Anal. Chem. 1996, 68, 2811-2817
Immobilization of Crown Ether Carboxylic Acids on Silica Gel and Their Use in Column Concentration of Alkali Metal Cations from Dilute Aqueous Solutions Matthew G. Hankins, Takashi Hayashita,† Stanislaw P. Kasprzyk, and Richard A. Bartsch*
Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061
Eight crown ethers with pendent carboxylic acid groups are immobilized on silica gel and utilized for column concentration of alkali metal cations from dilute aqueous solutions. The column concentration selectivity and efficiency are found to be strongly influenced by (1) the cavity size of the crown ether unit, (2) conformational positioning of the proton-ionizable side arm with respect to the crown ether cavity, and (3) capping of residual silanol surface groups with trimethylsilyl functions. By use of a chromatographic stripping technique, selective column concentration of Na+, K+, (Rb+ and Cs+), and Cs+ by different functionalized silica gels has been achieved.
Macrocyclic multidentate ligands, such as crown ethers, have been utilized extensively for the separation of alkali metal and alkaline earth metal cations, as well as some transition metal ions, from aqueous solutions.1,2 Polymerization of such macrocyclic ligands or their immobilization on support materials is important to the development of continuous separation processes and alleviation of the toxicity of some macrocyclic multidentate compounds.3 Crown ether-containing polymers have been utilized in cation and anion chromatography by several research groups.4-14 Bla† Present address: Department of Chemistry, Saga University, 1 Honjo, Saga 840, Japan. (1) Pedersen, C. J. J. Am. Chem. Soc. 1967, 89, 7017-7036. (2) Cation Binding by Macrocycles. Complexation of Cationic Species by Crown Ethers; Inoue, Y., Gokel, G. W., Eds.; Marcel Dekker: New York, 1990. (3) Hiraoka, M. Crown Compounds: Their Characteristics and Applications; Elsevier: New York, 1982; pp 265-272. (4) Blasius, E.; Adrian, W.; Janzen, K.-P.; Klautke, G. J. Chromatogr. 1974, 96, 89-97. (5) Blasius, E.; Janzen, K.-P.; Adrian, W.; Klautake, G.; Lorscheider, R.; Maurer, P.-G.; Nguyen-Tien, T.; Scholten, G.; Stockemer, J. Fresenius’ Z. Anal. Chem. 1977, 284, 337-360. (6) Blasius, E.; Janzen, K.-P.; Keller, M.; Lander, J.; Nguyen-Tien, T.; Scholten, G. Talanta 1980, 27, 107-126. (7) Blasius, E.; Janzen, K.-P.; Adrian, W.; Klotz, H.; Luzenburger, H.; Mernke, E.; Nguyen, V. B.; Nguyen-Tien, T.; Rausch, R.; Stockemer, J.; Toussanint, A. Talanta 1980, 27, 127-141. (8) Warshawsky, A.; Kalir, R.; Deshe, A.; Berkovitz, H.; Patchornik, A. J. Am. Chem. Soc. 1979, 101, 4249-4258. (9) Igawa, M.; Saito, K.; Tsukamoto, J.; Tanaka, M. Anal. Chem. 1981, 53, 1942-1949. (10) Grossman, P.; Simon, W. J. Chromatogr. 1982, 235, 351-363. (11) Frere, Y.; Gramain, P. Makromol. Chem. 1982, 183, 2163-2172. (12) Nakajima, M.; Kimura, K.; Shono, T. Anal. Chem. 1983, 55, 463-467. (13) Nakajima, M.; Kimura, K.; Shono, T. Bull. Chem. Soc. Jpn. 1983, 56, 30523056.
