Effects of specific anions on the relationship between the ion-adsorption properties of sulfobetaine gel and its swelling behavior

Article abstract of DOI:10.1016/j.polymer.2015.01.005

The adsorption of cations and anions from four different halide solutions, i.e., KF, KCl, KBr, and KI solutions, onto N,N-dimethyl(acrylamidopropyl)ammonium propane sulfonate (DMAAPS) gels prepared using various cross-linker concentrations was investigated at a variety of temperatures; the swelling degrees of the gel in these solutions were also examined. The amount of K⁺ adsorbed coincided with the adsorption of the anions (i.e., I^-, Br^-, Cl^-, or F^-) because of simultaneous adsorption of the cations and anions onto DMAAPS gel. The order of anions, which gave larger amount of K⁺ adsorbed onto the gel, was opposite that of the Hofmeister series, i.e., I^- > Br^- > Cl^- > F^-. Furthermore, the relationship between the degree of swelling of the gel and amount of K⁺ adsorbed onto the gel was elucidated. The data points laid on the same line for the same halide solution even at different cross-linker concentrations or temperatures. At smaller degrees of swelling, the amount of K⁺ adsorbed remained unchanged and then decreased gradually as the degree of swelling increased. In solutions containing anions with lower hydration ability, such as KI, the amount of K⁺ adsorbed decreased at larger degrees of swelling. In contrast, in solutions containing anions with higher hydration abilities, such as KCl, the adsorbed amount of K⁺ decreased at smaller degrees of swelling. Additionally, it was found that, in mixtures of these halide solutions, competitive adsorption of anions occurred. Anions with lower hydration abilities adsorbed more readily in the mixtures than in the single-anion solution. In contrast, anions with higher hydration abilities adsorbed less readily in the mixtures than in the single-anion solution.

Full text of DOI:10.1016/j.polymer.2015.01.005

Polymer 59 (2015) 144e154
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Effects of specific anions on the relationship between the ion-
adsorption properties of sulfobetaine gel and its swelling behavior
,
, *
Eva Oktavia Ningrum a b, Yasuhiro Ohfuka a, Takehiko Gotoh a, Shuji Sakohara a
a Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University, Kagamiyama 1-4-1, Higashi-Hiroshima 739-8527, Japan
b Department of Chemical Engineering, Faculty of Industrial Technology, Sepuluh Nopember Institute of Technology, Kampus ITS Sukolilo, Surabaya 60111,
Indonesia
a r t i c l e i n f o
a b s t r a c t
Article history:
The adsorption of cations and anions from four different halide solutions, i.e., KF, KCl, KBr, and KI so-
lutions, onto N,N-dimethyl(acrylamidopropyl)ammonium propane sulfonate (DMAAPS) gels prepared
using various cross-linker concentrations was investigated at a variety of temperatures; the swelling
degrees of the gel in these solutions were also examined. The amount of Kþ adsorbed coincided with the
adsorption of the anions (i.e., Iꢀ, Brꢀ, Clꢀ, or Fꢀ) because of simultaneous adsorption of the cations and
anions onto DMAAPS gel. The order of anions, which gave larger amount of Kþ adsorbed onto the gel, was
opposite that of the Hofmeister series, i.e., Iꢀ > Brꢀ > Clꢀ > Fꢀ. Furthermore, the relationship between the
degree of swelling of the gel and amount of Kþ adsorbed onto the gel was elucidated. The data points laid
on the same line for the same halide solution even at different cross-linker concentrations or temper-
atures. At smaller degrees of swelling, the amount of Kþ adsorbed remained unchanged and then
decreased gradually as the degree of swelling increased. In solutions containing anions with lower hy-
dration ability, such as KI, the amount of Kþ adsorbed decreased at larger degrees of swelling. In contrast,
in solutions containing anions with higher hydration abilities, such as KCl, the adsorbed amount of Kþ
decreased at smaller degrees of swelling. Additionally, it was found that, in mixtures of these halide
solutions, competitive adsorption of anions occurred. Anions with lower hydration abilities adsorbed
more readily in the mixtures than in the single-anion solution. In contrast, anions with higher hydration
abilities adsorbed less readily in the mixtures than in the single-anion solution.
Received 7 October 2014
Received in revised form
31 December 2014
Accepted 3 January 2015
Available online 8 January 2015
Keywords:
Sulfobetaine gel
Adsorption amount
Swelling degree
© 2015 Elsevier Ltd. All rights reserved.
1. Introduction
A zwitterionic betaine polymer that contains both anionic and
cationic active groups in the same polymeric repeat unit with an
Various zwitterionic betaine gels, such as phosphobetaine
[1e3], sulfobetaine [4e6], carboxybetaine [7,8], semi-
interpenetrating network (IPN) gel composed of amphiphilic gel
and zwitterionic betaine polymers [9,10], and chemicalephysical
cross-linked double network (DN) gel, which was obtained by
introducing ductile physical networks into the network of chemi-
cally cross-linked polysulfobetaine gels [11], have shown excellent
chemical stability [12], good biocompatibility [13], ultralow fouling
[14,15], and excellent mechanical properties [14,16]. Studies on the
properties of polymers or gels based on betaine have received
significant attention in the scientific community.
alkylene spacer group is neutral overall. It also offers potential
improvement of the ion selectivity toward adsorption because the
cations and anions in the solution can bond via both the negative
and positive charges located in the same repeating unit [17e19].
