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
1828 f 2 1825 f 2
1165 & 2 1165 f 2
Concn,
0.001
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).
BINDING OF COUNTERIONS TO POLYACRYLATE IN SOLUTION as evidence for site binding, while infrared absorption measurements6 provided no such evidence. Ultraviolet spectroscopy should be sensitive to the occurrence of intimate site binding, analogous to ion pairing, in solutions containing an absorbing counterion, but not to more diffuse binding. This technique has been used to study complexation of transition metal ions by polymeric carboxylates but in this work we studied the binding by polyacrylate of the hexaamminecobalt(II1) cation (M) , which should be extensively and intimately bound because of its high charge, but will not undergo inner-sphere coordination. Other workers have shown that the formation of ion pairs ("outer-sphere coordination") be tween R!I and an ions such as sulfate' or the halidess enhances the absorbance of M in the region of its charge-transfer band. Matthewsehas used this effect to study the association of XI with biological macroanions; he concluded that this is analogous to Langmuir adsorption behavior. However, this conclusion may not be valid since the equation which M a t t h e w used t o test his data contains an algebraical mistake; we were unable to derive his eq 6 from his eq 4. We have studied the binding of M to fully neutralized poly(acry1ic acid) (PAA) in solutions containing added electrolyte.
Experimental Section [Co(NH&]C13 was prepared by the method of Bjerrum and McReynolds, '0 and after two recrystallizations converted to the perchlorate by precipitation with HC104. A further recrystallization was then carried out from dilute HC104. (Anal. for NH3, 22.03% and for Co (by electrodeposition) , 13.17'%. Calculated for C O N ~ H ~ ~ C I"3, ~ O ~ 22.23%; ~: CO,12.82%.) PAA was prepared by the method of Kandanian.'l Fifty-seven om3 of glacial acrylic acid, distilled under reduced pressure, was dissolved in 325 em3 of butanone and 0.19 g of 2,2'-azo-i-butyronitrile added. Oxygenfree nitrogen was bubbled through the mixture for 1.25 hr and the temperature raised until polymerization commenced as seen by precipitation of the polymer. After 1 hr the mixture was cooled and the product collected and washed with butanone. The PAA was freeze-dried from water redistilled from KR4n04; yield, about 50%. The molecular weight was determined by viscosity measurements in l,.i-dioxane solution a t 30.0". A plot of C/qsp against C"' was linear, giving [q] = (71.7 i 1) dma kg-'. Hence, by comparison with published data,12weight-average molecular weight is 7.1 X 106. Aqueous solutions of PAA were neutralized to the phenolphthalein end point with NaOH or LiOH under a blanket of nitrogen. NaC104 and LiC104 were prepared as aqueous solutions by neutralizing NazCOa and LiOH with HC1o4. Six series of solutions were prepared containing 5 X
1447
V
I
io
20
30
LO
Figure 1. Spectrophotometric measurements on
NaPA-NaClO4-[Co(NHa)61(ClO& solutions. Ordinate, absorbance increment A; abscissa, (stoichiometric polymer concentration C,) x IO3,monomol dm-a; (-@-, C N ~ C I=O ~ 0.26; -+-, 0.41; -G, 0.15 mol dm-a).
10-4 mol dm-3, [CO(NH~),](CIO~)~, NaPA or LiPA a t (3-36) X monomol dm-3, and NaC104 or LiC104 to make the concentration of alkali metal 0.26, 0.41, or 0.51 mol dm-3. The pH of all solutions was between 7 and 8. The absorbance of each solution was measured at 235.8 nrn and 30.0" against 5 X low4mol dm-3 [ C O ( N H ~ ) ~ ] ( Cas ~ Oa~reference. )~ (The molar absorbance of the complex salt is not affected by added perchlorate at the concentrations used.) The absorbance of NaPA and LiPA was also measured in solutions of the same ionic strength. Small corrections for the absorbance of the perchlorate were made in all cases. The instrument used was a Hilger and Watts Uvispek, fitted with a circulating-water thermostat. The range of concentrations of added salt was restricted by precipitation of the polyanion complex cation adduct at lower concentrations, and of the hexaamminecobalt(II1) perchlorate at higher concentrations of added perchlorate. Under the conditions used no turbidity was observed, nor did the absorbance of the solutions change with time.
Results and Discussion The absorbance difference measured as described above is shown, after subtraction of the absorbance of the appropriate stoichiometric concentration of polymer, as a function of polymer concentration in Figures (6) J. C. Leyte, L. H. Zuiderweg, and H. J. Vledder, Spectrochim. Acta, 23A, 1397 (1967). (7) H. Taube and IF. A . Posey, J. Amer. Chem. SOC.,7 5 , 1463 (1953); 78, 15 (1956). (8) M. G. Evans and G. H. Nancollas, Trans. Faraday SOC.,51, 793 (1955). (9) M. B. Matthews, Biochint. Bwph,hys. Acta, 37, 288 (1960). (10) J. Bjerrum and J. P. McReynolds, Inorg. Syn., 216 (1946). (11) A. Y. Kandanian, Dissertation Polytechnic Institute of Brooklyn, Brooklyn, N. Y., 1964, p 18. (12) 5. Newman, W. R. Krigbaum, C . Laugier, and P. J . Flory, J . PoEym. Sci., 14, 451 (1954).
