J . Chem. SOC., Faraday Trans. I, 1982, 78, 29-36 Hydration and Ion-exchange Process in Carboxylic Membranes Part 1 .-Infrared Spectroscopic Investigation of the Acid Membranes BY LEON Y. LEVY, ANDRE JENARD AND HENRI D. HURWITZ* Laboratoire de Thermodynamique Electrochimique, Facult6 des Sciences, C.P. 160, Universitk Libre de Bruxelles, 50, av. F. D. Roosevelt, 1050 Brussels, Belgium Received 5th September, 1980 An infrared investigation of Teflon FEP and PTFE membranes grafted with poly(acry1ic acid) is performed. The changes in the principal absorption bands are examined as a function of the degree of humidity, the density of grafting and the nature of the polymeric matrix of the acid membrane. The prominent role played by the dimerisation of carboxylic groups which gives rise to a network of intermolecular hydrogen bonds is assessed.The specific properties of polymeric membranes containing ionogenic sites are to a large extent determined by ionic and polar interactions. The formation and function of the ionic hydration structures in the membrane also play a prominent role. The influence exerted on the conformational characteristics of these structures by the hydrophobic macromolecular membrane matrix has been assessed in previous investi- gations dealing with the infrared spectroscopy of sulphonic ion-exchange mem- branes.' Analysis of the solvent sorption and desorption pattern in sulphonic systems has led to the detection of an appreciable rearrangement of the solvent network after a certain degree of swelling has been attained.2 With carboxy-containing ion exchangers, the degree of s w e m is much smaller than with sulphonic ion exchangers, and it is well known that this swelling is fundamentally not controlled by ionic hydrati~n.~? This property of carboxylic resins and membranes is now exploited for the production of new types of membranes used in electrochemical cells.In addition, it is inferred from the nature of the fixed carboxy groups and from the influence of the pH on the properties of the membrane that the study of this material is appropriate for the physico-chemical understanding and modelling of biological membranes. The present series of investigation is aimed at elucidating the hydration and the ionic interactions in carboxy-containing ion exchange membranes by means of infrared spectroscopy performed on thin films of poly(acry1ic acid) grafted on different types of perfluorated matrices.This publication deals more specifically with the properties of these membranes in their acid forms as a function of their exchange capacity and water content. EXPERIMENTAL PREPARATION OF SAMPLES The characteristics of the membranes (supplied by Progil, France) are given in table 1. All i.r. spectroscopic measurements were carried out on samples of 32 mm radius with a Beckman 2930 I.R. INVESTIGATION OF ACID MEMBRANES TABLE 1 .-MEMBRANE CHARACTERISTIC thickness capacity membrane matrix h m /meq g-' nH* 0 /es-' 11A FEP 17 12A FEP 17 13A FEP 17 14 PTFE 9 ~ ~ ~ _ _ _ _ _ _ _ _ _ _ _ _ _ 0.97 2.23 1.25 2.92 0.64 1.93 0.69 2.62 I.R. 9 double-beam spectrophotometer. The successive experimental steps are as follows: (1) The membranes are placed successively in three fresh solutions of 1 mol dm-3 HC1, for four hours in each solution.After being removed from the solution, the membranes are washed with distilled water and carefully blotted with filter paper. (2) The membranes are fixed on a membrane holder and left to reach equilibrium with the laboratory atmosphere for one hour. (3) A first spectrum of the membranes is recorded. (4) The membrane remains exposed to the i.r. light in the sampling cell through which a flow of dry air (less than 4 ppm H,O at 25 "C) is passed. A spectrum of the membranes is recorded at regular periods during this drying process. DETERMINATION OF WATER CONTENT I N THE MEMBRANE Membranes at hydration equilibrium with the laboratory atmosphere at 25 OC [as used in step (3) of the experimental procedure] were titrated by the Karl-Fischer method.An automatic KF4 Beckman apparatus was modified slightly in order to satisfy the anhydrous conditions required for the titration agent and the titration cell during the experiment. Unfortunately, the very strong absorption of the C=O stretchingmode vibration at 1700 cm-l prohibits the use of the integrated absorbance of the water bending mode of vibration for determining the water content in the membrane, as can be done with the sulphonic films.5 DETERMINATION OF EXCHANGE CAPACITIES The exchange capacity of the samples prepared for the i.r. spectroscopic investigation was obtained through a coulometric multi-step microtitration carried out following the method described by Levy.