Betaine polymers are generally thermosensitive in aqueous solu-
tions [9,16,20]. The transition temperature, which is also referred to
as the upper critical solution temperature (UCST), of sulfobetaine
polymers varies over a wide range by altering the surrounding
conditions and polymer characteristics, such as length of the
alkylene spacer groups [16,20]. Interaction of the charged groups of
betaine and ions in aqueous solution strongly determines the
properties of the betaine polymer [21]. One of the unique proper-
ties of betaine is the ability of the fragments to form cyclic con-
formations of the cationic and anionic groups of neighboring
monomer residues (intra-group), form contacts between the
cationic and anionic groups of neighboring macromolecules (inter-
* Corresponding author.
E-mail address: sakohara@hiroshima-u.ac.jp (S. Sakohara).
http://dx.doi.org/10.1016/j.polymer.2015.01.005
0032-3861/© 2015 Elsevier Ltd. All rights reserved.

E.O. Ningrum et al. / Polymer 59 (2015) 144e154
145
chain), and undergo head-to-tail stacking within single macro-
molecules (intra-chain); these interactions result in betaine being
insoluble in pure water [21,22]. The solubility of betaine in solutions
depends on the nature of the anions and cations and the charge/
radius ratio of the ions, which is described by the Hofmeister series
and Pearson theory [21,23,24].
swelling behavior of DMAAPS gel in these halide solutions and their
relationship. In addition, to elucidate competitive adsorption, the
adsorption behavior of ions onto the DMAAPS gel was studied in
mixtures of these halide solutions.
2. Experimental section
Recently, numerous studies on the properties related to the
transition behaviors of both betaine polymers and gels and on the
adsorption behavior of ions onto betaine gels in various aqueous
solutions have been reported. A series of copolymeric gels con-
taining sulfobetaine was investigated by Lee et al. [25e28]. The
swelling behavior of these copolymer gels was strongly related to
their composition, chemical structure, and nature of ions in the
solutions. Neagu et al. [17] investigated the retention capacities of
divalent and trivalent heavy metals in zwitterionic ion exchangers
with carboxybetaine moieties based on 4-vinylpyridine: Divinyl-
benzene copolymers with two morphological structures, i.e.,
porous- and non-porous-type gels. Both types of gels synthesized
for their study retained metal ions and anions from aqueous solu-
tion; however, they did not adsorb alkaline earth metals. The effect
of the synthetic conditions of poly(DMAAPS) on polymer yield,
intrinsic viscosity, molecular weight, and gel fraction was investi-
gated by J. Ning et al. by varying the monomer concentration [20].
However, there are relatively few studies on the adsorption ca-
pacity of betaine gels and the correlation with its swelling ability in
solvents with various compositions, temperatures, and ion
strengths.
2.1. Synthesis of DMAAPS
DMAAPS was synthesized using the same method reported in
our previous study [29], which was proposed by Lee and Tsai [30];
this reaction involved the ring-opening of N,N-dimethylamino-
propylacrylamide (DMAPAA; KJ Chemicals Co., Ltd., Japan) and 1,3-
propanesultone (PS; Tokyo Chemical Industry Co., Ltd., Japan). A
mixture of PS (75 g) and acetonitrile (75 g) was added dropwise
with continuous stirring at 30 ꢁC for 90 min into a mixture of
DMAPAA (100 g) and acetonitrile (200 g). Stirring was continued for
16 h and then the solution was allowed to stand for 2 d. Precipitated
white crystals of DMAAPS were collected by filtration, washed with
500 mL of acetone, and finally dried under reduced pressure for at
least 24 h. The structure of DMAAPS is shown in Fig. 1.
2.2. Preparation of DMAAPS polymer and gel
DMAAPS polymer and gel were prepared by free-radical poly-
merization. Poly(DMAAPS) was used to evaluate the transition
behavior in response to temperature changes. The preparation
procedures were the same as those described in our previous study
[29]. N,N,N0,N0-Tetramethylethylenediamine (TEMED; Sigma-
eAldrich Co., USA) and ammonium peroxodisulfate (APS; Sigma-
eAldrich Co., USA) were used as a polymerization accelerator and
initiator, respectively. Furthermore, N,N0-methylenebisacrylamide
(MBAA; SigmaeAldrich Co., USA) was used as a cross-linker.
In the preparation of poly(DMAAPS), the concentrations of
DMAAPS, TEMED, and APS were 500, 2, and 2 mmol/L, respectively.
Initially DMAAPS and TEMED were dissolved in deionized water
and the solution volume was increased to 100 mL using deionized
water. This monomer solution was charged into a separable flask.
The solution was purged with nitrogen gas in order to remove any
dissolved oxygen and then 20 mL of APS solution that had also been
purged with nitrogen gas was added. Polymerization was carried
out under a nitrogen atmosphere for 6 h at 50 ꢁC. The resulting
polymer was purified over a period of one week by dialysis using a
membrane with a molecular weight cut-off of 12,000e14,000
(Cellu Step T3, Membrane Filtration Products, Inc.). Additionally,
the average molecular weight of the produced poly(DMAAPS) was
In our previous study, the effects of the temperature and prep-
aration conditions of the gels, such as the cross-linker and mono-
mer concentrations, on the adsorption of cations and anions onto
DMAAPS gel were investigated using Al(NO3)3, Zn(NO3)2, and
NaNO3 solutions with various concentrations [29]. The simulta-
neous adsorption of cations (Al3þ, Zn2þ, or Naþ) and anions (NO3ꢀ)
was confirmed. Furthermore, it was found that the amount of
cation (Zn2þ) adsorbed onto the DMAAPS gel remained unchanged
for the gel prepared with a higher cross-linker or monomer con-
centration even at higher temperatures. In contrast, the amount
adsorbed decreased significantly with increasing temperature for
the gel prepared at a lower cross-linker or monomer concentration.
