Indian Journal of Chemistry Vol. 31A,June 1992, pp. 317-322

Thermodynamic equilibrium constants of -hydrogen ion exchanges and related swelling free energies in perfluorosulphonate membrane (Nafion-117) in aqueous medium

Sita T Iyer, Deoki Nandan & R M Iyer" Chemistry Division, Bhabha Atomic Research Centre, Trombav. Bombay 400 085

Li + IH + , Na + IH t , K + IH + , Rb + IH + and Cs + IH + exchange equilibria on a perfluorosulphonate ex- changer (Nafion-1l7) have been investigated at 0.1 M ionic strength in aqueous medium and thermody-

namic equilibrium constants of 0.78 to 12.6 (Li + IH t to Cs + IH + ) have been evaluated and the alkali me-

tal selectivity sequence has been found to be Cs + > Rh + > K + > Na + > Li ". The various ionic forms of Nafion-1l7 involved have also been subjected to isopiestic water vapour sorption investigations and the hydration numbers and swelling free energies determined. The exchange selectivities have been found to be consistent with the sequence of ionic hydration and swelling free energies, the exchange free energies showing a linear relationship with the difference in free energies of hydration of the exchanging as per Eisenman's model. The expanded selectivity as well as swelling free energy ranges observed in Nafion- 117 compared to Dowex SOW type of resins have been interpreted in terms of hydrative and osmotic swelling behaviours of the two exchangers. Existence of a solvent shared ion pair in Nafion-1l7. i.e.

- SO; (H20)Cs + observed in an earlier study has been supported by present selectivity data.

Due to its exceptional chemical inertness and fa- have, however, emphasized that compared to PSS- vourable electrical properties, Nafion l-4 (a novel DYB exchangers, Nafion-120 exhibits greater selec- perfluorinated ion exchanger) membrane has been tivity spread for alkali metal ions which could be at- exploited for a number of chemical and electro- tributed to the cluster morphology of Nafion and chemical applications" including its use as a separa- the lower charge density on - SO; compared to tor in chlor-alkali cells. This ionomer of fluorocar- that in PSS-DYB. The authors supported these ob- bon backbone (structure I) has pendant side chains servations through satisfactory column chromatog- terminating with - S03 - H + exchange groups (m is raphic separation of alkali metals using powdered nearly unity and n varies between 5 to 11 thus gen- Nafion-120. erating an equivalent weight EW of 1000-1500). Al- In the recent past, a study from this laboratory' on. though, conventional polystyrene-DVB sulphonate the isopiestic water sorption isotherms of H + , Li + (PSS-DYB) exchangers+ such as Dowex 50, Am- and CST forms of Nafion-117 (EW -1100) mem- bcrlite IR-120 etc., too possess the same exchange brane has revealed larger swelling free energy group and have been thoroughly studied for their changes and swelling pressures generated from Naf- ionic selectivity behaviour in aqueous medium, ion-117 -water interactions involving H + and Li + there is only one study reported! (on the alkali metal forms. The hydration numbers (n +) for H +, Li + and alkaline earth ions versus H+) for Nation-120 were also deduced to be larger (6.0, 5.5 respect- (EW - 1200) and even this study reports selectivity ively) compared to 3.8 and 3.0 for PSS-DYB resins coefficients (Kc) at 50% exchanger loading (also no (hydration number for - SO; group was consid- attempt was made to incorporate solution phase ac- ered to be same, 1.0)3.Again, contrary to PSS-DYB tivity coefficients to obtain equilibrium constants (Cs "), Nafion-117 (Cs ") revealed the presence of which represent the true selectivity). The -authors? solvent shared ion pair [- SO; (H20)Cs + ]. Another investigation relating to deuteriumlhydrogen frac- tionation effects" revealed lesser hydrogen bonded (CFZCF2)n CFZCF- structure of water in Nafion-117. These data relating I to Nafion-117 membrane may indeed support the (OCFZCF) OCF CF -SO-Hi" I m Z 2 3 possibility of larger selectivity spread for alkali me- tals (and particularly high selectivity for Cs + in Naf- CF3 (I) ion-117 which can be a very useful consideration) 31H INDIAN J CHEM, SEe. A, JUNE Ig92

but suffer from the following (i) no water sorption is- 1L otherms are available for Na +, K + and Rb " forms (ii) no selectivity data are available concerning alkali metal - H + exchanges on Nafion-117. As the total water sorption data sct obtained for H + and alkali ion forms of Nafion-117 is apparently different from that for Nafion-120, the ionic selectivities in the two are also likely to be different. Whether a greater spread of selectivity also exists for Nafion- 117 thus remains to be explored. In view of above, equilibria in aque- ous medium involving Li + IH+, Na + /H+, K +/H+, Rb ' IH+ and Cs ' IH+ exchanges on Nafion-117 have been investigated in the present work. With the objective of getting a deeper insight into selectivi- ties, isopiestic water sorption isotherms have also been determined for Na +, K + and Rb + forms while similar isotherms for H +,Li + and Cs + forms have been reobtained under identical conditions, extend- ing them to lower water activity (aw) regions.

