J. Phys. Chem. 1986, 90, 4455-4461 finement is needed to both the theory and experiment before emulsions containing supercooled aqueous electrolyte solutions can be used to determine the conductivity of the aqueous phase.
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Glossary
*
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relative permittivity of disperse phase limiting low frequency relative permittivity
4455
limiting high frequency relative permittivity limiting high frequency conductivity of the emulsion conductivity of the disperse phase Kl molar conductivity of the disperse phase 4 volume fraction of disperse phase Registry No. KCI, 7447-40-7; heptane, 142-82-5. Kh
Vltrlfled Dilute Aqueous Solutions. 1. Infrared Spectra of Alkall Metal Nitrates and Perchlorates as Solutes Erwin Mayer Institut fur Anorganische und Analytische Chemie, Universitat Innsbruck, A 6020 Innsbruck, Austria (Received: December 18, 1985)
Vitrified dilute aqueous alkali metal nitrate and perchlorate solutions have been investigated by infrared spectroscopy and the spectra of the degenerate antisymmetric stretching modes of NO3- and Clod- are reported. For the alkali metal nitrates as solutes a group trend has been observed, the Au3 values decreasing with increasing mass of the cation. For K N 0 3 and RbN0, smaller Au3 values than those for “infinite dilution” at room temperature have been obtained. For the alkali metal perchlorates, complete loss of degeneracy of the u3(tz) mode of Clod- indicates strong perturbation. From a comparison with the spectra of salt vapors matrix isolated in glassy water an assignment in terms of two different types of ion pairs is given. The spectral changes observed in the vitrified solutes suggest that the various processes occurring during rapid quenching can be probed and their importance for the final immobilized state investigated. From this investigation and from comparison with spectra of matrix-isolated salt vapors it appears that in dilute aqueous solutions rapid quenching and vitrification favors the dominant formation of only one anion-containing structural element.
Introduction Until recently only “concentratedn aqueous electrolyte solutions could be vitrified1,*and investigated by vibrational spectro~copy.*~ In “dilute” solutions precipitation of crystalline ice and freeze concentration of the solute occur with the usual cooling methods. Recently four methods have been developed to vitrify pure liquid water and dilute aqueous solutions.G10 Three of theseG8 need a liquid cryomedium for heat transfer and are difficult to apply to spectroscopic investigations. The fourth method-rapid cooling of aqueous aerosol droplets on a solid cryoplate9-is ideally suited for spectroscopic in situ investigations. With this method it is possible to investigate the whole range from very dilute to concentrated vitrified aqueous solutions. The infrared spectrum of vitrified pure liquid water has been reported.” In this work the infrared spectra of vitrified alkali metal nitrate and perchlorate solutions are reported. These two anions have been widely used as spectroscopic probes to structural perturbation^.'^-'^ Only the antisymmetric stretching vibrations of N03-(u3,e’) and ClO&,tZ) (1) Vuillard, G. Ann. Chim. (Paris) 1957, 2, 23. (2) Angell, C. A.; Sare, E. J. J . Chem. Phys. 1970,52, 1058. (3) Hester, R. E.; Krishnan, K.; Scaife, C. W. J. J . Chem. Phys. 1968,49, 1 loo. (4) Kanno, H.; Hiraishi, J. Chem. Phys. Lett. 1979,62,82,1979,68,46, 1980,72,541,1980, 75,553,1981,83,452,J. Phys. Chem. 1982,86, 1488, 1983,87, 3664, 1984,88, 2781. ( 5 ) Carrick, M. T.; James, D. W.; Leong, W. H. Aust. J. Chem. 1983,36, 223. (6) Brtiggeller, P.; Mayer, E. Nature (London) 1980, 288, 569. (7) Dubochet, J.; McDowall, A. W. J. Microsc. (Oxford) 1981,124, RP3. (8) Mayer, E.; Briiggeller, P. Nature (London) 1982, 298, 715. (9) Mayer, E. J. Appl. Phys. 1985, 58, 663. (10) Mayer, E. J . Microsc. (Oxford) 1985, 140, 3. ( 1 1 ) Mayer, E. J . Phys. Chem. 1985, 89, 3474. (12) Irish, D. E.; Brooker, M. H. In Aduances in Infrared and Raman Spectroscopy, Clark, R. J. H., Hester, R. E., Ed.; Heyden: London, 1976; Vol. 2, Chapter 6. (13) Devlin, J. P. In Advances in Infrared and Raman Spectroscopy, Clark, R. J. H, Hester, R. E., Ed.; Heyden: London, 1976; Vol. 2, Chapter 5. (14) James, D. W. In Progress in Inorganic Chemistry, Lippard, S. J., Ed. Wiley: New York, 1985 Vol. 33, p 353.
0022-3654/86/2090-4455$01.50/0
hove been investigated because the other transitions are obscured by water absorption bands. Spectral changes in vitrified electrolyte samples can be caused by freeze concentration of the solute due to crystallization of ice I or by changes in the solute structure during quenching to low temperatures.I4-l6 So that the latter can be investigated it is therefore essential to preclude precipitation of ice I as completely as possible. The aerosol deposition method has been reported to give 85% vitrified material with pure liquid water.9 This method has been further improved to give about 95% vitrified material with pure water. These latter deposition conditions were used throughout in this work. A few percent precipitated ice I leads only to a minor increase in solute concentration, so that spectral features of the vitrified solutions are not caused by precipitation of ice I during quenching.
