DIELECTRIC PROPERTIES OF LIQUIDPROPYLENE CARBONATE dependence of the (0-0) absorption frequency or the line width of the vibrational fine structure, both of which depend on the refractive index. Also there is no observable solvent dependence in the 522 and the 914cm-' vibrational frequencies of the fluorescent state. The only clear empirical correlation for the fluorescence quenching was found with the (0-0) absorption intensity (Figure 3). The correlation suggests that the induced quenching depends on the same intermolecular forces that are responsible for the induced absorption. The latter are the average London-dispersion and dipole-induced dipole forces, as shown by Koyanagi. Is Little temperature dependence (100-300°K) was found in the (0-0) absorption intensity, a measure of the average intermolecular force. Thus the thermal quenching of the fluorescence in this same temperature region appears to arise from the thermal population of active vibrational levels above the zero-point level in the fluorescent state of benzene. In any case, only the total rate of all nonradiative transitions has been determined. Because the measured thermal quenching may arise from any number of thermally excited vibrat i o n ~a, single ~ ~ Arrhenius activation energy has only a limited quantitative significance. For this reason the
1443
authors have presented the tempbrature dependence of the fluorescence in graphical form (Figure 4). The (0-0)absorption depends on the volume fraction of components in several mixed solvents, so we conclude that no special conformation of benzene with solvent is necessary to explain the solvent-induced radiationless conversion, which correlates with the (0-0) absorption. However, the formation of complexes, which may occur in chloroform, can greatly enhance any effects that the induced-dipole and dispersion interactions exert on the nonradiative transitions. From the correlation between solvent-induced (0-0) absorption and fluorescence quenching (Figure 3) it is possible to estimate the fluorescence quantum yield of monomer benzene in liquid benzene at 25' as Qi =
0.040. Acknowledgments. The authors wish to acknowledge the helpful discussions of Drs. D. J. Meier, K. LOOS, F. S. Mortimer, and J. W. Otvos, as well as the persistent and careful experimental work of C. A. Augustin and R. Woolley. (19) S. H. Lin, J . Chem. Phys., 44,3759 (1966).
Dielectric Properties of Liquid Propylene Carbonate'" by Larry Simeral and Ralph L. AmeyIb Department of Chemistry, Occidental College, Lo8 Angelee, Califarnia 90041
(Received September 16, 1969)
The equilibrium dielectric permittivity, density, and refractive index of liquid propylene carbonate (4-methyl1,3-dioxolan-2-one) have been measured over the temperature range 220.15 5 T 5 293.15"K. The Kirkwood correlation factor is essentially unity over this temperature range. The results suggest that propylene carbonate behaves as a normal polar liquid with strong dipole-dipole interactions but with little or no specific association present. The effects of concentration and temperature on the ir and nmr spectra provide further evidence which is consistent with the dielectric results. Its ability to supercool is due to molecular asymmetry.
Introduction The nature and extent of association in aprotic solvents is of considerable interest and has been open to much discussion in recent literaturea2 Kempa and Lee, in determining the dipole moments of several cyclic carbonate^,^ suggested the possibility of intermolecular association in the pure compounds. As part of a general study of local structure in aprotic solvents of high dipole moment being conducted in this laboratory, an investigahion of several cyclic carbonates has begun. The usefulness of dielectric measurements to establish the extent of association in aprotic systems
has been shown both in this laboratory4 and elsewherens It has been demonstrated that little or no self-association occurs in liquid dimethyl s ~ l f o x i d e . ~Similar studies by Meighan, et al., of dimethylformamide have shown it to exhibit properties which are largely due to (1) (a) A portion of this work was presented at the 158th National Meeting of the American Chemical Society, New York, N. Y., Sept 8, 1969. (b) To whom correspondence should be addressed. (2) A. J. Parker, Chem. Rea., 69, 1 (1969). (3) R. Kempa and W. H. Lee, J. Chem. &e., 1936 (1958). (4) R. L. Amey, J. Phys. Chem., 72, 3358 (1968). (5) R. M. Meighan and R. H. Cole, ibid., 68, 509 (1964).