Materials and Methods Membranes- H +, Li + and Cs + forms were gen- erated as described earlier:'. Na ", K+ and Rb ' forms of Nafion-117 were generated from the H+ form using 0.5 M solutions (aq) of NaOH, KOH and RbCI following standard procedures. The mem- branes were stored in air dried (AD) forms. Ion ex- change capacities obtained on the totally dry basis are: H+ (0.94); Li+ (0.937); Na+ (0.915); K+ (0.900); Rb+ (0.870); Cs ' (0.845) meq/g. Fig. 1- Isopiestic water sorption isotherms for various ionic forms of Nafion-117 membrane (298 K) Isopiesticstudies- Water sorption as a function of water activity was investigated using two similar iso- piestic units fabricated earlier+? and aqueous elec- Table 1- Ionic hydration and swelling free energy data for trolyte solutions? of LiCI and H2S04, Sorption iso- Nafion-117 membranes (298 K) therms of various ionic forms of Nafion-117 were Ionic n -!!J.G,w obtained as n; (moles of water/equiv) versus a, ~ T n+ form (ala\\:= I) (integral) curves. Dry weights of the AD membranes for ob- kJ/mol taining n, were computed using another set of var- ious ionic forms which were heated in a vacuum ov- H+ 14.2 7,0 6.0 45,0 en at 413 K and the moisture contents determined. (3,8)* Ion exchange equilibria-Nearly 0.1 M solutions Li+ 13.6 6,5 5.5 36,1 of HCI, LiCI, NaCl, KCI, RbCl and CsCI were em- (3.0)* ployed, Batch method was used to investigate alkali 7.4 2.5 1.5 14.4 metal - H + exchange equilibria using approximate- Na+ ly 0.5 g each of membrane pieces (AD) and a total of (2.5)* 25 ml solution (HCI + MCI, M == alkali metal) in K+ 7.1 2.5 1.5 14.0 each erlenmeyer allowing 48 hrs for equilibrium at- (2.3)* tainment. Equilibrium concentrations of H + and Rb+ 4.1 1.3 0.3 7.3 M + ions in the outer solution were determined titri- Cs+ 3.2 1.0 0.0 6.1 metrically (NaOH, 0.05 M), and flame photometri- (2.0)* cally using Varian Techtron AN) model (Li, 670.8 nm; Na, 589.0 nm; K, 766.5 nm; Rb, 780.0 nm and *Values are for Dowex 50 resins (ref. 3,9). Cs, 852.1 nm) respectively. Quantities of H+ and IYER ('I III.: EXCHANGE EQUILIBRIA OF ALKALI IONS-HYDROGEN ION ON NAFION-117 31 »

SO~------~ Results Water sorption isotherms for H + and alkali metal _-----H+ ionic forms of Nafion-l17 are shown in Fig. 1. As can be seen, the ionic sequence of water sorption at any water activity is H+ >Li+ >Na+ >K+ >Rb+ > Cs + (Na + and K + forms isotherms are identical- L, + 35 upto a, - 0.9). Slight differences in the isotherms of H+ , Li + and Cs + can also be observed on compar-

)0 ison with those reported earlier" though their shapes remain essentially same. Differential swelling -0 E free energy (~Gsw) plots constructed from the data ~ are shown in Fig. 2, as computed using the relation- ship". 20 ~ VI 1'"

- 30 kJ/mol. Thus ~ Gsw support the above obser- K = mH+. XMN : .. (2) vation of larger range of ionic hydration in Nafion- c m + X M HN 117 as is actually expected owing to the fact that Equilibrium constants (K) were computed using the generally larger hydration effects also lead to larger relationships, osmotic (and thus total swelling) effects in ion ex- changer phase.

x MN-I Alkali metal ion-H" exchange selectivity coeffi- log K = log K; + log Yi' HO ... (3) cients obtained as a function of exchanger loading JXMN-O Y±MO (XMN) have been shown in Fig. 3. At any exchanger Y± being the mean molal activity coefficients of loading, the K, increases from Li +IH + to Cs +IH +. pure electrolytes in their aqueous solutions of 0.1 M Also K; increases for most exchanges as XMN dec- each. The ratio of molalities and molarities have been reases (a comparison of present plots with those of treated to be the same in the present study involving Nafion-120 reveals some significant differences dilute aqueous solutions (Eqs 2 and 3) of ionic which may be due to use of different EW exchang- strength 0.1 . ers as weU as ionic strengths). However, integral va- 320 [NOlAN J CHEM, SEe. A, JUNE 1992 ,I