Experimental Section Electrolytes solutions were prepared from analytical grade salts (LiN03, Mallinckrodt; N a N 0 3 and K N 0 3 , Merck; RbNO,, Ventron; NaC1O4-H20, Merck; LiC104.3H20, Fluka) and deionized water. The aqueous electrolyte solutions were vitrified in the same way as pure liquid water. Since this is described e1sewhere:JI it is only summarized: aqueous aerosol containing the electrolyte was produced with an ultrasonic nebulizer operating a t 3 M H z (LKB instruments, Model 108) and transferred with gaseous nitrogen as carrier gas through an electron microscope aperture with 200-~rndiameter into a high-vacuum system containing the low-temperature infrared cell. The aerosol droplet size was 13-gm diameter according to company specification. The carrier gas flow rate was 3 L min-I; the solutions were nebulized to aerosol at 0.5 mL m i d . The aerosol was led through tubing (1-m long, 23-mm diameter) to the aperture. The amount of water vapor was reduced by cooling the aerosol to approximately 273 K. The aerosol was deposited on a KBr window attached to a (15) Frost, R. L.; James, D. W. J . Chem. SOC.,Faraday Trans. I , 1982, 78, 3249. (16) Irish, D. E.; Jaw, T. Discuss. Faraday SOC.1977, 64, 95.
0 1986 American Chemical Society
4456
Mayer
The Journal of Physical Chemistry, Vol. 90. No. 18, 1986
Displex cryotip (Air Products, Model CS-208L), using 45' geometry. The distance between KBr plate and orifice was 40 mm; the vacuum during deposition was 2 X mbar. The deposition temperature was between 50 and 60 K. The infrared cell was constructed from a DN- 100-ISO-K vacuum flange (Bakers) adapted with various ports. The temperature was measured on the lower end of the cryotip. This temperature should be within a few degrees of the temperature of the KBr cryoplate because a vapor deposited HzO/N2 matrix annealed at the temperatures reported in the literature. Sufficient heat transfer was obtained by constructing the KBr sample holder from O F H C copper, and the KBr window was embedded in the holder with indium seals. The spectra were recorded in absorbance on a Pye Unicam SP 3-300 instrument with SP 3-050 data processing system and calibrated with COz or polystyrene film. The resolution was 3 cm-'. Deposition times between 10 and 25 s were chosen such that the absorbances of the reported band principal maxima were at most 1. Since the bands are still fairly broad even at 20 K, the resolved peak maximum values are believed to be accurate only to about f3 cm-'. Results General. Diffraction provides the most stringent test for determining small amounts of crystalline material in a vitrified sample. Pure liquid water can be vitrified by the aerosol deposition method with at most 5% ice I as impurity." Vitrified dilute aqueous solutions prepared in the same way contain even less ice I; for example, in a quenched 1 M KNO, solution the sharp reflections from ice I were not discernible any more above the broad signals from the vitrified material. Spectroscopic observations are in accord with the X-ray results: first, small amounts of ice I in vitrified liquid water can be most clearly observed in the decouled 0-H (0-D) stretching transition region." For that reason many of the aqueous electrolyte solutions have been vitrified containing 11 mol % HOD, and the 0-D stretching transition region has been investigated for ice I. No evidence was ever observed for the formation of ice I, and it is concluded that its concentration can have been at most a few percent. Differentiation between the decoupled 0-D bands of vitrified liquid water and ice I was facilitated in the vitrified solutions because the vitrified water band is shifted to high frequency by the solutes investigated so far. Second, no evidence for the formation of crystalline hydrates during quenching was ever observed. Formation of minor amounts of crystalline hydrate would have been discernible because some of their sharp 0-H stretching transitions occur at higher frequency than the broad band from the vitrified solvent. Only above 200 K crystalline hydrate formation was observed with some solutes. Most of the solutions were vitrified at least twice and the infrared spectra recorded. The reproducibility of the spectra with respect to band shape and positions and relative solvent/solute concentration was excellent. Only the thickness of the vitrified deposit varied somewhat from experiment to experiment, mainly due to small variations in aerosol density. The temperature dependence of the spectra between 20 and 130 K is barely perceptible; therefore only the spectra at 20 K are reported. To avoid (1 7) Our unpublished results: the essential point was to reduce the relative amount of water vapor in the aerosol/carrier gas mixture by precooling the aerosol to approximately 273 K and by using higher droplet density. The contribution of water vapor to the amount of crystalline material in the deposit was tested in a separate experiment where water vapor from saturated nitrogen was deposited on a cryoplate without water droplets, but with otherwise identical conditions. This gave 30% crystalline, mainly cubic, ice. The large amount of crystalline material from deposited water vapor in the presence of nitrogen is understandable by considering the form of the deposits as mentioned in ref 9: water droplets are deposited in form of a sharp disk, apparently sticking on the cryoplate at the first impact, whereas water vapor condensed evenly on the whole cryoplate, including the side opposite to the orifice. Apparently, water vapor reevaporates frequently on the cryoplate and thus has sufficient opportunity to nucleate and form crystalline ice in the cold nitrogen atmosphere above the cryoplate. The value of 95%vitrified material from aqueous aerosol is therefore a lower limit with respect to the water droplets because the vapor still present at 273 K aerosol temperature will contribute significantly to the remaining 5% crystalline ice.