S0003-2700(95)01159-0 CCC: $12.00
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sius and co-workers synthesized a variety of crown ether resins by condensation polymerization of dibenzocrown ethers with formaldehyde.4-7 Immobilization of macrocyclic ligands on silica gel was reported by Shono and co-workers12-14 and subsequently by Bradshaw et al.15-20 However, all of these studies involved polymeric material that contained neutral macrocyclic multidentate ligands.20 Therefore, complexation of metal cations was significantly affected by the identity of the concomitant anion(s). This complication would be avoided if an ion-exchange group were incorporated into the crown ether-containing polymer.21,22 Recently, we prepared novel crown ether resins that contain both ion-exchange and cyclic or acyclic polyether binding sites for metal ions by condensation polymerization of dibenzo polyether carboxylic acids with formaldehyde.23-25 It was found that the selectivity of alkali metal cation sorption was controlled by the size relationship between the crown ether cavity and the metal ion diameters, as well as the level of preorganization of the binding sites. In an analytical application, these proton-ionizable crown ether resins were utilized for the selective column concentration of alkali metal cations from dilute aqueous solutions.26,27 In (14) Nakajima, M.; Kimura, K.; Hayata, E.; Shono, T. J. Liq. Chromatogr. 1984, 7, 2115-2125. (15) Bradshaw, J. S.; Bruening, R. L.; Krakowiak, K. E.; Tarbet, B. J.; Bruening, M. L.; Izatt, R. M.; Christensen, J. J. J. Chem. Soc., Chem. Commun. 1988, 812-814. (16) Izatt, R. M.; Bruening, R. L.; Bruening, M. L.; Tarbet, B. J.; Krakowiak, K. E.; Bradshaw, J. S.; Christensen, J. J. Anal. Chem. 1988, 60, 18251826. (17) Bradshaw, J. S.; Krakowiak, K. E.; Tarbet, B. J.; Bruening, R. L.; Biernat, J. F.; Bochenska, M.; R. M.; Christensen, J. J. Pure Appl. Chem. 1989, 61, 1619-1624. (18) Izatt, R. M.; Bruening, R. L.; Tarbet, B. J.; Gritton, L. D.; Bruening, M. L.; Krakowiak, K. E.; Bradshaw, J. S. Pure Appl. Chem. 1990, 62, 1115-1118. (19) Bruening, M. L.; Mitchell, D. M.; Bradshaw, J. S.; Izatt, R. M.; Bruening, R. L. Anal. Chem. 1991, 63, 21-24. (20) Biernat, J. F.; Konieczka, P.; Tarbet, B. J.; Bradshaw, J. S.; Izatt, R. M. Sep. Purif. Methods 1994, 23, 77-348. (21) Bartsch, R. A.; Kim, J. S.; Olsher, U.; Purkiss, D. W.; Ramesh, V.; Dalley, N. K.; Hayashita, T. Pure Appl. Chem. 1993, 65, 399-402. (22) Bartsch, R. A.; Hayashita, T.; Lee, J. H.; Kim, J. S.; Hankins, M. G. Supramol. Chem. 1993, 1, 305-311. (23) Hayashita, T.; Bartsch, R. A. Anal. Chem. 1991, 63, 1847-1850. (24) Hayashita, T.; Goo, M. J.; Kim, J. S.; Bartsch, R. A. Talanta 1991, 38, 14531457. (25) Hayashita, T.; Lee, J. H.; Lee, J. C.; Krzykawski, J.; Bartsch, R. A. Talanta 1992, 37, 857-862. (26) Hayashita, T.; Lee, J. H.; Chen, S.; Bartsch, R. A. Anal. Chem. 1991, 63, 1844-1847. (27) Hayashita, T.; Lee, J. H.; Hankins, M. G.; Lee, J. C.; Kim, J. S.; Knobeloch, J. M.; Bartsch, R. A. Anal. Chem. 1992, 64, 815-819.
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Figure 2. Structures of crown ether ester precursors.
Scheme 1. Synthetic Procedure for Covalent Attachment of Crown Ether Esters (CE) with Pendent Terminal Vinyl Functions to Silica Gel
Figure 1. Structures of silica gel-bound crown ether carboxylic acids.
