IndianJournalof ChemicalTechnology
Vol. 1, May 1994,pp. 158-160
'r-'-4.
t
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, BhabhaAtomic ResearchCentre, Trombay, Bombay400 085,India
Wateruptake (nw)by the Mg2+,Ca2+, Sr2+, and &2 + ionic fonDSofNafion-l1? exchangermembraneas well as
y'-
Dowex 50WX8 resinhas beeninvestigatedasa 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 changeson 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 differencesbetweenthe 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
a factor of wider importance particularly in relation
to its kinetic and equilibrium exchange behaviour,2.
Our recent isopiestic water sorption investigations3,4, on proton and alkali metal ion forms of two
well known strong cationites namely Nation-117
(membrane form) and Dowex 50WX8 (granular
form) yielded swelling free energies (L\Gsw), and
cationic hydration (n+) for each ionic form, larger
spread being observed in Nation-I 17 with evidence
of solvent shared ion pair formation in its Cs + form.
standard procedure1,4 .These are then air dried and
stored. Known weights of air dried exchangers (~ I g
each) were heated (membranes 413 K; granules 363
K) under vacuum to determine their water content~
(Nation 0.053-0.090 gig; Dowex 50WX8 0.143-0.245
gig) followed by their capacity determinations by a
procedure described earlier3 -5. Based on these, dry
capacities have been found to be 5.02 (Mg2 +)-3.79
(Ba2+) for Dowex 50WX8 resins and 0.929 (Mg2+),
-0.876 (Ba2+) meqlg for Nation-1 17.
Water sorption (nw) as a function of water activity
This information
could be correlated with larger
spread of ionic selectivities in Nation-1 17 for alkali
metals3. Similar water sorption investigations for
alkaline
earth forms of Nation-117
(-thickness
0.0176 cm) and Dowex 50WX8 have presently been
carried out at 298.15:t I K with particular objective
of studying and comparing the hydration behaviour
of alkaline earth ions in these exchangers,
(ow) was investigated at 298.15:t I K using an
isopiestic unit fabricated earlier3,4 and aqueous
solutions of H2SO4 (low ow) and LiCI (high °v-).
Sorption isotherms -.£..onstructed from the data
yielding differential (L\Gsw)and integral swelling free
energies (L\G"w) for the two exchangers in various
ionic states following standard relationships 1,3.4.
Plots of differential free energies as a function of nw
yielded cationic hydration numbers (n + , assuming
hydration number for -: S03- group to the unity as is
generallydone1,3,4,7) following an approach adopted
earlier3,4,
Experimental Procedure
Nation-117{H+) and Dowex SOWX8 (H+) were first
treated with 2M HCI for their complete conversion
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,
CaCI2.2H2O,
Sr(N03)2 and Ba(OH)2.8H2O
following
The isothem1S obtained for Mg2+, Ca2+, Sr2+ and Ba2+
forms of Nation-l
17 and Dowex 50WX8 showed that
each ionic form of Nation-1 17 exhibited larger water
tThis paperwaspresentedin the 9th Indian MembraneSociety
Conferenceon "Integrated MembraneProcesses"held during
sorption (nw) throughout the water. activity range. In
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
\.
f'
I
~
t..;.-
1
..
---<~
I
.
IYEReta/.:NAFION-117EXCHANGERMEMBRANEAND VOWEX50WX8RESIN
Table I-Maximum
water sorption (nw), integral swelling free
en~rgies(~Gsw)
and 50WX8exchangers
hydration numbers
foralkaline
Nafion-111
membranes
andDowex
inthe
earth
Ionic
form
.Mg2
'-y
~ .M
+
Ca2+
Sr2+
Ba2+
Nafipn-117
membranes
12.4
58;0
6.0
11.0
38.1
5.4
10.6
33.4
4.9
9.4
28.0
4.1
Dowex50WX8granules
2+
g
~+'
Ca
10.5
(8 3)**
29.2
4.3
8.5
(7.9)**
23.9
3.6
Sr2+
8.05(7.5)**
23.6
3.3
..for
::~:::~:~~~
Na/ian-ll?
~=:::;;
2
5.0
4.4
3.9
3.1
I
Dowex
:t-
~--~---~~=:
-
SOW X 8
2+
Ca2+ 5,2+
2+
0
0.5 o.
I-I,
, '1
3.3
Fig: I-Plotsof~ationic h~dr~tionnumberasafunctionof crystal
(3 5)*
radius for alkaline earthIOnICforms of Nafion-117and Dowex
.50WX8
(298.15:::1:
I K)
2.6
(2.6)*
2.3
.(2.3)*
6.2
9.3
2.5
1.5
(5.8)**
(1.0)*
*Data fromref. (7)for 0.5%crosslinked
exchanger.
**Data from ref. (5)roundedoff to first decimalfigures
..3.9
5
ionic forms(All dataare on the equivalentbasis)
nw,
-;1G,w,
Total
Cationic
mol H2Ojequiv kJ(eqwv number,
hydration
hydration
nT number,n+ c+ 3
"Ba2+
~.
