J . Chem. SOC., Faraday Trans. I , 1982, 78, 17-28 Infrared Investigation of Ionic Hydration in Ion-exchange Membranes Part 2.-Alkaline Earth Salts of Grafted Polystyrene Sulphonic Acid Membranes BY LEON Y. J,EVY, MARC Muzzr AND HENRI D. HURWITZ* Laboratoire de Thermodynamique Electrochimique, Faculte des Sciences, C.P. 160, Universite Libre de Bruxelles, 50, av. F. D. Roosevelt, 1050 Brussels, Belgium Receiued 5th September, 1980 Infrared spectroscopic studies have been performed on thin ion-exchange membranes which consist of alkaline earth salts of polystyrene sulphonic acid grafted on a Teflon FEP matrix. The membranes were placed at isopiestic equilibrium with water vapour in the sampling cell. The water absorption isotherm at 25 O C has been determined by measuring the integrated absorbance of the sorbed-water bending vibration.In the case of Mg2+, Ca2+ and Sr2+ salts the isotherms exhibit an absorption step with a hysteresis. The frequency of the symmetrical stretching band of the SO; group passes through a minimum, and the OH stretching band of water in the membrane passes through a maximum at the relative water vapour pressure corresponding to the step displayed by the absorption isotherm. The dependence of the bending vibration band of water is also analysed in terms of solvent structure in the membrane. A model of hydration of alkaline earth salts of an FEP-PSSA membrane which emerges from the spectral analysis is suggested. The ionic interaction and hydration occurring with alkaline salts of polystyrene sulphonic acid grafted on a fluoroethylene propylene matrix (FEP-PSSA) were investigated in a previous pub1ication.l An infrared spectroscopic method was devised for examining these membranes while they were held under isopiestic conditions for the control of water absorption.In conjunction with these measurements, the water content of the film was determined using Karl-Fischer titrimetry.2 The validity of the i.r. spectroscopy applied to thin polyelectrolytic films was emphasized in the pioneering work of Z ~ n d e l . ~ This work and a previous contribution1 suggest that a comprehensive study of water absorption based on spectroscopic arguments should give access to a molecular interpretation of some of the phenomenological properties of ion-exchange membranes, such as swelling and selectivity. The results obtained with Teflon films provide further indication of the significant role played by the hydrophobic matrix in the formation of specific hydration structures.This aspect of the research is of particular interest due to similar effects which might occur in hydrophobic regions of biological membranes. In the case of alkaline earth ions, the shifts of the absorption band maxima of the symmetric stretching vibration vsoL of the sulphonate group and of the stretch- ing vibration vOH of water suggest that with increasing water uptake by the membrane, the cations are peripherally hydrated.l* Some unexpected features were found from the shift of BOH, the maximum of the scissor vibration band of water. This shift reveals the complex interdependence of ionic interaction and water configuration in the membrane pores.In the case of K+ and Cs+, at small water activity, the bending force constant of water is larger than in ice; conversely, with Li+ it is smaller than in liquid 1718 IONIC HYDRATION I N ION-EXCHANGE MEMBRANES - or lb io io i o 50 Qo ;o 80 sb l!O relative humidity (%) FIG. 1 .-Absorption isotherms at 25 OC for the alkaline earth salts of the FEP-PSSA membranes. (>, Mg2+; ., Ca2+; A, Sr2+; A, Ba2+. TABLE I.-NUMBER OF WATER MOLECULES ABSORBED IN THE THOROUGHLY DRIED FEP-PSSA MEMBRANE ion Mg2+ 3.1 Ca2+ 2.1 Sr2+ 1.8 Ba2+ 0.4 TABLE 2.-vIBRATION FREQUENCIES (Cm-') FOR POLYSTYRENE SULPHONIC ACID MEMBRANE AND PSSA GRAFTED ONTO FEP MEMBRANE AS A FUNCTION OF THE COUNTER-ION FOR A RELATIVE HUMIDITY OF 7% '0 H 60, vso; nHzO counter-ion PSSa FEP-PSSA PSSa FEP-PSSA PSSa FEP-PSSA FEP-PSSA Mg2+ 3394 3436 1640 1646 1050 1049 4.33 Ca2+ 3406 3433 1628 1638 1044 1048 2.36 Sr2+ 3412 3458 1625 1636 1039 1040 2.48 Ba2+ 3440 3455 1621 1633 1035 1038 0.86 a After Zundel, ref.(3).L. Y. LEVY, M. MUZZI AND H. D. HURWITZ loo- 19 " 3800 3400 3000 wavenumber/cm -' FIG. 2.-Infrared spectra of the OH stretching mode of the alkaline earth salts of the FEP-PSSA membranes at 7% relative humidity. ( * * * .) Mg2+; (---) Ca*+; (-) Sr2+; (-.--) Ba2+. water.l The specificity of the role of cations involved in water uptake by the membrane is thus clearly emphasized. We endeavour, therefore, in the present publication to extend the investigation to the alkaline earth salts of FEP-PSSA membranes. EXPERIMENTAL Membranes of 62 f 1 pm thickness of polystyrene sulphonic acid grafted on Teflon FEP were used.