YI
n
a I
I
I
I
I
I
I
precipitation of the solute in some of the droplets during transfer of the aerosol from the nebulizer to the cryoplate, solute concentrations close to saturation were avoided. For that reason CsN03, the least soluble of the alkali metal nitrates, was omitted. Alkali Metal Nitrates. Figure 1 shows the infrared spectra of vitrified aqueous alkali metal nitrate solutions from 2000 to 1000 cm-I at 20 K. The aerosol was produced from 1.0 M solutions throughout. The broad band centered at approximately 1650 cm-I is the deformation mode of the vitrified solvent H20, and the double peak at lower frequency is the u,(e') mode of the NO3- ion. With LiNO, and N a N 0 , as solutes a weak band at 1041 and 1047 cm-' was observed which is assigned as the symmetric NO3- stretching mode. A comparison with the H20 deformation mode intensity allows the determination of approximate relative concentrations. The u3 region of NO,- is expanded in Figure 2; increased ordinate gain values are given on this and the other expanded figures and refer to the corresponding spectrum without cm-' expansion. This allows comparison of spectra with and without expansion. For a 0.50 M KNO, solution, not shown in the figures, frequencies of the uj band components were at 1402 and 1367 cm-l. In Figure 3 the infrared spectra of vitrified aqueous LiNO, solutions are shown at different concentrations, using from a to c 0.50, 1.0 and 3.0 M solutions for aerosol formation. The spectrum of a 1.0 M LiNO, solution at room temperature is given for comparison in d. The u3 NO3- transitions are again expanded in Figure 4. The electrolyte concentrations refer to the solutions used for preparing the aerosols. There are certainly changes in solute concentration during the various stages such as ultrasonic nebulization, transport of aerosol to orifice, of passage of aerosol through orifice, of flow under vacuum to cryoplate, and of deposition on the cryoplate. A qualitative inspection of Figure 3 shows that increasing the concentration of the starting solution from a to c gives a corresponding increase of the NO,- band intensity relative to that of water. An estimate of the electrolyte concentration in the vitrified state can be obtained from a comparison of band areas of the antisymmetric stretching mode of NO3-with that of the librational band of HzOboth in the vitrified
The Journal of Physical Chemistry, Vol. 90, No. 18, 1986 4457
Vitrified Dilute Aqueous Solutions
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1
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Li NO3
x2
NaN O3
I .
0
m
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\ \
I V
m c m
n
0
m
m
n m
1500
cm-I 1450
1400
1350
1300
I
1250
Figure 2. The u3 NOo- infrared bands in Figure 1 expanded at higher gain, using the same numbering as in Figure 1. The ordinate gain values
are given in the figure and refer to the corresponding spectrum in Figure 1. The broken line in b indicates the peak maximum position in the spectrum of vapor-deposited KN03/H20matrix (from ref 45).
2000
1700
1400
I IS00
I
I
I
1450
1400
1350
I
cm-’ 1300
J 1250
Figure 4. The Y, NO< infrared bands in Figure 3 expanded at higher gain, using the same numbering as in Figure 3.
vitrified deposit. Since several of the electrolyte solutions were vitrified containing in addition 11 mol ?6 HOD, a second comparison with the band areas of the decoupled 0-D stretching transition is possible; this gives a value of 0.77, again for the concentration in Figure lb. The deformation mode of the solvent H 2 0 is not as suitable for evaluation because of tailing at low However, this band is quite useful as an indicator for relative approximate concentrations and is therefore included in this and other figures. The concentration estimate in the vitrified solution obviously presupposes first that the absorptivities of the various transitions are fairly independent of temperature,23comparing room temperature spectra with spectra at 20 K. Second, the vitrified samples are presumed to contain very little ice I and vapor-deposited amorphous solid water, H20(as), because both would contribute to the band areas of the solvent. As pointed out before, both X-ray diffractograms” and infrared spectra of the decoupled 0-D stretching transition show that at most a few percent of ice I can have been present. H20(as) as impurity, important in this context only because of its influence on concentration estimates, can be largely excluded, again by considering the infrared spectrum of the uncoupled 0-D stretching transition in vitrified pure liquid water;” there it was found that for the deposition conditions used in this investigation only minor amounts of H20(as) can have been codeposited. Alkali Metal Perchlorates. Figure 5 shows the infrared spectra of vitrified aqueous lithium and sodium perchlorate solutions, b and a, together with the spectrum of a 1.0 M LiC104 solution at room temperature for comparison (c). One molar solutions were used for aerosol formation. The deformation mode of the vitrified solvent is included again for assessing relative approximate solute concentrations. The u3(t2) transition of C104- is expanded in Figure 6 , using identical gain. In Figure 7 infrared spectra of the expanded u3 region of vitrified aqueous NaC104 solutions are
1100
Figure 3. Infrared spectra of vitrified aqueous LiNO, solutions at 20 K, using from (a) to (c) 0.50, 1.0, and 3.0 M LiNO, solutions for aerosol formation. In (d) the spectrum of a 1.0 M LiNO, solution at room
temperature is shown for comparison. state and in solution at room temperature. This was done for the concentration in Figure 1b and gave a value of 0.80, indicating a decrease of L i N 0 3 concentration from 1.0 to 0.80 M in the
Angeil, C. A.; Tucker, J. C. J . Phys. Chem. 1980,84, 268. Angell, C. A. Annu. Rev. Phys. Chem. 1983, 34, 593. Angell, C. A. In Water, A Comprehensive Treatise, Franks, F., Ed.; Plenum: New York, 1982;Vol. 7, Chapter 1. (21) Franks, F. In Water, a Comprehensive Treatise, Franks, F., Ed.; Plenum New York 1982; Vol. 7, Chapter 3. (22) Hagen, W.; Tielens, A. G. G. M.; Greenberg, J. M. Chem. Phys. 198‘1,56, 367. (23) Wyss, H. R.; Falk, M.Can. J. Chem. 1970,48, 607.