* A Pt electrode of 1 cm2 area was used as the hydroxyl ion source.The cell assembly was composed of a combined Ingold microelectrode (type HA405 MJNS) and a Pt counter-electrode in a separate compartment connected to the main compartment by means of a saturated K,SO, agar-agar bridge. A potentiostat (Tacussel PRT 3001) was used as a galvanostatic source providing current intensities of 250 pA with a precision better than 1 pA. An automatic set-up was designed to fix the pH in the well-stirred main compartment at a value of approximately 7. The current injection is stopped whenever pH 8 is reached and starts again whenever the pH falls beneath 6. The coulometry yields the amount of protons released by the membrane sample.RESULTS ASSIGNMENT OF THE INFRARED ABSORPTION BANDS The i.r. spectra of the membrane (1 1 A) in its acid form and salt form are shown in fig. 1. It is well known that five characteristic frequencies 2500-2700, 1700, 1400, 1200-1 300 and 900 cm-l can be ascribed to the functional group of carboxylic acids.' The same absorption bands are observed with our membranes in their acid forms. The most intense absorption band at ca. 1720 cm-l corresponds to the stretching vibrations of the carbonyl group vc=o in the cyclic dimeric or polymeric form ofL. Y. LEVY, A. JENARD AND H. D. HURWITZ 31 4 000 3000 200 0 1600 1200 800 wavenumber/cm-' I I 4000 3000 2000 1600 1200 800 wavenumber/cm-' FIG. 1.-(A) Infrared spectra of an FEP-PAA membrane (1 1A) after various times of drying.( 1 ) Initial hydrated state; (2) 2 h; (3) 24 h. (a) Shoulder at 3400 cm-I; (b) shoulder at 3600 cm-I. (B) Infrared spectra of Na+ salt of a FEP-PAA membrane (11A) after various times of drying. ( 1 ) Initial hydrated state; (2) 3 h; (3) 20 h; (4) 56 h. undissociated acid molecules. The formation of hydrogen bonds with the carbonyl group shifts the vc=o vibration by ca. 15-45 cm-l towards lower wave number^.^^ lo The broadness of this band may be attributed to the superposition of individual peaks due to the heterogeneity of the local molecular configuration around the carbonyl group. This is supported by the fact that the variation in the mutual distance between carbonyl groups affects the vc=o stretching vibration.For instance, oxalic acid absorbs very strongly in the region extending from 1720 to 1690 cm-l, while the spectrum of malonic acid shows two bands at 1710 and 1740 cm-l. With salts of the carboxylic acid, the difference between stretching vibrations with single-bond character at 1320-1210 cm-l (masked in our system by the Teflon absorption band) and with double-bond character at 1720 cm-1 disappears owing to the mesomeric effect and two new absorption bands are found which are then attributed to the carboxylate group.' The absorption band at 1410 cm-l is assigned to the symmetrical stretching vibration of the ionised group COO- whereas the absorption band at 1575 cm-l is assigned to the asymmetrical stretching vibration of COO-. However, the band at 1410 cm-l is very complex and is due also to the bending motion of the >CH, group in the a position of the carboxy group.Concerning the assignment of the broad absorption bands of low intensity near 2650 and 1970 cm-l, the available investigations32 I.R. INVESTIGATION OF ACID MEMBRANES systems (X representing various elements including carbon) indicate that on -X these bands are connected.lo-l3 They correspond to the A and B bands, respectively, in the nomenclature proposed by Sheppard14 and are observed in the case of dimer formation through hydrogen bonding. Their interpretation raises various hypotheses. Band A might belong to the combination vOH with overtone 2doH and to do, + yOH. Band B occurs through Fermi resonance of vOH with 2yOH. Hadzi and Kobilarov12 suggest that the participation of vOH is more important in band A than in band B, which causes a shift of band A towards smaller wavenumbers and a decrease of intensity with increasing strength of the hydrogen bond. Another band at 1280 cm-l can also be ascribed to the dimer formation by carboxylic groups but is masked in our case by the large absorption band at 1200 cm-l due to the CF, groups of the Teflon matrix.As for the very faint peak at 940 cm-l, it is attributed to the out-of-plane OH group vibration in the acid dimer.g A band at 850 cm-l is found only in the salt form of the membrane and corresponds to the bending deformation of the COO- group or the rocking deformation of the CH, group.15 Finally, the broad absorption band between 3000 and 3600 cm-l is due to the stretching vibration of OH groups belonging either to the water of hydration or to the carboxylic acid.Several bands have yet to be assigned. Two bands at 2940 and 2850 cm-l are ascribed to the asymmetric and symmetric stretching vibrations, respectively, of the CH, groups.' A band at 1454 cm-l arises from the combination of the scissoring vibration of CH, and of the coupling of the C-0 stretching vibration with the OH in-plane bending vibrati0n.l' The band at 1330 cm-1 corresponds to the wagging motion (out-of-plane bending) of the CH, group.' Bands at 2380, 1280, 1160, 980, 775, 750, 720, 628, 555 and 519 cm-l are associated with different motions of the CF, or CF, vibration groups as described in ref. (1 6) and (17). 