* A value of 1.15 meq g-l with a scattering of 2.5% was found for the exchange capacity of the various selected membrane samples. An assignment of the i.r.absorption bands found between 3700 and 770 cm-l is given in ref. (1). The accuracy in wavenumber determination reaches f 1 cm-l for the narrow i.r. absorption bands and extends to + 5 cm-l in the most unfavourable cases. The experimental set-up and methods are described in ref. (1). Note that the water content of membranes taken at equilibrium with the laboratory atmosphere was measured by means of a Karl-Fischer titration technique.2 In the case of membranes placed at equilibrium with water vapour in the i.r. spectrophotometric sampling cell, the determination of the amount of sorbed water was achieved by linear interpolation and extrapolation of the relationship which has been established between nHzO, the number of water molecules per equivalent of sites, and AdOH, the integrated absorbance of the water bending absorption band at ca.1640 cm-l. * Progil, France; ref. C50-7-70.20 IONIC HYDRATION IN ION-EXCHANGE MEMBRANES l o o 1 0 1700 1650 1600 1550 wavenumber/cm -' FIG. 3.-Infrared spectra of the OH bending mode of the alkaline earth salts of the FEP-PSSA membranes at 7% relative humidity. Key as in fig. 2. RESULTS The variation of AVOH, the integrated absorbance of the stretching vibration mode of the sorbed water molecules, has been plotted as a function of nHZ0 in fig.4 of ref. (1). This diagram stresses that alkali metal and alkaline earth salts of the FEP-PSSA membranes obey a different relationship. On the other hand, it is shown in fig. 5 of ref. (1) that both types of salts behave identically as regards the change of ABOH, the integrated absorbance of the bending vibration mode, with respect to nHtO. The linear function passes through the origin. The absorption isotherms which have been derived for alkaline earth salts of FEP-PSSA membranes from these results are shown in An estimate of n&,o, the lowest amount of water per equivalent retained in so-called ' thoroughly dried ' membranes prepared under drastic conditions of drying [see ref. (l)], is recorded in table 1.Positions of the maxima of the intense absorption bands corresponding to the symmetric vibration vSOh of the -SO; group, and the stretching vibration vOH and bending vibration do, of the hydration water molecules in FEP-PSSA membranes at 7% relative humidity and 25 O C are reported in table 2. Some group variations fig. 1.L. Y. LEVY, M. MUZZI AND H. D. HURWITZ 21 FIG. 4.-Dependence of the position of the symmetric stretching vibration of the sulphonate group on the degree of hydration (dotted lines represent values at constant relative humidity). Key as in fig. 1. I 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 5 10 15 n H 2 0 FIG. 5.-Dependence of the position of the OH stretching vibration of the water of hydration on the degree of hydration. Key as in fig. 1.22 FIG.6. on the degree FIG. 7.-Dependence nHzO of the half-width of the OH stretching vibration band of the water of hydration the degree of hydration. Key as in fig. 1 . onL. Y. LEVY, M. MUZZI AND H. D. HURWITZ 23 obtained by Zunde13 on salts of ungrafted PSS membranes are included in this table for comparison. Selected spectra are shown in fig. 2 and 3, corresponding to the vOH and do, vibration bands of water, respectively. They were obtained under equilibrium conditions with an atmosphere of relative humidity p/po = 7% at 25 OC. The frequency shifts of the maxima of the i.r. absorption bands for the various alkaline earth salts as a function of nHIO are shown in fig. 4, 5 and 6. Futhermore, fig. 7 shows the evolution of the half-width of the vOH band in terms of water uptake.DISCUSSION It has been seen' that the water absorption isotherms of alkali metal salts of FEP-PSSA membranes and PSSA resins cross-linked with 8 % DVB4 present roughly the same pattern. In contrast to this behaviour, there is a considerable difference in shape between isotherms of alkaline earth salts of FEP-PSSA membranes and PSSA resins. The membrane isotherms are much more dependent on the nature of the counter-cation. For most alkaline earth cations the water uptake at low relative water pressure p/po is also much larger whilst, as illustrated in fig. 1, from values o f p / p o x 0.25 onwards a jump in water content is observed. The wavy shape of the isotherm is accentuated in the presence of Mg2+ and is absent in the presence of Ba2+.In the latter case, the curve approximately conforms to the behaviour of PSS resins in the rangeplp, < 0.60. As sketched for Mg2+ in fig. 8, the rehydration of the membrane, starting from the vapour pressure of pure water, po, follows a desorption branch which is higher than the absorption branch in the region of the absorption step. Experimental points of both branches have been recorded in an identical way under conditions of isopiestic equilibrium of the membrane inside the i.r. sampling cell. The hysteresis observed with Mg2+, Ca2+ and Sr2+ salts might suggest either some phase transition of sorbed water molecules, undergoing a considerable mutual attraction, or some capillary condensation in the film.5 According to the experimental results for ' thoroughly dry membranes' it might be '0 1 10 20 30 40 50 60 70 80 90 100 relative humidity (%) FIG.8.