Investigation of ion adsorption properties of sulfobetaine gel and relationship with its swelling behavior

Article abstract of DOI:10.1016/j.polymer.2014.08.042

The adsorption of cations and anions in nitrate solutions on N,N-dimethyl(acrylamidopropyl)ammonium propane sulfonate (DMAAPS) gels prepared using various cross-linker and monomer concentrations was investigated. The influence of the temperature and nitrate concentration on the adsorption properties of the gel was evaluated, demonstrating simultaneous adsorption of cations and anions. The amount of Zn²⁺adsorbed on the gel in Zn(NO₃)₂solution increased as the cross-linker and monomer concentrations used in the gel preparation increased. For the gel prepared using a higher cross-linker or monomer concentration, elevation of the temperature did not induce any significant change in the amount of Zn²⁺adsorbed on the gel. Furthermore, for the gel prepared using a lower cross-linker or monomer concentration, the amount of Zn²⁺adsorbed on the gel decreased significantly as the temperature increased. In addition, an interesting correlation between the degree of swelling of the gel and the amount of Zn²⁺adsorbed on the gel was found. As the degree of swelling decreased, the adsorption amount increased to eventually achieve a constant value.

Full text of DOI:10.1016/j.polymer.2014.08.042

Polymer 55 (2014) 5189e5197
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Investigation of ion adsorption properties of sulfobetaine gel and
relationship with its swelling behavior
,
Eva Oktavia Ningrum a b, Yukiko Murakami a, 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 in nitrate solutions on N,N-dimethyl(acrylamidopropyl)ammonium
propane sulfonate (DMAAPS) gels prepared using various cross-linker and monomer concentrations was
investigated. The influence of the temperature and nitrate concentration on the adsorption properties of
the gel was evaluated, demonstrating simultaneous adsorption of cations and anions. The amount of
Zn2þ adsorbed on the gel in Zn(NO3)2 solution increased as the cross-linker and monomer concentrations
used in the gel preparation increased. For the gel prepared using a higher cross-linker or monomer
concentration, elevation of the temperature did not induce any significant change in the amount of Zn2þ
adsorbed on the gel. Furthermore, for the gel prepared using a lower cross-linker or monomer con-
centration, the amount of Zn2þ adsorbed on the gel decreased significantly as the temperature increased.
In addition, an interesting correlation between the degree of swelling of the gel and the amount of Zn2þ
adsorbed on the gel was found. As the degree of swelling decreased, the adsorption amount increased to
eventually achieve a constant value.
Received 15 May 2014
Received in revised form
14 August 2014
Accepted 17 August 2014
Available online 27 August 2014
Keywords:
Sulfobetaine gel
Adsorption properties
Swelling degree
© 2014 Elsevier Ltd. All rights reserved.
1. Introduction
the latter contain only negatively or positively charged functional
groups. Zwitterionic betaine polymers also exhibit ion selectivity
In the last five decades, polymers incorporating zwitterionic
species have been recognized as promising candidates for respon-
sive systems geared towards various potential applications such as
biosensors, catalysts, drug delivery systems, and separation media
[1,2]. The specific, sensitive, and instantaneous responsiveness
demonstrated by polyzwitterions has been the focus of consider-
able scientific research, and study of these materials has continued
to gain momentum in recent decades. The zwitterionic betaine
polymer containing both anionic and cationic active groups in the
same side chain of the polymeric repeat unit with an alkylene
group between them bears an overall neutral charge. Zwitterionic
polymers such as sulfobetaine [3e6], phosphobetaine [7,8], and
carboxybetaine [9e11] respectively contain sulfonate, phosphate,
and carboxylate species as the cationic active group with the qua-
ternary ammonium ion as the anionic active group. Zwitterionic
betaine polymers differ considerably from ionic polymers, where
because the ions can bond via both the negative and positive
charges located in the same repeat unit [12e14].
One of the unique characteristics of zwitterionic betaine is the
ability of the fragments forming a cyclic conformation of the
cationic and anionic groups of neighboring monomer residues
(intra-group), or cationic and anionic groups contact between
neighboring macromolecules (inter-chain), and head-to-tail stack-
ing within single macromolecules (intra-chain) which results in the
insolubility of zwitterionic betaine in pure water [15,16]. These
interactions depend on the flexibility of spacer between opposite
charges and length. This dependency determines the solubility,
phase, volume, ionization ability and conformational state of
polymeric betaine in aqueous solutions. Furthermore, zwitterionic
betaine polymers are generally thermosensitive in aqueous solu-
tions; that is, they are insoluble in water below the transition
temperature and soluble above the transition temperature. The
transition temperature is referred to as the upper critical solution
temperature (UCST). At temperatures below the UCST, the zwit-
terionic polymer is in a collapsed-coil state due to the intra- and/or
inter-chain interactions. However, at temperatures above the UCST,
* Corresponding author.
