J. Chem. SOC., Faraday Trans. I, 1982,78, 1001-1009 Hydration and Ion-exchange Processes in Carboxylic Membranes Part 2.-Infrared Spectroscopic Investigation of Alkali-metal Salt Membranes B Y LEON LEVY, MARC MUZZI AND HENRI D. HURWITZ* Laboratoire de Thermodynamique Electrochimique, Facultk des Sciences, C.P. 160, Universitk Libre de Bruxelles, 50, avenue F.D. Roosevelt, 1050 Brussels, Belgium Received 14th January, 198 1 An infrared investigation has been performed on totally neutralized membranes of poly(acry1ic acid) grafted on a Teflon FEP or PTFE matrix. The changes in the principal absorption bands characterizing the vibration modes of the carboxylate group and water in Li+, Na+ and K+ salt membranes are analysed as a function of the degree of humidity and of the exchange capacity.The strength of the hydrogen bonding between water and the carboxylate ion and the nature of the ionic,interactions in the membrane are discussed. Carboxy-containing polymeric ion exchangers have recently come under close scrutiny. The conformational characteristics of these systems are to a large extent determined by hydrogen bonds and strong ionic interactions. The assessment of these effects and the determination of their role will be pursued in this series of investigations using carboxy-containing polymeric grafted membranes and ionomers based on them. In a previous publication' we used infrared spectroscopy in order to investigate the acid form of thin membranes made of poly(acry1ic acid) grafted onto a perfluorated (FEP or PTFE) matrix.In the present publication, attention is focused on the totally neutralized form of these membranes. The neutralization of the carboxy-containing polymers by metal ions leads to the appearance of a series of specific properties. We have previously discussed the prominent role of the intensive network of interchain contacts via hydrogen bonding and of the dimerization of carboxy groups in the acid membrane.' This configuration confers a rigidity to the polyelectrolyte structure which explains the relatively small degree of swelling of the membrane and its stability towards the total ionic exchange of hydrogen ions by metal ions. Certain observations2~3 have shown that the neutralization process of grafted carboxylic acid membranes is limited when the membrane is equilibrated with alkali-metal or alkaline-earth halide solutions.The degree of dissociation of the carboxy groups has been estimated to be ca. 15% with the alkali-metal ions and to reach 30% in the presence of alkaline-earth ions. It is also well known that the ion-exchange process is rather slow and occurs essentially at high pH. Moreover, the presence of ionic contacts and ionic cross-linking between the ionomer macrochains leads to a change in the kinetics and thermodynamic parameters of the absorption of the ~olvent.~ The transition temperature in ionomers is also to a large extent determined by their content of acid groups and their degree of ne~tralization.~ Infrared spectroscopic data for ionomers suggest the formation of a whole set of ionic associated species of different structures.s~ The analysis of available data shows that the nature of the cation might affect the transition temperature.For I0011002 I.R. INVESTIGATION OF ALKALI-METAL SALT MEMBRANES the same degree of neutralization, the transition temperature in Na+ ionomers indicates stronger ionic cross-links than in Rb+ ionome~s.