Based on these results, an interesting correlation between the de-
gree of swelling of DMAAPS gel and amount of cation (Zn2þ
)
adsorbed onto the gel was observed using Zn(NO3)2 solutions of
various concentrations. The data points laid on the same line even
at different cross-linker concentrations or temperatures. For the gel
with a small degree of swelling, i.e., when the polymer concen-
tration in the gel was higher than ~180 g/L, the amount of Zn2þ
adsorbed remained unchanged. The adsorption amount is the
maximum adsorption amount of Zn2þ or NO3ꢀ onto the DMAAPS
gel. For the gel with a large degree of swelling, i.e., when the
polymer concentration in the gel was lower than ~180 g/L, the
amount of Zn2þ adsorbed decreased as the swelling degree
increased. Furthermore, at the same swelling degree, the amount of
Zn2þ adsorbed increased as the concentration of Zn(NO3)2
increased.
Based on the unique characteristic of DMAAPS mentioned
above, the effect of the anion species on the amount of ions
adsorbed onto DMAAPS gel and the relationship with its swelling
degree or polymer concentration in the gel was examined in this
study using solutions of halides, i.e., KF, KCl, KBr, and KI solutions.
These halide solutions were chosen because the halide anions in
these solutions have different hydration abilities and sizes that may
affect their interaction with the charged groups of DMAAPS. These
halide anions have been ordered in the Hofmeister series. The
purpose of the present study is to evaluate the influence of these
anion species on the adsorption behavior onto DMAAPS gel and the
estimated from the intrinsic viscosity [h], which was measured in a
0.1 M NaCl solution at 30 ꢁC using a Ubbelohde viscometer, to be
1.13 ꢂ 106 g/mol [29].
DMAAPS gels were prepared using the same procedures as those
described above for poly(DMAAPS) except that MBAA was used as
the cross-linker. A cylindrical gel was prepared to examine the
swelling properties. The synthesis was performed in a separable
flask containing glass tubes that were 2 mm in diameter and 30 mm
long. The gels were cut into pieces that were 2 mm long and rinsed
several times with deionized water. The gels were then slowly dried
over several days on a Teflon sheet that was spread on a Petri dish.
Fig. 1. Chemical structure of DMAAPS.

146
E.O. Ningrum et al. / Polymer 59 (2015) 144e154
The dish was covered with a thin plastic film containing small holes
to decrease the drying speed because the gels tended to break if
dried too quickly. Other gels were cut into small pieces, washed,
and then dried in an oven. Finally, the dried gels were ground into a
powder and sieved to greater than 180 mesh size for the adsorption
experiments. The synthetic conditions for the DMAAPS gels are
listed in Table 1.
the temperature reached 10 ꢁC. The swelling degree was calculated
from the following equation:
d3swell
d3dry
SD ¼
;
(2)
where dswell and ddry indicate the diameters of the swollen and dry
gels, respectively.
2.3. Measurement of the amount of cation and anion adsorbed onto
DMAAPS gel from the halide solutions
2.5. Measurement of the phase transition of poly(DMAAPS) in the
halide solutions
Ion-adsorption experiments were performed using the same
procedure described previously [29], as follows: Ground DMAAPS
gel (1 g) was added to 20 mL of the halide solutions, i.e., KF, KCl, KBr,
and KI solutions, or mixtures of the solutions in a glass bottle. The
concentrations of these halide solutions were fixed at 10 mmol/L.
The bottle containing the solution and gels was placed in a water
bath at the desired temperature and stirred gently for 12 h to allow
the gels to swell and reach adsorption equilibrium [29]. After
adsorption, the gels were removed from the solution using a
The phase transition of poly(DMAAPS) in response to temper-
ature changes was also measured using a reported procedure [29]
to elucidate the effect of the halide-solution concentration on the
transition temperature of poly(DMAAPS). The transition tempera-
ture was evaluated from the temperature dependence of the
transmittance at 600 nm of the poly(DMAAPS) solution using a
spectrophotometer equipped with a temperature control system
(V-530, Japan Spectroscopy Co., Ltd.). Poly(DMAAPS) is transparent
above the transition temperature because the polymer solution is
soluble in water and aqueous halide solutions. In contrast, the
polymer solution became opaque or milky white below the tran-
sition temperature because poly(DMAAPS) is insoluble in water and
aqueous halide solutions. The transition temperature was defined
as the value at 50% transmittance.
centrifuge at 3500 rpm and syringe filter (0.45
mm, Toyo Roshi
Kaisha, Ltd.) in order to measure the residual ion concentration in
the solution. The cation (Kþ) and anion (Fꢀ, Clꢀ, Brꢀ, and Iꢀ) con-
centrations were then measured using ion chromatography. The
measurements were performed using a SUS TSKgel Super IC-A/C
(Tosoh Corp.) column (6.0 mm I.D ꢂ 150 mm) with a solution
containing 6.0 mmol/L 18-crown-6 ether, 0.45 mmol/L 5-sulfosa-
licylic acid, 5.0 mmol/L L-tartaric acid, and 5% acetonitrile as the
3. Results and discussion
mobile phase. The adsorbed amounts of cation and anion were
calculated from their concentrations in the solution before and
after adsorption using the following equation:
3.1. Adsorption behavior of ions on DMAAPS gel in halide solutions
ðC0 ꢀ CÞV
In this section, the effect of the anion species in halide solutions,
i.e., KF, KCl, KBr, and KI solutions, on the amount of cation (Kþ) and
anions (Fꢀ, Clꢀ, Brꢀ, and Iꢀ) adsorbed onto DMAAPS gel was
examined. The gels were prepared using three different cross-
linker concentrations, i.e., 5, 10, and 30 mmol/L Fig. 2(a)e(d)
show the amount of Kþ adsorbed onto DMAAPS gel from these
halide solutions as a function of temperature. In our previous work,
it was confirmed that the DMAAPS gel simultaneously adsorbed
cations and anions from salt solutions [29]; thus, the amount of
cation (Kþ) adsorbed in this study coincided with the amount of
anion (Fꢀ, Clꢀ, Brꢀ, or Iꢀ) absorbed. Furthermore, the amount of Kþ
adsorbed depended strongly on the interactions between the anion
in the halide solution and Nþ of DMAAPS.