4458 The Journal of Physical Chemistry, Vol. 90, No. 18, 1986
Mayer
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I
a 0
I$
I
I
I
1260
1200
1150
1100
L
L
I cm-
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1050
1000
Figure 7. Expanded infrared spectra of the v3 CIO, mode of vitrified aqueous NaC10, solutions at 20 K, using from (a) to (d) 0.50, 1.0, 3.0, and 5.0 M NaC104 solutions for aerosol formation. The intensities are not comparable.
NaClOd
Figure 6. The u3 C104- region in Figure 5 expanded, using the same numbering and gain as in Figure 5 .
shown, using from a to d 0.50, 1.0, 3.0, and 5.0 M solution for aerosol formation. This figure demonstrates only the change in band shape with concentration and is not meant to provide a comparison in terms of relative intensity.
Discussion Irish and Davisz4have shown that even for dilute aqueous alkali metal nitrate solutions (0.12 to 0.50 M) the u3(e') mode of NO3(24) Irish, D. E.; Davis, A. R. Can. J . Chem. 1968, 46, 943
contains two components, the doublet structure having been attributed to lifting of the degeneracy and hydration of the anion. The curve-resolved infrared bands occur at 1349 and 1397 cm-'. For dilute solutions the band positions are independent of concentration and the effects attributed to cations are n o n s p e c i f i ~ . ~ ~ An inspection of Figures 3 and 4, where vitrified aqueous LiNO, solutions are compared with a 1.O M solution at room temperature, seems to indicate that at cryogenic temperatures only increased resolution of the two overlapping bands has occurred. However, further inspection of Figures 1 and 2 shows that additional effects beyond improved resolution must be involved because for the vitrified alkali metal nitrate solutions a cation effect is clearly observable, contrary to what has been reported for dilute solutions at room t e m p e r a t ~ r e :the ~ ~ band position of the high-frequency component at approximately 1400 cm-' is nearly constant, whereas the second component, observed a t 1340 cm-' in LiNO,, moves with increasing mass of the cation to higher frequency, thereby increasing the relative peak height of the high-frequency component. Band separations of 66 cm-' are obtained for LiNO,, 49 cm-I for NaNO,, and 34 cm-' for KNO,; for R b N 0 3 the band separation has even decreased further and curve resolution techniques would be necessary to determine the position of the low-frequency component. These changes in band separation have to be compared with the Au3 value of 48 cm-I at room temperature which for this concentration range is to a good approximation independent of catiomZ4 It is very likely that these drastic, cationdependent spectral differences reflect the response of the aqueous solution during quenching to cryogenic temperatures. Even with the extreme rate of cooling necessary to vitrify pure liquid water or dilute aqueous solutions, the structural state at room temperature is not frozen in, but the system responds to cooling by structural relaxation. The limiting structure obtained on cooling a liquid through the glass transition region has been characterized by Moynihan et al.z5926in terms of a limiting fictive temperature ( 2 5 ) Moynihan, C. T.; Easteal, A. J.; DeBolt, M. A,; Tucker, J. J . Am. Ceram. SOC.1976, 59, 12.
