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J. J. Thompson and E. Rutherford, Phi/. Mag., 42, 392 (1896). P. Kebarle, S. K. Searles, A. Zolla, J. Scarborough, and M. Arshadi, J. Amer. Chern. Soc., 89,6393 (1967). 91, 2827 (1969). (3) F. H.Field, J. Amer. Chem. SOC., (4) M. Arshadi, R. Yamdagni, and P. Kebarle, J. fhys. Chem., 74, 1475 (1970). M. DePaz. J. J. Leventhal, and L. Friedman, J. Chem. Phys., 51, 3748 (1969). I. Dzidic and P. Kebarle, J. Chem. fhys., 74, 1466 (1970). S. K. Searles and P. Mebarle, J. fhys. Chem., 72, 742 (1968). M. L. Vestal and J. H. Futrell, to be submitted for publication.
(9) M. L. Vestal. 20th Annual Conference on Mass Soectrometrv and Allied Topics, Dallas, Tex, June 1972. (IO) J. H. Futrell and M. L. Vestal, 20th Annual Conference on Mass Spectrometry and Allied Topics, Dallas, Tex, June 1972. (11) D. P. Beggs and F. H. Field, J. Amer. Chem. Soc., 93, 1576 (1971). (12) J. P. Briggs, R. Yamdagni, and P. Kebarle, J. Amer. Chem. Soc., 94, 5129 (1972). (13) J. D. Payzant, A. J. Cunningham, and P. Kebarle, Can. J. Chem., 51, 3242 (1973). (14) F. C. Fehsenfeldand E. E. Ferguson, J. Chem. Phys., 59, 6272 (1973). (15) P. Mulet, S. D. Peyerimhoff, and R. J. Buenker, J. Amer. Chem. Soc., 94, 8301 (1972). (16) H. Wincel, /nt. J. Mass. Spectrom. /on Phys.,
mics of Polycarboxylate Aqueous Solutions. 11. ilatometry and Calorimetry of Nickel and Barium Binding F. Belben* and S. Paoletti laboratorio di Chimica delle Macromolecoie. lstituto di Chimica. Universita di Trieste, Trieste, ltaly
(Received November 79, 1973)
Calorimetric and dilatometric data on the binding of Ni2+ and Ba2+ ions by three maleic acid copolymers in tetramethylammonium perchlorate aqueous solution a t 25" and at different neutralization degrees of the polyacids are reported. The results show a marked dependence of the thermodynamic binding parameters on the nature of the metal ion bound. The present data, and those previously obtained for the binding reaction between Cu2+ ions and the same copolymers, are compared.
Introduction In a recent paper from this laboratory the results obtained by means of calorimetric and dilatometric measurements on the protonation of, and Cu2+ ions binding by, three maleic acid (MA) copolymers have been rep0rted.l We wish to report here similar data concerning Ni2+ and BaZCions binding by the same copolymers (hydrolyzed maleic anhydride-ethylene, MAE; hydrolyzed maleic anhydride-propylene, MAP; and hydrolyzed maleic anhydride-isobutene, MAiB) in 0.05 M (CH3)4NC104 and at 25", and to discuss the observed differences in the set of thermodynamic binding parameters in a comparative way. Experimental Section
( a ) Matenals. Maleic acid-ethylene copolymer (MAE), maleic acid-propylene copolymer (MAP), and maleic acid-isobutene copolymer (MAiB) samples, all of the 1:l alternating type2,3 were received from the Monsanto Chemical Co Their molecular weight was about 105.2 Stock solutions of the three polyacids were prepared as previously d e ~ c r i b e d, .4~In the preparation of the polyelectrolyte solutions for both calorimetric and dilatometric experiments standardized (CH3)4NOH (a BDH product, employed without purification) solutions were used. Nickel perchlorate was prepared by allowing carbonate (Erba RP) to react with warm aqueous HClO4 (an Erba product, 70% w/w solution). The resultant solution was filtered and cooled. The metal perchlorate was collected and recrystallized twice from water; the titer of the nickel perchlorate solutions was determined using EDTA.5 BariThe Journai of Physical Chemistry. Vol. 78. No. 75. 1974
um perchlorate was prepared by allowing excess barium carbonate (Erba RP) to react with warm dilute HC104; the resultant solution was filtered and directly standardized using EDTA.6 Pure (CH3)&TCl04was prepared as previously described.l In all cases, freshly distilled water, obtained by a Heraeus quartz bidistillator, was used. ( b ) Methods. The calorimetric experiments were carried out at 25" using an LKB 10700-1 flow microcalorimeter, following a procedure already d e ~ c r i b e d .Also ~ the treatment of the calorimetric data follows previously reported pr0cedures.l (See paragraph at end of text regarding supplementary material.) Linderstroem-Lang dilatometers were used to measure volume changes, as described p r e v i o ~ s l y .The ~ water insoluble liquid used was n-heptane, pcrified as previously described .8 All measurements were carried out a t 25", with a constancy of bath temperature much better than 0.001 deg/ hr, following the dilatometric method successfully applied by Begala and S t r a ~ s s .Corrections ~ for effects due to dilution on mixing were found to be almost always negligible, All equilibrium dialysis experiments were carried out using cellulose tubings (Kalle AG, Wiesbaden), treated prior to use as previously described.l The metal concentration in the polyelectrolyte-free "external" solutions was determined by atomic absorption spectrometry (Perkin-Elmer 290-B, with acetylene as fuel) for barium, and by atomic absorption spectrometry or by EDTA for nickel. The final uncertainty in the calculated
Thermodynamicsof Polycarboxylate Aqueous Solutions fraction of bound metal ions should not exceed 2 and 5% for nickel and barium, respectively. Potentiometric titrations were performed in a thermostaked vessel at 25 & 0.05” using a Radiometer PH M 4d pH meter equipped wizh Radiometer “combination” electrode, GM-2301 C.