Q ¼
;
(1)
m
where Q is the amount of cation or anion adsorbed, C0 and C are the
concentrations of cation or anion in the initial solution and aqueous
solution after adsorption for a certain period, respectively, V is the
volume of the aqueous phase, and m is the weight of the dry sample
gels.
2.4. Measurement of the swelling degree of DMAAPS gel in the
halide solutions
The swelling degree of the DMAAPS gel was measured using a
reported procedure [29]. Initially, the diameter of the cylindrical
dry gel was measured using a cathetometer. The gel was then
immersed in water, one of the four halide solutions, i.e., KF, KCl, KBr,
and KF solutions, or mixtures of these solutions at 70 ꢁC. The gel
was allowed to swell for 24 h, which was sufficient for the gel to
reach its equilibrium swollen state [29]. The diameter of the
swollen cylindrical gel was then measured using a cathetometer.
The temperature was reduced in increments, and the swollen gel
diameter was measured again. This procedure was repeated until
In the KF solution, negligible amounts of Kþ adsorbed onto the
gels prepared at three different MBAA concentrations (i.e., 5,10, and
30 mmol/L) at temperatures in the experimental range of 10e70 ꢁC
(Fig. 2(a)). In other halide solutions, the amounts of Kþ adsorbed
onto the gels prepared at three different MBAA concentrations at
10 ꢁC were almost the same, regardless of the anion species
(Fig. 2(b)e(d)). At a lower cross-linker concentration (5 mmol/L),
the amount of Kþ adsorbed decreased significantly with increasing
temperature, especially at 50 and 70 ꢁC. In contrast, at 10 mmol/L
MBAA, there was a slight decline in the amount of Kþ adsorbed,
and, at 30 mmol/L, it remained unchanged with increasing
temperature.
The phenomenon above can be explained as follows. At low
temperature and in the gels synthesized with higher cross-linker
concentration, the electroneutrality of the charge groups results
from the intra-group ionic pairs formation [31]. An increase of the
cross-linker concentration increases the polymer concentration in
the gel and the network density in the gel. In this condition, the
expansion of the polymer network of DMAAPS gel is restricted and
the sulfonate groups become close to each other. In the halide
Table 1
Synthesis condition of DMAAPS gel.
Concentration
[mmol/L]
Monomer
Linker
: N,N-dimethyl(acrylamidopropyl)ammonium
propane sulfonate (DMAAPS)
1000
: N,N0-methylenebisacrylamide (MBAA)
5, 10, 30
Accelerator : N,N,N0,N0-tetramethylethylenediamine (TEMED) 10
Initiator
: Ammonium peroxodisulfate (APS)
0.5

E.O. Ningrum et al. / Polymer 59 (2015) 144e154
147
Fig. 2. Amount of Kþ adsorbed onto DMAAPS gel from 10 mmol/L (a) KF, (b) KCl, (c) KBr, and (d) KI solutions at various temperatures. The gels were prepared at three different
cross-linker concentrations.
solutions, only a part of the charged groups in the DMAAPS gel
interacts with Kþ and anions (Fꢀ, Clꢀ, Brꢀ, Iꢀ) because the inter-
action between the SOꢀ3 and Nþ groups of the DMAAPS gel that
forms an intra-group ionic pairing is quite strong; in other words
the amount of ions adsorbed onto the DMAAPS gel is limited and it
reached a constant value. This adsorption amount is regarded as
maximum adsorption amount of ions onto the gel. In contrary, at
high temperature and in the gels synthesized with lower cross-
linker concentration, the polymer network of DMAAPS gel swells
largely. Furthermore, the thermal motion weakens the interaction
between the Nþ and SOꢀ3 groups of DMAAPS. The breakages of as-
sociations caused by temperature increase are consistent with what
was observed for sulfobetaine gel [32] and sulfobetaine polymer
solids [33]. The thermal motion simultaneously weakens the ionic
pairings force between the charged groups in the DMAAPS gel and
the Kþ and/or anions (Fꢀ, Clꢀ, Brꢀ, Iꢀ) from the halide solutions.
These conditions reduce the adsorption of ions from the halide
solutions onto the DMAAPS gel, thereby decreasing the amount of
Kþ adsorbed on the gel.
The Hofmeister series is related to the anions' ability to directly
interact with the positively charged groups of DMAAPS, Nþ, and
their adjacent hydration shells [34e36]. The species at the left of
the Hofmeister series are referred to as kosmotropes or “water-
structure makers”. These anions are small and strongly hydrated
[36]. In contrast, the species at the right of the Hofmeister series are
referred to as chaotropes; these species are large and less hydrated
and are known as “water-structure breakers” [34,35]. The Clꢀ, Brꢀ,
and Iꢀ ions used in this study are chaotropes and exhibit a weaker
resistance to dehydration [37] in the following order: Clꢀ > Brꢀ > Iꢀ.
