Monovalent Selectivities and Secondary Interactions of

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Divalent/Monovalent Selectivities and Secondary Interactions of Multibasic Acids on Anion Exchange Resins Akihiro Inui, Chihiro Hama, Tomoaki Katsuragawa, Shuichi Iwata, and Akio Yuchi* Graduate School of Engineering, Nagoya Institute of Technology, Gokiso, Showa, Nagoya 466-8555, Japan S Supporting Information *

ABSTRACT: The ion exchange equilibria of Br− on three strongly basic anion exchange resins of different exchange capacities and different cross-linking degrees by five alkanoates (MCm), a phosphate, and eight alkanedioates (DCn; m,n = number of carbon atoms) were analyzed, and the selectivity coefficients for their monoanion and dianion, K11 and K21 were evaluated as a function of percent exchange (%E). The decrease in the K11 values of MCm with an increase in %E was attributed to the interference with hydration of MCm in the resin phase, while the increase in the K11 values of DCn to the intermolecular hydrogen bonding between exchanged acidic anions. The log K21 values of DCn at %E = 0 plotted against n showed the minimum rather than the maximum as described previously. The reason has been discussed in terms of the hydrophobic interaction of DCn with the polymer matrix, intramolecular hydrogen bonding, and the charge density.

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2( −R+,X−) + HA− = {( −R)2 ,A2 −} + 2X− + H+

INTRODUCTION Ion-exchange phenomena first discovered as early as 1850s still have a lot of properties to be uncovered from the scientific point of view.1−3 The ion-selectivity of this process is primarily governed by the electric charge of the ion. For example, anion exchange resins (AXRs) favor divalent anions like SO42− over monovalent ions like NO3−. Several attempts have been made to enhance or rather reverse this native divalent/monovalent anion selectivity.4−8 The effects of introducing a potentially hydrogenbonding group (type II) and of increasing the basicity of the nitrogen atom on AXRs have been examined by Gregory, Boari, and others.4,5 Clifford and Weber, based on the concept of “distance of charge separation”, proposed introduction of functional groups directly into the polymer backbone without any spacer, reduction in size of ammonium group, and decrease in cross-linking degree, in order to enhance the selectivity.6 Barron and Fritz succeeded in reversing the SO42‑/NO3− selectivity by using the AXR with a large tributylbenzylammonium group on ion chromatography of anions.7,8 These were fully discussed in a review.9 Later studies, however, demonstrated that the larger ammonium group favored less hydrophilic anions among monovalent ions.10,11 Increasing the size of the ammonium group not only changes the distance between functional groups but also the local charge density as well. Multibasic acids dissociate protons in a stepwise manner to yield species with different charges, which are respectively exchanged on an AXR in the X− form (−R+,X−). In the case of a dibasic acid H2A, for example ( −R+,X−) + HA− = ( −R+,HA−) + X−

(1)

2( −R+,X−) + A2 − = {( −R+)2 ,A2 −} + 2X−

(2)

The abundance of A2‑ in the resin phase is higher than that in the aqueous phase in the equilibrium state and is affected by the total concentration of the acid and pH {( −R+)2 ,A2 −} + H+ + HA− = 2( −R+,HA−)

(4)

Whereas the ion-exchange of inorganic multibasic acids, which cause acid dissociation reactions in proximity, has been well studied from both scientific and practical points of view,12−17 the ion-exchange of organic multibasic acids, which may cause acid dissociation reactions in more distant sites, have hardly been studied. Takahashi and others analyzed the exchange equilibria by alkanedioates on an anion exchange membrane.18 Subramonian and Clifford studied the exchange equilibria by alkanedioates on AXRs of different exchange capacities and indicated the optimal lengths of alkanedioate for the respective AXRs: oxalate for the AXR of 3.52 meq g−1, malonate for the AXR of 2.90 meq g−1, and adipate for the AXR of 1.72 meq g−1.19 The exchange capacity was, however, controlled by increasing the size of an alkyl group, which may involve other effects as described elsewhere.10,11 In this study, we have examined the exchange equilibria by phosphate, alkanoates and alkanedioates with varying alkyl chain lengths on a series of AXRs including that of a low exchange capacity prepared by introducing phenyl group without any substituent.20−22 The selectivity coefficients are discussed in terms of electrostatic interaction, hydrophobic interaction, and inter- and intramolecular hydrogen bonding.

