion The alkali metal trifluoroacetates show a significant degree of ion pairing, in contrast to the corresponding perchlorates which were found to be unassociated.2 Lithium chloride and bromide were found to be associated in PC, with K A values of 557 and 19, respectively.12 These results indicate that size and distribution of charge on the anion strongly influences association behavior in PC. Presumably the halide salts are associated due to small anion size, whereas the trifluoroacetates ion pair due to the localized charge on the oxygen atoms. In addition, the geometry of the carboxylate group would be expected to yield an ion of enhanced stability, particularly for smaller alkali metal ions. This may help to explain the trend of decreasing associakion from the lithium to the cesium salts. This trend in association behavior is opposite to that found for the alkali metal perchlorates in acetonitrile,13 which shows increasing association with increasing size. This was attributed to the lower ion solvation energies and consequent increased stability of ion pairs for larger cations. In contrast. the trend in association constants for aIkadi metal trifluoroacetates in PC indicates that ion-ion interactions predominate in determining the stability of Ion pairs, and changes in cation-solvent interactions are relatively unimportant. This trend in association constants is the same as those found for a variety of alkali metal carboxylate salts in ethyl methyl ketone.14 It appears that lithium ion derives greater stabilization from the carboxylate oxygens than could a larger cation such as cesium ion. Values of single ion mobilities for chloride, nitrate, and trifluoroacetate ions, 18.26, 20.42 and 17.87, respectively, were calculated from A0 values listed in Table I and cation mobilities listed in ref 2. The potassium salt was used to estimate the value for trifluoroacetate, for this salt was found to have the highest purity. These single ion mobilities, along with those of other ions in are plotted us. the reciprocal of estimated crystallographic radii in Figure 2. The higher anion mobilities have been discussed in terms of the ability of RC to solvate cations more strongly than anions.2 Of the anion mobilities reported here, even the relatively large trifluoroacetate ion is found to have enhanced mobility over cations of corresponding size. The
The Journal of Physical Chemistry, Vol. 78, No. 74. 1974
surprisingly large mobilities of the nitrate and thiocyanate ions might be ascribed to their nonspherical shapes and charge distributions, which could give rise to mechanisms of ion migration which are different from more spherical anions. Bromide is found to be the most mobile of the halide ions. In some dipolar aprotic solvents the mobilities of the halide ions decrease regularly with increasing ion size, although the reverse trend also occurs.15 Bromide ion is the most mobile halide ion in acetone.16 The observed maximum in halide ion mobility in PC could be a combination of two effects. A decrease in the anion size increases the mobility (normal Stokes law behavior), while an increase in anion-solvent interaction lowers the mobility. Acknowledgments. Financial support by the National Research Council of Canada and the University of Calgary is gratefully acknowledged. Supplementary Material Auailuble. Equivalent conductance data will appear following these pages in the microfilm edition of this volume of the journal. Photocopies of the supplementary material from this paper only or microfiche (105 x 148 mm, 24X reduction, negatives) containing all of the supplementary material for the papers in this issue may be obtained from the Journals Department, American Chemical Society, 1155 16th St., N.W., Washington, D. C. 20036. Remit check or money order for $3.00 for photocopy or $2.00 for microfiche, referring to code number JPC-741380.
References and Notes From the Ph.D. Thesis of M. L. Jansen, The University of Calgary, 1973. M. L. Jansen and H. L. Yeager. J. Phys. Chem.. 77, 3089 (1973). R. Payne and I. E. Theodorou, J. Phys. Chem., 76, 2892 (1972). Y.-C. Wu and H. L. Friedman, J. Phys. Chem., 70, 501 (1966). M. J. Baillie, D. H. Brown, K. C . Moss, and D. W.A . Sharp, J , Chem. Sac. A, 31 10 (1968). See paragraph at end of text regarding Supplementary material. R. M. Fuoss and K.-L. Hsia. Proc. Nat. Acad. Sci. U. S., 59, 1550 (1967). R. Fernandez-Prini, Trans. Faraday Soc,, 65, 331 1 (1969) R. M. Fuoss and F. Accascina, "Electrolytic Conductance," interscience, New York, N. Y., 1959. C . A. Kraus,d. Amer. Chem. Soc., 55,2387 (1933) C. B.Wooster, J. Amer. Chem. Soc., 59, 377 (1937). L. M. Mukheriee and D. P. Boden, J , Phvs. Chem.. 73, 3965 (1969) R L Kay, B J Hales and G P Cunningham, J Phys Chem 71, 3925 (1967) 5 Crisp, S R C Hughes and D F Price d Chem SOC A 603 1196RI B.-Kratochvil and ti. L. Yeager in "Topics in Current Chemistry," Voi. 27, Springer-Verlag, New York, N. Y., 1972. M. R. Reynolds and C . A. Kraus, J , Amer. Chem. Soc.. 70, 1709 (1948).