Therefore, when DMAAPS gel was immersed in a KI solution, the Iꢀ
ions strongly interacted with the Nþ groups of DMAAPS, which
resulted in greater adsorption. In contrast, Fꢀ ions are kosmotropes
[37]; therefore, this strongly hydrated species exhibits stronger
interactions with water molecules than the cohesive forces of water
molecules. Therefore, Fꢀ remained hydrated [38] resulting in less
interaction of the Fꢀ ions with the Nþ of DMAAPS, which led to
smaller adsorption.
The order of increasing adsorption from the halide solutions was
KI > KBr > KCl > KF, and the order of anion in these halide solutions,
i.e., Iꢀ > Brꢀ > Clꢀ > Fꢀ, was opposite that in the Hofmeister series.
The typical order of the anion series is:
3.2. Swelling behavior of DMAAPS gel in halide solutions
Fig. 3(a)e(d) shows the temperature dependence of the swelling
degree of DMAAPS gels prepared at three different MBAA concen-
CO23ꢀ >SO24ꢀ >S2O32ꢀ >H2POꢀ4 >Fꢀ >Clꢀ >BrꢀzNO3ꢀ >Iꢀ >ClOꢀ4 >SCNꢀ
(3)

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E.O. Ningrum et al. / Polymer 59 (2015) 144e154
Fig. 3. Swelling degrees of DMAAPS gel measured in 10 mmol/L (a) KF, (b) KCl, (c) KBr, and (d) KI solutions at various temperatures. The gels were prepared at three different cross-
linker concentrations.
trations in 10 mmol/L KF, KCl, KBr, and KI solutions. In each solution,
the swelling degree increased with increasing temperature and was
only significant for the gel prepared at a low MBAA concentration
(5 mmol/L). The addition of these halides into the poly(DMAAPS)
solution led to dissociation of Nþ and SO3ꢀ because of the new ionic
interactions between the ions in the halide solution and the
charged groups of poly(DMAAPS), i.e., SOꢀ3 and Nþ. For the DMAAPS
gel synthesized with lower cross-linker concentration, at equilib-
rium swelling the polymer network of the gel expands largely.
However, the degree of swelling of the gel prepared with a higher
cross-linker concentration (30 mmol/L) increased only slightly with
increasing temperature. Interesting phenomenon was found that
although the swelling degree increased at a higher cross-linker
concentration (30 mmol/L), the amount of Kþ adsorbed onto the
gel prepared at the same cross-linker concentration remained un-
changed as seen in Fig. 2. This result is thought to be due to the fact
that the increasing of the swelling degree is also influenced by the
enhancement of the thermal motion.
compared, as shown in Fig. 4(a)e(c). In water, the swelling degree
of the gel prepared at a higher MBAA concentration (i.e., 30 mmol/
L) increased gradually with increasing temperature; this increase
was not significant and increased from 3.5 at 10 ꢁC to only 6.6 at
70 ꢁC (Fig. 4(a)). Comparison of this swelling degree in water with
those in the halide solutions revealed that the swelling degrees in
the KF and KCl solutions were almost the same as that in water. This
similarity was observed at all three MBAA concentrations (i.e., 5, 10,
and 30 mmol/L). However, in the KBr solution, the swelling degree
was larger than that in water at the three different MBAA concen-
trations. Furthermore, the swelling degree in the KI solution was
much larger than those in water and the KF, KCl, and KBr solutions.
These results indicated that Kþ and Iꢀ ions interacted strongly with
the SOꢀ3 and Nþ of DMAAPS gel, respectively, as shown by the
amount of Kþ adsorbed (Fig. 3).
3.3. Transition behavior of poly(DMAAPS) in halide solutions
The order of decreasing the hydration of ability of the anions
was Fꢀ > Clꢀ > Brꢀ > Iꢀ, while the swelling degree of DMAAPS gel
decreased in the following order: KI > KBr > KCl > KF as seen in
Fig. 3. Furthermore, we observed that, at 10 ꢁC, the swelling degrees
of the gels prepared at three different MBAA concentrations were
almost the same in the KF and KCl solutions (Fig. 3(a) and (b));
however, in the KBr and KI solutions (Fig. 3(c) and (d)), the swelling
degrees of these three gels varied significantly at all temperatures.
The swelling degrees of DMAAPS gels prepared at three different
MBAA concentrations in the halide solutions and water were
The transition behaviors of poly(DMAAPS) in halide solutions
with various concentrations are shown in Fig. 5(a)e(d). The con-
centration of poly(DMAAPS) was fixed at 20 g/L. The results showed
that increasing halide concentration resulted in an initial increase
in the transition temperature at a specific halide concentration
followed by a decrease above that concentration.
At lower KI concentration of 0e1 mmol/L, the transition tem-
perature gradually increased from 36 to around 58 ꢁC (Fig. 5(d));
this increase might have been due to partial dissociation of Nþ and
SOꢀ3 pairing resulting from the new ionic interactions with Iꢀ and

E.O. Ningrum et al. / Polymer 59 (2015) 144e154
149
Fig. 4. Swelling degrees of DMAAPS gel prepared using (a) 30, (b) 10, and (c) 5 mmol/L of cross-linker measured in water and 10 mmol/L KF, KCl, KBr, and KI solutions at various
temperatures.
