J.C.S. Faruday I , 1980,76,2558-2574Infrared Investigation of Ionic Hydration inTon-exchange MembranesPart 1 .-Alkaline Salts of Grafted Polystyrene SulphonicAcid MembranesBY LEON Y. LEVY, ANDR~ JENARD AND HENRI D. HURWITZ"Laboratory of Electrochemical Thermodynamics, Faculty of Science,Universitk Libre, Brussels, BelgiumReceived 12th November, 1979Infrared spectroscopic measurements have been performed on thin ion-exchange membranes whichconsist of alkaline salts of polystyrene sulphonic acid grafted on a Teflon FEP matrix. The membranesunder investigation were placed in isopiestic equilibrium with water vapour in the sampling cell. Thewater content of these FEP-PSSA systems has been determined at 25°C by measuring the integratedabsorbance of the sorbed-water bending vibration as a function of the nature of the cation.The depen-dence of the position of the water stretching and bending vibration bands and of the symmetricvibration of the SO, groups with the nature of the cation and the water content has led to variousconclusions concerning the ionic interaction and hydration in the membrane. The peculiar behaviour ofLi' is due to its ability to interact strongly with the anion. A model of ion clustering has been proposedto explain the spectra of water in the presence of large alkali ions.Some infrared spectroscopic measurements on polystyrene sulphonic acid mem-branes were carried out by Zundel in 1969' in order to investigate the ion-solventinteraction in the polymer. Since then, few i.r.spectroscopic studies have beendevoted to the elucidation of the effect of water and of the exchangeable ion inion-containing polymeric membranes. This is surprising as a vast literature existsand the industrial and analytical applications of such ion-exchange materials haveincreased considerably. Among the polymeric ion-exchange membranes which areof special practical use to date, one finds films containing polystyrene sulphonicacid (PSSA) grafted onto fluorocarbon backbones? and so-called perfluorosul-phonic acid membranes made out of saponified copolymers of sulphonyl fluoridevinyl ether and tetrafluoroethylene.1 In our spectroscopic investigation, we havefocussed our attention on this type of material. The present publication describesthe i.r.spectroscopic analysis performed on the alkaline salts of PSSA grafted on aTeflon FEP matrix. The future publications will present spectroscopic measure-ments made on other salts of PSSA and on salts of perfluorosulphonic acid(Nafion) membranes.It is known from the investigations of Zunde12 and from several contributionsdealing with strong electrolytes that the vibrational spectra of H 2 0 3 and of SO:-may be deeply perturbed under the influence of the polarizing field produced by anearby cation. For instance, the OH stretching band and HOH scissoring band ofwater will both be shifted to different wavenumbers with ditferent magnitudes. Thusit is worthwhile to corfelate such effects with a model of ionic solvation andt AMF C322 PSSA grafted on polytrifluorochloroethylene (Kel F) or on fluorinated ethylene propy-$ Nafion (registered trade mark) produced by Dupont.lene copolymer (FEP).255FIG.l - - - ( ~ i ) : Electron micrograph of a FEP film surface ( x 15600). (h) Electron micrograph of the surfaceof a FEP membrane grafted with PSSA in uranyl salt form ( x 10300).To face page 2559L. Y . LEVY, A. JENARD AND H . D . HURWITZ 2559interaction. In the case of ion-exchange membranes, such an approach shouldprove particularly rewarding if the microscopic interpretation is successful inexplaining such macroscopic properties as equilibrium swelling, wetting and ionicselectivity. In this respect, the interpretation of the value of the ionic separationfactor of ion-exchange resins and membranes relies nowadays on various theories,some based on more or less crude statistical mechanical others onpurely thermodynamic deduction^.^ The complexity of combining several types ofionic interactions which are competing with hydration in the ionomeric materialalso produces models which are frequently restricted to qualitative and empiricaltheories by which the ability of the model to yield consistent counterion sequencesis emphasized more than its agreement with any statistical thermodynamic ormolecular computation.OSpectroscopic arguments might help to reconcile the various treatments. Conse-quently, we endeavour here to substantiate phenomenological properties (such asequilibrium wetting and ionic selectivity) in terms of molecular models of ionicsolvation and interaction in the polymeric membrane.Also, the specific influenceexerted on these effects by the inert macromolecular matrix will be