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Langmuir 1997, 13, 2980-2982
Water Sorption Isotherms and Cation Hydration in Dowex 50W and Amberlyst-15 Ion Exchange Resins R. S. D. Toteja,*,† B. L. Jangida,† M. Sundaresan,† and B. Venkataramani‡ Analytical Chemistry Division and Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India Received July 18, 1996. In Final Form: February 21, 1997X Water sorption isotherms of gel-type Dowex 50WX4 and Dowex 50WX8 and macroreticular Amberlyst15 resins in H+, Na+, and alkaline earth metal ionic forms have been determined at 298 ( 1 K using the isopiestic technique. The hydration numbers of cations derived from the analysis of the water sorption isotherms using the D’Arcy and Watt equation compare well with those reported for similar types of ion exchangers obtained using other techniques. An interesting feature that has emerged from the analysis is the presence of a constant amount of water having bulk structure in Amberlyst-15, irrespective of the nature of the cations, possibly those filling the pores of the exchanger. The present study, thus, indicates the structural differences between the water present in the gel phase and in the pores of the macroreticular Amberlyst-15.
Introduction Various models proposed to explain the selectivity and related characteristics, like swelling, of ion exchangers recognize the key roles played by the ion-water, counterion-ionogenic group, and water-water interactions in the ion exchanger phase.1-4 Moreover, it has been shown that Dowex 50W-type resins behave as single ion solutions,5 because the ionogenic groups are osmotically inactive and unhydrated.6,7 With a view to provide possible experimental evidence regarding the nature of the ion-pair formation, ionic hydration, mobility of the free water, etc., the state of water present in the ion exchanger has been investigated by a variety of techniques.5,8-18 It has been shown earlier19 that analyzing the water sorption isotherms of Dowex 50W resins in different ionic forms using the D’Arcy and Watt equation20 * To whom all correspondence should be sent. † Analytical Chemistry Division. ‡ Chemistry Division. X Abstract published in Advance ACS Abstracts, April 15, 1997. (1) Helfferich, F. Ion Exchange; McGraw Hill and Co.: New York, 1962. (2) Reichenberg, D. In Ion Exchange; Marinsky, J. A., Ed.; Marcel Dekker: New York, 1966; Vol. 1, p 227. (3) Sherry, H. S. In Ion Exchange; Marinsky, J. A., Ed.; Marcel Dekker: New York, 1969; Vol. 2, p 89. (4) Gupta, A. R. Ind. J. Chem. 1990, 29A, 409. (5) Gupta, A. R.; Venkataramani, B. Ind. J. Chem. 1995, 34A, 787. (6) Zundel, G. Hydration and Intermolecular Interactions; Academic Press: New York, 1966. (7) Marinsky, J. A.; Hoegfeldt, E. Chem. Scr. 1976, 9, 233. (8) Dinius, R. H.; Emerson, M. T.; Choppin, G. R. J. Phys. Chem. 1963, 67, 1178. (9) Frankel, L. S. Anal. Chem. 1970, 42, 1638; 1971, 43, 1506; J. Phys. Chem. 1971, 75, 1211. (10) Creekmore, R. W.; Reilley, C. N. Anal. Chem. 1970, 42, 570, 725. (11) Gough, T. E.; Sharma, H. D.; Subramanian, N. Can. J. Chem. 1970, 48, 917. (12) Sharma, H. D.; Subramanian, N. Can. J. Chem. 1971, 49, 457, 3948. (13) Levy, L. Y.; Jenard, A.; Hurwitz, H. D. J. Chem. Soc., Faraday Trans. 1 1980, 76, 2558; 1982, 78, 29. (14) Levy, L. Y.; Muzzi, M.; Hurwitz, H. D. J. Chem. Soc., Faraday Trans. 1 1982, 78, 17, 1001. (15) Glueckauf, E.; Kitt, G. P. Proc. R. Soc. London 1955, A228, 322. (16) Busch, M.; Goldammer, E. V. J. Solution Chem. 1982, 11, 777. (17) Samsanov, G. V.; Pasechnik, Y. A. Russ. Chem. Rev. 1969, 38, 547. (18) Nandan, D.; Gupta, A. R. Ind. J. Chem. 1974, 12, 808; J. Phys. Chem. 1975, 79, 180; 1977, 81, 1174. (19) Nandan, D.; Venkataramani, B.; Gupta, A. R. Langmuir 1993, 9, 1786. (20) D’Arcy, R. L.; Watt, I. C. Trans. Faraday Soc. 1970, 66, 1236.