Kþ, respectively. At these low KI concentrations, the amounts of
ions (Kþ and Iꢀ) available to disrupt the interactions of the Nþ and
SOꢀ3 groups in poly(DMAAPS) were restricted; thus, dissociation of
the ionic interactions occurred locally (i.e., partial dissociation).
Partial dissociation induced chain mobility, which led to enhanced
inter-chain interactions [39] leading to aggregation of the polymer
that inevitably increased the transition temperature. Upon further
increasing the KI concentration to 10 mmol/L, the transition tem-
perature decreased significantly to less than 20 ꢁC, which is out of
the experimental range; this can be explained by dissociation of the
ionic pairing of SOꢀ3 and Nþ, which occurred easier in highly
concentrated KI solutions because a significant number of ions (Kþ
and Iꢀ) were available for disruption [20].
In addition, it is seen in Fig. 5 that the transition temperature of
poly(DMAAPS) in KF and KCl of 0.25 mmol/L is lower than that in
KBr and KI at the same concentration. Furthermore, the difference
between the transition temperature of poly(DMAAPS) in water and
in 0.25 mmol/L halide solutions increased with decreasing the
hydration ability of the anions in the following order:
KI > KBr > KCl > KF. This order can be explained by the fact that in
solutions of lower concentration, ions from solution leads to the
partial dissociation of the ionic pairings as a result from the ionic
interaction with Kþ and anions (Fꢀ, Clꢀ, Brꢀ, Iꢀ) from halide solu-
tions. But since the ability of the chaotropes ions (Clꢀ, Brꢀ, Iꢀ) to
disrupt these pairing is greater than that the kosmotropes ions (Fꢀ),
the chain mobility of polymer in the solution containing chaotropes
ions is greatly enhanced especially for the ions having lower hy-
dration ability. As explained above, the chain mobility leads to
aggregation of the polymer that eventually increased the transition
temperature. As a consequence, the transition temperature of
poly(DMAAPS) in KI and KBr solutions of 0.25 mmol/L is higher
than that in KCl and KF solutions at the same concentration.
Very similar transition behavior was observed when poly(-
DMAAPS) was dissolved in KF, KCl, and KBr solutions, as shown in
Fig. 5(a)e(c). Comparison of the transition temperatures of poly(-
DMAAPS) in KCl, KBr, KI, and KF solutions revealed that the gap
between the maximum and minimum transition temperatures
within the experimental halide concentration range decreased in
the following order: KI > KBr > KCl > KF. Furthermore, the transi-
tion temperatures of poly(DMAAPS) in KF, KCl, KBr, and KI solutions
(10 mmol/L) were 50, 47, 31, and <20 ꢁC, respectively. The transition
behaviors of poly(DMAAPS) in halide solutions were strongly
related to the adsorption behavior of ions onto poly(DMAAPS), as
explained in our previous study [29]. Based on the adsorption and
transition behaviors, the transition temperatures of poly(DMAAPS)
in 10 mmol/L halide solutions decreased in the following order of
halides: KF > KCl > KBr > KI; in contrast, the amounts of Kþ
adsorbed showed the opposite trend. For the KF, KCl, KBr, and KI
solutions, the amounts of Kþ adsorbed onto DMAAPS gel in these
solutions were almost zero, 0.015, 0.03, and 0.047 mmol/g-dry gel,
respectively, as shown in Fig. 2. In addition, at a fixed halide con-
centration of 10 mmol/L, the anions with lower hydration ability,
such as Iꢀ, promoted dissociation of the Nþ and SOꢀ3 pairings
causing a shift in the transition temperature to a lower value. In
contrast, dissociation of Nþ and SOꢀ3 pairing was limited in solu-
tions containing anions with higher hydration abilities, such as Fꢀ;

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E.O. Ningrum et al. / Polymer 59 (2015) 144e154
Fig. 5. Transition behavior of poly(DMAAPS) (20 g/L) in (a) KF, (b) KCl, (c) KBr, and (d) KI solutions of various concentrations.
accordingly, the transition temperature of poly(DMAAPS) was
higher in these solutions.
DMAAPS gels prepared at three different cross-linker concentra-
tions, i.e., 5, 10, and 30 mmol/L, at various temperatures, i.e., 10, 30,
50, 70 ꢁC, in 10 mmol/L KF, KCl, KBr, and KI solutions. The re-
lationships are summarized in Fig. 6(a). The data points laid on the
same line for the same halide solution even at different cross-linker
concentrations and temperatures; this result was similar to that
obtained in our previous study [29]. At smaller swelling degrees,
the amount of Kþ adsorbed remained unchanged and decreased
gradually as the swelling degree increased. Although the trends
3.4. The relationship between the amount of ions adsorbed and
degree of swelling of DMAAPS gel in halide solutions
To elucidate the adsorption and swelling behaviors, similar to in
our previous study [29], we examined the relationship between the
swelling degree of the gel and amount of Kþ adsorbed onto the
Fig. 6. Relationship between the amount of Kþ adsorbed onto DMAAPS gel and (a) the swelling degree of the DMAAPS gel and (b) the polymer concentration in the gel. The gels
were prepared at three different cross-linker concentrations, and the amounts of Kþ adsorbed and swelling degrees were measured in KF, KCl, KBr, and KI solutions at various
temperatures.