explored.EXPERIMENTALPREPARATION OF SAMPLESMembranes of 62 f 1 pm thickness of PSS grafted on FEP" were used. The thickness of asample of non-grafted film was determined from its interference pattern in the i.r. spectrum. Itwas 50 pm thick within f 3%. All i.r. spectroscopic measurements were carried out on samplesof 32 and 15 mm radius.The membranes, initially in their acid form, were placed in 1 mol dmP3 solutions of alkalichloride for at least three periods of 4 h. All solutions were prepared with Merck pro anulysisalts. After being removed from the solution, the membranes were washed with triply distilledwater and carefully wiped with filter paper.DETER MI N ,\ T I O N OF EX C H A N GE C A P A C I T I E SThe exchange capacity of the samples prepared for i.r.spectroscopic studies were obtained bycoulometiic microtitration carried out following the method described by Sansoni.' A Ptelectrode of 1 cm2 area was used as hydrogen ion source. The cell assembly also contained acombined Ingold microelectrode (type HA 405 MJNS) and a Pt counterelectrode in a separatecompartment connected with the main compartment by means of a saturated K2S04 agar-agarbridge. A potentiostat (Tacwsel PRT 3001) was used as a galvanostatic source providingcurrent intensities ranging from 500 to 1000 pA with a precision better than 1 pA. The pH inthe cell was kept constant.The average value of the exchange capacity was 1.15 meq g- andthe scattering of the results was ~ 2 . 5 % for the various samples irrespective of the salt, whichindicates a small degree of inhomogeneity of the membrane. Such inhomogeneity, due to itsgrafting on the FEP film, could be studied by electronic microscopy. The membrane surfacewas observed using the technique of carbon replica shaded with silver at an angle of 15". Anelectron micrograph for an ungrafted film was taken in the same condition. Significant areas ofthe surface of an ungrafted film [fig. l(a)] and grafted film [fig. l(b)] are shown. By inspectionof the first micrograph, one: can exclude a microphase separation. Homogeneous distribution ofinterwoven chains is observed.The micrograph of the grafted membrane reveals the existenceof slightly swollen domains heterogeneously distributed on the surface. The diameter of these* Progil, France; ref. (2.50-7-702560 IONIC HYDRATION I N ION-EXCHANGE MEMBRANESdomains is up to 20.3 pm. These domains are presumably regions of high PSSA penetrationsurrounded by fluorinated backbones presenting a much lower graft density. The techniqueused did not detect any supermolecular structure as found by Ceynowa12 on a PTFE film.DRYING OF MEMBRANESThe membrane was dried in two successive operations. First, the membrane has been placedin a vacuum Torr) in the presence of P205 at 80‘C and, secondly, the dry membrane wasexposed to i.r. light at a temperature of 2 50‘C in the spectrometer sampling cell through whicha flow of dry air ( < 4 p.p.m.of H 2 0 at 25‘C) was delivered. The passage of dry air continueduntil no change was observed for a period of 24h in the continuously recorded i.r. waterabsorption band of the membrane. Such conditions of dryness were achieved after 4-6 daysdepending on the nature of the exchangeable cation. The final spectrum of the so-called“thoroughly dried membrane” indicated that there was still some residual hydration whichcould not be removed even after two or three weeks of a similar repetitive drying operations.DbSCRIPTlON OF THE I . R . SAMPLING CELLA sampling cell was designed to meet the following requirements: (i) the membrane underinvestigation must be placed in isopiestic equilibrium conditions in the cell; (ii) the cell mustdispose of a large thermal capacity and must be thermally well isolated; (iii) the saturatorscontaining solutions providing the selected water vapour pressures must be easily interchange-able.The body of the cell, as sketched in fig.2, is made of brass. It has cylindrical symmetry and iscomposed of two pieces (1 and 2) which can be combined by means of a ring screw and twoO-rings which insure a good vacuum seal. Inside the cell is placed a membrane holder of ironcovered with cadmium (3). The membrane (4) is fixed on its holder by means of a magneticribbon (5). Two nozzles (6) permit the circulation of thermostatic Auid through the concentricjacket of the cell. The inside of the cell is joined to a saturator and to the vacuum pumpthrough a nozzle (7). The two AgCl windows (8) have a PTFE frame (9) which insulates themFIG.2.