4 0 'OH DISCUSSION Comparison of the i.r.spectra displayed by carboxylic membranes with those displayed by sulphonic membranes shows a great difference in the pattern of the OH band. Unlike the carboxylic acid system, in the sulphonic acid system the bands are not resolved in the high-frequency region, which thus shows a continuum of absorption. This effect was interpreted by Zundel and Weidemann.18 For these authors, the continuous absorption, characteristic of the strong acid spectrum, appears whenever a hydrogen ion solvated structure like H,Oi or HgOi is formed. The formation of a H,Oi grouping implies necessarily that the acid is completely dissociated. This happens already, as shown by Zundel, for two water molecules per site in sulphonic acid membrane^.^^ From the previous consideration, our results lead us to conclude that in the carboxylic acid membranes the acidic groups of the vast majority of carboxylic acids are undissociated.As illustrated in fig. 1 (a), the gradual water uptake of the membrane causes only a slight enhancement of the absorbance in the range 3300-2800 cm-l. This weak increment probably results mainly from the swelling of the film. It is remarkable, however, that the hydration process gives rise, from the outset, to a shoulder at ca. 3600 cm-l and, at a larger water content (corresponding to the gain of approximately two water molecules per site), to a second shoulder at ca. 3400 cm-l. According to our previous investigations, the shoulders at 3600 and 3400 cm-l can be attributed to the stretching vibration mode of, respectively, the free OH groups and OH groups involved in hydrogen bonds linking together water molecules.l9 ' As regards theL.Y. LEVY, A. JENARD AND H. D. HURWITZ TABLE 2.-PRINCIPAL SPECTRAL PROPERTIES OF MEMBRANES 33 shoulder at capacity time of 3400 3600 band A band B vc=o membrane /meq g-l drying/h v,,/cm-l cm-l cm-l /ern-' /cm-l /cm-l 12 A 1.28 1 0 3102 yes yes 2672 weak 1720 2 2 3100 yes yes 2665 weak 1711 3 24 3104 weak yes 2662 1978 1706 4 48 3106 no no 2660 1980 1702 11 A 0.98 1 0 3142 no yes 2690 weak 2 3 3145 no yes 2684 weak 3 24 3146 no no 2681 1972 4 52 3143 no no 2680 1972 13 A 0.6 1 0 3173 no yes 2700 weak 2 2 3169 no no 2696 weak 3 24 3171 no no 2696 weak 716 709 707 706 715 710 708 4 48 3169 no no 2694 weak 1708 14 0.7 1 0 3127 yes yes 2680 weak 1716 2 4 3129 yes no 2679 weak 1709 3 24 3129 weak no 2675 weak 1708 4 48 3132 no no 2675 weak 1708 01 1800 1700 1( wavenumberlcm -' FIG.2,-Infrared spectra of the vc=o band of a FEP-PAA membrane (1 1 A) after various times of drying. (-) Initial hydrated state; (---) 2 h; (. * . 3) 24 h.34 I.R. INVESTIGATION OF ACID MEMBRANES position of the absorption band at ca. 3150 cm-l, it can thus be assigned almost exclusively to the hydroxy groups vibration vOH pertaining to the carboxylic acid. The lowering of the stretching vibration frequencies vOH and vc=o are an indication of the strength of the hydrogen-bond interaction. The data recorded in table 2 show that the humidity of the membrane does not affect the wavenumber of the absorption maximum of vOH.This makes it clear that all OH groups of the carboxylic acid are involved in hydrogen bonding, whatever the degree of humidity. As observed in table 2, in contrast to the behaviour of the vOH absorption band, the vcz0 band shifts abruptly towards smaller wavenumbers from the beginning of the dehydration process (fig. 2). Such a change reveals a strengthening of the hydrogen bonds acting on the >C=O oscillator. With regard to this shift of vco, the following is already known from extensive experimental and theoretical investigations :lo, 20-22 the v c=o vibration mode of a carboxylic group is more strongly affected by hydrogen bonding with an OH group pertaining to another carboxylic group than to water. For instance, in acrylic acid the absorption band of free C=O is found at 1725 cm-l, that of C=O involved in direct dimerisation of carboxylic groups at 1660 cm-l and that of C=O forming a dimeric structure mediated through hydrogen bonding with one water molecule per carboxylic group at ca.1690 cm-l (see, for example, the structure appearing on the right of case I in fig. 2).209 21 In the Raman spectrum of a perfectly dry poly(acry1ic acid), Simon et aI.l0 observe a broadening of the band at 1660 cm-l and a shift of the band at 1725 cm-l towards 1740 cm-l. In an aqueous solution of 80 % polyacrylic acid, the bands at 1660 and 1690 cm-l transform into a broad band extending between 1670 and 1740 cm-l.l0 In the i.r. spectrum of an aqueous solution of poly(acry1ic acid), this band is found at ca. 1723 cm-l.l0 On the grounds of these results, we believe that the low values of vcz0 frequencies ranging from 