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EXPERIMENTAL SECTION Resins and Reagents. Three strongly basic trimethylbenzylammonium-type AXRs of different exchange capacities and

While the reaction given by eq 2 is independently observed under the condition with A2‑ as a major species in the aqueous phase, the reaction given by eq 1 is accompanied with the protonreleasing reaction given by eq 3 under the condition with HA− as a major species in the aqueous phase.12 © 2013 American Chemical Society

(3)

Received: Revised: Accepted: Published: 16880

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Figure 1. Effects of %exchange on selectivity coefficients of MCm (m = 2−5) on three resins. Resin: (a) I-1×2, (b) I-3×2, ands (c) I-3×8. Anion: (□) MC2, (Δ) MC3, (ο) MC4, and (◇) MC5.

supernatant. In the case of DCn and phosphate, INA and OUTBr were different from each other. Since pH was controlled so that PO43‑ was absent both in aqueous and resin phases, phosphoric acid as well as DCn was conventionally expressed as H2A. Among the possible exchanged-species, (−R+,HA−) was counted singly in both INA and OUTBr, while {(−R+)2,A2−} was counted singly in INA but doubly in OUTBr. The chemical amounts of the anion as these two types of species in the resin phase, IN1− and IN2−, were thus calculable from INA and OUTBr by eqs 7 and 8.

different cross-linking degrees were examined (I-1×2, I-3×2, I3×8: the first digit indicates an approximate exchange capacity of Br− form and the second digit indicates a cross-linking degree by divinylbenzene). Commercially available resins (Dowex 1, 100 − 200 mesh) were used as I-3×2 and I-3×8, while I-1×2 was prepared using a Merrifield resin (TCI) as described previously.21 These AXRs were packed in columns and exchanged to the Br− form by introducing a 1 mol L−1 NaBr solution and sufficiently washing. After storage in a glovebox kept at 25 °C and at relative water vapor pressure of 50% to give constant weights, portions of the respective AXRs were weighed and the Br− contents were titrimetrically determined after exchange by 5 − 15 mmol L−1 NaClO4 solutions, in order to specify the exchange capacities (ECBr). It was found that the exchange capacity slowly decreased over a period of 6 months, probably due to deamination (from 0.87 to 0.82 for I-1×2, from 3.40 to 3.17 for I-3×2, from 3.09 to 2.91 for I-3×8). The decrease in exchange capacity was considered for the equilibrium analysis described below. Eight dioates [six alkanedioates with atom number n = 2−7 (DCn) between two negatively charged oxygen atoms, fumarate (DC4(trans)), and maleate (DC4(cis))], phosphate as an anion of n = 1, and five alkanoates with carbon number m = 1−5 (MCm) were subjected to equilibrium analysis. The corresponding acids of analytical grade were partially neutralized with NaOH to give stock solutions at respective pH values described below. Ion-Exchange by Anions. A portion (2−60 mg) of each accurately weighed AXR was shaken with each 6 or 25 mL portion of 0.1−300 mM anion solutions at 125 strokes min−1 for 12 h. In order to avoid complication of the reaction system, no supporting electrolyte was added. The ionic strength of the solution was changed from 0.001 to 0.030. The change in activity coefficient was estimated to be less than 0.1 in logarithmic scale and was negligible in equilibrium analysis. The supernatant obtained by filtration was subjected to pH measurement and ion chromatography. When a pH increase by the exchange was not negligible, shaking was performed again with addition of a small amount of NaOH by trial and error. Equilibrium Analysis. The chemical amount of the anion penetrated into the resin phase (INA) and that of Br− eluted from the resin phase (OUTBr) were calculated using eqs 5 and 6. INA = TAA − [A′]V −

OUTBr = [Br ]V

IN1 − = 2 × INA − OUTBr

(7)

IN2 − = OUTBr − INA

(8)