02
I kcslhanomolel
The microcalorimetj.ic data, obtained following closely the experimental procedures4 and the treatment1 described elsewhere, are plotted in Figure l. A ~ values T (A& i s a differential enthalpy of binding, in kcal/mol of metal bound, taken as the slope of the AHT us. R(M2+) curve at the limit of R(M2+) 0, where R(M2+) is the ratio of metal bound to monomeric units concentration) for N?+ and Ba2+ ions are reported in Table 1, together with. those of Cu2+ ions for the same experimental conditions.’ It is clearly seen that the AAT values associated to the binding reactions, a.t least for our experimental conditions, are positive and markedly decreasing passing from @uz--‘to Niz+ tc Ea2--,respectively. Two facts deserve attention here. First, weir calorimetric Ni2+-MAE binding t to agree with those obtained by other au0.97 kcal/rnol) .g This discrepancy depends, essentially on the different degree of neutralization, a,l0of the polymer chains, i . e . , cy is 1.0 in our case and is 0.5 for Purdii:, et al. This trend (increasing A es with increasing cy) has -Cu and MAiB-Cu bindbeen also reported for the ing, even for other experirn Second, the 4 8 7 for binding reaction decreases, for each polyelectrolyte, passing from Cu2+ to Ni2+ to Ba2+ ions, respectively. ‘This can be related to the decreasing ability, from Ci.n2+-to E a 2 + ions, to bind carboxylic groups yielding true site binding. In the case of the interaction between Cu2-- ions and partially neutralized polycarboxylic acids the existence of complexes has in fact been estabcontrary Ba2+-ions are known to form static bonds with charged polyelectrolytes.“ Our experimental data support the hypothesis that the Xi2+ ions can be bound by carboxylic groups to give a chelate intermediate between a site binding complex and an ion pair. Ilifferent binding depends on both the charge density to ionic volurntt ratio (and therefore on the hydration degree) and the electronic configuration of the different metal ions. Direct studies on the carboxylic groupmetal ion bond length could eventually support this hypothesis. Also cur dilatometric data can help us in this direction The dilatometric results are plotted in the Figures 2 and 3. The slope of thle curves a t R = 0, AVT, is the volume increase (XI mi/niol of “complex”) when the first metal ion is bound by Lhe polyelectrolyte chain. As can be seen from Tabie 1, the A ~ at T CY = 1.0 is, for each polymer-metal cornpiex, always lower than the corresponding value at a = 1.2 or 1.5. This evidence has been already discussed in terms of the hydration of the bicarboxylic groups.I Further evidences indicates that it is very difficult t o interpret. the dilatometric data relating them directly to the nature of metal ion and/or of polyelectrolyte cbain. We can observe. however, that, at least at cy = 1.2 and 1.5, there Is an. increase in the A ~ values T passing from MAE to MAP and to MAiB, respectively, for each metal bound. Purther:nore, our present results indicate
-
~
lkcsl~mwromala)
& / ,7 L 4 0 02
0
01
R “I* ____
02
Enthalpy changes on the addition of Ni(C104)z and Ba(C104)z respectively to tetramethylammonium polycarboxylates at a = 1.0 in 0.05 M (CH3)4NC104 at 25”.The abscissa, R ( M z + ) , denotes the moles of added Ni2+ or Baz+ bound per monomole of total polyacid. The ordinate, AH,, gives the total enthalpy change in kcal per monomole of total polyacid: (a) MAE; (b) MAP; (c) MAiB; (A)Ni2+; ( A )Ba’+. Figure 1.