Vitrified Dilute Aqueous Solutions which depends first on cooling rate and second on the structural relaxation time and its temperature dependence. It is likely that the quenched samples taken for Figures 1 and 2 have all experienced the same cooling rate or, better, distribution of cooling rates, considering the excellent reproducibility. N o attempt has yet been made to measure the cooling rates during quenching, and it is difficult to see how this can be done. However, from homogeneous nucleation theory U h l m a ~ ~ and n ~Fletcher28 ~ calculated that cooling rates in excess of lo7 or even 1O'O K s-l are necessary to vitrify a water sample of the size of a 1-pm layerz7 or droplet.28 This brings the cooling rates employed here for vitrifying the electrolyte solutions within the range typical for splat cooling of liquid metals and alloys.29 An analysis of structural relaxation processes in liquids cooled with extreme rates is quite complex. Angell and Torrel130 have analyzed the ion system Caz+-K+/N03- and have shown by a combination of experimental data and computer simulations that for extreme rates of cooling a broad dispersion range in temperatures will be observed where the system falls out of internal equilibrium, and that the fictive temperature concept which associates the glass structure with an internally equilibrated liquid structure must collapse.30 For the same system Howell et have argued from differences between the electric field and the shear stress relaxation times that, during cooling of an equilibrium fused salt into the high viscosity region, "elementary diffusive motions of the more mobile ions take place in the context of a frozen str~cture".~'It is important that in both studies cited the authors point out that these are probably general phenomena, not restricted to their particular system investigated. Therefore comparable effects might well be important for aqueous electrolyte solutions vitrified with extreme rates of cooling, and the dynamic aspects of ionic solvation have to be considered. The final immobilized state after rapid quenching might depend among others on differences between the hydrated alkali metal cations with respect to relaxation processes and dynamic solvent exchange3*J3 and the related fluidity changes brought about in water by the individual cations.34 These properties show group trends, and it is tempting to speculate and see some relation with the group trend observable in the spectra of Figures 1 and 2 with respect to band separation. Second, the anomalous properties of supercooled liquid water have to be taken into consideration. During rapid quenching and vitrification the aqueous electrolyte solutions have to pass a temperature region where the various thermodynamic and relaxational properties of supercooled liquid water show large and rapidly increasing temperature dependencies (263-233 K, at P = 1 atm), with a tendency to reach extreme values a little below the homogeneous nucleation t e m p e r a t ~ r e . ' ~ - ~ In ~ J binary ~ aqueous solutions these anomalies are known to disappear at high concentrations of so1ute.20,21*35-37They should, however, still be observable in the more dilute solutions investigated in this work, though less pronounced than in the pure solvent, and might contribute to the structure of the vitrified solute. A third aspect might be differences in the temperature dependence of the various association equilibria. Frost and James38*39 (26) Moynihan, C. T.; Macedo, P. B.; Montrose, C. J.; Gupta, P. K.; DeBolt, M. A.; Dill, F. J.; Dom, B. E.; Drake, P. W.; Eaststeal, A. J.; Elterman, P. B.; Mwller, R. B.; Sasabe, H.; Wilder, J. A. Ann. N . Y.Acad. Sci. 1976, 279, 15. (27) Uhlman, D. R. J. Non-Cryst. Solids 1972, 7 , 337. (28) Fletcher, N . H. Rep. Prog. Phys. 1971, 34, 913. (29) Jones, H. Rep. Prog. Phys. 1973, 36, 1425. (30) Angell, C. A.; Torrell, L. M. J. Chem. Phys. 1983, 78, 937. (31) Howell, F. S.;Bose, R. A.; Macedo, P. B.;Moynihan, C. T. J. Phys. Chem. 1974, 78, 639. (32) Bockris, J. 0. M.; Saluja, P. P. S. J. Phys. Chem. 1972, 76, 2140. (33) Petrucci, S. In Ionic Interactions, Petrucci, S., Ed.; Academic: New York, 1971; Vol. 11, Chapter 7, p 114. (34) Desnoyers, J. E.; Perron, G. J. Solution Chem. 1972, I , 199. (35) Lang, E. W.; LUdemann, H. D. Angew. Chem., In?.Ed. Engl. 1982, 21, 315. (36) Lang, E. W.; Fink, W.; Liidemann, H. D. J. Phys. Colloq. 1984, C7, 173. (37) Oguni, M.; Angell, C. A. J. Chem. Phys. 1980, 73, 1948.
The Journal of Physical Chemistry, Vol. 90, No. 18, 1986 4459 TABLE I: Comparison of Infrared Band Frequencies (cm-I) of Vitrified Aqueous Nitrate and Perchlorate Solutions with Those of Matrix-Isolated Vaporsa LiNO,/H,O -, -
matrix-isolated vaporsb
vitrified solution
1347 1412 65
1340 406 66
1348 1398
so
364 398 34
1065 1110 1150 85
065 111 151 86
1073 1112 1140 67
1076 1112 1136 60
aThe notation for the v3 component bands follows that used in ref 46 and 47. The NaC10, frequencies are only approximate because of poor resolution of the v3 components. bReferences 45 and 47.
have analyzed the profile of the band due to the symmetric stretching vibration in the Raman spectrum of aqueous alkali metal nitrate solutions for individual components, and from their concentration dependence they have made assignments as aquated ions, solvent-separated ion pairs, contact ion pairs, and ion aggregates. For a 2 M N a N 0 3 solution they have analyzed the variation in component-band intensity with temperature from 338 to 268 K (see Figure 5 in ref 39). The aquated NO3- is the dominant species at 338 K but approaches zero concentration already at 272 K; the concentration of both solvent-separated ion pairs and contact ion pairs increases with decreasing temperatures, the solvent-separated ion pair being the dominant species with 90% relative band intensity at the lowest temperature investigated. Comparable temperaturedependent data for the other alkali metal nitrates, possibly extended to even lower temperatures, would be of great help for estimates of the various nitrate species present in the vitrified solution. An extrapolation of the data for the N a N 0 3 solution seems to suggest that the solvent-separated ion pair is the dominant species even in a deeply supercooled solution. However, there is no reason why the trend toward decreasing ionic separation should not be continued at lower temperatures and contact ion pairs become the dominant species, the increase in association occurring with decreasing temperature in the face of an increasing dielectric constant. For dilute aqueous perchlorate solutions there seems to be agreement that, among the alkali metals, perchlorate association with lithium and sodium is negligible, while association with the heavier ions is small but s i g n i f i ~ a n t . ' ~ No J ~ ~evidence ~~ has been reported for perturbation and symmetry lowering of the degenerate ~3(t2)band of C104- in dilute solution, this degenerate antisymmetric stretching transition always being observed as a single symmetric band.12-41-43For dilute aqueous solutions of LiCIO, and NaC104 this has been confirmed recently by Raman line shape analysis of the nondegenerate symmetric stretching vibration of ClO; for band c ~ r n p o n e n t s . ' ~However, ~~ according to this band shape analysis, for all salts except LiC104, for higher concentrations ion association has been observed to be the rule rather (38) Frost, R. L.; James, D. W. J. Chem. Soc., Faraday Trans. I 1982, 78, 3223, 3235. (39) Frost, R. L.; James, D. W. J. Chem. Soc., Faraday Trans. I , 1982, 78, 3249. (40) Johansson, L. Coord. Chem. Rev. 1974, 12, 241. (41) Ross, S.D. Spectrochim. Acta 1962, 18, 225. (42) Jones, M. M.; Jones, E. A,; Harmon, D. F.; Semmes, R. T. J. Am. Chem. Soc. 1961,83, 2038. (43) Hester, R. E.; Plane, R. A. Inorg. Chem. 1964, 3, 769. (44) Frost, R. L.; James, D. W.; Appleby, R.; Mayes, R. E. J. Phys. Chem. 1982, 86, 3840.