The AXR of the mass w contained the total amount of the functional groups, TAR = ECBrw. The concentrations of each species in the resin phase were calculated using these quantities [−R+,Br −] = (TAR − OUTBr)/w

(9)

[−R+,HA−] = IN1 −/w

(10)

[( − R+)2 ,A2 −] = IN2 −/w

(11)

The percent exchange, %E as measures for the progress of the exchange and for the composition in the resin phase, was given by eq 12. %E = OUTBr /TAR × 100

(12)

In contrast, the concentrations of the species in the aqueous phase were given by eqs 13−15. [A2 −] = [A′]/(1 + K1[H+] + K1K 2[H+]2 )

(13)

[HA−]=K1[H+][A2 −]

(14)

[H 2A]=K 2[H+][HA−]

(15)

where K1 and K2 are the stepwise protonation constants available in literature (K2 and K3 for phosphate).23 The selectivity coefficients for the exchange of Br− by monoanions and dianions, K11 and K21, were calculated by eqs 16 and 17 with respect to each experimental point in each reaction system, respectively.

(5) (6)

K11 = [−R+,HA−][Br −]/([ − R+,Br −][HA−])

(16)

K 21 = [( −R+)2 ,A2 ‐][Br −]2 /([−R+,Br −]2 [A2 −])

(17)

A preliminary study on the ion-exchange of DC3 at varying pH values indicated that the most reliable values for these coefficients were obtained at pH around (log K1 + log K2)/2, where both (−R+,HA−) and {(−R+)2,A2‑} were appreciably

where TAA is the total amount of the anion, [Br−] and [A′] are the concentrations of bromide and the anion irrespective of the dissolution state in the supernatant, and V is the volume of the 16881

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Figure 2. Effects of %E on selectivity coefficients of phosphate and DCn (n = 2−4) on three resins. Resin: (a−d) I-1×2, (e−h) I-3×2, and (i−l) I-3×8. Anion: (a,e,i) phosphate, (b,f,j) MC2, (c,g,k) MC3, and (d,h,l) MC4. ■, log K11; □, log K21.

for MC2. The disadvantage was overcome for MCm with a longer alkyl chain by the hydrophobic interaction with the phenyl ring as shown in Figure S-1; the log K11 values at %E = 0 plotted against m had the slope of 0.39 on I-1×2, 0.08 on I-3×2 and 0.13 on I3×8 for m ≥ 2. The selectivities of MCm on I-3×8 were lower than those on I-3×2 by around 0.5, due to the higher crosslinking degree. Formic acid (MC1) showed much higher selectivities than those expected from the correlation (Figure S-1). This may be attributed to the higher charge density than those of the other MCm. Such a unique behavior of MC1 was also reported in the ion-exchanges on the same type of an AXR with 10% crosslinking and on the cetyltrimethylammonium micelles.24,25 Increase in K11 of DCn at High Loading by Intermolecular Hydrogen Bonds. The selectivity coefficients of phosphate and DCn (n = 2 − 4) on three AXRs were determined over a wide range of %E (Tables S-16−S-27) and their logarithmic values are given in Figure 2. The log K11 values of DC3 on three AXRs and those of phosphate and DC2 on I-1×2 were independent of %E (Figure 2a−c,g,k), while those of the others were constant up to %E = 70 and increased at the higher % E by about unity. The coefficients of DC4(cis) were constant, while those of DC4(trans) showed an increase at high loading (Tables S37−S-42 and Figure S-2). These are in remarkable contrast to MCm showing linear decreases in log K11 at high loading on the AXRs of the higher exchange capacities. The intermolecular hydrogen bonding between HA− having both hydrogen-bond donors and acceptors is expected to give such positive cooperation between exchanged species.26,27 The difference IR spectra with reference to the Br−-form were recorded in order to characterize the chemical state of DCn in the resin phase. Although the water vapor pressure and thereby the swelling degree of AXRs in the IR measurement were different

formed. Thus, for each of other acids, the exchange equilibrium was analyzed at a fixed pH around (log K1 + log K2)/2. After equilibration, pH decreased by the acid-forming reaction given by eq 3. In the case of monocarboxylates, pH was adjusted to be appreciably larger than log K1. INA and OUTBr agreed with each other within experimental errors. The selectivity coefficients K11 were calculated according to eq 16. The coefficients for MCm, phosphate, and DCn had standard deviations of ±0.1 in logarithmic scale. Infrared Spectroscopy. The difference infrared spectra of the resins were recorded using FTIR (type 460 Plus, Jasco) with reference to the Br− form by the KBr disk method.