Volume 74, Number 7 April 2, 1970
LARRY SIMERAL AND RALPH L. AMEY
1444 nonspecific dipole-dipole interactioms These results are consistent with the findings of infrared studies.6 The related properties and extensive liquid range (224.3-515°K) of propylene carbonate (4-methyl-1,3dioxolan-2-one) have led us to begin with an examination of its dielectric behavior. Because of its increasing use as a solvent and reaction medium, an understanding of its local liquid structure should prove valuable. Experimental Section
Materials. Propylene carbonate was obtained from Matheson Coleman & Bell and dried by passage through a 1-ft column of Linde type 4A molecular seive under a nitrogen atmosphere. The effluent was vacuum distilled in a 3-ft spinning-band column at a pressure of 4-7 Torr. The middle 70% was collected under dry nitrogen and subsequently used for study. Purity was determined by gas chromatography; matched 4.5-ft (1/8 in. 0.d.) columns filled with Poropak Q (100-120 mesh) were used with dual thermal conductivity detectors in a Beckman GC 5 gas chromatograph. The water content, determined by standard additions method, was found to be less than 20 ppm. Permittivity Measurements. Capacitance measurements were made on a General Radio 1615A capacitance bridge in conjunction with a Rhode & Schwarz tunable indicating amplifier and a Hewlett-Packard 200 C D wide-range oscillator. The dielectric cell used was a three-terminal guarded concentric electrode assembly attached to the bridge with 0.085-in. 0.d. Teflon coaxial cable. The cell was immersed in propylene carbonate contained within a jacketed vessel. The latter was thermostated by circulating methanol as the refrigerant. Temperature control was maintained by a Lauda TK-30 Ultra Kryomat to *0.02". The cell constant was determined over the temperature range of interest. The temperature of the liquid sample in immediate thermal contact with the cell was determined
by a calibrated copper-constantan thermocouple and recorded on a 1-mV recorder. Densities were determined with a Sprengle-Ostwald pycnometer calibrated with water and absolute methyl alcohol. Refractive indices were determined with an Abbe precision refractometer. The latter was calibrated with Cargille master calibration liquids. Other Measurements. Kmr data were obtained with a Jeolco C-60HL spectrometer operated in the external lock frequency sweep mode. Variable-temperature controller Model JESVT-3 was used to obtain the desired temperature operation. Chemical shifts relative to the methyl proton signal were measured with a Beckman Model 7360RU frequency counter. Infrared measurements were made with a Perkin-Elmer Model 237 spectrophotometer. Sample temperature was approximately 25" during each run. Results The density of liquid propylene carbonate was determined over the temperature range 220.15 _< T 5 293.15"K. A relation of the following form was obtained from the data by least-squares analysis p =
1.541
- 1.148 X
10-3T
where T is the Kelvin temperature and p is in g cc-l. The uncertainty in p is estimated to be +0.001 g cc-'. The refractive index was found to behave linearly over the temperature range 220.15 5 T 5 293.15"K. The corresponding relation is n D = 1.5314 3.752 X lO-4T, with a maximum uncertainty in nD of h4 X 10-4. The dielectric permittivity was measured at 10 kHz and was found to be a nonlinear function of temperature. Some representative values are listed in Table I. A plot of permittivity vs. temperature is shown in Figure 1,
-
Table I: Static Dielectric Permittivity for Propylene Carbonate a t Several Temperatures
5
eo
Temp, OK
CO
Temp,
OK
61. 7a 66.1 f0 . 3 6 8 . 4 -+: 0 . 1 71.0 f 0 . 2 73.9 -+: 0 . 1
313.15 293.15 283.15 273.15 263.15
7 6 . 9 i0 . 1 80.0 5k 0 . 1 83.0 f 0 . 1 86.1 f0.1 8 9 . 3 f0 . 3
253.15 243.15 233.15 223.15 213.15
From ref 3.
Discussion No subambient data have been reported in the literature for liquid propylene carbonate. However, Sax' 1
I
-60
I
I
-40
I
I
-20
I
I
0
I
i
i
20
Figure 1. Dielectric permittivity of liquid propylene carbonate as a function of temperature. The Journal of Physical Chemistry
J
(6) A. Allerhand and P. Schleyer, J. Amer. Chem. SOC.,85, 1715 (1963). (7) N. I. Sax, "Dangerous Properties of Industrial Materials,'' 2nd ed, Reinhold Publishing Corp., New York, N. Y.,1963,p 1139.