E-mail address: sakohara@hiroshima-u.ac.jp (S. Sakohara).
http://dx.doi.org/10.1016/j.polymer.2014.08.042
0032-3861/© 2014 Elsevier Ltd. All rights reserved.

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E.O. Ningrum et al. / Polymer 55 (2014) 5189e5197
the polymer adopts an extended conformation because the thermal
motion of the polymer molecules overcomes the intra- and/or
inter-chain interactions [17]. Moreover, the UCST of zwitterionic
polymers increases with increasing polymer concentration, which
is attributed to an increase in the intra- and/or inter-chain pairings.
Consequently, more thermal energy is required to overcome these
interactions [18]. The interaction of the charge groups of zwitter-
ionic betaine and aqueous salt solution also strongly determines
the zwitterionic betaine properties [15]. The solubility of zwitter-
ionic betaine in salt solutions depends on the nature of the anions
and cations and is well described by the charge/radius ratio, Hof-
meister series and Pearson theory [15,19,20].
2.2. Synthesis of DMAAPS
DMAAPS was synthesized by the ringeopening reaction of
DMAPAA and PS [29]. A mixture of PS (75 g) and acetonitrile (75 g)
was added dropwise to a mixture of DMAPAA (100 g) and aceto-
nitrile (200 g) with continuous stirring at 30 ꢁC for 90 min. Stirring
was then continued for 16 h, and the solution was allowed to stand
for 2 d. The precipitated white crystals of DMAAPS were collected
by filtration and washed with 500 mL of acetone. Finally, the ob-
tained DMAAPS crystals were dried under reduced pressure for
24 h. The structure of DMAAPS is shown in Fig. 1.
Numerous studies of zwitterionic sulfobetaine type polymers
and gels have been reported in recent years, most of which have
placed emphasis on the synthesis and solution properties of the
polymers, such as the thermosensitive properties mentioned
above, or the swelling degree of the gels, which varies with the
monomer concentration [21], cross-linker concentration [22e24],
salt concentration [18,25,26], ionic strength [27,28], etc. In contrast,
there are relatively few studies on the adsorption of cations and
anions on such polymers and gels, although the adsorption
behavior is recognized to affect the swelling and transition be-
haviors. Moreover, there is very little information on the effects of
the conditions utilized in the preparation of the gels, such as the
cross-linker and monomer concentrations, on the amount of ions
adsorbed on the gel. The adsorption behavior of zwitterionic sul-
fobetaine type polymers and gels is primarily determined by the
interaction between the charged groups in the sulfobetaine (SO3ꢀ
and Nþ) group and the ions (cation and anion) in the solutions.
To address this paucity of information, the adsorption of cat-
ions and anions on the sulfobetaine type N,N-dimethyl(acrylami-
dopropyl)ammonium propane sulfonate (DMAAPS) gels in various
nitrate solutions was investigated in this study. The effects of the
temperature and the conditions employed in the preparation of
the gel, i.e., cross-linker and monomer concentrations, on the
extent of adsorption of cations and anions on the DMAAPS gel
were evaluated. The adsorption behavior of cations and anions on
the DMAAPS gel is discussed herein by considering the degree of
swelling of the gel as well as the polymer concentration in the gel.
NaNO3, Zn(NO3)2, and Al(NO3)3 were selected as the target
solutions.
2.3. Preparation of the DMAAPS polymer and gel
The DMAAPS polymer and gel were prepared by free radical
polymerization using TEMED and APS as the polymerization
accelerator and initiator, respectively. In the preparation of the
DMAAPS polymer [poly(DMAAPS)], the concentrations of DMAAPS,
TEMED, and APS were 500, 2, and 2 mmol/L, respectively. DMAAPS
and TEMED were dissolved in 100 mL of deionized water and
charged into a separable flask. The dissolved oxygen in the solution
was removed by purging with nitrogen. A 20 mL aliquot of APS
solution, from which the dissolved oxygen had already been
removed, was then added to the mixture. Polymerization was car-
ried out for 6 h at 50 ꢁC under a nitrogen atmosphere. The resulting
polymer was purified by dialysis using a membrane with a mo-
lecular weight cut off of 12,000e14,000 (Cellu Step T3, Membrane
Filtration Product, Inc.) over a period of one week.