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RESULTS AND DISCUSSION Decrease in K11 of MCm at High Loading by Hydration. The log K11 values of MCm (m = 1−5) on three AXRs were determined over a wide range of %E (Tables S-1−S-15), and those of MCm (m = 2−5) are plotted against %E in Figure 1. The coefficients of MCm on I-1×2 were practically independent of % E, while those of MCm of m ≥ 3 on I-3×2 and those of MCm of m ≥ 2 on I-3×8 decreased linearly with an increase in %E. The higher hydration properties of MCm than Br− and thereby the swelling of the resin were counteracted by cross-linking of the polymer to reduce the selectivities of these anions at high loading.1 The trend was more remarkable at a higher crosslinking of 8% than at 2% at a fixed ECBr of 3 mmol g−1. The effect of an alkyl chain length of MCm on the coefficients was remarkable on I-1×2, compared with that on I-3×2, due to the styrene residue with no functional group introduced to decrease the exchange capacity. The lipophilic phenyl rings interfered with the electrostatic interaction between exchange groups and hydrophilic carboxylate groups to give a lower selectivity, e.g., 10−1.8 on I-1×2 compared with 10−0.8 on I-3×2 16882

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Figure 3. Difference IR spectra of resins. Sample: DC4 form of I-3×8 at %E = 40 (a) and 100 (b); DC2 (c), DC3 (d), and DC4 form of I-1×2 at %E = 100.

showed some broadening at 1000 cm−1 as well as at 1500 cm−1 (Figure 3c,e). These agreed with that log K11 was constant for DC2 but increased for DC4 at high loading (Figure 2b,d). Thus, intermolecular hydrogen bonding is possible for the long DC4 but not for the short DC2 on the AXR of the lower exchange capacity. Increase in K11 of DCn Even at Low Loading by Intramolecular Hydrogen Bonds. The selectivity coefficients of the longer DCn (n = 5−7) were determined only at low loading (Tables S-28−S-36 and Figure S-3). The average values of the selectivity coefficients of phosphate and all DCn (n = 2−7) at low loading are summarized in Table S-43 and are plotted against n in Figure 4a−c, which also includes those of DC4(trans) and DC4(cis). All the coefficients on I-3×2 were larger than those on I-1×2 and on I-3×8 as in the case of MCm. The log K11 value linearly increased for DCn of n ≥ 5 with a slope of 0.35 on I-1×2 and for DCn of n ≥ 4 with a slope of 0.13 on I-3×2 and with a slope of 0.25 on I-3×8. These slopes were in good agreement with those for MCm. The coefficients of DC4(trans), which may have only the extended conformation, were fit in this correlation for three AXRs including I-1×2. Thus, monoanionic species of these DCn with the longer spacer adopt the extended conformation without any intermolecular interaction at low loading and are benefitted by hydrophobic interaction with the polymer matrix (Scheme 1b). The 1:1 species of DCn with the shorter spacer showed the higher stabilities than those expected from this correlation. The high charge density of DC2 may be responsible for the larger K11 on any AXR as in the case of MC1. In order to elucidate the reason for the larger K11 of DC3, difference IR spectra of DC3 were recorded on I-1×2 (Figure 3d). Another type of broadening was observed at around 1500 cm−1 and was assigned to the intramolecular hydrogen bonding.28 A similar spectrum was also observed for DC4(cis) even at %E = 40. In the crystalline state, the

from those in equilibrium analysis, the previous studies on the hydration indicated that the strong interactions were commonly observed.21,22 The spectra of DC4 on I-3×8 at %E = 40 and 100 are given in Figure 3a,b. Two sharp bands were clearly observed at low loading, while two broad bands centered at 1700 cm−1 and especially at 1000 cm−1 overlapped with these sharp bands at high loading. Such broadening was attributed to the intermolecular hydrogen bonding (Scheme 1a).28 An increase in log K11 (Figure 2i) and the appearance of similar bands for phosphate on I-3×821 indicated that even such small anions were located close to each other so as to form intermolecular hydrogen bonding on the AXRs of the higher exchange capacities. On I1×2, DC2 at %E = 100 showed only sharp peaks, while DC4 Scheme 1. Interactions of Dicarboxylates within the Resin Phase