-Absorption isotherms at 25 O C for the Mg2+ salt of the FEP-PSSA membrane as a function of hydration (a) and dehydration (H) in terms of relative humidity.24 IONIC HYDRATION I N ION-EXCHANGE MEMBRANES 10 c 0 10 20 30 40 50 60 70 80 90 100 relative humidity (%) FIG. 9.-Absorption isotherm corrected from the initial water uptake as a function of relative humidity. Key as in fig. 6. taken for granted that the initial water content corresponds to a physical absorption process which proceeds differently from any further water uptake and also involves a maximum of molecules, as recorded in table 1. A weaker absorption process sets in after the steep initial rise. Consequently we have subtracted ngz0, the number of water molecules absorbed in the thoroughly dried membrane, from the values of the ordinate of fig.1. The resulting curves drawn in fig. 9 indicate that the isotherms of all alkaline earth systems have a nearly identical slope at a vanishing value of AnHzO = nHZO -ng20. Thus, we might infer that they possess a similar AGE, the initial free energy of absorption for the water molecules involved in this absorption step. An approximate value of AGE can be computed from the slope of AnHz0 against the water activityplp, at vanishingp/p,, as shown in fig. 9. We therefore use the Henry’s law approximation of the Langmuir adsorption isotherm, where a is the fraction of sites accessible in this absorption step. Therefore AnH2,/a is the degree of coverage of the accessible sites.If one assumes that one-half (a = $) or one third (a = i) of the sites are accessible, AGE becomes, respectively, -8.8 and -9.8 kJ mol-l. When compared with the value of ca. - 10 kJ mol-1 obtained for AGE by Glueckauf6 with Li+ salts of PSSA resins, it becomes plausible that an average of of the overall exchange capacity of the grafted PSSA membrane remain available for this absorption process. Furthermore, it is apparent from fig. 9 that the isotherms of Mg2+, Ca2+, Sr2+ and Li+ reproduce the same curve up to AnFIz0 = 3.5. At larger values of water uptake, the absorption step sets in for the alkaline earth salts. We now turn to a spectral analysis of the absorption bands vso;, vOH and vOH. This will provide insight into the physical mechanism responsible for the pattern of the isotherms.L.Y. LEVY, M. MUZZI A N D H. D. HURWITZ 25 SYMMETRIC VIBRATION B A N D : Vso; As already pointed in the most stable ion-pair conformation the centre of the cation is placed in the direction of the SO bond axis of the sulphonate ion. The stronger the ion pairing, the higher the rise in frequencies of vsoT beyond 1040cm-l. As shown in fig. 4, the increase in strength of the interaction between the divalent cation and the fixed anion follows, at any water content, the sequence Ba2+ < Sr2+ < Ca2+ < Mg2+. We also infer from fig. 4 that the interionic bond strength decreases to a minimum with increasing hydration of the membrane for Sr2+, Ca2+ and Mg2+. Note that for the values ofp/p, plotted in fig.4 the minimum appears in the region 0.25 < (p/po)* < 0.33, corresponding to the rise of the absorption step in the isotherm, where (p/po)* is the value ofp/p, at the minimum. STRETCHING VIBRATION B A N D : VOH Notwithstanding the complexity of this band extending from 3200 to 3600 cm-l, some important conclusions can be drawn concerning the OH groups involved in hydrogen bonds. The fact that the membrane contains a negligible number of free water molecules is supported by the absence of any absorption peak at frequencies above 3600 cm-l. The faint shoulder detected at ca. 3600 cm-l is indicative of the small amount of free OH groups pertaining to water molecules hydrogen bonded by their second OH group to the sulphonate ion or to another water molecule absorbed within the membrane.These considerations are valid even at the lowest degree of hydration. On the other hand, the contribution of OH groups involved in hydrogen bonds is very important, as shown by the broadness and intensity of the vOH band. The width