IO
4
8 2
U0 4 -
2 -
e
0 -
4
Figure 2. Volume changes on the addition of Ni(C104)2 to tetramethylammonium polycarboxylates at various degrees of neutralization, cy, in 0.05 M ( C H 3 ) 4 N C 1 0 4 at 25”.The abscissa, R ( N i 2 + ) , denotes the moles of added Niz+ bound per monomole of total polyacid. The ordinate, AVT, gives the total volume change in milliliters per monomole of rota1 polyacid: (a) M A E ; (b) MAP; (c) MAiB; (0)a = 1.00; ( 0 )a = 1.50.
that the behavior of Ni2+ is intermediate between those of Cu2+ and Ba2+, respectively, a t the above-mentioned values of cy (see Table I). The not well-understandable trend of A ~ atT a = 1.0 is likely to be due to a superposition of various effects, some of which have been mentioned above. It is worth pointing out, however, that our data agree with those already published by other author^.^ In order to evaluate the variation in entropy associated with the binding process (which could contribute, together with the dilatometric results, to a better structural interpretation of the binding phenomenon) we tried to calThe Journai of Physicai Chemistry. Vol. 78. No. 75 1974
F. Delben and S. Paoletti
1488
TABLE I: T h e r m o d y n a m i c s of Cu2f,Ni2+, and Ba2 Binding by Three Hydrolyzed Maleic Anhydride Copolymers +
Polymer
Metal
A ~ T kcal/mol ,
AGE',^ kcal/mol
of complex
of complex
AS
4.4 4.4
7.6
40
2.0
6.4
28
0.6
5.9
22
4.0 4.0 2.0
8.9
43
6.4
28
0.6
6.1
22
4.0 4.0
10.1
47
1.8
8.1
33
0.7
7.2
27
(I
AVT,ml/mol of complex
__I_____
MAE
1.00 1.20 1.50 1.00 1.50 1.00 1.50 1.00 1.20 1.00 1.50 1.00 1.50 1.00 1.20 1.50 1.00 1.50 1.00 1.50
CU2+
Ni2 +
Ba2+ MAP
CUZ”
Niz +
Ba2+
MAiB
CUZ+
NiZ + Ba2+ a pK, and pKnfor
-
AGE evaluation are respectively MAE, 4.4,7.1; MAP 4.1,8.0;MAiB, 3.4,9.4.AGB’ areextrapolated to R
AH-
+ M’?
= 0 AGB
GHM+
32 41 52 13 36 24 24 26 52 20 51 25 39 31 85 85 23 52 13 40 values (see text).
(4)
or similar ones, the stoichiometric or apparent equilibrium constant of the “pure” binding reaction 2 is the following
i
‘r
a
I
I
0.2
0.3
R.3.
Figure 3. Volume changes on t h e addition of Ba(C104)nto tetramethylammonium polycarboxylates at various degrees of neutralization, a. in 0.05 IVI (CH3)4NC104 at 25“. The abscissa, R(Ba2+),denotes the moles of added Ban+ bound per monomole of total polyacid. The ordinate, AVT, gives the total volume change in milliliters per monomole of total polyacid: (a) MAE; (b) MAP; (c) MAiB, ( 0 )01 = 1.00: ( 0 )a: = 1.50. culate the free-energy changes associated to the process. Our effort, althocgh not fully successful, gives some interesting indications. If AH2, A H - , and A2- stand for discharged, monodissociated, and bidissociated monomeric unit, respectively, M2+ represents the divalent chelated counterion, and MA is a “monomole” of complex, we can picture the whole binding reaction, at 01 = 1.0, as follows
-
,413A7- +
A2r
M2’
+ H+
-
(1)
MA (2) AH‘, (3) From this scheme and if in a first approximation we do not take into account the possibility of the following reacl,iong
W’
+ AH-
----t
The docirnal of Physical Chemistry. Voi. 78, No. 15. 1974
[M2+] is known from dialysis equilibrium experiments, by analyzing the polyelectrolyte-free ‘‘external’’ solutions (see Experimental Section). [MA] is knowmfrom the total concentration of metal, e,,,, and [M2+]; [MA] = C,,, [ M 2 + ] ; finally, [A2-] can be evaluated from the titration curves of each single polyelectrolyte. The K M A a p P values are of limited reliability for three principai reasons (besides the fact that the quantities in the expression are the stoichiometric concentrations, and not the activities): (1) the uncertainty in the dialysis equilibrium data (h27’0 for Cu2+ and Ni2-’-,and 4 ~ 5 %about for Ba2i ions concentrations values, respectively) do not permit an exact calcuiation of bound to total metal ratio, particularly at the highest ratio values; (2) K l a P P and K z a p P , the first and second polyacid dissociation constants, respectively, which must obviously be known in order to calculate [A2-], are not necessarily the same in the absence and in the presence of divalent metal ions. Furthermore, one can easily verify that a nonexact choice of these values can lead to macroscopic errors in the evaluation of A2- groups concentration; (3) dialysis equilibrium measurements can in principle give the “exact” (see above) amount of metal ions that are surely not bound at all to the