~ At large degrees of neutralization, ionic cluster frameworks and supermolecular configurations might significantly stabilize the ionomer structure.8, It is thus expected that the infrared spectroscopic investigation presented here will provide information on the properties of PAA grafted membranes caused by the interplay of hydrogen bonding and ionic interactions. This work deals with membranes totally neutralized by alkali-metal ions.Such neutralization is achieved only with Li+, Na+ and K+ halides. EXPERIMENTAL PREPARATION OF SAMPLES To convert the membranes into their salt form, they are placed in a 0.1 mol dm-3 alkali-metal carbonate and 1 mol dm-3 alkali-metal chloride solution for at least seven periods of 24 h. After this first operation, the same successive experimental steps are used as described for the membranes in their acid form.' The characteristics of the membranes (supplied by Progil, France) are as follows: membranes 1 1 A, 12A and 13A have a FEP matrix and a thickness of 17 pm; membrane 14 has a PTFE matrix and a thickness of 9 pm. The respective capacities expressed in meq g-l are 0.97 (llA), 1.25 (12A), 0.64 (13A) and 0.69 (14). RESULTS All i.r. spectroscopic measurements are carried out on samples of 32 mm radius with a Beckman I.R.9 double-beam spectrophotometer.Table 1 shows the main results obtained with totally neutralized carboxylic membranes. The spectra are displayed in Analysis of the doublet situated at 1420-1450 cm-l shows that the intensity of the peak at 1420 cm-l depends neither on the amount of water nor on the ionic species (the transmission is ca. 7%) while the intensity of the peak at 1450 cm-l is higher the larger the ratio of the ionic charge Ze to the ionic radius a, Ze/a. Furthermore, the intensity of this peak decreases during the drying process of the membrane, with the exception of Li+ salts where it remains constant. In the case of neutralized membranes, the absorption bands A and B at, respectively, 2650 and 1970 cm-l disappear. Note that these bands are associated with dimer formation through strong hydrogen bonding of the carboxylic groups.' The peak situated at 860 cm-l is not observed for a degree of neutralization below 70%.Some authorslo have assigned this band to the bending vibration of the carboxylate group COO-, although it is generally ascribed to a splitting of the rocking vibration of the CH, group found at 790 cm-l.ll Dehydration of the membrane does not change the intensity of this band. Another peak appears at ca. 1330 cm-l in the spectra of membranes in their salt form and is not observed in the acid membrane. It is assigned1' to the wagging vibration mode of the CH, group and is more intense as the electric field (thus the ratio Ze/a) at the surface of the ion is increased. This band is situated at 1330 cm-l for Li+, 1324 cm-l for Na+ and 1320 cm-l for K+.Its intensity decreases with drying of the membrane and it transforms into a faint peak for Li+ salts, a strong shoulder on the CF, band for Na+ and a weak shoulder for K+ . It is known that the bending vibration do, of water in the membrane yields a band near However, in the case of the systems considered here, this absorption peak cannot be isolated. Quite probably the very strong absorption band fig. 1-3.TABLE 1 .-PRINCIPAL SPECTRAL CHARACTERISTICS OF VARIOUS ALKALI-METAL SALT MEMBRANFS vs coo- vAS COO- dCHo + YOH (4 duration of drying /h 12A 11A 13A 14 12A 11A 13A 14 12A 1 l A 13A 14 Li+ 1 0 1424 1420 2 3 1424 1420 3 18 1424 1420 4 80 1424 1420 Na+ 1 0 1415 1410 2 3 1415 1410 3 20 1415 1410 405 1421 1577 1575 1553 1574 1450 1459 1435 1450 405 1421 1578 1578 1555 1576 1450 1459 1435 1451 405 1421 1577 1578 1555 1576 1451 1458 1435 1450 405 1421 1577 1578 1555 1575 1451 1457 1435 1450 399 1412 1575 1570 1551 1570 1461 1457 1445 1458 399 1412 1575 1573 1553 1571 1461 1457 1445 1458 399 1412 1575 1573 1554 1571 1461 1457 1446 1458 4 80 1415 1410 1399 1412 1575 1573 1553 1571 1461 !457 1446 1458 K+ 1 0 1410 1405 1392 1408 1579 1575 1558 1572 1460 1452 1440 1457 2 3 1410 1405 1392 1408 1584 1578 1558 1572 1460 1452 1440 1457 3 24 1410 1405 1392 1408 1586 1578 1558 1572 1460 1452 1440 1457 4 80 1410 1405 1392 1408 1586 1578 1558 1572 1460 1452 1440 1457 (b) vAscoo- of complex duration dCOO- Or PCHl or do, of water '0 H of drying /h 12A 11A 13A 14 12A 11A 13A 14 12A 1 l A 13A 14 ~~~~ ~ Li+ 1 2 3 4 Na+ 1 2 3 4 K+ 1 2 3 4 0 3 18 80 0 3 20 80 0 3 24 80 860 862 864 864 856 857 858 858 852 848 84 1 840 858 859 860 86 1 854 856 857 857 848 846 842 84 1 857 858 859 860 854 855 855 856 