16883

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Figure 4. Effects of n on selectivity coefficients of DCn (n = 2 −7) at lower %E (a−c) and on divalent/monovalent selectivity of phosphate and DCn (d− f). Resin: (a,d) I-1×2, (b,e) I-3×2, (c,f) I-3×8. Closed symbol, K11; open symbol, K21; diamond, DC4(trans); triangle, DC4(cis).

lammonium micells.37 Both the charge density and the hydrophobicity of alkyl chain contribute to the log K21 values. Since the log K11 and log K21 values may not be directly comparable, because of their different dimensions, an imaginary equilibrium given by eq 18 was adopted as a measure of stabilization of A2− relative to HA− in the resin phase in comparison to the aqueous phase.

monoanionic species of DC3 and DC4(cis) are known to adopt the planar conformation by intramolecular hydrogen bonding.29−31 Thus, the intramolecular hydrogen bonding of DC3 and DC4(cis) makes the monoanions more rigid and more compact to enhance the electrostatic interaction with the exchange groups in the resin phase, irrespective of %E (Scheme 1c). This is why o-phthalate and DC4(cis) are favorably used as a powerful eluting reagent in ion chromatography of anions.32 The convex feature for log K11 value of DC4 on I-1×2 may also be attributed to such intramolecular hydrogen bonding, because of the larger void available. Because of these three contributions (high charge density of DC2, intramolecular hydrogen bonding of DC3 and partly DC4, and hydrophobic interaction with the polymer matrix for DCn (n ≥ 4)), the log K11 value at low loading in Figure 4a−c seemingly had minima at n = 5 or 4. K21 and Divalent/Monovalent Selectivities of DCn. The log K21 values, in Figure 2, were relatively independent of %E, except for a few cases (f,g,h,j) where the interaction between (−R+,HA−) and {(−R+)2,A2‑} may be involved. Although the high stability of the 1:1 species interfered with evaluation of the selectivity coefficients of the 2:1 species of DC4(cis) on I-1×2 and I-3×2, the log K21 value on I-3×8 was close to those of DC4 and DC4(trans). This is in contrast to that the corresponding value of DC4(trans) was much larger than that of DC4(cis) on layered double hydroxide (LDH) exchangers.33,34 The ion exchange groups on AXRs were located at varying directions and distances to allow the electrostatic interaction (Figure 1d). The log K21 values are plotted against n in Figure 4a−c. The former study using the same type of AXRs with varying exchange capacities had suggested their maxima in such relations and claimed recognition of the distance between two negative charges of DCn,19 in analogy with molecular recognition by ditopic anion receptors in dilute solutions.35,36 In this study, however, the log K21 values did not vary so much and showed minima rather at n = 4 or 5 on any AXR. This means that the distance between two negative charges was not recognized by AXRs. Such a minimum was similarly observed in the ion exchange by DCn on the cetyltrimethy-

2( −R+,HA−) + A2 − = {( −R+)2 ,A2 −} + 2HA−

(18) 2

The exchange constant (K) was calculated as K21/(K11) . The log K values for DCn are given in Table S-43 and are plotted against n in Figure 4d−f, together with phosphate at n = 1. The change in log K with n is simple, compared with those in log K11 and log K21, because of cancellation of the effects of a few factors. The decrease in log K with an increase in n indicated that the interaction of DCn at both ends with ion exchange groups on AXRs became more difficult for the DCn of the large n. In addition, a remarkable difference was observed between DCn with odd n values and those with even n values. This may be related to the native orientation of two carboxylates. The different behaviors between DCn of odd n values and DCn of even n values were also reported in the intercalation in LDH.38