1702 to 1708 cm-l, as displayed by the membranes at their lowest degree of humidity, suggest the formation of cyclic acid dimers without the involvement of water molecules.The low values of frequencies might be explained on the grounds that the hydrophobic matrix restrains the movement of the carboxy groups and thereby gives rise to stronger contacts at the intermolecular level. The sharpening displayed by bands A and B and the band at 940 cm-l as a function of drying provides further evidence for the existence of a carboxylic acid dimer configuration. The consideration developed above allows us to propose fig. 3 as a model of the distribution of intermolecular bonds.Structure (111) illustrates the presence of an extensive network of interchain contacts via hydrogen bonds. The passage from structure (111) to structure (11) is consistent with the observation of a sudden increase of the absorption shoulder at 3600 cm-l and the fact that the position of the vOH band remains nearly unaffected by the degree of humidity. The transition from structure (11) to structure (I) demonstrates the tendency of the carboxylic dimers to open and accept hydrogen-bonded water molecules. This model agrees with the appearance of the shoulder at 3400 cm-l, with the weakening of bands A and B and the band at 940 cm-l and with the increase of the vce0 band frequencies. The latter increase is also found in the above-mentioned Raman spectroscopic observation on acrylic and polymeric acids.It is also inferred from the evolution of bands A and B and the vco and vOH absorption bands recorded in table 2 that the hydrogen bonds linking together the carboxylic groups in the dimer configuration are getting stronger as the exchange capacity of the membrane increases. The reason for this tendency is found in the fact that the dipolar interaction between polarizable bonds is more pronounced in the case of a large graft density, which causes a reinforcement of the hydrogen bonds. The aggregation of carboxylic groups in networks of very strong interchain contact and cyclic dimerisation is thus promoted. The membranes 13A and 14 both possess anFIG. 3.-Model L. Y. LEVY, A. JENARD AND H. D. HURWITZ 0 H, I of distribution in intermolecular f--", H ,O ....H- O..-.- H-0, 0-H ..... 0 - H .... 0 ,R -R-C, *C - R .c\o H' ;I bonds as a function of water content of 35 membrane. equivalent capacity but differ in their Teflon matrix. The results of table 2 suggest that the existence of a lateral -CF, group in the FEP matrix exerts a steric hindrance which significantly weakens the hydrogen bonds linking two carboxylic groups. With respect to this observation, it shows that, whatever the nature of the matrix or the graft density, it is essentially vOH, and thus the hydrogen-bond donor property of the carboxylic group, which is affected. CONCLUSIONS The information resulting from the i.r. spectroscopic investigation has lead to an understanding of some of the properties of the carboxy-containing ion exchangers.The building-up of a network of hydrogen bonds crosslinking the polymeric chains affords some flexibility of the PAA polymers between their graft sites on the Teflon matrix. Moreover, the carboxylic groups must possess some rotational freedom in order to orient themselves favourably for hydrogen bonding. The dimer association of the acid groups c o n k s to the system a rigidity prohibiting, for example, the out-of-plane wagging vibration mode of the CH, groups which absorb at36 I.R. INVESTIGATION OF ACID MEMBRANES ca. 1330 cm-l. The presence of the extensive lattice of hydrogen bonds hinders the diffusion of water in the membrane and the hydration of the membrane occurs in such a way as to affect the intermolecular association of the polymeric chains to only a very slight degree.This restraint explains the small water uptake of the membrane. Since the carboxylic resins show an equivalent small degree of swelling, one might suppose that hydrogen bonding of the functional groups is an essential characteristic of solid carboxy-containing It is thus understandable that any ionic exchange can happen only in as much as it destroys this network of hydrogen bonds. Consequently, the selectivity of the ion exchangers towards the various counter-ions is usually weak compared with their affinity for the hydrogen ion. This point will be considered in a forthcoming publication. We thank the ‘Fonds National de la Recherche Fondamentale Collective’ and the ‘Fonds National de la Recherche Scientifique’ of Belgium for having supported this work.L. Y. Levy, A. Jenard and H. D. Hurwitz, J. Chem. Soc., Faraday Trans. I , 1980, 76, 2558. L. Y. Levy, M. Muzzi and H. D. Hurwitz, J. Chem. Soc., Faraday Trans. 1, 1982, 78, 17. V. I. Soldatov and L. V. Novitskaya, Russ. J. Phys. Chem., 1965, 39, 1453. H. P. Gregor, L. B. Luttinger and E. M. 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