of the band is caused by variations in strength and length of the hydrogen Accordingly, it is suggested that broad absorption bands predict a strong induced dipole-ion interaction between the hydrogen bond and the cation. It is furthermore well known that the stronger the hydrogen bond, the larger the shift of the band maximum towards lower frequencies. One also observes in fig. 7 that, at a similar degree of hydration, the ionic interaction with the hydrogen bond decreases as the ionic radius increases.Moreover, the band displays a minimum half-width at (p/po)*, the relative humidity corresponding to the rise of the absorption step in the isotherms. The shifts of vOH plotted in fig. 5 stress the fact that at any given value of nHZq the hydrogen-bond strength also decreases as the ionic radius increases. It is striking that the hydrogen-bond strength passes through a minimum at the same critical value of relative humidity (PIP,,)* as found for the half-width of vOH, vsoT and the isotherms. BENDING VIBRATION B A N D : OH The interaction between a cation and its water of hydration affects predominantly the do, band.13 In order to describe this influence for an electrolytic solution it is necessary to identify two main ionic hydration effects: the building-up effect of a peripheral hydration sheet around the cation and the solvent-restructuring effect in the bulk electrolytic solution beyond the effective radius of the hydrated entity.The magnitude of the decrease in do, is therefore not simply indicative of water molecules bound more firmly to the cation than to other water molecule^.^^ Even though Ba2+ is less peripherally hydrated than Mg2+, it exhibits a more pronounced decrease in do, in electrolytic solutions. This apparent anomalous behaviour parallels some viscosity results13 proving that Ba2+ serves as a centre of disturbance for the structural order of the bulk solvent. Such concepts are also relevant in the case of ionic hydration in the membrane. As shown in fig. 6, the decrease of 2 FAR 126 IONIC HYDRATION IN ION-EXCHANGE MEMBRANES the maximum frequency do, follows the sequence Mg2+ < Ca2+ < Sr2+ Ba2+.It keeps the scissoring vibration frequencies at a value which is higher than that of free water molecules (ca. 1595 cm-l) but for Ca2+, Sr2+ and Ba2+ beneath that of liquid water (ca. 1645 cm-l). Recalling the tendency of these ions to weaken the hydrogen bonding, according to the sequence mentioned above, it becomes apparent that these ions act as entities with increasing ability to disorder water structure, thus producing a collapse around the ion of the ice-like structure of water. Unlike its divalent congeners, Mg2+ produces at low water content a bending vibration which is approximately the same as in liquid water. With increasing hydration, do, has values much above that observed in ice (at 1650 cm-l).This demonstrates a strong ordering effect presumably yielding, in the presence of a sufficient amount of water, a transition towards a highly rigid structure of the surrounding solvent network. We now note the remarkable observation that Li+ and Mg2+ both follow a similar variation in dOH up to frequencies below that of ice within the range 0 < AnH20 < 4-5. MODEL OF HYDRATION OF ALKALINE EARTH SALTS OF FEP-PSSA MEMBRANES Given the fact that water molecules remain absorbed in the dry membrane, we are inclined to adopt the hypothesis that the membrane contains, in limited regions of high graft density, a number of sulphonate sites crosslinked by divalent cations in such a manner as to trap a given amount of water into poorly accessible channels or pores. This view is supported by the strong inhomogeneity of grafting as revealed by electron micrographs,l and the almost irreversible hydration character of the film.Thus, as suggested from the evaluation of AGO,, if ca. 4 of the sulphonate sites remain accessible to reversible water absorption, there are $ of the sites serving to encage the water in the pores. Consequently, the number of water molecules pertaining to the pores per ionic crosslink amounts to an average of 9.3 in the case of Mg2+ and 1.2 in the case of Ba2+. The very weak contribution of free OH group vibration which has been detected suggests that nearly all these water molecules are organized in aggregates of associated molecules. The existence of such aggregates is also confirmed by the values of dOH recorded in the dry membrane and by the value of vOH which predicts that some water molecules undergo hydrogen bonding with the -SO; sites belonging to the neigh- bouring PSSA chains.The tightness of the occluded pores is correlated with the strength of ion association. Simultaneously, the hydrogen-bond