Aug., 1954
ALKALINEEARTH CHELATES
WITH
HYDROLYZED M A L E I C ANHYDRIDE COPOLYMERS
619
CHELATION OF ALKALINE EARTH IONS BY HYDROLYZED MALEIC ANHYDRIDE COPOLYMERS BY H. MORAWETZ, A. M. KOTLIAR AND H. MARK Polytechnic Institute of Brooklyn, Brooklyn 1 , New York Received April I D . ID64
The chelation of Mg, Ca, Sr and Ba with hydrolyzed maleic anhydride-styrene and maleic anhydride-vinyl ethyl ether copolymers was studied by the effect of low concentrations of alkaline earths salts on the titration curve of the polymer in strong potassium nitrate solution. Bjerrum’s method of calculating chelate formation Constants was modified t o take account of the variation of the effective ionization conatant with the charge of the polyelectrolyte. Apparent chelate formation constants increased, in general, with the charge of the polyanion in the case of the styrene copolymer, while the reverse w a ~ found with the vinyl ether copolymer. The results were interpreted as due to a superposition of an electrostatic and a saturation effect.
The interactions of polyelectrolytes with their counter-ions have been studied intensively in recent years. In view of the high charge of the polymeric ion, the counter-ions are subjected to strong electrostatic forces, which manifest themselves in the characteristic titration behavior of polymeric acids,l low activity coefficients of the ions in such solutions2~3and the electrophoretic transport of part of the counter-ions with the p ~ l y i o n . ~ Most of the work to date was restricted to systems with monovalent counter-ions, although the effect of divalent counter-ions on the titration6 and solution viscosity6 of polyelectrolytes was briefly mentioned, and the gelation of polycarboxylic acids with divalent cations was studied in some detail.’,* The interpretation of the behavior of such systems is complicated by the fact that complex formation of divalent cations with carboxyl groups may be superimposed on the electrostatic effect. Gregor and Luttingerg have observed a pronounced shift in the titration curve on addition of small concentrations of cupric salt to polyacrylic acid in strong salt solution and interpreted this finding as evidence of chelation. Before such data can be translated into chelate formation constants, a modification of Bjerrum’s procedure’o consistent with polyelectrolyte theory is required. The study of the chelation of alkaline earth cations with hydrolyzed maleic anhydride copolymers offers two advantages for a preliminary study of this problem: there is no uncertainty about the number of ligands bound by the chelating ion, and formation constants of analogous succinic acid chelates, similar to the complex formed by the polyacid but without the complications peculiar to polyelectrolyte equilibria, have been reported.“
Results and Discussion Two hydrolyzed copolymers of maleic anhydride (1) A. Katchalsky and J. Gillis, Rec. trav. chim., 6 8 , 879 (1949). (2) W. Kern, Malcromol. Chem., 2, 269 (1948). (3) A. Katchalsky and S. Lifson, J . Polymer Sci., 11, 409 (1953). (4) J. R.Huisenga, P. F. Grieger and F. T.Wall, J. A m . Chem. Soc., 78, 2636, 4228 (1950). (5) A. Katchalsky and P. Spitnik, J . PoEymer Sci., 2, 432 (1947). (6) T.Alfrey, R. M. Fuoss, H. Morawetz and H. Pinner, J. A m . Chem. Boc., 74, 438 (1952). (7) F. T. Wall and J. W. Drenan, J . Polymer Sci., 7 , 83 (1951). (8)H.Deuel and H. Solms, KoEEoid-Z., 124, 65 (1951). (9) H.P. Gregor and L. Luttinger, private communication. (10) J. Bjerrum, “Metal Ammine Formation in Aqueous Solukion,” P. Haase and Son, Copenhagen, 1941. (11) A. E. Msrtell and M. Calvin, “Chemistry of the Metal Chelate Compounds.” Prentice-Hall, Inc., New York, N. Y.,1952,p. 617.