S0743-7463(96)00711-1 CCC: $14.00
(DWE) gives hydration numbers that are realistic and consistent with the results obtained from other studies. The physical structure of macroreticular resins (example, Amberlyst-15) is fundamentally different from that of the gel-type resin (Dowex 50W). Macroreticular resins have a gel phase, where the ionogenic groups are located, and a phase composed of large pores.9,21-26 The gel phase is usually of higher cross-linking than conventional geltype resins. The large pores imbibe more solvent (including nonaqueous solvents) than conventional gel-type resins.21,25,26 Because of the structural differences, the kinetics of exchange24 and ion exchange selectivities27 show variations as compared to those of gel-type resins. The hydrated porosity measurements using mercury porosimetry indicated that approximately half the total water in a macroreticular resin was in the pores with the rest being in the gel microsphere.9,23 The water present in the gel phase could not be distinguished from that present in the pores by NMR spectroscopy, due to the rapid rate of exchange of water between the two phases.9 The main objective of the present study is to get information on the state of water present in Amberlyst15, by analyzing the water sorption isotherms by the DWE, and compare it with that obtained for Dowex 50W resins and use these results to provide a possible explanation for the observed variation in the selectivities involving alkaline earth metal ion-Na+ exchanges on Dowex 50WX8 and Amberlyst-15, reported earlier.27 Experimental Section Dowex 50WX4 and Dowex 50WX8 (J. T. Baker), 100-200 mesh, and Amberlyst-15 (Rohm and Haas), 20-25 mesh, were used for the present work. The various ionic forms of the resins were generated and their capacities determined by the standard procedures.1 From the moisture contents, the ion exchange capacities of the fully dried resins were determined and are given in Table 1. (21) Kunin, R.; Meitzner, E. F.; Bortnick, N. J. Am. Chem. Soc. 1962, 84, 305. (22) Kunin, R.; Meitzner, E. F.; Oline, J. A.; Fischer, S. A.; Frisch, N. Ind. Eng. Chem. Prod. Res. Dev. 1962, 1, 140. (23) Kun, K. A.; Kunin, R. J. Polym. Sci., Part C 1967, 16, 1457. (24) Miller, J. R.; Smith, D. G.; Marr, W. E.; Kressman, T. R. E. J. Chem. Soc. 1963, 218, 2779. (25) Pietrzyk, D. J. Talanta 1966, 13, 209, 225; 1969, 16, 169. (26) Wilks, A. D.; Pietrzyk, D. K. Anal. Chem. 1972, 44, 676. (27) Toteja, R. S. D.; Jangida, B. L.; Sundaresan, M. Ind. J. Chem. 1991, 30A, 322.
© 1997 American Chemical Society
Water Sorption Isotherms and Cation Hydration
Langmuir, Vol. 13, No. 11, 1997 2981
Table 1. Capacities, Water Content (nw at aw ) 1), and Parameters of the D’Arcy and Watt Equation for Different Ionic Forms of Dowex 50WX4, Dowex 50WX8, and Amberlyst-15 ionic capacity nw form (mequiv/g) (mol/equiv) npa nca nma
K
k
ESSb
H+ Na+ Mg2+ Ca2+ Sr2+ Ba2+
5.07 4.53 4.76 4.38 3.99 3.68
Dowex 50WX4 20.98 1.9 2.7 18.02 1.0 2.4 15.20 2.9 7.0 14.83 2.2 5.1 13.67 2.3 5.4 9.28 2.4 5.5
H+ Na+ Mg2+ Ca2+ Sr2+ Ba2+
4.95 4.52 4.86 4.49 4.27 3.82
Dowex 50WX8 10.80 1.9 0.3 8.6 8.72 1.0 0 8.0 8.27 3.0 2.5 11.1 7.88 2.4 0 13.4 7.49 2.5 2.1 10.2 5.52 2.2 2.8 5.9
14 52 21 27 20 22
0.72 0.72 0.75 0.74 0.83 0.79
0.0007 0.0017 0.0003 0.0004 0.0005 0.0002
H+ Na+ Mg2+ Ca2+ Sr2+ Ba2+
4.84 4.26 4.27 4.23 3.83 3.25
Amberlyst-15 11.64 2.7 0.2 11.11 1.7 0.6 11.60 5.2 0.5 11.54 3.2 1.9 11.27 3.9 0.6 11.23 4.2 0
6 8 10 13 9 10
0.93 0.93 0.94 0.95 0.94 0.94
0.0056 0.0047 0.0045 0.0053 0.0040 0.0031
16.4 25 0.93 14.6 40 0.95 20.4 52 0.95 22.4 54 0.95 19.7 748 0.95 10.6 34 0.97
8.7 8.8 17.4 17.7 17.9 18.2
0.0029 0.0016 0.0081 0.0014 0.0008 0.0004
a n , n , and n are the amounts of water associated with different p c m types of water sorption sites at aw ) 1, expressed as moles per mole b of the ion. ESS ) error sum square, eq 3.