845 843 842 840 857 1660e 858 859 860 853 165Oe 855 166Oe 856 167Oe 857 1669e 849 1654e 847 1662 843 1668 842 1670 - - - 1660e 16% 1662e 1670e 167Oe 165Oe 1660e 1668 1669 1650e 1650e 3381 3368 3357 3380 I - 3365 3346 3340 3354 - - 3330 3327 3327 3333 - - 3313 3309 3311 3328 16% 1650e 3375 3362 3351 3370 - - 3341 3330 3330 3341 - - 3326 3300 3312 3328 - - 3306 3289 3293 3318 165Oe 165Oe 3316 3320 3315 3314 1655e - 3305 3307 3308 3305 - - 3299 3300 3300 3301 - - 3290 3295 3294 3296 CI 0 0 w1004 I.R.INVESTIGATION OF ALKALI-METAL SALT MEMBRANES I 4000 3600 3200 2800 2LOO 2000 1000 1600 l L O O 1200 1000 800 600 waven umber/ cm - I FIG. 1.-1.r. spectra of Li+ salt of a FEP PAA membrane (1 1A) after various times of drying. (-) Initial hydrated state; (---) 3 h ; ( . . . ) 80 h. LO00 3600 3200 2800 2LOO 2000 1800 1600 1400 1200 1000 800 600 wavenumberf cm-' FIG. 2.-1.r. spectra of Na+ salt of a FEP PAA membrane (1 1A) after various times of drying. (-) Initial hydrated state; (- . - .) 3 h ; (---) 20 h; (. . .) 80 h. 4009 3600 3200 2800 2600 2000 1800 1600 :LO0 1200 1000 800 600 wavenumber/cm-' FIG. 3.--I.r. spectra of K+ salt of a FEP PAA membrane (1 1A) after various times of drying. (--) Initial hydrated state; (- . - .) 3 h; (---) 24 h ; (.. . ) 80 h.L. LEVY, M. B. M U Z Z I AND H. D. HURWITZ 1005 of the COO- group at 1575 cm-l overlaps with the a,, absorption band in the region around 1640 cm-l. The contribution of do, produces a strong asymmetry in hydrated membranes on the higher-frequency side of the vAs band of COO-. With decreasing water uptake in the membrane a surprising result is observed. In the case of Li+, the asymmetry (or intensity of the shoulder) is reduced. In the case of Na+, the asymmetry also first decreases and then the band is changed into an absorption step in the dryest membrane. In the case of K+, the asymmetry is first reduced, then at lower water content enhanced to the extent of taking the shape of a peak. Note that the vibration frequencies of the methylene group at 2940-2860 cm-l (stretching), 1330 cm-l (wag- ging) and 790-860 cm-l (rocking) depend on the nature of the counter-ion and the water content in the membrane.DISCUSSION The neutralization procedure, as performed in this investigation, leads to complete dissociation of the carboxylic groups. The total replacement of acid groups by salt groups is confirmed by the disappearance of the vco absorption band at ca. 1700 cm-l,ll corresponding to the stretching vibration of the carbonyl group in the undissociated carboxylic acid. Furthermore, the absence of the A and B absorption bands near, respectively, 2650 and 1970 cm-l and of the peak at 940 cm-l is noteworthy. It indicates that dimers with carboxylic groups cannot be formed in the membrane.l The band at ca.3300 cm-l can be ascribed to the OH stretching vibration vOH of water molecules linked through hydrogen bonds to the carboxylate ion. In the alkali-metal salt membranes, the vOH band intensity and wavenumber depend on the degree of hydration of the membrane. This contrasts with the behaviour observed for the absorption band vOH at ca. 3150 cm-l in the acid membrane,l where vOH was attributed to the OH vibration pertaining to the carboxylic acid. In all systems investigated here, the wavenumber of the maximum of the band at ca. 3300 cm-l decreases with increasing drying. From this shift it is inferred that the hydrogen bonds between the water and the carboxylate anions are strengthened as a function of the decreasing amount of absorbed water. Note that this vOH vibration at ca.3300 cm-l is found at much lower wavenumbers than the equivalent vOH vibration observed at 3400-3500 cm-l in the sulphonic- salt-containing membranes.l2* l3 Obviously, the carboxylate groups are more strongly hydrogen bonded to the water than the sulphonic groups. This can be regarded as a consequence of the basic character of the COO- ion which strongly interacts with the hydrogen ion or with small cations with a rare-gas