Nafion-117) and Dowex 50WX8 Cationites in Aqueous Medium T

Indian Journal of ChemicalTechnology Vol. 1, May 1994,pp. 158-160

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Integral swelling free energies of alkaline earth ionic forms of perfluorosulphonate exchanger membranes (Nafion-117) and Dowex 50WX8 cationites in aqueous medium t . Sita T Iyer, Deoki Nandan & R M Iyer Chemistry Division, Bhabha Atomic ResearchCentre, Trombay, Bombay400 085,India Water uptake (nw)by the Mg2+, Ca2+, Sr2+, and &2 + ionic fonDSofNafion-l1? exchangermembrane as well as y'- Dowex 50WX8 resinhas beeninvestigated as a function of water activity (aw).The isothermsconstructed reveal " . appreciable differences for eachionic form, larger water uptake being seenfor Nation-II? particularly at low water activities. Thus much larger integral free energy changes on water sorption (AGsw)haw beenfound for Nation-II? compared to that for Dowex 50WX8, Mg2 + form revealing a factor of two while 8a2 + form AGsw involves a factor closerto three. Analysis further shows that the differencesbetween the two exchangersemanate due to different hydration behaviour of each cation caused by different exchangermorphologies and ionic concentrations. Preliminary data obtained on the alkaline earth ion/H+ exchangesappear to support above conclusions.

The degree of solvent sorption by an ion exchanger is standard procedure1,4 .These are then air dried and a factor of wider importance particularly in relation stored. Known weights of air dried exchangers (~ I g \. to its kinetic and equilibrium exchange behaviour,2. each) were heated (membranes 413 K; granules 363 f' I Our recent isopiestic water sorption investiga- K) under vacuum to determine their water content~ tions3,4, on proton and alkali metal ion forms of two (Nation 0.053-0.090 gig; Dowex 50WX8 0.143-0.245 ~ well known strong cationites namely Nation-117 gig) followed by their capacity determinations by a (membrane form) and Dowex 50WX8 (granular procedure described earlier3 -5. Based on these, dry form) yielded swelling free energies (L\Gsw), and capacities have been found to be 5.02 (Mg2 +)-3.79 cationic hydration (n+) for each ionic form, larger (Ba2+) for Dowex 50WX8 resins and 0.929 (Mg2+), spread being observed in Nation-I 17 with evidence -0.876 (Ba2+) meqlg for Nation-1 17. of solvent shared ion pair formation in its Cs + form. Water sorption (nw) as a function of water activity This information could be correlated with larger (ow) was investigated at 298.15:t I K using an spread of ionic selectivities in Nation-1 17 for alkali isopiestic unit fabricated earlier3,4 and aqueous metals3. Similar water sorption investigations for solutions of H2SO4 (low ow) and LiCI (high °v-). t..- alkaline earth forms of Nation-117 (-thickness Sorption isotherms -.£..onstructed from the data 0.0176 cm) and Dowex 50WX8 have presently been yielding differential (L\Gsw)and integral swelling free ;.- carried out at 298.15:t I K with particular objective energies (L\G"w) for the two exchangers in various 1 of studying and comparing the hydration behaviour ionic states following standard relationships 1,3.4. of alkaline earth ions in these exchangers, Plots of differential free energies as a function of nw yielded cationic hydration numbers (n + , assuming Experimental Procedure hydration number for -: S03- group to the unity as is Nation-117{H+) and Dowex SOWX8 (H+) were first generallydone1,3,4,7) following an approach adopted treated with 2M HCI for their complete conversion earlier3,4, into H+ follll. Mg2 +, Ca2 +, Sr2+ and Ba2+ ionic foll11Sof these exchangers were generated from H+ form using Results and Discussion 0.5M aqueous solutions of MgCI2.6H2O, The isothem1S obtained for Mg2+, Ca2+, Sr2+ and Ba2+ ..I CaCI2.2H2O, Sr(N03)2 and Ba(OH)2.8H2O following forms of Nation-l 17 and Dowex 50WX8 showed that ---<~ each ionic form of Nation-1 17 exhibited larger water I tThis paperwas presentedin the 9th Indian MembraneSociety sorption (nw) throughout the water. activity range. In Conferenceon "Integrated Membrane Processes"held during Table l, nw values (at Ow = 1) listed for the two 6-? February 1992,at IICT, Hyderabad. exchangers show the differences at this activity. At . IYEReta/.: NAFION-117 EXCHANGER MEMBRANE AND VOWEX50WX8 RESIN 159