--Exploded view of the i.r. sampling cellL . Y . LEVY, A . JENARD A N D H . D . HURWITZ 256 1from the metal body of the cell. This frame is pressed on the cell by means of a Teflon flatO-ring screw (10 and 11). The path length from window to window is 5 cm. The saturatorconsists of simple Pyrex tube sealed on the entry of a three-way stopcock connected to thepump and to nozzle (7).DETERMINATION OF WATER CONTENTHydrated membranes were titrated by the Karl Fischer method. For this purpose sn auto-matic K.F.4 Beckman apparatus was modified slightly in order to satisfy the anhydrous con-ditions for the titration agent and the titration cell during the experiment.The membranes aresubjected to a fast spontaneous rehydration or converse slow dehydration whenever theselected isopiestic water absorption equilibrium conditions are enforced, which happens duringthe transport of the membrane to the Karl Fischer cell. These processes prohibit accuratedetermination of the degree of wetness by the Karl Fischer titration unless the membrane istaken at equilibrium exchange with the laboratory atmosphere at 25°C. For this reason, it wasnecessary to conceive an indirect way of water content determination in membranes which areplaced at equilibrium at a relative humidity different from the laboratory atmosphere. Thismethod will be described later in the results section.SP E C TR 0 S C OP 1 C ME A SUR EME N T S U N D ER I SOP I E S T I C CON U 1 T I ON SSamples previously tested, under identical conditions, for their water content and exchangecapacity were thoroughly dried in the i.r.sampling cell. A progressive rehydration was carriedout stepwise by placing saturated electrolytic aqueous solutions of increasing water vapourpressure in the saturator. It is assumed that the water absorption equilibrium is reached whenthe water i.r. absorption bands remain unchanged for 24 h after connecting the saturator andthe cell, both at 25°C. The spectra of the membranes were recorded with a Beckman i.r. 9double-beam spectrophotometer. An identical cell, without the membrane but set at the samevapour pressure, was inserted in the reference beam.In order to obtain good accuracy in bandfrequency determination, a slow scanning of 20 cm-' min-' was used and the bands wereenlarged using an amplification factor of 4 or, in some cases (OH stretching absorption band), afactor of 2. This extends the accuracy to z 2 I cm-' for the narrow bands and yieldsin the most unfavourable cases (the very large water stretching bands or shoulders). It shouldbe stressed that for the hydrated membranes at equilibrium with the laboratory atmosphere at2 5 T , only the first of the recorded spectra was retained since a slow process of dehydrationcould occur under the influence of the i.r. radiation.Each complete set of investigations, as a function of the membrane wetting and the nature ofthe countercation, was made on the same sample in order to avoid the effect of inhomogeneitiesin the film.The accuracy of frequency measurements has, however, been checked by repeateddetermination on several samples cut from different films.5 cm-ASSIGNMENT OF THE 1 . R . ABSORPTION BANDSIn fig. 3(u) is shown a selected spectrum of the K + salt of a FEP-PSSA film takenat equilibrium with an atmosphere of relative humidity p / p o = 0.98 at 25'C. Thespectrum of the FEP film taken at the same equilibrium conditions is given forcomparison in fig. 3(h). The assignment of the most important spectral bands are asfollows :The broad band in the region 3700-3300cm-1 is ascribed to the stretchingvibrations of H20. In the case of the ion-exchange membrane, the position of themaximum of absorption and the intensity of this band depend on the nature of thecation as well as the degree of hydration.Thus this band can be attributed tomolecules involved in ionic solvation inside the membrane. The presence of freeOH groups, thus not involved in a hydrogen bond, might produce some shouldersat z3600cm-'2562 IONIC HYDRATION I N ION-EXCHANGE MEMBRANES1 I I I I I lOOr I I I II I , I 14000 3600 2 800 2000 1600 1200 800 400wavenumber/cm -FIG. 3.--(a) 1.r. spectrum of a K' salt of the FEP-PSSA membrane at 98:< relative atmospherichumidity. (b) 1.r. spectrum of a FEP film.The bands at 3060 and 3025 cm-' are caused by the stretching vibrations of the>CH groups in the benzene ring.These bands cannot be related to the individualvibrations of these groups since they are coupled with other vibrations appearing inthe system.I3The bands at 2924 and 2851 cm-' are assigned, respectively, to the asymmetric andsymmetric stretching vibrations of the -CH2- group. l4The band at 2398 cm- ' is characteristic of the first harmonic of the CF2 stretchingvibration. The shape and intensity of this band depends on the type of ion and thewater content in the membrane. It has been suggested that the thickness