Water sorption studies were carried out at 298 ( 1 K in a locally fabricated isopiestic setup, similar to the one described in detail earlier.18 LiCl solutions of varying concentrations were employed to study the water sorption at different water activities. Equilibrium between the water vapor and the resinates was obtained in 3-4 days at higher activities and in 7 days at lower water activities. The water activity, aw, corresponding to the equilibrium molality of the LiCl solution was obtained from the literature.28 The maximum water uptake was determined by soaking the air-dried resin in distilled water to attain swelling equilibrium. It was later surface dried, weighed, and dried at 393 K to constant weight. The water contents, nw (moles of water sorbed per equivalent capacity), of the fully swollen resins in different ionic forms are given in Table 1.
Results
l
KiKi′aw
∑ i)11 + K a
i w
+ Caw +
number of different types of primary sorption sites. C is a constant assigned for the linear form of the sorption isotherm.20 It is possible to get more information on the nature of the primary sorption sites (first term in eq 1) and also about the second term in eq 1 by modifying the DWE (eq 1), as discussed in detail earlier.19,29 When the isotherms are analyzed by setting i ) 1, the total amount of water associated with strong primary sites, np (contribution from the first term), associated with weak sites, nc (contribution from the second term), and present in the multilayer, nm (contribution from the third term), at aw ) 1 (fully swollen resin) is given by
nT ) np + nc + nm
(2)
nT, np, nc, and nm are all in moles of water per mole of the ion. A computer program based on nonlinear least square analysis was used to fit the equation to the sorption data. The program calculates the water sorbed at various aw’s using the optimized parameters. The error in fitting the experimental data to the computed values for a certain set of the parameters is given by n
error sum square (ESS) )
∑i {wi(obsd) - wi(calcd)}2
(3)
The water contents of the fully swollen Dowex 50WX4 and Dowex 50WX8 resins vary with the nature of the ionic form, while all the ionic forms of Amberlyst-15 have more or less the same water content (Table 1). Analysis of the Water Sorption Isotherms. Water sorption isotherms (nw vs aw) were sigmoid in shape, indicating multilayer formation. As typical examples, the isotherms for Amberlyst-15 are shown in Figure 1. The general form of the D’Arcy and Watt equation (DWE)20 used to analyze the water sorption isotherms is
w)
Figure 1. Water sorption isotherms of Amberlyst-15 (aw vs nw, mol per equiv) in different ionic forms: (O) H+; (4) Na+; (0) Mg2+; (b) Ca2+; (2) Sr2+; (9) Ba2+.
kk′aw 1 - kaw
Discussion
(1)
where w is the amount of water (g/g of dry resin). In eq 1, the first term refers to the Langmuir-type sorption sites (primary strong sites), the second term to the sorption sites which can be approximated to a linear isotherm (weak sites), and the third term to the multilayer formation. In eq 1, Ki′ and k′ are the primary and multilayer sorption site densities, respectively, and Ki and k are the interaction parameters related to the heats of sorption in the primary sorption site and the multilayer, respectively. l is the (28) Robinson, R. A. Trans. Faraday Soc. 1945, 41, 756.
where n is the number of data points. The program optimizes the parameters by minimizing the above function (eq 3). The amounts of water actually associated with different types of sites at aw ) 1 for Dowex 50WX4, Dowex 50WX8, and Amberlyst-15 are given in Table 1. The interaction parameters (K and k) of different types of sorption sites and the error in fitting the data to the computed values, ESS (eq 3), are also included in Table 1.
Vapor pressure isotope effect (VPIE) studies during the dehydration of ion exchangers have shown the presence of the three water subsystems (primary and secondary hydration shells and free or bulk water) in alkali metal ionic forms of Dowex 50W resins.5 On the basis of these findings5 and the earlier conclusions on the analysis of water sorption isotherms of Dowex 50W using the DWE,19 the amount of water associated with strong and weak sorption sites (np and nc, respectively) could be thought of as constituting the total hydration shell around the cation and nm, the water present in the free state. The water present in the hydration shell of Amberlyst-15 (29) Venkataramani, B. Langmuir 1993, 9, 3026.
2982 Langmuir, Vol. 13, No. 11, 1997
Toteja et al.