polyelectrolyte backbone; it is not possible however to know the exact fraction of the remaining ions really “site bound” to the polyelectrolyte and that quantity surrounding the polymeric backbone and interacting with it more weakly by long-range electrostatic forces. Keeping these difficulties in mind, we have calculated the apparent equilibrium constants for the binding reaction at 01 = 1.0; these were found in each case to depend on R. We cannot say if this trend has physical significance or if it is fictitious (due, for example, to the omission of the electrostatic potentials and/or activity coefficients). In Table I the corresponding roughly approximate free-
Thermodynamics cf Polycarboxylate Aqueous Solutions energy variations AGBo (AGBo is the free-energy value extrapolated to R .- 0) and the associated A S values are reported. We are well aware of the fact that our treatment needs some criticisms. First of all, let us consider again the complete process of binding at cy = 1.0. It is immediately understood that our calorimetric data are comprehensive not only of the second stage (the “pure” binding reaction), but also of the other two stages (partial second disscciation and partial first proton association). A correction for the mentioned dissocintion enthalpies should be performed; this is very d-fficult however for two principal reasons. First, the enthalpy of proton exchange is not a fixed value for each polymer chain, but depends on the dissociation degree of‘ the chain itself; second, it does not seem reasonable to assume that such enthalpy can be insensitive to the preseince of bound metal ions. We can say, however, according to enthalpy of protonation data collected in our laboratory,l that these corrections must not be able to change considerably the above mentioned AHT and, of course, the associated A S values. If we compare the A S with the AVT data (see Table I) we can immediately observe that there is a good (see above) correlation b e t w e n the A S and AVT trends. If we attribute these variations to the hydration phenomena, it is possible to affirm that, independently of the particular polyacid considered the number of water molecules lost in the binding reaction by each metal ion decreases in the order Cu2+ > N i 2 ~> a2+. This fact can be qualitatively related with !be decreasing positive AH, values we obtained passing from CuZL to Ni2+ to Ba2+ ions, respectively. Ah1 these considerations do not take into account that in the polymer chain an important role is played by the confoimational and configurational situation of each monomeric unity. In order to better evaluate the influence of local configuration on the thermoc ynamics of both dissociation and divalent metal ion binding for nearest neighbor carboxyl-
1489
ic groups, we are beginning a study of some appropriate “model” monomeric compounds.
Acknowledgments. This work has been sponsored by the Italian Consiglio Nazionale delle Ricerche. The authors are indebted to professor V. Crescenzi for many helpful discussions. Supplementary Material Auailable. A complete list of dilatometric, calorimetric, and dialysis equilibrium experimental 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 $3.000 for photocopy or $2.00 for microfiche, referring to code number JPC-74-1486. References and Notes V . Crescenzi, F. Delben, S. Paoletti, and J, Skerjanc, J. Phys. Chem., 78, 607 (1974) E. Bianchi, A. Ciferri, R. Parodi, R. Rampone, and T. Tealdi, J. Phys. Chem., 74, 1050 (1970). P. L. Dubin and U. P. Strauss, J. Phys. Chem., 74, 2842 (1970) V . Crescenzi, F. Delben, F. Quadrifoglio, and D. Dolar, J. Phys. Chem., 77, 539 (1973). H. Flaschka, Mikrochemie, 39,38 (1952). G. Anderegg, H. Flaschka, R. Sallmann, and 6 .Schwarzenbach, Helv. Chim. Acta, 37, 113 (1954). A. J. Begala and U. P. Strauss. J. Phys. Chem.. 76, 254 (1972). S. Katz and T. G. Ferris, Biochemistry, 5, 3246 (1966). B. J. Felber. E. M. Hodnett, and N. Purdie, J . Phys. Chem., 72, 2496 (1968). The value of (Y equal to 2 corresponds, in our notations, to complete neutralization of the polydicarboxylic acids. H. P. Gregor, I.. 6.Luttinger, and E. M. Loebl, J. Phys. Chem., 59, 366 (1955). A, M. Kotliar and H. Morawetz, J. Amer. Chem. SOC., 77, 3692 (1955), H. Morawetz, J. Polym. Sci.. 17, 442 (1955). F. T. Wall andS. J. Gill,J. Phys. Chem., 58, 1128 (1954)
The Journal of Physical Chemistry. Vol. 78. No. 15. 7974