donor ability of OH groups is enhanced by their polarization occurring in the strong electric field generated by small divalent ions. Both arguments support an increase of water uptake from Ba2+ to Mg2+. The model suggests further that the equilibrium absorption of the exchangeable water sets in on the remaining accessible sulphonate sites which initially bear no solvent molecules, this being equivalent in the process of absorption irrespective of the amount of water already fixed in the pores.This explains the similarity in slope and pattern of the initial branch of the water absorption isotherms for Mg2+, Ca2+, Sr2+, Li+ and to a lesser extent for Ba2+. As regards the inability of Ba2+ and K+ to reproduce exactly the same behaviour, we assume that a steric effect might impair the water absorption in the case of Ba2+ and note that ionic clustering has been advocated in order to describe the ion-solvent interaction of Na+, K+ and cs+. With increased hydration of the membrane, it is supposed that water molecules form bridges connecting several anionic sites, as depicted in fig. lO(a). In this configuration, the bending vibration mode of water is stiffened and tends towards that of liquid water.Simultaneously, the appropriate orientation of water molecules for hydrogen bondingL. Y. LEVY, M. MUZZI AND H. D. HURWITZ 27 'I' (cF~\.-:-IcF,). F FIG. 10.-Hydration model of the Mg2+ salt of the FEP-PSSA membrane: (a) at a low degree (b) at a degree of hydration corresponding to the absorption step. of hydration; with the sulphonate sites is hindered by the interaction with the cation linking those sites together. Hence a partial cationic hydration shell is built up at the cost of the hydrogen-bond donor properties of the OH groups of water. Similar conclusions have been drawn from the study of alkali metal salts of our grafted membrane9 and of salts of PSSA ungrafted membrane^.^ Zunde13 has developed the argument that the polarizing effect of the cation on the OH band is reduced if spread over several water 2-228 IONIC HYDRATION I N ION-EXCHANGE MEMBRANES molecules.Conversely, the formation of the cationic hydration shell leads to a progressive loosening of the ionic SOT-(cation)-SO; crosslink. It is thus expected that the ion-triplet configuration becomes unstable upon further water uptake. At a given water activity or critical value (p/po)*, the swelling of the membrane coincides with a transition to a more stable conformation. In the building up of the new hydration structure, the decrease in the overall free energy of hydration and the increase in the absorption of water are associated. The absorption isotherm steps rather abruptly to a larger value.For the physical interpretation of this process, one might assume that water molecules insert themselves between the divalent cation and the anion, and contribute to the ionic crosslinking as depicted in fig. 10(b). Such an interpretation of the ionic interaction relies, for instance, on a type of Robinson-Harnedl* localized hydrolysis. This effect leads to the formation of a kind of ion-triplet but differing in that polarized water molecules act as intermediary. The greater the strength of the ionic bond, the more polarized the solvent molecules will be, so the effect would decrease from Mg2+ to Ba2+. This conclusion is consistent with the experimental minima values for the shift of vso, in fig. 4. As such an interaction would enhance the hydrogen-bond donor property of the water molecules, it would also explain why in presence of a proton acceptor like SO; the shift of vOH decieases (fig.5) and the half-width of the band increases (fig. 7) for p / p o > (p/po)* with increasing water uptake. In this respect, it is worth comparing the behaviour of Li+ and Mg2+ salts. At low membrane water contents, it might be suggested that two Li+ and two SO; ions form an ion-pair doublet acting as an ionic crosslink. However, the hydration sheet which is formed around the monovalent cation disrupts the ionic pairs without fostering, at larger water contents, any new type of ion association or hydrogen bonding. The case of Li+ compared with Mg2+ in this work and with K+ in ref. (1) is a good example of the complex behaviour which can be met in ionomeric membranes or resins. We thank Prof. G. Zundel for helpful discussions. 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Stepanov, Nature (London), 1946, 157, 808. l3 R. E. Nightingale, in Chemical Physics of Ionic Solutions, ed. B. E. Conway and R. G. Barradas l4 R. A. Robinson and H. S. Harned, Chem. Rev., 1942, 28,419. C. A. Coulson and G. N. Robertson, Proc. R. Soc. London, Ser. A, 1974, 337, 167. (J. Wiley, New York, 1966), p. 87. (PAPER O / 1377)