The average molecular weight of the synthesized poly(DMAAPS)
was 1.13 ꢂ 106 g/mol, as estimated from the intrinsic viscosity [
h]
that was measured in 0.1 M NaCl solution at 30 ꢁC using a Ubbe-
lohde viscometer. The weight-average molecular weight ½Mwꢃ was
calculated using the following equation, which has previously been
proposed by Ning et al. [21]:
 Ã
h ¼ 2:3 ꢂ 10ꢀ5Mw0:78
(1)
The DMAAPS gels were prepared using the method presented for
the synthesis of poly(DMAAPS), with the exception that MBAA was
used as a cross-linker. The synthesis of the DMAAPS gels was carried
out in a separable flask containing glass tubes (2 mm in diameter and
30 mm in length) to prepare the cylindrical gels. The gels prepared in
the glasstubes were cutinto pieces of 2 mm in lengthandrinsed with
deionized water. The gels were then slowly dried over several days.
During the drying process, the gels were placed on a Teflon sheet that
was spread on a Petri dish. Because the gels break if they are dried
quickly, the dish was covered with a thin plastic filmwith small holes
to decrease the drying speed. These gels were used to measure the
degreeofswelling. Othergelswere cut into smallpieces, washed, and
then dried in an oven. The dried small gels were then ground into
powder and sieved to over 180 mesh size. These ground gels were
used for the adsorption experiments. The conditions employed in the
synthesis of the DMAAPS gels are summarized in Table 1.
In addition, to elucidate the role of the ionic interactions in the
adsorption behavior of the DMAAPS gel relative to the copolymer
gel, the gel comprising a negatively charged sodium 2-(acryl-
amido)-2-methylpropanesulfonate (NaAMPS) and
a positively
charged N,N-dimethylaminopropylacrylamide methyl chloride
quaternary (DMAPAA-Q) unit was also prepared. The DMAPAA-Q-
co-NaAMPS gel network also consists of Nþ and SO3ꢀ groups.
However, these ions have Clꢀ and Naþ as counter ions, respectively.
2. Experimental section
2.1. Materials
2.4. Preparation of the copolymer gel of NaAMPS and DMAPAA-Q
1,3-Propanesultone
(PS)
and
2-(acrylamido)-2-methyl-
propanesulfonate (AMPS) were purchased from Tokyo Chemical
Industry Co., Ltd. and used as received. N,N-Dimethylaminopropy-
lacrylamide (DMAPAA) and DMAPAA-Q were kindly supplied by
Kohjin Film & Chemicals Co., Ltd. DMAPAA was purified by vacuum
distillation and DMAPAA-Q was used as received. N,N0-Methyl-
enebisacrylamide (MBAA), N,N,N0,N0-tetramethylethylenediamine
(TEMED), and ammonium peroxodisulfate (APS) were purchased
from SigmaeAldrich Co. (USA) and used without further purifica-
tion. Acetonitrile and acetone were purchased from Kanto Chemical
Co., Inc. and Nacalai Tesque, Inc., respectively.
The chemical structures of DMAPAA-Q and NaAMPS are shown
in Fig. 2. NaAMPS was prepared by neutralization of the monomer
Fig. 1. Chemical structure of N,N-dimethyl(acrylamidopropyl)ammonium propane
sulfonate (DMAAPS).

E.O. Ningrum et al. / Polymer 55 (2014) 5189e5197
5191
The concentrations of Naþ, Zn2þ, and NOꢀ3 were measured using
ion chromatography. The Naþ and Zn2þ assay was carried out using
a 4.6 mm (ID) ꢂ 10 cm (L) TSKgel IC-Cation 1/2 HR column (Tosoh
Corp.) with 2 mmol/L HNO3 as the mobile phase. The NOꢀ3 con-
centration was determined using a 4.6 mm (ID) ꢂ 100 mm (L)
Shim-pac IC-A1column (Shimadzu Corp.) with 2.5 mmol/L phthalic
acid as the mobile phase, and the Al3þ concentration was deter-
mined by using inductively coupled plasma (ICP) atomic emission
spectrometry (Seiko Instruments Inc. SPS. 3000).
Table 1
Synthesis condition of DMAAPS gel.
Concentration
[mmol/L]
Monomer N,N-dimethyl(acrylamidopropyl)ammonium
500, 750, 1000
propane sulfonate (DMAAPS)
Linker
N,N0-methylenebisacrylamide (MBAA)
5, 10, 30
Accelerator N,N,N0,N0-tetramethylethylenediamine (TEMED) 10
Initiator Ammonium peroxodisulfate (APS) 0.5
The adsorption of the cation and anion was measured after 12 h
of contact time. A period of 12 h was sufficient to reach the equi-
librium adsorption and was further used in investigating the
amount of ions adsorbed on the DMAAPS or copolymer gel.
in acid form, AMPS, with an equimolar amount of sodium hy-
droxide (NaOH) in methanol in the same manner as outlined in our
previous study [30]. In the preparation of the copolymer gel, the
concentrations of DMAPAA-Q, NaAMPS, MBAA, TEMED, and APS
were 500, 500, 30, 10, and 0.5 mmol/L, respectively. The method
used for preparation of the gel is similar to that used for the
DMAAPS gel.