contrast to liquid-liquid extraction, this column concentration method eliminates the need for organic diluents, an attractive feature from an environmental perspective. Despite a high ion-exchange capacity (1-3 mmol/g of resin), these formaldehyde condensation resins suffer from compaction in high-pressure column separation systems. Therefore, we have synthesized the new crown ether-containing polymers 1-9 (Figure 1), in which crown ethers with pendent carboxylic acid groups are immobilized on silica gel. Alkali metal cation sorption and column concentration from dilute aqueous solutions by these novel polymeric materials are now reported. EXPERIMENTAL SECTION Reagents. Inorganic and organic compounds were reagentgrade commercial products and were used as received. Deionized water was prepared by passing distilled water through three Barnstead D8922 combination cartridges in series. Preparation of Crown Ether Carboxylic Acids Immobilized on Silica Gel. Crown ether esters with pendent vinyl groups 10-17 (Figure 2) were prepared according to procedures reported elsewhere.28 The crown ether compounds were attached to silica gel to give polymers 1-9 by the four-step synthetic route shown in Scheme 1.15,20 A mixture of the crown ether ester with a pendent vinyl group (0.50 mmol), diethoxy(methyl)silane (1.00 mmol), chloroplatinic acid (0.20 mmol), and 6 mL of dry benzene was refluxed for 20 h. The mixture was filtered and evaporated in vacuo. The crown ether-containing diethoxysilane was dissolved in chloroform and added to activated silica gel (60-200 mesh, Aldrich) with a ratio (28) Goo, M.-J.; Kasprzyk, S. W.; Bartsch, R. A. J. Heterocycl. Chem., submitted.
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of 1 part of crown ether compound to 10 parts of silica gel by weight. The chloroform was evaporated in vacuo, and the solid was heated at 120 °C for 24 h in an oven to attach the crown ether to the silica gel. After cooling, the solid was washed with toluene, ethanol, and methanol and then dried at 60 °C. A mixture of the crown ether ester-functionalized silica gel (1 part by weight) and chlorotrimethylsilane (4 parts by weight) was refluxed for 1 day and evaporated in vacuo. The crown ether ester-functionalized, trimethylsilyl-capped silica gel was washed with methanol and water, and the ester functions were hydrolyzed with KOH (the mole ratio of crown ether ester units to KOH was 1:6) in 25 mL of 25% water-75% dioxane (v/v) for 24 h at room temperature without stirring. The functionalized silica gel was immediately washed with 3% HCl (200 mL) and then with deionized water until the pH of the eluent was 6.5. The functionalized, capped silica gel was dried in an oven at 60 °C for 24 h. Apparatus. Concentrations of alkali metal cations in aqueous solutions were determined with a Dionex Model 2000i ion chromatograph and a Hewlett Packard Model 3394A integrator. The pH of sample solutions was measured with a Fisher Accumet 825 pH meter and a Corning combination pH semimicroelectrode (Model 476541). To prevent metal contamination, all glassware was soaked in 5% HNO3 for 24 h and rinsed with deionized water before use. The column assembly (Figure 3) included four eluent reservoirs, a Rheodyne Model 5012 six-position switch, a Milton Roy minipump (P/N 920148-03), a Dionex pressure gauge (0-2000 psi range), a Dionex air-operated, four-way valve (P/N 35914),
concentration factor (CF):
CF )
Figure 3. Column concentration apparatus assembly: 1, acid wash solution; 2, stripping solution; 3, sample solution; 4, deionized water; 5, six-way valve; 6, pump; 7, four-way valve; 8, column; 9, fraction collector; 10, pressure gauge; 11, syringe; and 12, waste container.
and a 20-cm Dionex Tefzel column. The resin was dry packed into the Tefzel column and held in place with 35-µm bed supports. Standard 1/4-28 Dionex plastic male nuts were used to attach the tubing and system components. Teflon tubing (1/8 in.) was employed to transport the sample, washing, and eluent solutions from the reservoirs to the pump. Tefzel tubing (1/16 in.) was used to carry solutions from the high-pressure side of the pump to the column. The stripping solution was collected with a Bio-Rad Model 2110 fraction collector. Procedure. A column containing 0.50 g of the functionalized silica gel was washed with 25 mL of 0.10 M HCl and then with 100 mL of deionized water at a flow rate of 185 mL/h. For the competitive sorption of alkali metal cations, 1.00 L of an aqueous sample solution containing the five alkali metal hydroxides (2.00 × 10-5 M in each, pH ) 9.87-9.94) was pumped through the column. After this sorption step, the column was washed with 100 mL of deionized water to remove any uncomplexed alkali metal salts from the silica gel bed. Residual deionized water was removed from the column and connecting tubing by air pressurized with a plastic syringe, which was attached by tubing to the Dionex air-operated, four-way valve. The alkali metal cations bound to the functionalized silica gel were stripped with 0.010 M HCl at a flow rate of 18.5 mL/h. The stripping solution was collected in 0.25-mL fractions and analyzed by ion chromatography after appropriate dilution. For each column of functionalized silica gel, three runs were conducted. The column concentration behavior for alkali metal cations was evaluated in terms of the