andcrystallization of the Teflon film may be determined by means of this band.15-17 Inthe case of a grafted film, this band might, ,however, hide a weak absorption banddue to the sorbed hydrogen-bonded water molecules.The four weak bands at 1938, 1845, 1792 and 1725 cm-' (mixed with an interfer-ence pattern) are due to combination vibrations and overtones of the out-of-planebending vibration of the > CH group of the benzene ring.'The band at 1640 cm- ' corresponds to the scissor vibration of water. By analysis ofcationic influences on this band, specific characteristics of ionic hydration phenom-ena in the membrane are detected.Bands at 1599, 1494 and 1411 cm-' are caused by the skeletal stretching vi-brational of the benzene The band at 1445 cm-' is the scissor vibration ofthe -CH2- group.14The very intense band ranging from 1100 to 1300 cm- ' corresponds to the stretch-ing vibration of the CF2 group.This band is masking the antisymmetric stretchingof the SO, ion (thoroughly analysed by Zundel').The band at 1040cm-' is attributed to the symmetric stretching vibration of theSO, ion in mesomeric form of C30 symmetry.The position of this band isinfluenced by the cation.The band at 1011 cm-' is ascribed to the ion plane bending vibration of the >CHgroup of benzene." This peak is shifted towards 1000 cm-' as a function of thedegree of dryness of the membrane.The band at 981 cm-' is characteristic of Teflon FEP and might be due to the-CF3- group asymmetric stretching vibration.The band at 900 cm-' is due to the stretching vibration of the single S-0 bondappearing in the SO, non-mesomeric form or HS03 acidic form. It is observedwith alkaline salts but its very weak intensity does not allow any significant inter-pretation.2L .Y . LEVY, A. JENARD AND H . D. HURWITZ 2563The band at 831 cm-l is due to the out-of-plane bending vibrations of the twopairs of > CH groups on each side of the para-disubstituted benzene ring. l9The assignment of the peak at 773 cm-' is still unsettled. By comparison with thespectra of ortho-toluene sulphonic acid, Zundel has suggested that this band corre-sponds to the out-of-plane deformation of four CH groups in the benzene skele-ton.' Note that this band is found at the same location as an absorption peak ofTeflon FEP.RESULTSINFLUENCE OF WATER CONTENT ON THE INTENSITY OF THE WATER I.R.ABSORPTION BANDSIn the case of hydrated membranes taken at equilibrium with the laboratoryatmosphere, the maximum absorbances and integrated absorbances of the waterstretching and bending vibration bands have been plotted in fig.4 and 5 as afunction of the amount of water measured by the Karl Fisher titration method. Acomputer program has been used to integrate the absorbances as a function ofnH2OFIG. 4.-OH stretching vibration integrated and maximum absorbance of the hydration water as afunction of the degree of hydration of the membrane in its alkaline and alkaline-earth salts. 0, Li; @, Na;0, K ; U, Cs; @, Mg; 8, Ca; +, Sr; 0, Ba2564 I O N I C HYDRATION I N ION-EXCHANGE MEMBRANESlTH 0FIG. 5.-OH bending vibration integrated and maximum absorbance of the hydration water as afunction of the degree of hydration of the membrane in its alkaline and alkaline-earth salts.Symbols asin fig. 4.wavenumber. This computation is made by subtracting from the integrated valuesthe contributions of, respectively, the shoulder at 3025 cm-I and the bands at 3060,2924, 2851 and 1600cm-'. The optical integrated densities due to these variouspeaks are calculated from the spectrum of the Cs+ salt of the thoroughly driedFEP-PSSA membrane, for which a minimum of wetting is observed. The deconvo-lution in this system is based on the assumption of a gaussian shape for thestretching and scissoring vibration bands of water. As a result of these calculations,we have ascertained that a linear relationship exists between the integrated absorb-ance of the bending vibration mode Ad and the amount of absorbed water, irrespec-tive of the nature of the cations.An equivalent simple behaviour is exhibited neitherby the integrated absorbance of the stretching vibration mode A , , which shows twodifferent linear segments depending on the charge of the countercation, nor by themaximum absorbances of both modes.21Using these results we have computed the relative water content of any samplesubjected to various hydration conditions, by measuring its integrated absorbanceAh and assigning a relative value of 100 to the A6 value of the membrane withME2+ salt taken at its maximum degree of swelling. The results, presented in theform of absorption isotherms, are depicted in fig. 6 for alkaline saltsL . Y . LEVY, A . JENARIJ AND H . D. HURWITZ 2565relative humidity (x)FIG. 6.