12

10

e

u >C 6 , 6

2 • + + o L i I H 0 0'0 0·2 04 06 0·8 1·0

Fig. 3-Selectivity coefficient (Kc) versus exchanger loading plots for alkali metal ion - H + exchanges on Nafion-1l7 at 0.1 M ionic strength (298 K) lues arc of greater relevance (first term in Eq. 3; nut water around fixed charges and counterions is ex- available for Nafion-120) which are summarized in pected to lead to important variation 11 in the overall Table 2 for present exchanges. After incorporating swelling (hydration + osmotic) behaviour as well ion activity coefficient corrections (second term in Eq. exchange selectivity in the two types of exchangers 3), thermodynamic equilibrium constants (K) are as is actually witnessed above in terms of expanded found to range from 0.78 (Li+ /H+) to 12.6 (Cs+ I ranges for hydration numbers, integral swelling free H +) yielding free energies of exchange (~Gex) from energies and ion exchange equilibrium constants for +0.617 (l.i+/H+) to -6.29 (Cs+/H+) kl/mol Nafion-117 compared to Dowex 50 type of ex- (Table 2). These data clearly show that compared to changer (8% DVB is taken as an example). It is re- H+ , all alkali ions exhibit greater selectivity except- cognised that due to the presence of in the ing Li +, the overall selectivity sequence being Cs + Nafion backbone, the electric field strength on the > Rb > K + > Na + > Li + which is' exactly op- sulphonate fixed group is lowered/ thereby reduc- posite to the sequence observed for ionic hydration ing its interaction with water (JR studies 12, however, as well as swelling free energies, as expected based confirm that it does interact with water though the on Eisenman's model, as well as Gregor's mechanis- hydration number could not be determined and tical model of ion exchangers (although the latter therefore is assumed to be unity! as is the case with model is evolved for crosslinked resins such as PSS- PSS-DVB exchangers). This lowered hydration ap- DYB but is applicable to other exchangers). How- parently promotes greater hydration of counterions ever, it is clear that present selectivity range is much with high charge density (H ", Li +) due to lesser larger than that of PSS-DYB resin such as Dowex competition for the available water while ions of 50WX8 (included in Table 2, 0.79-2.31), which lower charge density (Na +, K +, Rb +) are hydrated should lead to better resolution in the alkali metal- to a lesser extent. The cluster morphology of Naf- ion separation as observed by Yeager et at', ion-Il7 must also be responsible. The presence of solvent separated ion pair in Nafion-117 (Cs ") has Discussion been reconfirmed in the present study. As integral Compared to a three dimensional crosslinked and free energies are mainly derived from free energies somewhat rigid and heterogenous hydrocarbon of hydration of ions, a larger range of ~ Gsw is also structure of PSS-DVB exchangers!", Nafion is con- observed for Nafion-117 compared to PSS-DVB sidered to be a dynamic type of polymer of high mo- resins. lecular weight with a cluster morphology+':". Owing The interrelationship between the swelling behav- to differences in the backbone material, ionic con- iour and the ionic selectivity is well recognised-P centration (capacity) and morphology, the state of and reflected in several theoretical models of ion ex- IYER 1'1 al.: EXCHANGE EQUILIBRIA OF ALKALI IONS-HYDROGEN ION ON NAFION-117 32]

-r Table 2-Selectivity coefficients, equilibrium constants (K) and free energies of ion exchange (ti C,,) for •• alkali metal ion - H + exchanges

Exchange 10gK K - tiCex system j""IOgK, Iog--Y,HCI (kl/rnol) XMN-~O Y'MCI u- IH+ -0.1ll 0.003 -0.108 0.78 -0.617 (0.79)* Na+ IH+ +0.295 0.010 0.305 2.02 1.74 ( 1.49)· K+/H+ +0.636 0.014 0.650 4.47 3.71 (2.09)* Rb+ IH+ +0.708 0.Ql8 0.726 5.32 4.15 (2.29)· Cs+ IH+ +0.979 0.022 1.101 12.6 6.29 (2.31)*

"Values are for Dowex 50WX8 exchanger (ref. 2. 8).

changers':". Thus Gregor's theoryv'" considers the 12 difference between the partial hydrated volumes of the exchanging ions as the origin of selectivity while 1 0 l that of Eisenmann-v-" considered to be a superior theory explains selectivity in terms of free energies a e of hydration of ions and the coulombic interactions <."J 06, using the relationship 0