concn of M+ in the stripping solution initial concn of M+ in the sample solution
(1)
RESULTS AND DISCUSSION Preparation of Crown Ether Carboxylic Acids Immobilized on Silica Gel. Crown ether esters 10-17 with pendent vinyl groups (Figure 2) were attached to silica gel to give resins 1-9 by the four-step synthetic route shown in Scheme 1. Since silica gel itself is a nonselective chelating material, the residual silanol groups in the functionalized silica gel were capped with trimethylsilyl groups by reaction with chlorotrimethylsilane. Mild basic hydrolysis of the immobilized crown ether ester groups with KOH in aqueous dioxane for 24 h at room temperature without stirring, followed by acidification, gave the silica gel-bound crown ether carboxylic acid. This mild hydrolysis procedure was found to produce well-shaped silica gel particles. If the hydrolysis mixture of the functionalized silica gel was stirred or heated, the silica gel product after acidification exhibited serious compaction during the column experiments, which is attributed to deterioration of the silica gel matrix. In the series of capped, silica gel-bound crown ether carboxylic acids 1-5, the crown ether ring size is systematically increased by ethyleneoxy units from 12-crown-4 (12C4) to 15-crown-5 (15C5) to 18-crown-6 (18C6) to 21-crown-7 (21C7) to 24-crown-8 (24C8), respectively. For structurally related 6, the ring size is 14-crown-4 (14C4). In a second series of capped, silica gel-bound crown ether carboxylic acids 7 and 9, the polyether rings are dibenzo-14crown-4 (DB14C4) and dibenzo-16-crown-5 (DB16C5). The same crown ether carboxylic acid units are present in both 8 and 9, but there was no capping of residual silanol groups for 8. Sorption of Alkali Metal Cations by Silica Gel-Bound Crown Ether Carboxylic Acids 1-9. For competitive sorption of alkali metal cations by silica gel-bound macrocycles 1-9, the sorption behavior for each functionalized silica gel was evaluated by determining the amounts of alkali metal cations sorbed per gram of resin. The sorption values were calculated by summing the numbers of moles of all alkali metal cations recovered in all effluent fractions and dividing by the weight of dry resin packed into the column. The resultant sorption values presented in Table 1 are averages from three consecutive sorption experiments conducted with a single column. Cavity dimensions for the various crown ethers and the alkali metal cation diameters are compared in Table 2. Plots of the sorption values against the ionicdiameters of the alkali metal cations are presented in Figures 4 and 5. For comparison, sorption values for capped silica gel that does not contain a crown ether unit are also shown in Table 1 and in the figures. For trimethylsilyl-capped, silica gel-bound crown ether carboxylic acids 1-5, in which the crown ether ring size is systematically varied, the sorption results are shown in Figure 4a. Sorption of the five alkali metal cations by resin 1, which has 12C4 units, is almost identical to that for observed for trimethylsilyl-capped silica gel. It is apparent that the crown ether unit does not contribute appreciably to the binding of any alkali metal cation and that sorption is due to the chelating capabilities of the silica gel support. Based on the cavity size of 15C5 (Table 2), functionalized silica gel 2 would be expected to have a somewhat greater affinity for Analytical Chemistry, Vol. 68, No. 17, September 1, 1996
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Table 1. Competitive Sorption of Alkali Metal Cations by Capped Silica Gel and Silica Gel-Bound Crown Ether Carboxylic Acids 1-9 functionalized crown ether (ring size)
Li+
nonea 1 (12C4) 2 (15C5) 3 (18C6) 4 (21C7) 5 (24C8) 6 (14C4) 7 (DB14C4)c 8 (DB16C5)d 9 (DB16C5)
2.1 ( 0.5b 2.1 ( 0.3 2.7 ( 0.6 2.0 ( 0.5 1.6 ( 0.2 2.0 ( 0.4 3.4 ( 0.1 6.7 ( 0.2 3.2 ( 0.3 2.2 ( 0.2
sorption (µmol/g of dried resin) Na+ K+ Rb+ 2.5 ( 0.9 2.8 ( 0.4 4.6 ( 0.6 2.8 ( 0.4 2.1 ( 0.3 2.4 ( 0.4 3.5 ( 0.2 13.8 ( 0.1 10.7 ( 1.0 10.9 ( 0.2
3.4 ( 0.7 4.0 ( 0.4 8.2 ( 1.3 15.5 ( 0.6 6.3 ( 0.9 5.4 ( 0.4 5.9 ( 0.3 7.6 ( 0.5 7.1 ( 0.8 5.2 ( 0.3
4.2 ( 1.1 4.8 ( 0.3 7.7 ( 0.9 8.1 ( 0.5 10.3 ( 1.6 7.6 ( 0.7 7.6 ( 0.6 7.6 ( 1.3 7.9 ( 0.7 5.8 ( 0.3
Cs+ 5.3 ( 1.3 6.4 ( 0.5 8.2 ( 1.0 7.2 ( 0.7 11.7 ( 1.8 9.3 ( 1.1 10.4 ( 1.6 9.2 ( 0.9 9.8 ( 0.8 8.1 ( 0.5
a Trimethylsilyl-capped silica gel. b Average value and standard deviation from triplicate runs conducted with a single column. c DB, dibenzo. d Uncapped, functionalized silica gel.