2.6. Measurement of the degree of swelling of the DMAAPS gel
The degree of swelling was determined according to the
following procedures. The cylindrical dry gel, the diameter of which
was predetermined using a cathetometer, was immersed in water or
nitrate solution at 70 ꢁC, and the gel was allowed to swell for 24 h;
this time was sufficient for the gel to reach its equilibrium swollen
state. The diameter of the swollen gel was then measured with a
cathetometer. The temperature was reduced in increments and the
diameter of the swollen gel was measured again. This procedurewas
repeated until the temperature was reduced to 10 ꢁC. The degree of
swelling, SD, was calculated from the following equation:
2.5. Measurement of the amount of cation and anion adsorbed from
the nitrate solutions on the ground gels
The adsorption experiments were performed using nitrate so-
lutions, i.e., NaNO3, Zn(NO3)2, and Al(NO3)3 solutions. One gram of
the ground gel was placed in a glass bottle containing 20 mL of the
select nitrate solution of the desired concentration. The bottle was
placed in a water bath at the desired temperature, and the solution
containing the gels was gently agitated for 12 h using a magnetic
stirrer to allow the gels to swell and attain the equilibrium
adsorption state. In order to measure the concentration of the
cation (Naþ, Zn2þ, or Al3þ) or anion (NOꢀ3 ) in the solution after
adsorption, the gels were removed from the solution by centrifu-
gation at 3500 rpm for 10 min and subsequently filtered using a
d3swell
d3dry
SD ¼
(3)
where dswell and ddry respectively indicate the diameter of swollen
gel and dry gel.
syringe filter (0.45
mm, Toyo Roshi Kaisha, Ltd.). The amount of
2.7. Measurement of the phase transition of poly(DMAAPS)
cation or anion adsorbed on the gel was calculated from the dif-
ference between the initial and the final concentration of the cation
or anion in the solution by using the following equation:
The UCST of the DMAAPS polymer was evaluated from the
temperature dependence of the transmittance through the poly-
mer solution. The transmittance was measured at 600 nm using a
spectrophotometer equipped with a temperature control system
(V-530, Japan Spectroscopy Co., Ltd.). Above the UCST, the polymer
solution is transparent because poly(DMAAPS) is soluble in water
or aqueous nitrate solution. On cooling to below the UCST, the
polymer solution becomes opaque or milky white because poly(-
DMAAPS) is insoluble in water or aqueous nitrate solution.
ðC0 ꢀ CÞV
Q ¼
(2)
m
where Q is the amount of cation or anion adsorbed, C0 is the con-
centration of cation or anion in the initial solution, C is the con-
centration of cation or anion in the solution after adsorption for a
certain period, V is the volume of the solution, and m is the weight
of the dry ground gels.
3. Results and discussion
3.1. Time dependence of adsorption
Fig. 3 shows the time course of the amount of Zn2þ adsorbed on
the DMAAPS gel at 50 ꢁC, which was measured in the Zn(NO3)2 so-
lution at the initial concentration of 5 mmol/L. The figure showed a
high adsorption rate within the first 6 h followed by a much slower
rate, and the adsorption ultimately achieved equilibrium at 8 h. This
result leads to the conclusion that 12 h of contact time is enough to
reach the equilibrium adsorption and further was further used in
investigating the amount of ions adsorbed on the DMAAPS gel.
3.2. Simultaneous adsorption of cation and anion on the DMAAPS
gel
To gain insight into the process of simultaneous adsorption of
the cation and anion on the prepared DMAAPS gel, adsorption
experiments were conducted using NaNO3, Zn(NO3)2, and Al(NO3)3
Fig. 2. Chemical structures of N,N-dimethylaminopropylacrylamide methyl chloride
quaternary (DMAPAA-Q) and sodium 2-(acrylamido)-2-methylpropanesulfonate
(NaAMPS).

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E.O. Ningrum et al. / Polymer 55 (2014) 5189e5197
Fig. 5. Relationship between the amounts of the cations (Naþ, Zn2þ, and Al3þ) and
anion (NOꢀ3 ) adsorbed on the DMAAPS gel in NaNO3, Zn(NO3)2, and Al(NO3)3 solutions
of various concentrations.
Fig. 3. Time dependence of the amount of Zn2þ adsorbed on the DMAAPS gel at 50 ꢁ
in 5 mmol/L Zn(NO3)2 solution.