Table 2. Hydration Numbers of Cations in Ion Exchangers ion exchangera PSS-DVB (0.5%)1 Dowex 50W2 X2 X4 X8 X12 Dowex AG 50W2 X2 X4 X8 X16 FEP-PSS3,4 Dowex 50WX45 Dowex 50WX85 Amberlyst-155
H+ Na+ Mg2+ Ca2+ Sr2+ Ba2+ 3.9
2.9
1.5 3.5 3.0 2.9 2.8
7.0
5.2
4.7
2.0
ref 15 10
2.0
6.7
4.6 2.2 2.9
6.4 4.8 3.6 2.2 0.5 3.4 1.1 2.4
changes in the amount of watera (mol/equiv) exchange system
11, 12 0.6
Table 3. Equilibrium Constant (Kex) and Changes in the Amount of Different Types of Water during Alkaline Earth Metal Ion-Na+ Exchange at 298 ( 1 K on Dowex 50WX8 and Amberlyst-15 in Aqueous Solutions
6.4 7.0 3.1 9.9 5.5 5.7
2.1 7.2 2.4 5.1
1.8 7.7 4.6 4.5
0.4 7.9 5.0 4.5
13, 14 b b b
a Technique: (1) water sorption; (2) NMR; (3) FEP-PSSs polystyrene sulfonate grafted to perfluoro matrix (Teflon); (4) water sorption, IR; (5) (np + nc). b Present work.
interacts weakly with the cations (parameter K) as compared to that in Dowex 50W (Table 1). A value for the interaction parameter k close to 1 indicates that the structure of water in the multilayer is similar to that of bulk water. Thus, the water in the multilayer of Dowex 50WX8 is more structured than that in Dowex 50WX4 and Amberlyst-15 (Table 1). A significant feature emerging from the analysis is the fact that a nearly constant amount of water, 8-9 mol/ equiv, is present in a multilayer of Amberlyst-15, irrespective of the nature of the counterion (Table 1), which could be the water filling the pores of the resin. The structure of the water, nm, is similar to that of bulk water (k ) 0.92-0.94). The present study, thus, indicates the differences in the nature of the water present in the gel phase and in the pores of the macroreticular resin. The values of the total hydration numbers, that is (np + nc), deduced from the analysis of the water sorption isotherms by the DWE for different cations in the three ion exchangers (Table 2) compare well with those reported by other investigators for cations in an ion exchanger matrix10-15 (Table 2) and for electrolyte solutions.30,31 NMR studies have shown that the acid strength of a 4% DVB resin is higher than that of a 8% DVB resin.8 As a consequence ion-pairing increases and hydration decreases with an increase in cross-linking. This is borne out in the present study as well as the reported results (Table 2), keeping in mind that the gel phase of Amberlyst15 is more cross-linked than conventional gel-type resins. In an earlier study,27 it was observed that the selectivity of Amberlyst-15 was lower than that of Dowex 50WX8 for exchange equilibria involving alkaline earth metal ion(30) Burgess, J. Metal Ions in Solutions; Ellis Harwood: Chichester, 1978. (31) Ohtaki, H.; Radnai, T. Chem. Rev. 1993, 93, 1157.
water in hydration shell
free water (nm)
Kexb
Mg2+-Na+ Ca2+-Na+ Sr2+-Na+ Ba2+-Na+
Dowex 50WX8 1.8 0.3 1.4 1.5
-2.5 -1.4 -3.0 -5.1
9.43 14.74 27.01 66.04
Mg2+-Na+ Ca2+-Na+ Sr2+-Na+ Ba2+-Na+
Amberlyst-15 0.4 0.1 -0.2 -0.4
0.4 0.6 0.6 0.8
5.04 15.37 19.84 43.59
a
From Table 2. b From ref 27.
Na+
in aqueous solutions. Simultaneous calorimetric measurements on the above system27 suggested that a net gain in entropy was the driving force for the exchange process. Apart from the total amount of water released during the process, an increase in entropy could also result from the state of water present in the exchanger.3 On the basis of the present study, there is a net gain in the water associated with hydration shells of the cations and a net loss in the structured free water (nm) during the alkali earth metal ion-Na+ exchange on Dowex 50WX8 (Table 3). The loss in the free water is more than the gain in the water associated with the hydration shell. Thus, a net loss of structured free water contributes to the gain in the entropy. On the other hand, the gain in the water due to hydration shell and free water during a similar exchange on Amberlyst-15 is very marginal (Table 3). Thus, on the basis of the changes in the state of water alone, there is no net gain in the entropy during ion exchange on Amberlyst-15. It is also known that the selectivity depends on the way the polyvalent ion orients itself with respect to the ionogenic group.12,32 The rigid gel structure of Amberlyst15 as compared to the more swellable structure of Dowex 50WX8 possibly affects the optimum orientation of the alkaline earth metal ion in Amberlyst-15, thus lowering its overall selectivity as compared to that of Dowex 50WX8 (Table 3). Acknowledgment. The authors wish to thank Mr. T. S. Krishnamoorthy, formerly Head, Analytical Chemistry Division, Dr. R. Parthasarathy, Analytical Chemistry Division, and Dr. J. P. Mittal, Director, Chemistry Group, B.A.R.C., for their constant encouragement during the course of the investigation. The authors also wish to thank the reviewers for their valuable comments and suggestions. LA9607114 (32) Kaminsky, M. Discuss. Faraday Soc. 1957, 24, 171.