Volume 74, Number 7 April 8 , 1970
R. J. ELDRIDGE AND F. E. TRELOAR
1448
If now the binding of each trivalent ion can be regarded as outer-sphere coordination by three carboxylate groups (ie., we assume n = 3), and is accompanied by the release of three univalent cations from the polymer domain, giving for the binding reaction Ma+ S MS 3M+ then we can define an association constant
+
+
3C'M (1
tf
- f>(cp- 3 f C M S f )
(4)
neglecting the activity coefficients y. We believe that this neglect is justified since y 3 for a singly charged cation should be approximately equal to y for a triply charged cation and because the activity coefficients of the polyanion including bound complex ion or including bound singly charged cation should be very similar. Equation 4 assumes that the carboxylate groups binding each trivalent ion are attached to the one polymer molecule. If this assumption is valid then a plot of f/(l - f) against [COO-] = Cp - 3 f C ~ ~ ~ wbei llinear. l Equation 4 was tested in two wa,ys. Figure 3 shows f / ( l - f ) as a function of Cp - 3 f C ~ ~ + feach o r of the six series; the gradient m of the best line through the origin for each series and mCM+3 are shown in columns 2 and 3 of Table I.
Figure 2. Spectrophotometric measurements on LiPA-LiCIO, [Co(NH3)~] (c10& solutions. Ordinate: absorbance increment A ; abscissa: (stoichiometric polymer concentration C,) X 108, monomole dmW3; -e-, Cl;io~o~ = 0.26; -+-, 0.41; -0-, 0.51 mol dm-8).
1 and 2. We conclude from the magnitude of the absorbance increment that extensive site binding of the hexaamminecobalt(II1) ion occurs. At higher polymer concentrations the absorbance increment A approaches a constant value for all six series of solutions, showing that the trivalent ion is virtually completely bound. Increasing concentrations of added electrolyte decrease the degree of binding, and LiC104 is obviously more effective than NaC104 in doing this. This is in accord with the observation of other workers that the extent of binding of the alkali metal cations to the Table I : Test of the Dependence of the Degree of Site Binding anions of weak polyacids increases with decreasing of [Co(NH3)6]8 + on Polymer and Alkali Metal Concentration radius of the unhydrated This fact has Reciprocal been attributed to intimate binding accompanied by Gradient gradient Concn Cnrt disruption of the hydration sheath of the c a t i ~ n . ~ J ~ J ~ of added m of plot 77%' of plot of eq 4 , mCay+, of eq 5 , m'Cang+, electrolyte, I n order to calculate the extent of binding of [Codma mol-' mol2 d m 4 dma mol-' moll dm-8 mol dm-a (N€13)6]3+, we assume the binding reaction to be repre1280 22.5 26.1 1480 0.26 NaClOa nCOO11s where Ma+ represented by Ma+ 316 21.8 330 22.8 0.41 NaC104 sents a "free," and MS a site-bound [Co(NH3),J8+ion. 181 24.1 25.4 191 0.51 NaC104 The absorbance increment will then be 722 12.7 824 14.5 0.26 LiClOa ~~
+
+
0.41 LiClOa 0.51 LiC106
A = [MSIEMS [ M 3 + ] € ~ 4a+
240 131
16.6
228
17.4
114
15.7 15.3
[COO-]Ep - CM*+%*t- c p 6 p (1) where the E are molar absorbances, the C stoichiometric concentrations, and the subscript p indicates polyacrylate. If a fraction f of the M 3 + ions are site-bound f E
[Ms]/C~at
(2)
and A
=:
fc~s+{- E M S + EMS
-
%Ep]
(3)
Thus f can be found for any Cp by dividing the observed A by the limit of A at high C,, wheref approaches unity. The Journal of Phqsical Chemietry
Equation 4 also lea8dsto (5)
This relation is tested in Figure 4 which shows l/f plotted against (C, - 3 f C ~ * + ) - *the ; reciprocal m' of the gradient of the best line with unit intercept and (13) U. P. Strauss, D. Woodside, and P. Wineman, J . Phys. Chem., 61, 1363 (1957). (14) U. P. Strauss and P. D. Ross, J . Amer. Chem. Soc., 81, 5296, 5299 (1959). (16) J. Bourdais, J. Chim. Phys., 56, 194 (1959).
1449
BINDINGOF COUNTERIONS TO POLYACRYLATE IN SOLUTION
600 I
10
I
20
200
Figure 3. Test of eq 4. Ordinate: f / ( l - f); abscissa: (concentration of “free” carboxylate groups, (c, 3fC~st)X 108, monomole dm-8; (A: -0-, 0.51 mol dm-3 LiClO,; 0.41 mol dm-a LiCIOd; e-, 0.51 mol dm-* NaClO,; -x-, 0.41 mol dm-8 NaClOd; B: -0-, 0.26 mol dm-* LiClO,; -@-, 0 . 2 6 mol dm-3 NaCIOd).
-
-+-,
nt’Canft are shown for each series in columns
4 and
5 of Table I. Because of the constancy of the figures obtained for each alkali metal, we conclude that three univalent ions are indeed released from the polymer domain during the binding of one trivalent ion, and because the “association constant” for [Co(NH3)6I3+ is greater in the case of Na+, we further conclude that the alkali metal cations are also site bound. (The stronger binding of Li+ to the polymer cannot be explained by a model based on the interaction of the counterion’s charge with the electrostatic potential well of the polyanion. An explanation of the observed dependence on ionic radius of the binding of the alkali metal cations requires the assumption that binding is to one or perhaps a few of the polymer chain’s charged groups, and involves partial dehydration of the cation. There is experimental evidence for the occurrence of such a process in a number of systems. Thus the volume changes6 accompanying the binding of Li+, Na+, I