C
NOꢀ3 adsorbed as a function of the Zn(NO3)2 equilibrium concen-
trations and Fig. 6(b) shows the amount of Al3þ and NO3ꢀ adsorbed
as a function of the Al(NO3)3 equilibrium concentrations. The
amount of Zn2þ and Al3þ adsorbed increased significantly, as the
concentration of the solution increased, and reached a constant
value at higher concentration. On the other hand, the adsorption of
NOꢀ3 was insignificant. These results imply that the adsorption of
Zn2þ and Al3þ on the copolymer gel occurs by ion exchange with
Naþ, which is the counter ion of SOꢀ3 in the NaAMPS groups in the
copolymer gel. However, ion exchange between NOꢀ3 and Clꢀ,
which is the counter ion of Nþ in the DMAPAA-Q groups in the
copolymer gel, hardly occurs. Thus, the amount of Naþ released due
to ion exchange was also measured, and the results are presented in
Fig. 6 along with the amount of NOꢀ3 and Zn2þ or Al3þ adsorbed. The
amount of Naþ released was almost twice or three times higher
than the amount of Zn2þ or Al3þ adsorbed, respectively. Figs. 4(bec)
and 6(aeb) illustrate that the amount of Zn2þ or Al3þ adsorbed on
the copolymer gel was larger than that adsorbed on the DMAAPS
gel at the same equilibrium concentration. These results are
attributed to the fact that the adsorption of Zn2þ and Al3þ on the
copolymer gel occurs via ion-exchange with Naþ, whereas that on
the DMAAPS gel is controlled not only by the affinity between Zn2þ
or Al3þ and SO3ꢀ but also the affinity between NOꢀ3 and Nþ in the
DMAAPS gel due to the simultaneous adsorption of Znþ or Al3þ and
NOꢀ3 .
solutions. The amounts of cation (Naþ, Zn2þ, and Al3þ) and anion
(NOꢀ3 ) adsorbed on the DMAAPS gel in these nitrate solutions of
various concentrations at 50 ꢁC are shown in Fig. 4(a), (b), and (c),
respectively. The cation and anion were both adsorbed on the
DMAAPS gel, and the amount of each type of ion adsorbed
increased as the concentration of the solution increased. In the case
of the NaNO3 solution (Fig. 4(a)), the amounts of Naþ and NO3ꢀ
adsorbed, expressed in units of [mmol/g-dry gel], were almost the
same. In the case of the Zn(NO3)2 solution (Fig. 4(b)), the amount of
NOꢀ3 adsorbed was larger than that the amount of Zn2þ adsorbed.
Furthermore, in the case of the Al(NO3)3 solution, the amount of
NOꢀ3 adsorbed was markedly higher than the amount of Al3þ
(Fig. 4(c)). The observed order of adsorption was as follow:
NaNO3 < Zn(NO3)2 < Al(NO3)3. This order can be explained by the
fact that the fixed ionic groups of the DMAAPS gel interact more
strongly with ions having larger radius, and higher valence [20].
Fig. 5 shows the relationship between the amount of cation and
anion adsorbed. The amount of cation adsorbed is proportional to
the adsorbed anion for each nitrate solution, and the slopes of the
plots in Fig. 5 are inversely proportional to the valence of the cation.
This result implies that the cation and anion are simultaneously
adsorbed on the DMAAPS gel. Therefore, the amount of anion
adsorbed can be simply determined by the amount of cation
adsorbed, or vice-versa.
3.3. Effect of the cross-linker concentration on adsorption and
swelling behaviors
The adsorption capacity of the DMAAPS gel in the Zn(NO3)2
solution was compared with that of the DMAPAA-Q-co-NaAMPS gel
in Zn(NO3)2 and Al(NO3)3 solutions. This gel also contains SOꢀ3 and
Nþ ions in the side chains. Fig. 6(a) shows the amount of Zn2þ and
Fig. 7(a) shows the effect of the concentration of cross-linker
used in the preparation of the DMAAPS gel on the amount of
Fig. 4. Amount of the cation and anion adsorbed on the DMAAPS gel in (a) NaNO3 solution, (b) Zn(NO3)2 solution, (c) Al(NO3)3 solution of various concentrations.

E.O. Ningrum et al. / Polymer 55 (2014) 5189e5197
5193
Fig. 6. Adsorption of Zn2þ, Al3þ and NO3ꢀ on the copolymer gel of DMAPAA-Q-co-NaAMPS in (a) Zn(NO3)2, (b) Al(NO3)3 solutions of various concentrations and amount of Naþ
released from the copolymer gel.