Table I-Maximum water sorption (nw), integral swelling free 6 membranesen~rgies(~Gsw) and Dowex and 50WX8exchangershydration numbersin the foralkaline Nafion-111earth 5 ::~:::~:~~~Na/ian-ll? :t- ionic forms (All dataare on the equivalentbasis) Ionic nw, -;1G,w, Total Cationic form mol H2Ojequiv kJ(eqwv number, hydrationnT number, hydrationn+ c+ 3 ~=:::;; ~--~---~~=:- Dowex SOW X 8 Nafipn-117membranes 2 .Mg2 + 12.4 58;0 6.0 5.0 Ca2+ 11.0 38.1 5.4 4.4 I 2 + Ca2+ 5,2+ 2+ Sr2+ 10.6 33.4 4.9 3.9 Ba2+ 9.4 28.0 4.1 3.1 0 0.5 o. I-I, '-y Dowex50WX8 granules , ' 1 ~ .M 2+ 10.5 29.2 4.3 3.3 Fig: I-Plotsof~ationic h~dr~tionnumberasa function of crystal g (8 3)** (3 5)* radius for alkaline earthIOnIC forms of Nafion-117and Dowex ~+' .50WX8 (298.15:::1:I K) Ca 8.5 23.9 3.6 2.6 (7.9)** (2.6)* Sr2+ 8.05- 23.6 3.3 2.3 Thus,~Gswvaluesforthetwoexchangersinalkaline (7.5)** .(2.3)* earth ionic forms (being reported for the first time to "Ba2+ 6.2 9.3 2.5 1.5 the best of our information) show greater free energy ~ (5.8)** (1.0)* decrease on water sorption by Nafion-II7 for each ~: *Data from ref. (7)for 0.5%crosslinked exchanger. individual ionic form the largest difference is :: 1 r th M +'2 r h. h 1 h ' b.t :- **Data from ref. (5)rounded off to first decimalfigures 0 b VIOUS. Y lor e g lorm w lC a so ex lIS \, largest n+ and nw values. Thus, cationic hydration ~: lower aw values, the differences were particularly numbers, nw aQd decrease in free energy on water ; ;. prominent. The total hydration numbers (nT) in the sorption (-,- ~Gw)' all exhibit the following ionic .:; exchanger phase obtained from differential swelling sequence: .' free energy vsnw plots have also been summarised in Mg2+ > Ca2+> Sr2+> -Ba2+ ...(I) Table 1. From these, cationic hydration numbers The ~Gswvalues are expected to be accurate within (n+) have been obtained on the equivalent basis. As :t4%. While deriving cationic hydration numbers, can re ~ n+ valuesof 3.3(Mg2+), 26 (Ca2+),23 (Sr+) the above experimental accuracy in free energy was and 1.5 (Ba2 +) obtained for Dowex 50WX8 agree taken into account. reasonably well with those reported by Glueckauf The above sequence of cations is in conformity and Kitt7 for a very low crosslinked Dowex 50 resin with their hydration tendencies9.10.The decrease in based on enthalpies and entropies of water sorption free energies on swelling are found to be in the same determined by them. This agreement supports the sequence as the free energies of hydration of these ~. methodology3.4 of obtaining n+ from swelling free al~alineearth ions10.The n+ values reported earlier3 energyplots. Thus, n+ valuesof 5.0 (Mg2+), 4.4 (Ca2+), forH+ and alkali metal ionic forms of Nafion-117 are ..3.9 (Sr+) and 3.1 (Ba2+)derived for Nafion-I 17for the as follows: H+ (6.0); Li+ (5.5); Na+ (1.5); K+ (1.5); first time (Table 1) are clearly much higher than those Rb+ (0.3) and Cs+ (0.0). Inclusion of these numbers ..for Dowex 50WX8 resins. Plots of n + as a function of yield the following overall sequence for cationic hy- crystal radius are good linear plots as expected for dration in Nafion-117 both exchangers (Fig. 1) but the striking feature is H+>Li+>Mg2+>Ca2+>SrZ+>8a2+>Na+>K+>Rb+>Cs+ nearly a constant difference in the n+ values for the.. .(2) two exchangers irrespective of ionic form (slopes are which is identical with the sequencerelated to free nearly the same). eneregiesof hydration of theseions on the equiyalent It is known from batch to batch of an exchanger,nw basis9. values could differ by as much as :t 10%. This is The reasons behind greater hydration of alkali confirmed by comparing presentnw values for Dowex metal ions in Nation-I 17compared to that in Dowex 50WX8 exchangers with those recently reported by 50WX8 apparently lie in (i) differences in the Toteja et af.5 (Table 1). As far as swelling free energies backbone and morphological structure of the two are concerned,the only other study reported is that by exchangers. Due to fluorine in the Nafion-II7 Gregor et af.8 for a Dowex 50 resin of 10% backbone, the electrical charge density on -SO,- crosslinking, which cannot be strictly compared. group is lower compared to that in Dowex 50WX8. 160 INDIAN J. CHEM. TECHNOL., MAY 1994