4460
The Journal of Physical Chemistry, Vol. 90, No. 18, 1986
than the exception. In contrast to dilute solutions at room temperature, the Vitrified dilute aqueous LiC104 and NaC10, solutions depicted in Figures 5 and 6 show clear evidence for three components within the region of the v3(tz)transition of C104-. This can be interpreted either in terms of several C104- species with different band positions, where symmetry and degeneracy are still partly or completely retained, or in terms of complete loss of symmetry, this implying a bidentate-like pairing with the ClO,ion. It is not possible to distinguish solely on the basis of these spectra between the various possibilities. But for reasons mentioned below the latter interpretation, complete loss of degeneracy, is preferred. The band shape is concentration dependent as shown in Figure 7 for vitrified NaC10, solutions. The same factors discussed above for the vitrified alkali metal nitrate solutions will be of importance for the final immobilized state in the vitrified alkali metal perchlorate solutions, producing as a rather surprising result a strongly perturbed C104- anion. A better understanding of the various processes occurring during rapid cooling and vitrification of an aqueous solution might be obtained from a comparison with spectra of matrix-isolated vapors. ~ ~ investigated S ~ ~ , ~ the ~ ~infrared ~ spectra Devlin and C O - W O ~ ~ have of vapors of molten oxyanion salts isolated among others in a glassy water matrix at cryogenic temperatures. This allows comparison of the glassy materials obtained by matrix isolation from the vapor phase with those from rapid quenching of the solution for details of solvation, and partly for this comparison the solutes were selected. Table I shows the peak maximum values of the systems investigated so far by both methods. For the vitrified solutions the values were from 1 M solutions. For the matrix-isolated vapors the salt concentration was not reported. Therefore, this comparison is only qualitative, especially for NaC104, where in Figure 7 a marked concentration dependence of the band shape was observed. However, for matrix-isolated L i N 0 3 / H 2 0 and K N 0 3 / H 2 0 the v3 band components were reported to be nearly independent of matrix ratio and sample thickne~s!~,~Despite these shortcomings, the agreement between the peak maximum values and the Av3 splitting for the L i N 0 3 / H 2 0 and LiC104/H20 systems is remarkable, though the band components in vitrified solution spectra seem to be somewhat better resolved. For NaC104/H20 poor resolution of the composite band does not allow exact evaluation of peak maxima without curve-resolving techniques, but an estimate gives similar values. For K N 0 3 / H 2 0 ,however, the spectra are clearly different: in Figure 2b the peak maximum value of the low-frequency band component from the vapor deposit is indicated with a broken line, showing clearly the much larger Av3 separation obtained for the matrix-isolated vapors. If one excludes artifacts such as partial decomposition of the KNO, vapor or matrix isolation of salt aggregates, it follows that at least for this system a different state of solvation has been obtained by the two methods. It is unlikely that this difference is caused by differences in KNO, concentration because in the matrix the K N 0 , / H 2 0 ratio has been ~ a r i e d , ~and . , ~ we have obtained nearly identical peak component frequencies from 0.50 M vitrified solution. Devlin et a1.’3345-49have interpreted the loss of degeneracy and splitting of the antisymmetric stretching frequencies of NO3- and C104- in glassy water matrices in terms of contact ion pair formation, relying for this interpretation on the very careful investigation of stepwise formation of solvates in argon matrices. For C104- they have interpreted the complete loss of degeneracy of the ~3(tz)mode in terms of bidentate or bidentate-like coordination of the cation with the anion4’,& and, because of the good agreement between the two sets of spectra, we follow their assignment. For L i N 0 3 / H 2 0 matrices they have suggested the pentahydrate of the contact ion pair, (HzO)5.Lif-N03-,to be the final stage of hydration.49 The close correspondence between the two sets of indication frequencies and the Avj values in Table I is a very. good (45) (46) (47) (48) (49) 1167.