- Absorption isotherms at 25'C of the Li' (0) and K' (U) salts of the FEP-PSSA membrane.DEPENDENCE OF BAND POSITIONS ON THE NATURE OF THECATION AND ON THE WATER CONTENTPositions of the maxima of the intense bands corresponding, respectively, to thesymmetric vibration v, (SO;) of the SO, group, the stretching vibration vOH andbending vibration aOH of the hydration water molecules in FEP-PSS membranes at7% relative humidity and 25°C are recorded in table 1.With our results we alsoshow some group vibrations obtained by Zundel' for salts of ungrafted PSS mem-branes. The shift in frequency of the maxima of the absorption bands recorded as afunction of the water content for the various alkaline salts of FEP-PSS membranesis given in fig. 7-9.As regards the effect of water content on other vibrational group absorbances, letus note that some influence has been detected on a shoulder which exists to thelow-frequency side of the CH bending vibration of the benzene ring atw 1000 cm-'.DISCUSSIONThe FEP films possess a high mechanical resistance and are extremely hydro-phobic. Hence we do not find any trace of hydration, as revealed by i.r.waterabsorption bands, in pure FEP films kept for three weeks in contact with anTABLE 1 .-VIBRATION FREQUENCIES FOR POLYSTYRENE SULPHONIC ACID MEMBRANE AND PSSAGRAFTED ONTO FEP MEMBRANE AS A FUNCTION OF THE COUNTERION FOR A RELATIVE HUMIDITY OF7%VOH hOH VSSOT n H 2 0 PSS" FEP-PSS PSS" FEP-PSS PSS" FEP-PSS ds FEP-PSSLi + 3458 3453 1631 1643 1041 1041 2.3Na+ 3459 3455 1629 1651 1036 1042 1.4Kf 3460 3454 1631 1655 1035 1044 1.1c s + 3459 3455 1637 1657 1030 1043 0.5" After Zundel, ref.(1)2566 I O N I C HYDRATION I N ION-EXCHANGE MEMBRANESFIG. 7.-Dependence of0 5 10nH 2 0the position of the symmetric stretching vibration bandon the degree of hydration. Symbols as in fig. 4.of the sulphonateatmosphere of 98% humidity. The grafting of these films with PSSA maintains ahigh degree of rigidity as compared with PSS ungrafted membranes and the mem-brane swelling is kept at a relatively low value. Thus, the ratio between the thick-ness of the FEP-PSSA film and the ungrafted FEP film, both films being placed in3500-- -I\ j 3475-5$EFIG. 8.-Dependence of'H 2 0the position of the OH stretching vibration band of thethe degree of hydration.Symbols as in fig. 4.water of hydration oL. Y . LEVY, A . JENARD AND H . D. HURWITZ16402567FIG. 9.- -Dependence of1655 btof hydration on theequilibrium with the laboratory atmosphere, is <1.2. The number of water mol-ecules per equivalent, t ~ ~ ~ ~ ~ , absorbed in the Li’ salt of a FEP-PSSA membranereaches a maximum of 10 at p o , the vapour pressure of pure water at 25°C. Thisvalue is z 1 water molecule below the water content of Li+ salts of PSSA resinscontaining 8% DVB.” An estimate of the lowest amount of water, I Z ~ , ~ , in themembrane follows from our i.r. spectroscopic investigation of thoroughly driedmembranes. On the whole, except for Cs’ and perhaps K+ salts, some watermolecules are very firmly bound to the membrane and cannot be removed.Theirnumber, as it is reported in table 2, reaches an average value of I Z ~ , ~ 2 1 in the caseof Li+ salts.Fig. 5 shows a linear relationship at 25°C between flHZo, the amount of ab-sorbed water molecules, and A,, the integrated do,, absorbances. Such simple be-haviour, which is valid irrespective of the nature of the counterion, can be obtainedTABLE 2.-NUMBER OF WATER MOLECULES ABSORBED IN THE THOROUGHLY DRIED FEP-PSSAMEMBRANEion Lif Naf K+ Csfng,o 0.9 0.5 0.3 0.2568 IONIC HYDRATION I N ION-EXCHANGE MEMBRANES'lH2OFIG. 10.-Dependence of the OH stretching vibration integrated absorbance of the hydration water onthe degree of hydration water in alkaline and alkaline-earth salts of the membrane. Symbols as in fig.4.only after deductions of the peaks due to the polystyrene skeleton in this spectralregion. The hypothesis of constant absorption coefficient of the bending mode,irrespective of ntl,() (thus below n1120 = 3.5 for Ba2+ and 1.7 for Cs') and of thenature of the cation, is justified by the fact that in fig. 5 the linear extrapolation ofthe straight line segment, obtained at equilibrium with the laboratory atmosphere,passes through the origin.As revealed in fig. 4, the conclusion regarding the behaviour of A,,,, the inte-grated vOH absorbances, is quite different. The two slopes of the linear segments canbe interpreted in terms of absorbance coefficients characteristic of the countercatio-nic charge.Furthermore, the plot of A,,, with respect to nHzO in fig. 10 indicates achange in the absorbance coefficients as a function of the water content at lowdegrees of hydration.If