Table 2. Diameters for Crown Ether Cavities and Alkali Metal Cations crown ether
cavity sizea (Å)
cation
ionic diameterb (Å)
12C4 15C5 18C6 21C7 24C8 14C4 DB14C4c DB16C5c
1.2 1.8 2.7 3.3-3.5 4.1-4.3 1.2 1.2 1.9
Li+ Na+ K+ Rb+ Cs+
1.48 2.02 2.76 2.98 3.70
Figure 5. Sorption of alkali metal cations by functionalized silica gels 7-9.
a Estimated from space-filling models. b From crystallographic results for a coordination number of 6.29 c DB, dibenzo.
Figure 6. Profile of alkali metal cation elution from trimethylsilylcapped silica gel.
Figure 4. Sorption of alkali metal cations by functionalized silica gels 1-6.
Na+ than for K+. However, 2 exhibits greater sorption of K+, Rb+, and Cs+ than of Na+. The Rb+ and Cs+ sorptions are partly due to the enhanced binding of these two ions by trimethylsilylcapped silica gel itself. When this effect is considered, 2 appears (29) Shannon, R. D. Acta Crystallogr. 1976, A32, 751-767.
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to possess a slight selectivity for K+. This result is consistent with thermodynamic data for Na+ and K+ complexation by 15C5 in water, which show a slightly higher log Ka value for K+ association than for Na+.30,31 The polyether cavity and cation sizes correlate well for K+ and 18C6 (Table 2). Functionalized silica gel 3 shows a pronounced sorption selectivity for K+. The ordering of stability constants for alkali metal complexes with 18-crown-6 in water30,31 is in good agreement with the sorption results. Among the functionalized silica gels used in this study, 3 exhibits the greatest K+ selectivity. Functionalized silica gels 4 and 5, which contain 21C7 and 24C8 rings, respectively, show sorption selectivities consistent with a best match between the polyether cavity and cation sizes. The 21C7 ring in 4 is well suited for forming a nesting32 complex with Rb+ and a perching32 complex with Cs+, and sorption selectivity for these two large alkali metal cations is observed for 4. The larger cavity size associated with the 24C8 rings in 5 should allow nesting complexes to be formed with both Rb+ and Cs+. In agreement, the two best sorbed alkali metal cations by 5 are Rb+ and Cs+. (30) Izatt, R. M.; Pawlak, K.; Bradshaw, J. S.; Bruening, R. L. Chem. Rev. 1991, 91, 1721-2086. (31) Hoeiland, H.; Ringseth, J. A.; Brun, T. S. J. Solution Chem. 1979, 8, 779792. (32) Cram, D. J.; Trueblood, K. N. In Host Guest Complex Chemistry. Macrocycles; Vo ¨gtle, F., Weber, E., Eds.; Springer-Verlag: New York, 1985; p 128.
Figure 7. Profile of alkali metal cation elution from functionalized silica gels (a) 1, (b) 2, and (c) 3.
Figure 8. Profile of alkali metal cation elution from functionalized silica gels (a) 4, (b) 5, and (c) 6.