with vinyl ethyl ether (VEE/MA) and styrene (ST/MA), respectively, were titrated with sodium hydroxide in 1 N potassium nitrate solution. Since it is known that maleic anhydride adds to styrene and vinyl ether radicals very much more rapidly than to its own radical, the hydrolyzed copolymers contain pairs of carboxyl groups isolated by the comonomer from similar pairs. Titration curves of such “polydicarboxylic acids” should have a break at half-neutralization, since the double ionization of a carboxyl pair is improbable while there remain any un-ionized carboxyl pairs. Ferry and his collaborators12 reported such a break in the titration curve of hydrolyzed styrene-maleic anhydride copolymers, but found Iittle evidence of it in titrating a hydrolyzed copolymer of maleic anhydride and vinyl ethyl ether, which is much more expanded in aqueous solution. When small amounts of alkaline earth nitrates were added, the second half of the titration curve was displaced to lower pH values. The total salt concentration being very much larger than that of the alkaline earth salt added, the effect could not be due t o a change in the electrical free energy of ionization and had to be ascribed to complex formation. Let us denote by A the stoichiometric concentration of carboxyl pairs, (HA-) and (A‘) being the concentrations of the singly and doubly ionized species and (X) the concentration of carboxyl pairs complexed with an alkaline earth ion. Since complexing is observed only in the second half of the titration curve, we may assume that the concentration of un-ionized carboxyl pairs is negligible so that for a degree of neutralization of the second carboxyl 02 (HA-) A(l Aaz = (A-1
- a*)+ ( O H - )
+ (X) + ( O H - )
(1) (2)
We shall assume that the electrical free energy of dissociation AFe1’ depends only on the average charge Z per carboxyl pair, Le., that as long as Z is held constant, AFe1 is independent of the number of ions chelated. Thus
where 2 =1
+ ap - 2 ( X ) / A - ( O H - ) / A
(4)
(12) J. D. Ferry, D. C. Udy, F. C. Wu, G. E. Heckler and D. F. Fordyce, J . CoEEoid Sci., 6 , 429 (1951).
H. MORAWETZ, A. M. KOTLIARAND H. MARK
620
By proper substitution from (l),(2) and (4) into equation (3) we obtain The function f(2) is known from titration data obtained in the absence of chelating ions, and 2 can be evaluated from ( 5 ) , leading to (X) and (A') by the relations (4) and (2). The chelate formation constant
can now be calculated,.since the free metal concentration (M++) is the difference between the stoichiometric metal concentration and the chelate concentration (X). Tables I and I1 give the titration data obtained with copolymers VEE/MA and ST/MA in the presence and absence of alkaline earth cations. Figure 1 gives plots of Ka as a function of 2 for the two copolymers. It can be seen that the apparent dissociation constant of the styrene copolymer falls off continuously with increasing charge on the polyion as would be expected. The apparent initial increase in the Ka values of the vinyl ether copolymer with increasing degree of neutralization is undoubtedly due to the error introduced in neglecting the presence of un-ionized carboxyl pairs at lorn values of aZ,and the Ka values obtained in this region were not used in subsequent calculations. It has been pointed out previously12 that the differences in the titration behavior of these two polyelectrolytes are due to the tighter coiling of the styrene copolymer.
I 1.0
I
I
1.2
1.4 1.6 1.8 Z. Fig. 1.-Apparent dissociation constants of VEE/lMA and ST/MA as a function of copolymer charge density.
The results of the calculations of chelate formation constants are represented on Figs. 2 and 3.
Vol. 58
TABLE I EFFECT OF ALKALINE EARTHIONS ON TF~E TITRATION OF COPOLYMER VEE/MA IN 1 N POTASSIUM NITRATEAT 25' CE++ B a + + Sr++ 0.0036 0.0018 0.0036 0.0018 0.0036 0.0036 M M M M M M
6.21 6.48 6.70 6.93 7.14 7.36 7.60 7.87 8.18 8.57
0.19 0.08 0.17 0.06 .25 .ll .21 .10 .29 .15 .25 .ll 0.10 0.09 .14 .39 .15 .19 -29 .16 .45 .23 .34 .19 .18 * 19 .23 .51 .28 .25 .39 .23 -30 .29 .58 .31 .45 .27 .36 .37 .32 .65 .35 .52 -44 .45 .74 .40 ..59 .35 .72 .40 .66 .51 .50 .36
PH
ai
0.042 .128 .215 .302 .389 ,476 .562 .649 .736 .823
c:??
Blaqk titration
TABLE I1 EFFECTOF 0.0036 ;1.I ALKALINE EARTHIONS ON THE TITRATION OF COPOLYMER ST/MA IN 1 N POTASSIUM NITRATE AT 25' a2
Blank titration PH
0.083 .167 .260 .333 .417 .500 .583 .667 750 833
6.76 7.17 7.55 7.93 8.27 8.64 8.98 9.30 9.62 9.88
Ca++
Mg+TApHBa++
Sr++
0.18 .22 31
0 09 .12 .18 .29 -37 .51 .61 .70 .76
0.06
.44
.53 .70 .86
.94 .98 .96
.68
0.07 09 .16 19 .29 .34 .41 .48
. .