electronic configuration. Such behaviour is exemplified in homogeneous solutions of alkali-metal acetate as the activity coefficients increase from Li+ to Cs+. Accordingly, Diamond14 has classified the acetate ion as among the water-structure-promoting ions and similarly Gurney15 has assumed that the acetate ion might be linked to a small cation by means of strongly polarized water molecules.From the frequenciesrecorded in table 1 (b)it can be suggested that the hydrogen-bond strength increases following the sequence Li+ < Na+ 6 K+. No significant effect is observed as a function of the ion-exchange capacity during the evolution of the drying process. In the wettest membranes, however, the vOH frequencies are shifted towards smaller values with decreasing exchange capacities in the presence of Li+ and Na+. In the cases of these two salts, one also observes some influence due to the matrix. The presence of the hydrophobic -CF3 group in the FEP membrane enhances the strength of the hydrogen bonds as compared with the PTFE matrix.1006 I.R. INVESTIGATION OF ALKALI-METAL SALT MEMBRANES It is assumed that the observed asymmetric stretching frequency vAS of the carboxylate ion is closely dependent on the interaction between a cation and the COO- group.Indeed, the bonding between a metallic ion and a ligand like -COT or -SO, lowers the symmetry of the latter and therefore enhances the complexity of the vibrational spectra.l6 Since the symmetry of the free carboxylate group is already low (C2,) and all its vibration modes are active in the infrared, the bonding of this group with a particle might have a marked effect on the values of the absorption frequencies. However, the formation of the bond will keep the number of absorption bands constant, contrary to the behaviour observed in the case of the sulphonate groups of C,, symmetry. The formation of a metal-oxygen bond in a structure of type I M - 0 \ O//C-R affords a redistribution of the electron densities between the CO bonds in the ionized carboxy group With increasing strength of the M-0 bond, the asymmetric COO- stretching frequency vAS will rise towards the frequency characteristic of the double-bonded C=O group, whereas the symmetrical COO- stretching frequency vs will fall towards the value characteristic of the single-bonded C-0.As a consequence, the difference Av, = vAS - vs yields a measure of the interaction of the counter-ion with the fixed carboxylate sites." Sawyer and Paulsen18 have suggested in an i.r, investigation of EDTA complexes that, for a difference Avl larger than 225 cm-l, the metal-oxygen bond should be predominantly covalent and that for a smaller difference the bond should be of prevailing ionic character.On the other hand, contrary to the properties of a structure of type I, a characteristic feature of the formation of a metal-oxygen bond in a structure of type I1 and of type I11 0 0 M/ \ C-R \ .;;;i M - 0 M - 0 *\ gc-R (11) (111) consists of the occurrence, in both cases, of two equivalent C-0 bonds which are similarly affected by the metal ion. Therefore, in many complexes of types I1 and 111, the replacement of one metal ion by another will lead to the shift of vAs and vs in the same direction. * From the values recorded in table 1 it is observed that vs and vAS behave quite differently as functions of the nature of the ion. Furthermore, the values of Avl given in table 2 are lower than 225 cm-l but increase as a function of the ionic radius.Therefore, it can be inferred that structure I is formed, the prevailing ionic character of the metal-ion-carboxylate bond being larger the smaller the counter-ion. No significant effect on Avl is observed as a function of the ion-exchange capacity and the nature of the matrix. * It has been established,'" for example, that the nickel, zinc and copper acetate salts form, respectively, structures of types I, 11 and 111. The v, wavenumbers of the bidentate copper acetate complex and zinc acetate complex are, respectively, 