the linear dependence of AdOH with nHzO is taken for granted, water absorptionisotherms can be readily derived from combined i.r. and isopiestic measurements.The isotherms recorded in fig. 6 broadly present the classical morphology usuallyencountered in polyelectrolyte systems. A comparison with the absorption isothermin PSSA resins cross-linked with 87; DVB22.23 indicates similar behaviour for K+salts. As already emphasized in table 2, however, one notices with membranes asteep initial rise and a sharp bend. For Li+ salts, H : , , ~ 5 1. Thereafter, a graduallydecreasing sloping curve extends as far as p / p o = 0.40.It follows a moderatelyrising part yielding at p / p o = 1 a smaller water content than in the case ofPSSA-8:l; DVB resins.In order to reach a molecular interpretation of the sorption process of water inthe membrane, it is worth focussing attention on the frequency shift of the principaL. Y . LEVY, A . JENARD A N D H . D . HURWITZ 2569i.r. absorption band maxima with respect to hydration and to the nature of theexchangeable cation.THE SYMMETRIC VIBRATION, Vso3The strength of the ionic interaction between the fixed SO, anion and thecountercation can be evaluated from the position of this absorption band. Con-sideration of the electronic distribution in the direction of one of the s-0 bonds inthe sulphonate ion indicates that the bond energy is larger when the centre ofcationic charge is located on the S-0 bond axis, in contact with the oxygen, thanif the counterion affects all three S-0 bonds together.' The increased probabilityof the existence of the S-0- (cation) structure produces a shift of the maximumof the spectroscopic stretching vibration band to frequencies > 1040 cm-'.* Zundel'has analysed the antisymmetric stretching vibration of SO, in PSSA ungraftedmembranes, but he has not considered the change in the position of the symmetricstretching vibration maximum.On Teflon films, the antisymmetric vibration isobscured by the broad and intense Teflon band at 1200 cm-' ; however, thechanges in the symmetric vibration band become very important.In all alkalinesalts (fig. 7), the shift of the maximum of the vsSoF band with increasing hydrationindicates a weakening of the ion-pair bond strength. At low water content, thestrongest interaction is observed for the Li'-SO, pair, the maximum lying at2 1050 cm-'. At nt120 > 5, however, the binding strength of Li' becomes smallerthan that exerted by other cations. This difference is not too significant because ofthe high intensity of the SO, stretching vibration band.THE STRETCHING VIBRATION B A N D V O HThe vOH stretching vibration band of water at z 3400 cm-' is broad and fairlycomplex. It overlaps with the overtone 280t, of the scissor vibration of H 2 0 at2 3250 cm- ', with the stretching vibration of the CH groups in the benzene ringand partially with symmetric and asymmetric vibrations of the -CH- groups ofpolystyrene.A faint shoulder at z 3600 cm-' can be assigned to the stretchingvibration of the free OH groups pertaining to water molecules linked by theirsecond OH group to an oxygen atom belonging to the sulphonate site or to thewater network contained in the pores of the membrane. The fact that no distinctpeak or shoulder is found at a higher frequency, which could correspond to the OHstretching modes of free water,24 confirms that the number of free water moleculesis negligibly small at any degree of wetness. Due to the weakness of the band at3600 crn-l, the number of free OH from molecules of water of hydration should bevery small, even at the lowest water content.Note that our consideration relies onthe assumption that the absorption coefficient of the free OH group is not too lowcompared to the absorption coefficient of the hydrogen-bonded OH. We have somearguments proving this assertion, since some i.r. spectra of other polyelectrolyticmembrane systems (Nafion) exhibit a sharp band at 2 3600 cm-', associated with asignificant amount of free OH groups in the systems.25The contribution of the OH groups involved in hydrogen bridges is shown by thebroadness and intensity of the band at 3460 cm- An essential cause of the broad-ness of the band is the variation of strength and length of the hydrogen bondswhich link the two OH groups of the water molecules to neighbouring hydrogen907 cni-'.