.44
.os
.I3 .24 .34 .42 .50 .59 .56
They may be interpreted as due t o two opposing factors: the electrostatic free energy of chelate dissociation would tend to produce Kr values increasing with the charge of the polyion. A superimposed saturation effect hinders the production of chelates with high densities of alkaline earth ions bound to the chain. With the less expanded styrene copolymer the electrostatic factor predominates, while the reverse is true of the vinyl ether copolymer. The slight increase in the Kf of magnesium and VEE/MA at any given 2,when the metal concentration was reduced by one-half, is in qualitative agreement with this interpretation. It should also be pointed out that solvation effects, which are difficult to evaluate, were not taken into account in assuming that the chelate formation constant depends only on the charge density of the polymer. Table 111gives a comparison between the values of K t of alkaline earth complexes with the polyelectrolytes used in the present study (at an arbitrarily chosen value of Z = 1.3) and the analogous complexes with succinic acid." At this charge the chelate formation constant of VEE/MA is between TABLE I11 COMPARISON OF CHELATE FORMATION CONSTANTS OF ALKALINE EARTHS WITH VEE/MA, AND SUCCINIC ACID . . ST/MA . log Kf Ca++
VEE/MA(Z = 1.3) 2.45 ST/MA(Z 1.3) 2.11 Succinic acid 1.16 i
Mg++
2.30 1.74 1.02
Ba++
Sr++
2.00 1.36 .0.97
1.96 1.46 0.75
TITRATIOS OF POLYELECTROLYTES AT HIGHER IONIC STRENGTHS
Aug., 1954
62 1
-
2.2
t I
1.2
I
I
1.2
I
1.4 2.
I
I
1~
1.6
Fig, 2.-Chelate formation constant of copolymer ST/MA with 0.0036 Af alkaline earths: @, CaN03; 0, SrNOs; 9, MgNOs; 0, BaN03.
ten and twenty times as high as the corresponding value for succinic acid, while somewhat lower values of Kf were obtained for copolymer ST/MA. Experimental Preparation of Polymers.-Forty grams of maleic anhydride and 20 g. of the comonomer were dissolved in 400 ml. of butanone and 100 mg. of azo-bis-isobutyronitrile initiat;r was added. The polymerization was carried out at 70 , the copolymer was precipitated in hexane, dissolved in hot water, dialyzed and freeze dried. The vinyl ethyl ether copolymer (VEE/MA) contained 62 mole % of vinyl ethyl
1.6 2. Fig. 3.-Chelate formation constants of copolymer VEEIMA: @, 0.0018 Jl CaNOa; 0, 0.0036 M BaNOs' 0 , 0.0018 M MgN03; 0 , 0.0036 N SrNOa; e, 0.0036 MgNOa.
1.4
id
ether, the styrene copolymer (ST/lMA) 54 mole % of styrene as calculated from titration data. The ma1Fic anhydride copolymers were fully hydrolyzed, since boiling with alkali before dialysis did not alter their titration behavior. Titration .-The pH determinations were carried out with a Cambridge Instrument Co. research model pH meter with external shielded electrodes. During titration, solutions were held a t 25' to within 0.lo, stirred and protected by a stream of nitrogen from atmos heric COZ. Four minutes were allowed for equilibrium after each addition of base; longer time intervals produced no further pH change. All titrations were carried out in the presence of 1 N potassium nitrate.
Acknowledgment.-This study was supported by a research grant from the Monsanto Chemical Company.
TITRATION OF POLYELECTROLYTES A T HIGHER IONIC STRENGTHS BY R. A. MARCUS Polytechnic Institute of Brooklyn, Brooklyn 1, New York Received April 19, 1954
The titration behavior of polyelectrolytes at higher ionic strengths is treated on the basis of the nearest neighbor interaction between the fixed ions. This point of view suggests a quantitative comparison of the behavior of polymeric and dibasic acids. The application of these considerations to existing work on the correlation of viscosity and titration curves of polymeric acids is briefly discussed.
Introduction.-The correlation of various properties of polyelectrolytes, such as viscosity and titration behavior, has been the subject of a number of recent theoretical treatments.' These approaches have had varying degrees of success and while differing in a number of important respects, have in common the assumption that both the fixed and mobile ions in the polyelectrolyte system may be treated as a continuous charge distribution. The purpose of the present note is to examine a Katohalsky, 0. Kunale and W. Kuhn, J . Polymer Sci., (I, 283 (1950); J. Hermans and J. T. G. Overbeek, Rec. trou. chim., 67, 761 (1948); G. E. Kirnball, M. Cutler and H. Sanielson, THISJOURNAL, 66. 57 (1948). ( 1 ) A.
different approach to the titration behavior of these systems, one which does not involve the latter assumption but instead assumes that the important interactions between the fixed ions on the polyelectrolyte are nearest neighbor interactions. It is expected that the repulsions between the immobile ions will vanish rapidly with distance, R , a t the higher ionic strengths, in the first approximation as e-KR/Rwhere K is the reciprocal Debye length. On this basis nearest neighbor interaction will begin to predominate when KR 1 where R is the diftance between nearest neighbors. When R 5 A. this will occur when the salt concentration exceeds 0.4 M.
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