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CONCLUSIONS The selectivity coefficients were determined over a wide range of %E on the ion exchange of Br− by MCm, phosphate, and DCn on three AXRs of different exchange capacities and different crosslinking degrees. The greater difference in coefficients of MCm or DCn was observed on the resin of the low capacity and the lower cross-linking degree, because the hydrophobic environment weakened the electrostatic interaction and differentiated anions by the hydrophobic interaction. This may be the reason for the high performance of ion chromatography, which uses the stationary phase with the lower exchange capacities, compared with the conventional ion exchange chromatography.32 Intermolecular hydrogen bonding between monoanionic species enhanced the selectivity at high loading, while intramolecular hydrogen bonding specifically enhanced the selectivity 16884

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(2) Korkisch, J. Handbook of Ion Exchange Resins Their Application to Inorganic Analytical Chemistry; CRC Press: Boca Raton, FL, 1989; Vol. 1. (3) Moyer, B. A.; Bonnesen, P. V. In Supramolecular Chemistry of Anions; Bianchi, A., Bowman-James, K., Garcia-Espana, E., Eds.; WileyVCH: Weinheim, Germany, 1997. (4) Gregory, J.; Dhond, R. V. Anion Exchange Equilibria Involving Phosphate, Sulphate and Chloride. Water Res. 1972, 6, 695−702. (5) Boari, G.; Liberti, L.; Merli, C.; Passino, R. Exchange Equilibria on Anion Resins. Desalination 1974, 15, 145−166. (6) Clifford, D.; Weber, W. J., Jr. The Determinants of Divalent/ Monovalent Selectivity in Anion Exchangers. React. Polym. 1983, 1, 77− 89. (7) Barron, R. E.; Fritz, J. S. Effect of Functional Group Structure on the Selectivity of Low-Capacity Anion Exchangers for Monovalent Anions. J. Chromatogr. 1984, 284, 13−25. (8) Barron, R. E.; Fritz, J. S. Effect of Functional Group Structure and Exchange Capacity on the Selectivity of Anion Exchangers for Divalent Anions. J. Chromatogr. 1984, 316, 201−210. (9) Calmon, C. Recent Developments in Water Treatment by Ion Exchange. React. Polym. 1986, 4, 131−146. (10) Bonnesen, P. V.; Brown, G. M.; Alexandratos, S. D.; Bavoux, L. B.; Presley, D. J.; Patel, V.; Ober, R.; Moyer, B. A. Development of Bifunctional Anion-Exchange Resins with Improved Selectivity and Sorptive Kinetics for Perchlorate: Batch-Equilibrium Experiments. Environ. Sci. Technol. 2000, 34, 3761−3766. (11) Gu, B.; Ku, Y.-K.; Brown, G. M. Sorption and Desorption of Perchlorate and U(VI) by Strong-Base Anion-Exchange Resins. Environ. Sci. Technol. 2005, 39, 901−907. (12) Horng, L.-L.; Clifford, D. The Behavior of Polyprotic Anions in Ion-Exchange Resins. React. Funct. Polym. 1997, 35, 41−54. (13) Sengupta, A. K.; Clifford, D. Some Unique Characteristics of Chromate Ion Exchange. React. Polym. 1986, 4, 113−130. (14) Sengupta, A. K.; Clifford, D. Chromate Ion Exchange Mechanism for Cooling Water. Ind. Eng. Chem. Fundam. 1986, 25, 249−258. (15) Yoshimura, K.; Miyazaki, Y.; Ota, F.; Matsuoka, S.; Sakashita, H. Complexation of Boric Acid with the N-Methyl-d-Glucamine Group in Solution and In Crosslinked Polymer. J. Chem. Soc., Faraday Trans. 1998, 94, 683−689. (16) Lackner, K. S. Capture of Carbon Dioxide from Ambient Air. Eur. Phys. J. Spec. Top. 2009, 176, 93−106. (17) Takahashi, H.; Kikuchi, K. Sorption of Phosphoric Acid by AnionExchange Membrane. J. Ion Exch. 2010, 21, 392−396. (18) Takahashi, H.; Ohba, K.; Kikuchi, K. Sorption of Di- and Tricarboxylic Acids by an Anion-Exchange Membrane. J. Membr. Sci. 2003, 222, 103−111. (19) Subramonian, S.; Clifford, D. Monovalent/Divalent Selectivity and the Charge Separation Concept. React. Polym. 1988, 9, 195−209. (20) Matsuura, T.; Ohnaka, K.; Takagi, M.; Ohashi, M.; Mibu, K.; Yuchi, A. Coadsorption of Trivalent Metal Ions and Anions on Strongly Acidic Cation-Exchange Resins by Bridge Bonding. Anal. Chem. 2008, 80, 9666−9671. (21) Yuchi, A.; Kuroda, S.; Takagi, M.; Watanabe, Y.; Nakao, S. Effects of Exchange Capacity and Cross-Linking Degree on Hydration States of Anions in Quantitative Loading onto Strongly Basic Anion-Exchange Resins. Anal. Chem. 2010, 82, 8611−8617. (22) Watanabe, Y.; Ohnaka, K.; Fujita, S.; Kishi, M.; Yuchi, A. Effects of the Spaces Available for Cations in Strongly Acidic Cation-Exchange Resins on the Exchange Equilibria by Quaternary Ammonium Ions and on the Hydration States of Metal Ions. Anal. Chem. 2011, 83, 7480− 7485. (23) Serjeant, E. P.; Dempsey, B. Ionisation constants of organic acids in aqueous solution; IUPAC chemical data series, No. 23; Pergamon Press: New York, 1979. (24) Chu, B.; Whitney, D. C.; Diamond, R. M. On Anion-Exchange Resin Selectivities. J. Inorg. Nucl. Chem. 1962, 24, 1405−1415. (25) Lissi, E.; Abuin, E.; Ribot, G.; Valenzuela, E.; Chaimovich, H.; Araujo, P.; Aleixo, R. M . V.; Cuccovia, I. M. Ion Exchange between nAlkyl Carboxylates and Bromide at the Surface of Cetyltrimethylammonium Micelles. J. Colloid Interface Sci. 1985, 103, 139−144.