1604 and 1550 cm-', whereas for the monodentate nickel acetate complex vAS = 1530 cm-l.L. LEVY, M. B. MUZZI AND H. D. HURWITZ 1007 The absorption band found at ca.1450 cm-l has been assignedlg to two overlapping peaks corresponding first to the scissoring vibration SCHp of the methylene groups and secondly to the coupling of the C-0 stretching vibration with the out-of-plane bending mode yOH of the OH groups belonging to water molecules linked to COO- sites via hydrogen bonds. This last contribution is responsible for the fact that the intensity of this band is stronger the larger the water uptake by the membrane. TABLE 2.-DIFFERENCE BETWEEN THE VIBRATION FREQUENCIES OF THE CARBOXYLATE GROUP FOR DIFFERENT ALKALI-METAL IONS A v ~ = v A S - V S Av2 = ( ~ c , , + Y O H ) -vs 12A 11A 13A 14 12A 11A 13A 14 Li+ 153 158 150 154 27 28 30 29 Na+ 160 163 155 159 46 47 47 46 K+ 174 173 166 164 50 47 48 49 The intensity of the S,,2+yo, band increases following the sequence K+< Na+ < Li+ at any degree of swelling of the membrane, which leads us to believe that, even in the dryest specimen, the ionic pair COO-Lit is created with the Li+ ion at least partially hydrated.This assumption is in accordance with the ionic interaction model suggested by Gurney15 and Harned and Robinson.20 As regards the difference Av, = (dCH2 + yOH) - vs shown in table 2, it appears that its value depends neither on the exchange capacity nor on the type of matrix, but that it is strongly influenced by the type of counter-cation. At the present stage of our research, it is still difficult to elaborate more on the meaning of Av,. The step and peak of absorption, respectively, which take the place of the So, absorption band contribution at 1669 cm-l for the Na+ and K+ salt membranes at their lowest degree of swelling, raise some problems of interpretation.Concerning this particular morphology of the spectra, two hypotheses can be proposed, each of which tries to explain the absorption band pattern at low water content. First, the idea can be put forward that some specific hydration structure arises yielding a new contribution to SOH. Secondly, the occurrence of a splitting in the vAS stretching vibration can be suggested which ensures the presence of a new peak. If we take the first assumption for granted, we might suggest that conformational changes in the grafted polyelectrolyte occur during the drying process of membranes neutralized by large cations. In this respect it is significant that the pattern of the different ,CH, vibrations (stretching, bending, wagging, rocking, etc.) are influenced by the degree of hydration and the type of cation.The wagging vibration band at ca. 1320 cm-l is strongly attenuated in the presence of K+ and affected to a lesser extent in the presence of Na+ salts of the membrane at their lowest degree of swelling. The absence of this band in the acid membrane is indicative of the high rigidity of the PAA chains.l In view of these conformational changes, the residual water molecules should be compelled to organize into highly rigidly associated structures. Strong hydrogen bonding of water molecules in the environment of hydrocarbon chains is well known.,' Under such conditions the do, bending frequency is shifted towards higher values.This might determine the appearance of a step or a peak at ca. 1669 cm-l. A similar result for the So, of water has been observed with Na+, K+ and Cs+ salts of PSSA mernbranes.l3 Alternatively, it might be suggested that a framework of ionic clusters \1008 I.R. INVESTIGATION OF ALKALI-METAL SALT MEMBRANES appears which significantly stabilizes the ionomeric structure at low water content of the membrane. The concept of aggregation of counter-ions and carboxylate ions (clusters and multiplets) in carboxylate-containing ionomers constitutes the basis of the interpretation of a large number of experimental data in recent years.