E.O. Ningrum et al. / Polymer 59 (2015) 144e154
151
remained the same, the adsorbed amount of Kþ decreased at larger
degrees of swelling in solutions containing anions with lower hy-
dration abilities, such as KI. However, in solutions containing an-
ions with higher hydration abilities, such as KCl, the amount of Kþ
adsorbed decreased at smaller degrees of swelling. The amount of
Kþ adsorbed from the KI, KBr, and KCl solutions decreased when the
swelling degree reached about 35, 23, and 16, respectively; how-
ever, it was difficult to determine the swelling degree at which the
adsorption amount decreased in the KF solution because almost no
Kþ was adsorbed from the KF solution.
concentrations in the gel at which the maximum adsorbed amount
of Kþ were obtained were about 31, 70, and 111 g/L for KI, KBr, and
KCl solutions, respectively. Furthermore, the maximum adsorption
amount of Kþ depended strongly on the anion species. These results
were attributed to the anion size, which decreases in the order of
Iꢀ > Brꢀ > Clꢀ > Fꢀ, and that penetration of into the side chain of
DMAAPS was more difficult for larger anions than smaller anions.
Furthermore, the order of decreasing interaction with Nþ in
DMAAPS of the anions was Iꢀ > Brꢀ > Clꢀ > Fꢀ, as obtained from the
Hofmeister series shown in Eq. (3).
To clearly elucidate the relationship between the amount of ions
adsorbed and degree of swelling, similar to in our previous study
[29], the concentration of polymer in the gel (Cp) at the equilibrium
swollen state was determined from the swelling degree of the
DMAAPS gels, as follows:
In order to clarify the behavior of the gel over a wide concen-
tration range, the adsorption and swelling behaviors of DMAAPS gel
were further investigated in concentrated solutions of KI i.e.,
50 mmol/L (8.3 g/L) and 100 mmol/L (16.6 g/L) as shown in Fig. 7. KI
solution was chosen to represent the adsorption and swelling be-
haviors in the concentrated solutions because the maximum
adsorption amount of Kþ onto the gel in this solution was the
highest among four kinds of halide solutions. Furthermore, the
DMAAPS gel was prepared at a 30 mmol/L cross-linker concentra-
tion and was measured at 10 ꢁC to obtain the maximum adsorption
amount of Kþ onto the gel.
W0
V0
1
SD
Cp ¼
ꢂ
;
(4)
where W0 is the weight of the dry sample gel, V0 is the volume of
the dry sample gel, and SD is the swelling degree of the gel, which is
defined by (swollen gel diameter)3/(dry gel diameter)3.
Fig. 7(a) shows the maximum adsorption amount of Kþ at a
certain KI equilibrium concentrations. It is seen that the maximum
adsorption amount of Kþ increased linearly with equilibrium con-
centration of Kþ over a wide range of experimental concentrations.
The value of adsorption amount can be obtained by the following
equation:
The relationship between the amount of Kþ adsorbed and
polymer concentration in the gel is shown in Fig. 6(b). The amount
of Kþ adsorbed increased with increasing polymer concentration
until it reached a constant maximum amount. This trend was the
same as those described in our previous study [29]. The constant
maximum value was regarded as the maximum adsorption amount
of Kþ onto the DMAAPS gel in the specific halide solution.
Qmax ¼ 0:0057 Ce
(5)
This phenomenon was explained as follows in our previous
study [29]: Gels with high polymer concentrations feature gel
networks that are dominated by intra-group ionic pairing in-
teractions [31]. Since the interaction between the SOꢀ3 and Nþ
groups of the DMAAPS gel that formed intra-group ionic pairings
was quite strong, the amount of ions adsorbed by DMAAPS gel was
limited [29]. However, at low polymer concentrations, thermal
motion weakened the interactions between ions in the halide so-
lution and the charged groups of poly(DMAAPS), i.e., SOꢀ3 and Nþ.
In addition, for the halide solutions containing chaotropic an-
ions with higher hydration abilities, such as the KCl solution, the
amount of Kþ adsorbed reached a constant maximum amount at
higher polymer concentrations in the gel. However, for halide so-
lutions containing chaotropic anions with lower hydration abilities,
such as the KI solution, the maximum adsorbed amount of Kþ
occurred at a lower polymer concentration in the gel. The polymer
where Qmax is the maximum adsorption amount of Kþ by DMAAPS
gel and Ce is the equilibrium concentration of Kþ. This result leads
to the conclusion that the maximum adsorption amount of Kþ is
proportional to the concentration of KI solution in the experimental
range. In addition, as seen in Fig. 7(a) the maximum adsorption
amount of Kþ onto the gel in 100 mmol/L of KI solution of
0.49 mmol/g-gel was estimated to be ~13.7% of the sulfonate con-
tent in the gel. This means that the sulfonate content in the gel to be
used to interact with Kþ from halide solution was still low even if
the KI concentration was relatively high. This can be explained by
the fact that the ionic pairings between SOꢀ3 and Nþ groups of the
DMAAPS gel is quite strong. From this result, we can conclude that
the selectivity of the ions from halide solutions (KF, KCl, KBr, and KI)
toward the charge groups of DMAAPS; or in other words the order
of the adsorption was unchanged with increasing the solutions
Fig. 7. Relationship between KI equilibrium concentrations and (a) maximum adsorption amount of Kþ onto DMAAPS gel for Kþ and (b) degree of swelling of the DMAAPS gel in
diluted concentrations of KI solutions at 10 ꢁC. The DMAAPS gels were prepared at a 30 mmol/L cross-linker concentration.

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E.O. Ningrum et al. / Polymer 59 (2015) 144e154
concentrations in our experimental range. In addition, the swelling
degree of the DMAAPS gel in concentrated solutions of KI is shown
in Fig. 7(b). The degree of swelling of the gel increased with
increasing KI equilibrium concentrations and was significant for KI
concentration, i.e., 100 mmol/L. The addition of relatively high
concentration of KI solution into the gel promoted more dissocia-
tion of Nþ and SOꢀ3 because of the new ionic interactions between
the ions in the halide solution and the charged groups of as a result
polymer network of the gel expanded and swelling degree
increased.