Smyrl, N.; Devlin, J. P. J . Phys. Chem. 1973, 77, 3067. Ritzhaupt, G.; Devlin, J. P. J . Phys. Chem. 1975, 79, 2265. Ritzhaupt, G.; Devlin, J. P. J . Chem. Phys. 1975, 62, 1982. Draeger, J.; Ritzhaupt, G.; Devlin, J. P. Inorg. Chem. 1979, 18, 1808. Ritzhaupt, G . ; Consani, K.; Devlin, J. P. J . Chem. Phys. 1985, 82,
Mayer that with Li+ as cation the same state of solvation of the NO3anion is achieved for the vitrified solution and the matrix-isolated vapors. For K N 0 3 / H 2 0matrices Ritzhaupt and Devlin have reached a conclusion similar to that for the L i N 0 3 / H 2 0 matrix, although based on less extensive experimental work, that “contact ion pairs are the dominant species regardless of matrix c ~ m p o s i t i o n ” .In ~~ the vitrified KNO, solution spectrum, however, the low-frequency component of the v 3 doublet is shifted to higher frequency in comparison to the matrix spectrum, this resulting in a smaller Av3 separation. If it is accepted that contact ion pairs are the dominant species in a matrix-isolated KNO!/H,O sample, it simply follows that in the rapidly quenched solution a large fraction of the NO< anions has some other state of solvation, not contact ion paired, with a smaller Av, value. It has been shown that the Av, splitting of the v3(e’) transition of NO3- is a sensitive indicator for perturbation of the anion by the polarizing power of the cation, smaller values of Av, corresponding to less pert~rbati0n.I~ For the series of vitrified alkali metal nitrate solutions shown in Figures 1 and 2, the Av, values parallel the polarizing power (in terms of charge/mass ratio) of the alkali metal cations and indicate within the interpretation above decreasing perturbation of the nitrate anion with increasing cation mass. Since the AV, value of NO3- in vitrified aqueous KNO, solution is clearly smaller than the value from the K N 0 3 / H 2 0 matrix (34 vs. 50 cm-I, Table I), it is necessary to look for an associated NO3- species with less anion perturbation than present in the Kf.N03- contact ion pair in the matrix (thereby taking as a basis Ritzhaupt and Devlin’s a ~ s i g n m e n t ~The ~ ) . most obvious way of reducing the polarizing power of a cation and of perturbation of an anion is insertion of a solvent molecule between cation and anion.I4 Therefore it is suggested that in vitrified aqueous KNO, solution soluent-separated ion pairs, K+.H,O.NO,-(aq), are the dominant species, in contrast to a K N 0 3 / H 2 0 matrix prepared by deposition of the vapors. This assignment applies also to vitrified aqueous R b N 0 3 solutions with an even smaller Av, value. A specific cation dependence of the Av3 values, expected for solvent-separated ion pairs,24is observable from a comparison of the spectra in Figures 1 and 2, a and b. Further support for two different types of NO,--containing species in vitrified alkali metal nitrate solutions comes from the observation of a weak symmetric stretching transition with Li+ and Na+ as cations. This transition, which is infrared inactive in the “free” NO< anion with D3,, symmetry, has often been observed in the infrared spectra of concentrated aqueous solutions and melts, and its intensity has-like the Av3 splitting-been used as an indicator of strong NO3- p e r t ~ r b a t i o n . ~ ~InJ the ~ , ~vitrified ~ KNO, and R b N 0 3 solutions this band was much less intense than in the LiNO, and NaNO, solutions, having been barely recognizable above the background. An argument against the assignment in terms of solvent-separated ion pairs has been discussed by Ritzhaupt and D e ~ l i n , ~ and is repeated here: they have argued that the Av, value of 50 cm-’ in dilute aqueous alkali metal nitrate solutions which is caused by nitratewater interactionsN is a lower limit such that “regardless of the magnitude of the cation distortion of the anion, the v 3 splitting in a pure glassy H 2 0 matrix cannot be reduced below 50 cm-’”.46 Since according to their assignment the same Av3 value of 50 cm-l has been observed for K+.N03- contact ion pairs in a KNO,/H,O matrix, they have further argued that a solventseparated ion pair should have the same value and not be distinguishable in the v3 region. This argument does not necessarily hold, considering the uncertainties about the origin of the v 3 doublet structure in dilute solutions as discussed recently by Irish and Brooker.’, Within the Av3 concept for nitrate perturbation, a value of 34 cm-l for the postulated K+.H20.NO3-ion pair seems to imply that the nitrate ion experiences a smaller peiturbation by the field of the solvent-separated cation than by the water solvation shell only in a dilute solution at room temperature, which is at least surprising. However, it is debatable whether room (50) Irish, D.E.; Walrafen, G . E. J . Chem. Phys. 1967, 46, 378.
J . Phys. Chem. 1986, 90, 4461-4464 temperature values are really transferable to a vitrified solution, considering the anomalous properties of supercooled liquid water and dilute aqueous solutions. In this context it should be pointed out that a very similar Av3 value of 30 cm-' had been observed for K N 0 3 isolated in glassy KC103 or KC104 rnatrice~.~'The assignment in terms of solvent-separated ion pairs obviously is only speculative at present. However, it should be possible by a combined investigation of solutes in vitrified solution, in matrix-isolated vapors, and in supercooled solutions to put this assignment on a firmer basis. The most direct evidence of multiple discrete environments will come from the analysis of nondegenerate bands in the Raman spectrum where more than one component will indicate more than one anion e n ~ i r o n m e n t . ~ From ~ . ' ~ such studies information on the factors important for the final immobilized state after rapid quenching might be obtained on a molecular level. From the assignment above follows that the alkali metal cations can be separated into two groups, Li and Na+ forming contact ion pairs with both C104- and NO), and K+ and Rb+ solventseparated ion pairs with NO). It is certainly premature to generalize on the basis of these few systems investigated so far. However, it is striking that the alkali metal cations are classified into the same subgroups when investigated for the dynamics of water exchange according to S a m o i l o or ~ ~for~ fluidity ~ ~ ~ changes (51) Ritzhaupt, G.; Devlin, J. P. J . Chem. Phys. 1976, 65, 5246. (52) Samoilov, 0. Ya. Discuss. Faraday SOC.1957, 24, 147. (53) Conway, B. E. In Ionic Hydration in Chemistry and Biophysics, Elsevier: New York, 1981; Chapter 33, p 672.