* In hulphonic acid, the S=O and S-0 stretching vibrations are, respectively, located at 1172 an2570 IONIC HYDRATION I N ION-EXCHANGE MEMBRANESbond acceptors.6-2 As has been ~ t r e s s e d , ~ ' - ~ ~ the uncoupling of the vibration ofthe two OH groups produces a broadening of the absorption band. Furthermore,the increase in strength of the hydrogen bond leads to a shift of the maximum ofthe band towards smaller wavenumbers. Hence the behaviour of vOH in fig. 8denotes a continuous decrease in the hydrogen-bond strength with respect to thedegree of hydration. With Li+ salt, one observes a constant value of frequencyabove nIlLo = 5. Furthermore, the association of water molecules with the SO,sites or with each other is progressively enhanced in passing from Cs' to Li'.THE BENDING VIBRATION B A N D 601,1.r.spectral analysis of aqueous solutions of electrolytes has shown that theinteraction between the catiop and the solvent affects essentially the BOH band.33This influence is confirmed in fig. 9 where a drastic difference appears between thebehaviour of the Li' salt and the other alkaline systems. With increasing watercontent, the band maximum with Li' shifts towards larger wavenumbers. WithnHzO > 5, the increment in dOH decreases and the force constant of the H 2 0 scissorvibration reaches at water saturation a value of the same order of magnitude as inice. Contrary to this effect, one observes in dry membranes, following the sequenceNa' < K+ < Cs', a considerable increase in the maximum frequency of hoHabove the value found in ice.This implies, in passing from Na' to Cs', an impor-tant increase in the rigidity of the water hydration molecules. Such phenomenacannot be interpreted as being in a simple relationship with the hydrogen bondingsince the strength of the hydrogen bond decreases along the same ionic sequence.Let us note that such high frequencies have been detected in crystals containingH 3 0 + groups (1700 cm-1)34 or H501 groups (1675 cm-1).35 The addition ofwater to the membrane produces a progressive decrease of the dOH frequenciestowards the value found in liquid water at z 1645 ern-'.MODEL FOR THE HYDRATION OF ALKALINE SALTS OFFEP-PSSA MEMBRANESOn the basis of the spectroscopic results reported here we will attempt to de-scribe a molecular model for hydration of the membrane.The low frequencies ofthe vgH band in the dry membranes (fig. 8) reveal that the sulphonate sites stronglybind the first absorbed water molecules. These molecules possess almost no OHgroups free of hydrogen bonding and thus will bridge two anionic sites, as shown infig. 11. The more water molecules there are absorbed in the membrane, the weakerare the hydrogen bonds formed by the water molecules and the electrostatic inter-action linking cation and anion together. Thus a cationic hydration shell may bemore or less completely built up at the cost of the hydrogen-bond donor proper-ties of the OH groups of the water and at the cost of the strength of the alkalinesulphonate ion-pair association. Conclusions of the same type have been reachedby Zundell for PSSA ungrafted membranes and this author developed the argu-ment that the polarizing effect of the cation on the OH bond is reduced if spreadover several water molecules.Closer examination of our experimental results raises the question as to how thewater molecules are affected by the type of cation fixed in their neighbourhood.Theprofound discrepancy found between the behaviour of the bending vibration doH inthe presence of Li+ on the one hand and the larger alkaline ions on the otherprecludes any simple and general model of cationic hydration. In this respect, thL. Y. LEVY, A. J E N A R D A N D H. D. HURWITZ 257 1IC,Fz In- P S -C - FlCF21nK'PisICF,),- C I - ICF, InFIP SIICFZJn-PS -c -II(CF2)nI\CFJ)n-CdCF2)nFFIG.1 1 . 4 ~ ) Hydration models of the Kf salt of the FEP-PSSA membrane. (b) Hydration model of Lifsalt of the FEP-PSSA membrane.dramatic increase in rigidity of the water molecules following the sequenceNa' < K+ < Cs' and the accompanying decrease in frequency as a function ofhydration is a feature not encountered in PSSA ungrafted membranes. In thesesystems the shift of the scissor vibration under the influence of the alkaline cation isonly 3 cm-'.' Faced with this peculiar behaviour, we will focus our attention firston the large cations and suggest that some clustering of ionic multiplets composedof SO, and alkaline cations should occur, according to the following parameters:(a) their sizes, (b) their mutual repulsions, (c) the vicinity of the anionic sites and (d)the rigidity of the hydrophobic polymeric matrix.A model of stable molecularconfiguration of polyelectrolytes arising through ionic aggregation has already beenproposed in some similar cases. Note, for example, that a dynamic mechanicalstudy of perfluorosulphonic copolymers has yielded different results with Li + salt2572 IONIC HYDRATION IN ION-EXCHANGE MEMBRANEShydrophobic 1 region0 -FIG. 12.