Zn2þ adsorbed on the gel in Zn(NO3)2 solutions of various con-
centrations at 50 ꢁC. The gels were prepared using three different
cross-linker concentrations of 5, 10, and 30 mmol/L. The amount of
Zn2þ adsorbed increased significantly as the concentration of the
solution increased, and the use of a higher cross-linker concen-
tration also increased the amount of ions adsorbed.
concentration in the gel. In addition, at equilibrium swelling the
polymer network of DMAAPS gel swells largely and eventually in-
creases the distance between the sulfonate groups. The reduction
in the amount of Zn2þ adsorbed on the DMAAPS gel indicates the
weakening of the interaction between SOꢀ3 groups of the DMAAPS
gel and the Zn2þ from the Zn(NO3)2 solution due to the expansion
of the gel network.
The swelling properties of the gels in Zn(NO3)2 solutions of
various concentrations were also examined. Fig. 7(b) shows the
degree of swelling of the DMAAPS gel measured at 50 ꢁC. The de-
gree of swelling of the gel prepared using a cross-linker concen-
tration of 30 mmol/L increased gradually as the Zn(NO3)2
concentration increased, although this increase was not significant.
Reducing the cross-linker concentration to 5 mmol/L increased the
degree of swelling significantly; increasing the Zn(NO3)2 concen-
tration produced a similar effect. For polyzwitterionic betaine gel
synthesized with higher cross-linker concentration, the electro-
neutrality of the charge groups results from the intra-group ionic
pairs formation [31]. Furthermore, an increase of the cross-linker
concentration increases the polymer concentration in the gel and
the network density in the gel. As the DMAAPS gel shrinks the
polymer network of DMAAPS gel become close to each other and
the distance between sulfonate groups decreases. This condition
promotes the formation of cation bridges between two neighboring
sulfonate groups; thereby increasing adsorption of more Zn2þ ions
from the Zn(NO3)2 solution on the DMAAPS gel. In contrast,
decreasing the cross-linker concentration reduces the polymer
It is well known that the DMAAPS gel exhibits thermosensitive
properties [21,32] as described above. Thus, the ability of this gel to
adsorb Zn2þ in 10 mmol/L Zn(NO3)2 solution was studied by con-
ducting different sets of experiments at various temperatures.
Fig. 8(a) shows the effect of varying the temperature on the amount
of Zn2þ adsorbed on the DMAAPS gel prepared using three different
cross-linker concentrations, i.e., 5, 10, and 30 mmol/L. At 10 ꢁC, the
amount of Zn2þ adsorbed on these gels was almost the same. With
the use of a low cross-linker concentration of 5 mmol/L, the amount
of Zn2þ adsorbed on the gel decreased significantly as the tem-
perature increased from 10 to 70 ꢁC. At a higher cross-linker con-
centration of 10 mmol/L, the amount of Zn2þ adsorbed decreased
gradually. However, with the use of a higher cross-linker concen-
tration of 30 mmol/L, there was no significant change in the amount
of Zn2þ adsorbed at temperatures in the range of 10e70 ꢁC. This
phenomenon can be explained as follows. For the DMAAPS gel
synthesized with lower cross-linker concentration, at equilibrium
swelling the polymer network of the gel expands largely. This
simultaneously increases the distance between the neighboring
Fig. 7. (a) Amount of Zn2þ adsorbed on the DMAAPS gel and (b) the degree of swelling of the DMAAPS gel in Zn(NO3)2 solutions of various concentrations at 50 ꢁC. The gels were
prepared using different cross-linker concentrations.

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E.O. Ningrum et al. / Polymer 55 (2014) 5189e5197
Fig. 8. (a) Amount of Zn2þ adsorbed on the DMAAPS gel and (b) degree of swelling of the DMAAPS gel in 10 mmol/L Zn(NO3)2 solution at various temperatures. The gels were
prepared using different cross-linker concentrations.
sulfonate groups of DMAAPS. Moreover, the thermal motion
weakens the interaction between the charged groups in the
DMAAPS gel and the Zn2þ and NO3ꢀ ions from the Zn(NO3)2 solu-
tion. As a result the adsorbed amount of Zn2þ decreased with
increasing temperature, particularly for the gel synthesized at
lower cross-linker concentrations.
The effect of temperature on the swelling properties of the gels
in 10 mmol/L Zn(NO3)2 solution was also determined as shown in
Fig. 8(b). The gel prepared using a lower cross-linker concentration
(5 mmol/L) swelled significantly with increasing temperature.
However, the degree of swelling of the gel prepared with a higher
cross-linker concentration (30 mmol/L) increased only slightly with
increasing temperature.
with the ions in the Zn(NO3)2 solution does not increase. A similar
trend was also observed for each solution irrespective of the
concentration.