of DC3 and DC4(cis) on any AXR and of DC4 on I-1×2, irrespective of %E. Several factors including the intramolecular hydrogen bonding, the stronger electrostatic interaction with carboxylates of the higher local charge densities and the stronger hydrophobic interaction of carboxylates with the higher lipophilicity caused a minimum selectivity at low loading as a function of the carbon number of carboxylates. The distance between two negative charges was not recognized by AXRs because the ion-exchange groups are densely located at various directions and distances. The divalent/monovalent selectivity in the resin phase relative to the aqueous phase decreased with an increase of the spacer length.

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ASSOCIATED CONTENT

S Supporting Information *

Additional figures showing the log K11 at %E = 0 in ion exchange of Br− by MCm and the plot of log K11 and log K22 against %E for DCn (n = 5 − 7), DC4(trans), and DC4(cis) and additional tables showing ion exchange of Br− by MCm and DCn and selectivity coefficients. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research from Ministry of Education, Culture, and Technology, Japan (No. 23550093).

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NOMENCLATURE AXR = anion exchange resin DCn = dicarboxylate with carbon number of n DC4(cis) = fumarate DC4(trans) = maleate ECBr = exchange capacity of Br− form H2A = dibasic acid INA = chemical amount of anion penetrated into the resin phase IN1− = chemical amount of monoanion in the resin phase IN2− = chemical amount of dianion in the resin phase K1 = first protonation constant K2 = second protonation constant K11 = selectivity coefficient of monoanionic species K21 = selectivity coefficient of dianionic species MCm = monocarboxylate with carbon number of m OUTBr = chemical amount of bromide eluted from the resin phase −R+,X− = anion exchange resin of X−-form TAA = total chemical amount of anion TAR = total chemical amount of functional group in the resin phase V = volume of solution w = weight of resin %E = percent exchange

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REFERENCES

(1) Helfferich, F. Ion Exchange; McGraw-Hill Book Company, Inc.: New York, 1962. 16885

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dx.doi.org/10.1021/ie402117b | Ind. Eng. Chem. Res. 2013, 52, 16880−16886