-Biphasic molecular configuration in a supermolecular structure.on one hand and the remaining alkaline ions36 on the other. For this system,some experimental arguments based on stress relaxation and small-angle X-rayscattering have led to the suggestion of ion clustering. Such a model of ion cluster-ing, advocated in this context by E i ~ e n b e r g , ~ ~ is not just a local multiplet ionaggregation leading to electrostatic cross-linking in the polymer, but introduceslarge-scale order incorporating a large number of regions of ionic grouping separ-ated by the non-ionic polymeric material.It is difficult to assess how far such amolecular picture of supermolecular ionomeric aggregation can be adapted to ourpresent results. Nevertheless, Hopfinger et al. ’** have conceived a molecular modelfor the case of perfluorinated copolymer membranes of aqueous solvated polymerswith mobile ions and water present in the pores, as depicted in fig. 12. The mem-brane is envisaged as being composed of spherical pores coated by a polymeric skinand drawn together to form supermolecular structures.Inside each pore the anionand countercation interact forming ionic multiplets intermeshed with water. Aninteresting feature predicted by the model is the loss of individual ionic hy-dration shells, the remaining water molecules being accommodated within theinterstices of the ionic network near the sulphonate sites. The rigidity of the poly-meric backbone limits the extension of volume occupied by the ionic multiplets. Itfollows that at low water content the elastic deformation exerted on the packing ofthe hydrogen-bonded water molecules attached to the SO, sites and located in theionic network increases with the size of the countercation.If the relative humidityin the membrane rises, the water molecules, in order to satisfy the free energybalance, will increasingly absorb in the vicinity of the cations and thereby decreasethe lifetime of the cluster [see fig. ll(a)]. This kind of behaviour is in agreementwith the observation of stress relaxation changes in perfluorinated copolymers37 onaddition of water and is also suggested by the position and shift of thevibration recorded in fig. 9.The peculiar behaviour of the small cation Li+ certainly pertains to its ability tointeract strongly with the anion. In some way, the Li+-SO, pair should proveunable to form more than ionic dipole doublets or quadruplets. Thus it will notcontribute to a large-scale cluster organization. The initial water moleculesattached to the SO, sites are therefore strongly polarized by the electric field of the* We thank Dr.L. Bourgeois of Solvay (Belgium) who kindly made this communication known to usL. Y . L E V Y , A . JENARD A N D H. D. HURWITZ 2573nearby cation. This greatly enhances the hydrogen-bond donor properties of theOH group and in addition decreases the energy of absorption of the water molecule[fig. ll(b)]. On increasing the water content in the membrane and as a consequenceof the strong interaction with the cation, a hydration shell is established whichloosens the hydrogen bonds and also enhances the value of the force constant of thebending vibration mode of the water towards that of liquid water, The final settingup of a constant aO1, frequency of the order of magnitude of that of ice, which islarger than the values found for Na' and K', appears consistent with observationson electrolytic aqueous solutions and characteristic of a water structure orderingion with peripheral hydration.33 That the frequencies do not approach a con-stant value at any value of nHzO clearly indicates that even at a relative humidity of80% no definitive hydration configuration of the anion and cation is reached in themembrane.CON CLU S 1 0 N SThe use of i.r. spectrophotometry under isopiestic conditions in conjunction withmore classical techniques, such as microcoulometric titration and Karl Fischertitration, has proved to be a very valuable means of investigating the hydration ofthin ion-exchange membranes.Our results indicate that the presence of a Teflon FEP matrix yields an elasticconstraint exerted on the grafted polyelectrolyte, on the counterion and on waterwithin the pores which might lead, in the case of the large alkali cations, to thesetting up of an ionic cluster configuration. 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King, Ion Containing Polymers and PIzysical Properties and Structure, in Polymer3 8 A. J. Hopfinger, K. A. Moritz and C. J. Hora, communication at the meeting of the Electrochem. SOC.,I. Kampschultre-Scheuring and G. Zundel, J . Phys. Chern., 1970, 74, 2363.N.Y., 19661, p. 87.Physics, ed. R. S. Stein (Academic Press, 1977), vol. 2.Oct. 1977, Atlanta, U.S.A.(PAPER 911812