This behavior can be explained as follows. At low temperature
and high polymer concentration i.e., in the gels prepared using a
high cross-linker concentration, the interaction is dominated by
intra-group ionic pairs [31]. However, because the interaction
between the SOꢀ3 and Nþ groups of the DMAAPS gel that forms
an intra-group ionic pairing is quite strong, in the Zn(NO3)2 so-
lutions, only a part of the charged groups in the DMAAPS gel
interacts with Zn2þ and NO3ꢀ; in other words the amount of ions
adsorbed by the DMAAPS gel is limited. Furthermore, at high
temperature and low polymer concentration i.e., in the gels
prepared with a low cross-linker concentration, the thermal
motion weakens the interaction between the Nþ and SO3ꢀ groups
of DMAAPS. The breakages of associations caused by temperature
increase is consistent with what was observed for sulfobetaine
hydrogel [33] and sulfobetaine polymer solids [34]. Simulta-
neously, the thermal motion also weakens the ionic pairings
force between the charged groups in the DMAAPS gel and the
Zn2þ and NO3ꢀ ions from the Zn(NO3)2 solution. These conditions
reduce the adsorption of ions from the Zn(NO3)2 solution by the
DMAAPS gel, thereby decreasing the amount of Zn2þ adsorbed on
the gel.
It is well known that the interaction between the charged
groups in the sulfobetaine (SOꢀ3 and Nþ) and the ions (cation and
anion) in the solutions have a direct impact on the transition
temperature of polysulfobetaine. Thus, the UCST of poly(-
DMAAPS) was measured to verify the effect of the concentration
of Zn(NO3)2 on the amount of Zn2þ adsorbed as shown in
Fig. 9(a) and (b). Fig. 10 shows the temperature dependence of
the transmittance through the 100 g/L poly(DMAAPS) solution in
Zn(NO3)2 solutions of various concentrations. Increasing the
Zn(NO3)2 concentration resulted in a gradual decrease of the
UCST of poly(DMAAPS) from 47 to 38 ꢁC. This phenomenon is
attributed to dissociation of the Nþ and SO3ꢀ pairings resulting
from the new ionic interactions of the ions in the Zn(NO3)2 so-
lution, i.e., Zn2þ and NO3ꢀ, with the charged groups of poly(-
DMAAPS), i.e., SOꢀ3 and Nþ [21]. Increasing the solution
concentration enhances the dissociation of the chain pairings
because the amount of ions (Zn2þ and NO3ꢀ) adsorbed on pol-
y(DMAAPS) also increases. At higher dissociation of the chain
pairings of poly(DMAAPS), the UCST shifted to lower value. This
result is congruent with the dependence of the adsorption
amount on the solution concentration observed in Fig. 9(a)
and (b).
The relationship between the amount of Zn2þ adsorbed and the
degree of swelling of the DMAAPS gels prepared with three
different cross-linker concentrations, i.e., 5, 10, and 30 mmol/L,
measured at various temperatures, i.e., 10, 30, 50, and 70 ꢁC, and in
various concentrations of Zn(NO3)2 solutions, i.e., 2.5, 5, 7.5, and
10 mmol/L, is summarized in Fig. 9(a). The data reveal an inter-
esting correlation; the data points fall on the same line when the
Zn(NO3)2 concentration is the same. Furthermore, when the degree
of swelling was lower, the amount of Zn2þ adsorbed remained
unchanged and decreased gradually at a higher degree of swelling.
The line depended on the Zn(NO3)2 concentration, and at the same
degree of swelling, the amount of Zn2þ adsorbed increased as the
concentration of Zn(NO3)2 increased.
In addition, the relationship between the amount of Zn2þ
absorbed and the degree of swelling was considered by converting
the degree of swelling to the polymer concentration in the gel,
which could be calculated using the following equation:
W0
V0
1
SD
Cp ¼
ꢂ
(4)
where Cp is the concentration of the polymer in the gel, W0 is the
weight of the dry sample gels, and V0 is the volume of the dry
sample gels.
Fig. 9(b) shows the relationship between the amount of Zn2þ
adsorbed on the DMAAPS gel and the concentration of polymer in
the gel. When the concentration of polymer in the gel was lower,
i.e., at a higher degree of swelling, the amount of Zn2þ adsorbed
increased with increasing polymer concentration. In contrast, when
the concentration of polymer in the gel was higher than about
180 g/L, the amount of Zn2þ adsorbed remained unchanged with
increasing polymer concentration. This implies that the number of
charged groups in the DMAAPS gel that are available to interact

E.O. Ningrum et al. / Polymer 55 (2014) 5189e5197
5195
Fig. 9. Relationship between the amount of Zn2þ adsorbed on the DMAAPS gel and (a) the degree of swelling of the DMAAPS gel, (b) the polymer concentration in the gel. The gels
were prepared using different cross-linker concentrations; the amount of Zn2þ adsorbed and the degree of swelling were measured in the Zn(NO3)2 solutions of various con-
centrations (2.5, 5, 7.5, and 10 mmol/L) and at various temperatures (10, 30, 50, and 70 ꢁC).