O-L InK=~[(~) -(~) +&G2-&GI] RT rA+ r1 rA+ r2 NQ+ 02 ... (5) a-a where rA, r 1, r 2 are radii of anion and cations (for a cation exchanger), and &G2, &G are hydration - 0-2 1 130 150 170 190 210 free energies of exchanging ions. In the case of a ca- AG HYDRATION (M+)-AG HYDRATION (H+) tion exchanger with a larger size fixed grouping of low field strength (such as Nation-U7), the only Fig. 4- Plot of log K versus difference of hydration free energies term which will mainly contribute to In K will be for the exchanging ions for Nafion-117 (298 K) (~G2 - & G1)· In the present work, the swelling free energies of Nafion-Ll? exhibit the same sequence as here that observations of linear variation of ion ex- the hydration free energies of the ions reported 16,17. change free energies (or log K) with water contents Also excepting the special case of Cs + form of Naf- [e.g.n, or n, (exchange)3.S)]is mere coincidental and ion-II7, the hydration number sequence of ions also has no theoretical basis. Thus, the parameters of im- is consistent with the swelling free energies. Thus, it portance are free energies of hydrations. IS , swelling could be expected that log K values in the present free energies or the volume swellability'':". The an- work should exhibit a linear relationship with alysis of water sorption isotherms in terms of hydra- [~G(M + ) - & G(H+)] excepting probably the case of tion numbers etc works as an aid to visualize any de- Cs + /H + exchange which is likely to show unusually viations from theory (Cs +/H+) and to understand high selectivity (K) due to solvent shared ion pair higher selectivities and the expanded ranges (while formation. This expectation is clearly borne out Li ' IH+ exchange selectivity is same in Nafion-117 (Fig. 4) from the plot of log K versus l& Ghhlralio" and Dowex 50Wx 8, Na+ /H+,K+ /H+ and Rb+ / (M + ) - & GhYdration (H + )]. It should be emphasized H + selectivities are higher in Nafion-l l? due to low- 322 INDIAN J CHEM, SEe. A, JUNE 1992

er hydration numbers of Na + , K + , Rb + ). The pres- 4 Sondheimer S J, Bunce N J & Pyfe C A. JMS-REV, MQC~ ent data (Fig. 4) also emphasize that in the selectiv- mol chem Phys,C26(3) (1986) 353. ity, inclusion of solution phase activity coefficients is 5 HeHferich F, Ion uchange(McGraw Hill, Nell York) 1962. 6 Pushpa K K, Nandan D & Iyer R M, J cltem Soc, FfUtIIiay essential. T1fIILf, 86 (1990) 409. Acknowledgement 7 NandanD& Gupta AR,JphysChem, 81 (1977) 1174. The authors thank Dr S. Ganapathy and his col- 8 NandanD& GuptaAR,JphysChem, 79(1975) 180. 9 Nandan D & GuptaAR, IndianJ Chern,12 (1974) 808. leagues for making the flame photometer arrange- 10 Goldring L S, in "Ion uchange(VoJ. Ir,edited by J A Mar- ment available and for all cooperation. S11 and DN insky (Marcel Dekker, New York) (1966) p. 205. also thank Dr J.P. Mittal, Associate Director, Chem- 11 Tasaka M, Suzuki S, Ogawa Y & Kamaya M, J memb Sci, 38 istry Group for all encouragement during the pres- (1988) 175. . 12 Nandan D, Pushpa K K, Kartha V B, Wahi P K & Iyer R M, ent investigations. [Communicated], 13 BraudC & Selegny E, SepSci, 9 (1974) 13; 9 (1974) 2l. Refere.ces 14 ReicbenbergD,Rd.l0,p-227. 1 Yeager H L & Eisenberg A, ACS Symp. Ser. No. 180; Perflu- 15 Eisenman G, in Membrane transport and metabolism (edit- onnated ionomer membranes, edited by A Eisenberg and H ed by A Kleinzeller and A Kotyk), (Academic Press, New L Yeager, (Amchem Soc, WasbingtonDC, 1982),p.1. York) 1961, pp 163-179; BiophysJ. 2, part 2(1962) 259. 2 Yeager H L & Steck A, Anal Chem, 51 (1979) 862. 16 Franks F, Water, A compnhensive tmltise, (Plenum, New 3 Pushpa K K, Nandan D & Iyer R M, J chem Soc, FfUtIIiay York) (1973) Vol. 3, Chapter l. T1fIILf, I.84 (1988) 2047. 17 JainDV S, IndianJ Chem; 3 (1965) 466.