I This obviously reduces the competition for hydration Associate Director, Chemical Group, BARC for between the s'ulphonate group and alkaline earth ions encouragement given while carrying out the present leading to greater hydration of the latter ions, and investigations. (ii) differences in the ionic concentration in the fully swollen Nafion-117 and Dowex 50WX8. As per the Rcfcrcnccs information available on the equival~nt volu.mes of I HelfferichF, Ion exchange(McGraw Hill, New York), 1962. the dry and water wet forms of certaIn alkali ~et~l 2 Marcus Y, in Ion exchange and solvent extraction, edited by forms of these exchangers 11',12, the catiOnIC Marinsky J A and Marcus Y (Marcel Dekker, New York), concentration in the swollen Dowex 50WX8 is larger YQI 4 (1973)1-119. .. than that in Nafion-117. This should obviously lead 3 Iyer S T, NandanD & Iyer R M, IndianJ Chern,31A (1992) to greater hydration of cations in Nafion-1I7, 317. ... 1 beha ' ur fior alkall ' ne ea rth 4 PushpaK K, Nandan D & Iyer R M, J ChernSoc, Farada.v assumIng slml ar VIO Trans1,84 (1988)2047. . forms. 5 Toteja R S D, JangidaB L & SundaresanM, Indian J Chern, '; Preliminary data obtained on the ion-exchange 30A (1991)322. , . selectivities of alkaline earth ions (against a reference 6 Nandan D & Gupta A R, IndianJ Chern,20A (1981)974. ion, H +) show that selectivities move in a narrower 7 GlueckaufE & Kitt G P, Proc Royal Soc London,Ser A, 228 range for Dowex 50WX8 ionite compared to that for (1955)322. h. b ...l" . h 8 Gregor H P, Sundheim B R, Held K M & Waxman M H, J Nafion-I~7.T ISO serv~tlonlsmconlormltywlt Co//oidSci,7(1952)511. the h swelling (T free bI energies 1) observed for the two 9 Franks F, Water,a comprehensIve(realtlse, Vol 3 (Plenum, exc angers a e .New York), 1973. 10 Jain D V S, Indian J Chern,3 (1965)466. Acknowledgement II NandanD, Mohan H & Iyer R M, J Membr Sci,71 (1992)69. Two of us (STI and DN) thank Dr J P MittaI, 12 Nandan D & Gupta A R, J Phys Chern,81 (1977).1174. '. .

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