** 9 9 l3 The building of these clusters is prevented by a large amount of water and by strong ionic pairing of the fixed anions with small cations like Li+.The ionic clustering is strongest with Cs+ salts and K+ salts of the polyelectrolytes. The model of clustering predicts further the loss of individual ionic hydration shells, the remaining water molecules being accomodated within the interstices of the ionic network in the vicinity of the charged sites. There the joint action of the hydrophobic matrix and of the polar groups ensures strong hydrogen bonding. We proceed now with the second assumption. In this case, some reasonable argument must be found in order to account for the splitting of the vAs vibration mode of the carboxylate ion notwithstanding the low symmetry of this group. The C,, symmetry prevents the spectral splitting of this band unless there exist simultaneously two or more molecular structures sufficiently different so as to be characterized by different wavenumbers.It might be suggested that the band at 1669 cm-l corresponds to a molecular structure existing to a lesser degree than the -COO--metallic-ion pair which is related to the absorption band at ca. 1570 cm-l. In this respect, it becomes extremely significant that the difference Avl corresponding to this new configuration will be 254 cm-l. This means that a small proportion of the carboxylate ions forms bonds of type I, as considered above. The number of these bonds increases along the sequence Li+ < Na+ < K+. Note that Chuveleva et aZe2, have detected in the i.r. spectroscopic investigation of uranyl ion complexes with carboxylic resins a band at ca.1640 cm-l in addition to that located at 1540 cm-I. The two bands have been related to two different types of bonds between the carboxylate group and the uranyl ion. It has also been suggested by these authors that the appearance of a band at 860 cm-l (after 65% conversion of the resin to uranyl form) might be interpreted as \ a splitting of the rocking mode of the ,CH, group (at 790 cm-l) caused by complex ion formation. However, it is worth stressing the fact that this band at 860 cm-l ought to be assigned to the bending motion of the carboxylate group. This attribution is all the more pertinent in view of the presence of the band at 860 cm-l, independent of the existence of the band at 1669 cm-l, and on account of the strong influence of the nature of the counter-cation on this band.CONCLUSIONS The neutralization of the carboxylic groups in the membrane determines the formation of a set of different ionic associated species. At this stage of our investigation it is difficult to assess whether ionic clustering occurs in our membranes. Some complementary experiments with D,O have to be performed in order to ascribe precisely the peak appearing at 1669 cm-l either to the bending vibration mode of water or to the vAS of the carboxylate ion linked covalently to the metal ion. However, note the important role played by the water with respect to the ionic configuration. The same is true for the exchange capacity which enhances considerably the ability of the ions to interact by means of strong bonds. As regards the hydrogen-bond strength between the water and the carboxylate ion, it is observed that it is significantly increased in the presence of the K+ ion.L.LEVY, M. B. MUZZI AND H. D. HURWITZ 1009 I L. Y. Levy, A. Jenard and H. D. Hurwitz, J. Chem. SOC., Faraday Trans. I , 1982, 78, 17. E. Selegny, communication presented at the Advanced Study Institute on Charged and Reactive Polymers, Forges-les-Eaux, 1973. L. Y. Levy, Ph.D. Thesis (Universite Libre de Bruxelles, 1979). L. M. Kalyuzhnaya, A. N. Krasovskii,Yu. N. Panov, A. G. Zam, I. S. Lishanksiiand S. Ya. Frenkel, Vysokomol. Soedin., Ser. A, 1975, 17, 993. S. R. Rafikov, Yu. B. Manakov, I. A. Ionova, G. P. Gladyshev, A. A. Andrusenko, 0. A. Ponomarev, A. I. Vorobeva, A. A. Berg, L. F. Antonova, E. I. Ablyakimov, M. F. Sisin and A. A. Smorodin, Vysokomol. Soedin., Ser. A, 1973, 15, 1974. Yu. N. 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