Effects of specific anions on the relationship between the ion-adsorption properties of sulfobetaine gel and its swelling behavior
Graphical abstract
Introduction
Various zwitterionic betaine gels, such as phosphobetaine [1], [2], [3], sulfobetaine [4], [5], [6], carboxybetaine [7], [8], semi-interpenetrating network (IPN) gel composed of amphiphilic gel and zwitterionic betaine polymers [9], [10], and chemical–physical cross-linked double network (DN) gel, which was obtained by introducing ductile physical networks into the network of chemically 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.
A zwitterionic betaine polymer that contains both anionic and cationic active groups in the same polymeric repeat unit with an 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 [17], [18], [19]. Betaine polymers are generally thermosensitive in aqueous solutions [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 properties of betaine is the ability of the fragments to form cyclic conformations of the cationic and anionic groups of neighboring monomer residues (intra-group), form contacts between the cationic and anionic groups of neighboring macromolecules (inter-chain), and undergo head-to-tail stacking within single macromolecules (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].
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 containing sulfobetaine was investigated by Lee et al. [25], [26], [27], [28]. 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: Divinylbenzene 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 solution; 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 investigated by J. Ning et al. by varying the monomer concentration [20]. However, there are relatively few studies on the adsorption capacity of betaine gels and the correlation with its swelling ability in solvents with various compositions, temperatures, and ion strengths.
In our previous study, the effects of the temperature and preparation conditions of the gels, such as the cross-linker and monomer 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 simultaneous adsorption of cations (Al3+, Zn2+, or Na+) and anions (NO3−) was confirmed. Furthermore, it was found that the amount of cation (Zn2+) adsorbed onto the DMAAPS gel remained unchanged for the gel prepared with a higher cross-linker or monomer concentration 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 degree of swelling of DMAAPS gel and amount of cation (Zn2+) 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 concentration in the gel was higher than ∼180 g/L, the amount of Zn2+ adsorbed remained unchanged. The adsorption amount is the maximum adsorption amount of Zn2+ or NO3− 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 Zn2+ adsorbed decreased as the swelling degree increased. Furthermore, at the same swelling degree, the amount of Zn2+ 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 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.
Section snippets
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-dimethylaminopropylacrylamide (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
Adsorption behavior of ions on DMAAPS gel in halide solutions
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 (K+) and anions (F−, Cl−, Br−, and I−) 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)–(d) show the amount of K+ adsorbed onto DMAAPS gel from these halide solutions as a function of temperature. In our previous work, it was confirmed that the DMAAPS gel
Conclusion
The effect of the anion species in the halides, i.e., KF, KCl, KBr, and KI, on the amount of cation (K+) or anions (F−, Cl−, Br−, or I−) adsorbed onto DMAAPS gel was investigated. The order of increased adsorption of the anion onto DMAAPS gel was opposite that in the Hofmeister series, although almost no K+ and F− were adsorbed from the KF solution. 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
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