DIELECTRIC PROPERTIES OF LIQUID PROPYLENE CARBONATE
1445
a t 20°, Kempa and Lee13Harris,* Kronick and FUOSS,~ and Wu and Friedmanlo a t 25” have all reported density values which agree with our corresponding high-temperature results. Similarly, Kempa and Leea and others” have published refractive index data a t 25 and 20”, respectively, which are somewhat lower than values reported here. TO C The static dielectric permittivity at 25” has been reported by several w ~ r k e r s , ~ ~the ~ l value l ~ - l ~by FUOSS, -60 -40 -20 0 20 et al., being in closest agreement with ours. A value Figure 2. Kirkwood correlation factor of liquid propylene reported by Kempa and Lee3 a t 40” agrees very well carbonate as a function of temperature. with our extrapolated value at that temperature. Examination of association character through dibelieved to occur. l 9 However, these instances are rare electric behavior is particularly useful in that local and to our knowledge are consistent with models which liquid structure can be inferred from appropriate calculagenerate rather small equilibrium constants. For tions based on dielectric m e a s u r e r n e n t ~ . ~The ~ ~Kirk~~ if the Dannhauser-Cole model for n-mer example, wood equation16 association is applied to our propylene carbonate data, an equilibrium constant of 9.8 X 1. mol-‘ at 20’ is obtained. From the temperature dependence of K , thermodynamic functions may be calculated : AHo permits a description of those polar liquids having = 4-2.2 kcal/mol, AS” = -1.4 eu/mol, AGO = specific short-range forces which hinder rotation of the +2.6 kcal/mol. The sign of AHo is obviously inconmolecule. The limiting dielectric permittivities a t low sistent with a reasonable model for associationz0 and and high frequencies are €0 and E , , respectively, po is the leads us to conclude that if self association is occurring permanent dipole moment of the free molecule, and V it is not of the linear n-mer type. is the molar volume measured at temperature T. Because of the sensitivity of spectroscopic techniques The correlation factor g is a measure of the short-range to the presence of association species in liquids both ir effects which hinder orientation of a molecule with its and nmr measurements were made on propylene carsurrounding neighbors. For systems in which specific bonate. The ir data were obtained on the pure liquid intermolecular forces orient neighboring dipole vectors and on several dilute carbon tetrachloride solutions. in a parallel fashion, g is greater than unity; for an Due to the limited solubility of propylene carbonate in antiparallel configuration of dipoles, g is less than unity. nonpolar solvents it was necessary to compromise in For systems with nonspecific intermolecular forces, g of an “inert” solvent for the study. The the choice equals unity and Kirkwood’s equation reduces to the Nmr measurements results are shown in Table 11. Onsager expression for a normal polar 1 i q ~ i d . l From ~ were made on the pure liquid over the temperature a calculation of the correlation factor with available range 373 5 T 5 303. These are reported in Table dielectric data as a function of temperature, it is thus I11 relative to the high-field peak of the methyl proton possible to provide a measure of the association present doublet. As can be seen from the data, essentially in a polar liquid. Figure 2 shows the results of such a calculation of g for propylene carbonate over the temperature range of this study. It can be seen that within (8) W. Harris, Report UCRL-8381, USAEC, Berkeley Radiation Laboratory, 1958. the estimated experimental error the correlation factor (9) P. Kronick and R. Fuoss, J . Amer. Chem. Soc., 77, 6114 (1955). remains near unity. This temperature dependence is (10) Y. \Vu and H. Friedman, J . Phys. Chem., 70, 501 (1966). consistent with the behavior of a polar liquid with very (11) Technical Bulletin, Jefferson Chemical Co., Houston, Texas, strong, but nonspecific, dipole-dipole attractive forcesm4 1960, p 1. It should be noted that the value of g is sensitive to (12) M . Watanabe and R . Fuoss, J . Amer. Chem. Soc., 78, 527 (1956). the E , chosen for calculation of eq 1. For comparison (13) R. Seward and E . Vierira, J. Phys. Chem., 6 2 , 127 (1958). purposes, e, was represented a t each temperature by the T. 3, 103 (1961). (14) J. B. Hasted, P T O ~Dielectrics, usual but arbitrary approximation, em = l.ln2, where (15) A . D. Buckingham, Discuss. Faraday Soc., 43, 205 (1967). n is the refractive index measured at optical frequencies. (16) J. G . Kirkwood, J . Chen. Phys., 7 , 911 (1939). It is readily shown that larger assumed values for e, (17) R. H. Cole, PTOgT. Dielectrics, 3, 70 (1961). will result in somewhat reduced g values.ls A notable (18) J. Middlehoek and C. J. F . Bottcher in “Molecular Relaxation Processes,” Academic Press, London, 1966, p 69. characteristic of most associated liquids is a strongly (19) W. Dannhauser and A. F. Flueckinger, J . Phys. Chem., 6 8 , temperature-dependent correlation factor. Correlation 1814 (1964). factors with small temperature coefficients have been (20) G. C. Pimentel and A. L. McClellan, “The Hydrogen Bond,” observed for some liquids in which association is W. H. Freeman and Co., San Francisco, Calif., 1960, p 210. t
I
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,
,
,
Volume 7.4, Number 7 April 9,1970
R. J. ELDRIDGE AND F. E. TRELOAR
1446 ~~
Table I1 : Infrared Absorption Frequencies of Propylene Carbonate in Carbon Tetrachloride
c=o
mol 1.-1
am-1
Skeletal atr, om -1
Neat
1805 f.2
1176 f 2
0.2 0.04
1825 f 2 1825 =k 2
1165 f 2 1165 ic 2
0.02
1825 f 2
1165 f 2
0.004 0.001
1828 f 2 1825 f 2
1165 & 2 1165 f 2
Concn,
str,
Table 111: Nmr Chemical Shifts" for Liquid Propylene Carbonate at Various Temperatures
C-H atr, om-1
2940 ic 5 2930 f.5
... 2940 f 5 2920 f 5 2940 f 5 2920 f 5
.*. 2940 f 5 2925 5
*
no shifts are observed in either spectrum. The slight shift in the carbonyl stretch near 1820 cm-* and the skeletal stretch near 1160 cm-' in the ir spectra can be attributed to solvent interaction.21 Again we are led to believe that this indicated the presence of little or no specific association in the liquid. Recent nmr analysis of propylene carbonate has shown that its molecular skeleton is planar in the liquid state.22 This is consistent with the observed dielectric
a
Temp,
Vat
vbl
YO,
OK
Hz
Hz
H5
373 348 328 303
215 f 2 214 f 2 215 f 1 214 f 1
191 f 1 190 1 190 f.1 192 f 1
158 3t 1 158 f 1 158 f 1 160 f 1
*
Relative to the high-field peak of the methyl proton doublet.
permittivity, refractive index, and density results, all of which show smooth behavior through the melting point into the supercooled liquid region. Thus the supercooled state in propylene carbonate is not the result of specific association, but rather, is due simply to the low symmetry of the planar skeleton.
Acknowledgment. Acknowledgment is made to the donors of the PetroIeum Research Fund, administered by the American Chemical Society, the Research Corp., and the National Science Foundation for partial support of this work.
c.
(21) L.Angell, Truns. Faraday Soc., 52, 1178 (1956). (22) H. Finegold, J . Phys. Chem., 72, 3244 (1068).
Binding of Counterions to Polyacrylate in Solution by R. J. Eldridge and F. E. Treloar Physical Chemistry Department, University of Melbourne, Purkville, V k t o r h , 8068,Australia (Received September 86,I#@)
Ultraviolet spectrophotometric measurements on solutions containing [Co(NH3)6](C104)3,fully neutralized poly(acry1ic acid) of molecular weight 7 x 105, and added electrolyte show the occurrence of intimate binding of the trivalent cation, analogous to ion pairing. The extent of this binding is inversely proportional to the cube of the concentration of added LiC104 or NaC104, and LiC104 reduces the binding more effectively than NaC104. This leads to the conclusion that the alkali metal cations compete with [Co(NH&]*+for binding sites on the polyanion. Since the hypothesis of site binding in polyelectrolyte solutions was first introduced, evidence has been obtained that association of this nature does in fact occur in many cases. Mande12 has pointed out that more than one type of ion-association can occur in a given polyelectrolyte solution and that experimental methods differ in their sensitivity to the different kinds of association. He emphasized that a variety of techniques must be used before the nature of the counterion-polyion interaction The Journal of Physical Chemistry
in any polyelectrolyte system can be made clear. Thu and dilatometrics measurements on solutions of alkali metal polyacrylates have been interpreted (1) F. E. Harris and 8.A. Rice, J . Phys. Chem., 58, 725, 733 (1954). (2) M. Mandel, J. Polym. Sci. ( C ) , 16, 2955 (1967). (3) S. J. Gill and G. V . Ferry, J. Phys. Chem., 66, 995, 999 (1962). (4) L. A. No11 and 8. J . Gill, ibid., 67, 498 (1963). (5) U. P. Strauss and Y . P. Leung, J. Amer. Chem. SOC.,87, 1476 (1965).