Hofmeister series shown in Eq. (3). Furthermore, the amount of Iꢀ
adsorbed in the KI þ KF, KI þ KCl, and KI þ KBr mixtures was larger
than that in the solution of KI (Fig. 8(a)e(c)), and the amounts of Clꢀ
or Brꢀ adsorbed were smaller than that in the single solution of KCl
or KBr, although the amount of Fꢀ adsorbed was too small to be
compared with that in the solution of KF. In addition, the amounts
of Brꢀ adsorbed from the KBr þ KF and KBr þ KCl solutions were
also larger than that from the KBr solution (Fig. 8(c) and (d)), and
the amount of Clꢀ adsorbed was smaller than that from the KCl
solution of KCl; the amount of Fꢀ adsorbed was too small to be
compared with that from the KF solution. These results were
attributed to the fact that the concentrations of Kþ cations in the
mixtures were double that in the single-halide solutions. The
higher concentration of Kþ promoted adsorption of Kþ, which
promoted adsorption of the anions because of simultaneous
adsorption of cations and anions. However, adsorption of anions
from the mixtures was competitive: The anions on the right side of
the Hofmeister series shown in Eq. (3) tended to adsorb in larger
amounts. To compensate for this increase, the anions on the left
side of the Hofmeister series shown in Eq. (3) adsorbed less.
Therefore, the amounts of anions on the right side of the Hof-
meister series, such as Iꢀ, adsorbed from the mixtures were higher
than that from the single-halide solutions, as shown above;
accordingly, the amounts of anions on the left side of the Hof-
meister series adsorbed from the mixtures were lower than that
from the single-halide solutions.
3.5. Competitive adsorption of anions onto DMAAPS gel
Competitive adsorption of anions onto the DMAAPS gel from
these halides was investigated using KI þ KF, KI þ KCl, KI þ KBr,
KBr þ KF, and KBr þ KCl mixtures; the results are summarized in
Fig. 8. The concentration of each halide was 10 mmol/L. The gels
were prepared at 30 mmol/L MBAA, and the amounts of Kþ and
anions (Iꢀ, Brꢀ, Clꢀ, and Fꢀ) adsorbed onto the gels were measured
at various temperatures, i.e., 10, 30, 50, and 70 ꢁC. The amount of Kþ
adsorbed was equal to the total amount of both anions adsorbed
onto the DMAAPS gel because of the simultaneous adsorption of
anions and cations onto DMAAPS gel, as was shown in our previous
study [29].
In all the mixed-halide solutions, i.e., KI þ KF, KI þ KCl, KI þ KBr,
KBr þ KF, and KBr þ KCl, the anion of the halide on the left side
adsorbed better than on the right side; accordingly, the observed
order of decreased adsorption ability of the anions was
Iꢀ > Brꢀ > Clꢀ > Fꢀ, although the amount of Fꢀ adsorbed was almost
zero. This order is opposite that of the anion species in the
In addition, Fig. 8(a)e(c) shows the effect of temperature on the
amounts of ions adsorbed from mixtures containing KI. As shown in
Fig. 2, in single-component solution and for gels prepared at a
cross-linker concentration of 30 mmol/L, increasing the
Fig. 8. Amounts of ions adsorbed onto DMAAPS gel in (a) KI þ KF, (b) KI þ KCl, (c) KI þ KBr, (d) KBr þ KF, and (e) KBr þ KCl mixtures at various temperatures. The concentration of
each halide solution was 10 mmol/L. The DMAAPS gels were prepared at a 30 mmol/L cross-linker concentration.

E.O. Ningrum et al. / Polymer 59 (2015) 144e154
153
Fig. 9. Swelling degrees of DMAAPS gels measured in (a) a KI þ KF mixture and (b) KI þ KF, KI þ KCl, KI þ KBr, KBr þ KF, and KBr þ KCl mixtures at various temperatures. The
concentration of each halide solution was 10 mmol/L. The DMAAPS gels were prepared at 30 mmol/L cross-linker concentration.
temperature did not induce any significant change in the amount of
Kþ adsorbed onto the gel. However, in the mixture, the amount of
Kþ adsorbed decreased gradually as the temperature increased
from 10 to 70 ꢁC (Fig. 8(a)e(c)). In the presence of competitive
adsorption between two less-hydrated anions in a mixture, such as
the KI þ KBr solution, the amount of Kþ adsorbed decreased
significantly with increasing temperature, as shown in Fig. 8(c). In
contrast, in mixtures containing more strongly hydrated anions,
such as the KI þ KCl solution, the amount of Kþ adsorbed only
decreased gradually with increasing temperature, as shown in
Fig. 8(b). Additionally, in the mixture composed of KI and KF, the
amount of Kþ adsorbed was almost constant and only decreased at
70 ꢁC, as shown in Fig. 8(a).
decreased, and the amount of Kþ adsorbed increased to a constant
maximum adsorption amount, i.e., the maximum adsorption
amount, above polymer concentrations of 31, 70, and 111 g/L in KI,
KBr, and KCl solutions, respectively. Additionally, adsorption of
anions was competitive in mixtures of these halide solutions. In the
mixtures, the anions on the right side of the Hofmeister series
adsorbed in larger quantities than in the single-components solu-
tions, while those on the left side of the Hofmeister series adsorbed
in smaller quantities than in the single-component solutions.
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4. Conclusion
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