3.4. Effect of monomer concentration used in gel preparation on the
adsorption and swelling behaviors
Fig. 12(a) shows the effect of temperature on the gels at a
Zn(NO3)2 concentration of 10 mmol/L. At 10 ꢁC, the amount of Zn2þ
adsorbed on the gels was almost the same. The amount of Zn2þ
adsorbed decreased slightly as the temperature increased from 10
to 70 ꢁC when the gels that were prepared using lower monomer
concentrations, i.e., 500 and 750 mmol/L were immersed in the
Zn(NO3)2 solution. Moreover, the amount of Zn2þ adsorbed
remained unchanged in the case of the gel prepared with a higher
monomer concentration of 1000 mmol/L, as shown previously in
Fig. 8(a). In addition, the degree of swelling of these gels increased
with increasing temperature and was significant for the gel pre-
pared with a lower monomer concentration, i.e., 500 mmol/L, as
shown in Fig 12(b).
As explained previously, the amount of cation adsorbed on the
DMAAPS gel may be correlated to the degree of swelling as well as
to the concentration of polymer in the gel (see Fig. 9). Therefore the
relationship between the degree of swelling and the amount of
Zn2þ adsorbed on the gels prepared with various monomer con-
centrations was examined (Fig. 13(a)); the Zn(NO3)2 concentration
was 10 mmol/L. A similar correlation was found despite variation of
the monomer concentration used in the preparation of the gel. The
correlation between the polymer concentration and the amount of
Zn2þ adsorbed is shown in Fig. 13(b). These results were similar to
those previously described in Fig. 9, regardless of the monomer
concentration. The amount of Zn2þ adsorbed increased with
increasing polymer concentration when the polymer concentration
in the gel was lower. Furthermore, at a polymer concentration
higher than about 180 g/L, the amount of Zn2þ adsorbed remained
unchanged with increasing polymer concentration.
Fig. 11(a) shows the effect of the concentration of the DMAAPS
monomer used in the preparation of the gel on the amount of Zn2þ
adsorbed after immersion of the gel in Zn(NO3)2 solutions of
various concentrations. The gels were prepared by using three
different monomer concentrations of 500, 750, and 1000 mmol/L at
a fixed cross-linker concentration of 30 mmol/L. The amount of
Zn2þ adsorbed increased with increasing Zn(NO3)2 concentration.
Furthermore, the amount of Zn2þ adsorbed on the DMAAPS gel
prepared using a higher monomer concentration was larger than
that prepared using a lower monomer concentration. The swelling
behavior of these gels is shown in Fig. 11(b). The gel prepared using
a lower monomer concentration can swell to a greater extent than
that prepared using a higher monomer concentration.
4. Conclusion
The adsorption of cations and anions on DMAAPS gel was
investigated using NaNO3, Zn(NO3)2, and Al(NO3)3 solutions. Cat-
ions and anions were simultaneously adsorbed from the nitrate
solutions by the DMAAPS gel. That is, the amount of cation adsor-
bed was proportional to the amount of anion adsorbed and the
slope of the plot of the amount of anion adsorbed vs. the cation
adsorbed was inversely proportional to the valence of the cation.
Furthermore, the adsorption amount increased with increasing
valence of the cations. The amount of Zn2þ adsorbed on the
Fig. 10. Transition behavior of poly(DMAAPS) (100 g/L) in Zn(NO3)2 solutions of
various concentrations.

5196
E.O. Ningrum et al. / Polymer 55 (2014) 5189e5197
Fig. 11. (a) Amount of Zn2þ adsorbed on the DMAAPS gel and (b) degree of swelling of the DMAAPS gel in Zn(NO3)2 solutions of various concentrations at 50 ꢁC. The gels were
prepared with various monomer concentrations.
Fig. 12. (a) Amount of Zn2þ adsorbed on the DMAAPS gel and (b) degree of swelling of the DMAAPS gel in 10 mmol/L Zn(NO3)2 solution at various temperatures. The gels were
prepared with various monomer concentrations.
Fig. 13. Relationship between the amount of Zn2þ adsorbed on the DMAAPS gel and (a) the degree of swelling, (b) the polymer concentration in the gel. The gels were prepared with
different monomer concentrations; the amount of Zn2þ adsorbed and the degree of swelling were measured in Zn(NO3)2 solutions of various concentrations and at various
temperatures.

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5197
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