6
159
Thus,~Gswvaluesforthetwoexchangersinalkaline
earth ionic forms
(being reported
for the first
time
to
lower aw values, the differences were particularly
prominent. The total hydration numbers (nT) in the
exchanger phase obtained from differential swelling
the best of our information) show greater free energy
decrease on water sorption by Nafion-II7 for each
individual ionic form the largest difference is
b.t
. 1Y rlor the M g +'2 rlorm wh.lCh a1so exh lIS
0bVIOUS
largest n+ and nw values. Thus, cationic hydration
numbers, nw aQd decrease in free energy on water
sorption (-,-~Gw)' all exhibit the following ionic
sequence:
free energy vsnw plots have also beensummarised in
Table 1. From these, cationic hydration numbers
(n+) have been obtained on the equivalent basis. As
canre ~ n+ valuesof 3.3(Mg2+),26 (Ca2+),23 (Sr+)
and 1.5 (Ba2 +) obtained for Dowex 50WX8 agree
reasonably well with those reported by Glueckauf
and Kitt7 for a very low crosslinked Dowex 50 resin
based on enthalpies and entropies of water sorption
determined by them. This agreement supports the
methodology3.4 of obtaining n+ from swelling free
energyplots. Thus, n+ valuesof 5.0 (Mg2+), 4.4 (Ca2+),
(Sr+) and 3.1 (Ba2+)derived for Nafion-I 17for the
first time (Table 1) are clearly much higher than those
Dowex 50WX8 resins.Plots of n+ as a function of
crystal radius are good linear plots as expected for
both exchangers (Fig. 1) but the striking feature is
nearly a constant difference in the n+ values for the..
two exchangersirrespective of ionic form (slopes are
nearly the same).
It is known from batch to batch of an exchanger,nw
values could differ by as much as :t 10%. This is
confirmed by comparing presentnwvalues for Dowex
50WX8 exchangers with those recently reported by
Toteja et af.5(Table 1). As far as swelling free energies
are concerned,the only other study reported is that by
Gregor et af.8 for a Dowex 50 resin of 10%
crosslinking, which cannot be strictly compared.
Mg2+> Ca2+> Sr2+> -Ba2+
...(I)
The ~Gswvalues are expected to be accurate within
:t4%. While deriving cationic hydration numbers,
the above experimental accuracy in free energy was
taken into account.
The above sequence of cations is in conformity
with their hydration tendencies9.10.The decrease in
free energies on swelling are found to be in the same
sequenceas the free energies of hydration of these
al~alineearth ions10.The n+ values reported earlier3
forH+ and alkali metal ionic forms of Nafion-117 are
as follows: H+ (6.0); Li+ (5.5); Na+ (1.5); K+ (1.5);
Rb+ (0.3) and Cs+ (0.0). Inclusion of these numbers
yield the following overall sequence for cationic hydration in Nafion-117
H+>Li+>Mg2+>Ca2+>SrZ+>8a2+>Na+>K+>Rb+
>Cs+
.(2)
which is identical with the sequencerelated to free
eneregiesof hydration of theseions on the equiyalent
basis9.
The reasons behind greater hydration of alkali
metal ions in Nation-I 17compared to that in Dowex
50WX8 apparently lie in (i) differences in the
backbone and morphological structure of the two
exchangers. Due to fluorine in the Nafion-II7
backbone, the electrical charge density on -SO,group is lower compared to that in Dowex 50WX8.
'
~
~:
::
\,
~:
;
;.
.:;
.'
:-
160
I
INDIAN J. CHEM. TECHNOL., MAY 1994
This obviously reduces the competition for hydration
between the s'ulphonate group and alkaline earth ions
leading to greater hydration of the latter ions, and
(ii) differences in the ionic concentration in the fully
swollen Nafion-117 and Dowex 50WX8. As per the
information available on the equival~nt volu.mes of
the dry and water
forms
of
these
wet forms of certaIn alkali ~et~l
exchangers 11',12, the
catiOnIC
Associate Director, Chemical Group, BARC for
encouragement given while carrying out the present
investigations.
Rcfcrcnccs
I HelfferichF, Ion exchange(McGraw Hill, NewYork), 1962.
2 Marcus Y, in Ion exchangeand solvent extraction, edited by
Marinsky J A and Marcus Y (Marcel Dekker, New York),
concentration in the swollen Dowex 50WX8 is larger
than that in Nafion-117. This should obviously lead
to greater hydration
of cations in Nafion-1I7,
...
1 beha ' ur fior alkall ' ne ea r t h
assumIng slml ar
VIO
forms.
Preliminary data obtained on the ion-exchange
selectivities of alkaline earth ions (against a reference
ion, H +) show that selectivities move in a narrower
range for Dowex 50WX8 ionite compared to that for
h. b
...l"
. h
YQI 4 (1973)1-119.
3 Iyer S T, NandanD & Iyer R M, IndianJ Chern,31A (1992)
317.
4 PushpaK K, Nandan D & Iyer R M, J ChernSoc,Farada.v
Trans1,84 (1988)2047.
5 Toteja R S D, JangidaB L & SundaresanM, Indian J Chern,
30A (1991)322.
6 Nandan D & Gupta A R, IndianJ Chern,20A (1981)974.
7 GlueckaufE & Kitt G P,Proc RoyalSoc London,Ser A, 228
(1955)322.
the h swelling (Tfree
bI energies
1)
observed
exc angers
a e
.New
9 Franks F, Water,a comprehensIve
(realtlse,Vol 3 (Plenum,
York), 1973.
Nafion-I~7.T
ISO
serv~tlonlsmconlormltywlt
for the two
Acknowledgement
Two of us (STI and DN) thank Dr J P MittaI,
8 Gregor H P, Sundheim
Co//oidSci,7(1952)511.
B R, Held K M & Waxman
..
.
';
,.
M H, J
10 Jain D V S, Indian J Chern,3 (1965)466.
II NandanD, Mohan H & Iyer R M, J Membr Sci,71 (1992)69.
12 Nandan D & Gupta A R, J Phys Chern,81 (1977).1174.
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