4461
broght about in water by the individual cations.34 The differences between the two subgroups are interpreted in terms of decreased or increased mobility of water in the hydration shell of the cations in comparison with that of bulk water and have been considered in terms of so-called structure-making and structure-breaking effects on the solvent lattice.53 It is quite possible that Samoilov dynamic exchange is one of the important parameters for the structure of the vitrified solutions, and that it causes the observed spectral differences between the two alkali metal subgroups. From the four systems investigated so far by matrix isolation of the vapors and by vitrification of the solutions, three give nearly identical spectra in the u3 region of the nitrate ion (Table I). This is at least surprising and seems to suggest the dominant formation of just one anion-containing structural element, despite the experimental differences between the two methods. This could well be a general phenomenon, the various processes during rapid quenching of a solution favoring primarily one structural element. It might, however, be restricted to dilute solutions because in concentrated aqueous solutions additional ion-ion interactions become important as shown in Figure 7d by additional band broadening in a vitrified 5 M NaC104 solution.
Acknowledgment. This work was supported by the "Fonds zur Forderung der wissenschaftlichen Forschung" of Austria. I am very grateful to Doz. R. Abermann for his help in constructing the infrared cell, and to Prof. B. M. Rode and Dr. M. Probst for discussions. Registry No. LiNO,, 7790-69-4; NaNO,, 763 1-99-4; KN03, 775779-1; RbNO,, 13126-12-0; LiC104, 7791-03-9; NaClO,, 7601-89-0.
Equilibrium Studies by Electron Spin Resonance. 16. Effect of Charge Density upon I on Association Thermodynamics and 'Kinetics in Liquid Ammonia Gerald R. Stevenson,* Dean J. Lovett, and Richard C. Reiter Department of Chemistry, Illinois State University, Normal, Illinois 61 761 (Received: December 23, 1985; In Final Form: April 7 , 1986)
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The solvated anion radicals of a series of para-substituted nitrobenzenes in liquid ammonia were found to react rapidly with the sodium cation to form the ion pair (X-PhN02'- + Na+ X-PhNOO-, Na+). The rate constant for ion association ( k f ) does not vary with the choices of the para substituent (X). However, the rate constant for ion pair dissociation decreases by more than a factor of 2 from its value of 2.3 X lo7 s-I with X = OCH, when X is changed to NO2. Since kf is invariant and encounter controlled (kf= 1 X 10" M-I s-I), kd controls the magnitude of the ion association constant (KA). Plots of the log K A vs. the u value for X are linear and lead to a p value of -0.49.
Introduction Liquid ammonia has the unique property of having the ability to dissolve the alkali metals, the alkaline earth metals, and the lanthanides Eu and Yb.' Many of the numerous investigations into the nature of these metal solutions originate from the fact that they exhibit a fascinating variety of electronic behaviors as the concentration of the metal is increased. Further, at the lower concentrations, the solutions behave as electrolytes owing to the presence of solvated metal cation and solvated electron. It has been stated in this journal2 that progress toward describing an adequate and physically meaningful model of the solvated electron is also important for the development of a general theory of condensed matter. The situation is equally important in the case of anion radicals in liquid ammonia, where the solvated electron has been captured by an organic 7-system, which is in turn solvated by the ammonia. A prevailing tendency of polar anion radicals ~
~
~
~
(1) White, T. R.; Hsu,S. P.;Mobley, M. J.; Glaunsinger, W. S . J. Phys. Chem. 1984.88, 3890. (2) Brodsky, A. M.; Tsarevsky, A. V. J. J . Phys. Chem. 1984,88, 3790.
in liquid ammonia is to undergo ion association with alkali metal cations. However, how the chemistry and nature of these ion pairs vary with the polarity of the organic substrate remains an unexamined problem. The para-substituted nitrobenzene (X-PhN02) anion radicals would be of particular value in unraveling this problem, as the para substituents can be used to vary the electron density on the N O z group that is involved in ion a s s ~ c i a t i o n . ~ In hexamethylphosphoramide (HMPA), which cannot form hydrogen bonds to the anion, the free energy of ion association (reaction 1) increases as the electron-withdrawing nature and spin and
charge density on the NO2group, q(NOz), are d e ~ r e a s e d .This ~ is the expected result if one considers only the Coulombic attraction ___
_ _ _ _ ~ _ _ _ _
___
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(3) Stevenson, G. R.; Echegoyen, L ; Hidalgo, H. J . Phys. Chem. 1975, 79, 152. (4) Stevenson, G. R.; Echegoyen, L. J . Phys. Chem 1973, 77, 2339.
0022-3654/86/2090-4461$01.50/00 1986 American Chemical Society