In 6, the 14C4 ring has two three-carbon bridges. Sorption of alkali metal cations by 6 is quite different from that for resins 2-5, in that the sorption affinities appear to be unrelated to the polyether cavity dimensions. Unexpectedly, sorptions of Li+ and Na+ are just slightly higher than those for these metal cations by trimethylsilyl-capped silica gel, while sorption of K+, Rb+, and Cs+ increases appreciably as the cation size is enhanced (Figure 4b). The Cs+ sorption efficiency was only slightly lower than that for 4, which contains 21C7 rings. Examination of the Corey, Pauling, and Kortum (CPK) space-filling model suggests that the side arm of the crown ether units in 6 is not long enough to extend over the polyether cavity. Therefore, the space between the carboxylate oxygen and the polyether cavity center may be suitable for binding of larger cations in perching32 complexes. The functionalized silica gels 7-9 are distinguished from 1-6 in that they contain dibenzocrown ether units and oxyacetic acid side arms rather than crown ether units with salicylic acid side arms. For 7, which has DB14C4 rings, the sorption behavior is shown in Figure 5a. Pronounced Na+ selectivity is noted. The solid state structure of lithium sym-dibenzo-14-crown-4-oxyacetate monohydrate has been determined by X-ray diffraction. In this complex, Li+ is perched above the plane of the crown ether oxygens, and there is a bridging water molecule between the complexed Li+ and the carboxylate group in the side arm. A perching complex is also assumed for the Na+ complex formed in sorption by 7. The Na+ complex may be more stable than the Li+ complex because the spacing of the ionizable side arm permits direct coordination between a larger perching32 Na+ and the anionic oxygen of the carboxylate group. The cavity size of the
DB16C5 ring is well suited for complexation of Na+ (Table 2). In agreement, the sorption value for Na+ by 9 is much higher than those for Li+, K+, Rb+, and Cs+ (Figure 5a). The sorption behavior of a functionalized silica gel that has not been capped by reaction with chlorotrimethylsilane is shown in Figure 5b. Functionalized silica gels 8 and 9 are identical except that the residual silanol groups in the former were not capped with trimethylsilyl functions. Both 8 and 9 are Na+ selective, but 9 exhibits higher selectivity due to reductions in the sorption level of Li+, K+, Rb+, and Cs+ compared with 8. Selective Column Concentration of Alkali Metal Cations by Silica Gel-Bound Crown Ether Carboxylic Acids 1-9. The performance of resins 1-9 in selective column concentration of alkali metal cations may be assessed from their elution profiles (Figures 6-9). The profiles are constructed by plotting the concentration factor (CF) as a function of the volume of stripping solution. Metal ion separations are enhanced by this column concentration method due to chromatographic partitioning of metal cations between the mobile and stationary phases during the stripping step. Strongly bound cations elute from the column later, while weakly bound ions emerge rapidly. If the functionalized silica gel exhibits selectivity in metal ion sorption, the selected metal species may be collected in a highly enriched form during the latter stages of the elution. Among the alkali metal cations, trimethylsilyl-capped silica gel shows a slight preference for Cs+ sorption (Figure 4a). As shown in Figure 6, it exhibits an elution ordering for alkali metal cations based on cation size (and hydrophobicity), with Li+ being Analytical Chemistry, Vol. 68, No. 17, September 1, 1996
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Figure 9. Profile of alkali metal cation elution from functionalized silica gels (a) 7, (b) 8, and (c) 9.
Figure 10. Elution profiles for three consecutive runs conducted with a single column of functionalized silica gel 5.
the first to elute completely and Cs+ the last to emerge from the column. An elution profile very similar to that obtained for the capped silica gel was observed for functionalized silica gel 1, which has 12C4 units (Figure 7a). This indicates that the 12C4 macrocycle is a very weak binder of alkali metal cations. The elution profile for 2 with 15C5 rings (Figure 7b) is quite different from that for the capped silica gel. Elution of K+, Rb+, and Cs+ occurs with very similar retention volumes, which provides no separation of these later-eluting, larger cations from each other. For functionalized silica gel 3, which has 18C6 units, the elution ordering is Li+, Na+ > Rb+ > Cs+ > K+ (Figure 8c). In the last three effluent fractions, which contained appreciable levels of metal ions, only K+, Rb+, and Cs+ are detected, and the proportion of K+ is 75%, compared with 20% in the sample solution. Thus, 3 has potential for use in the separation of K+ from the other alkali metal cations. For 4, which has 21C7 rings, considerable potential for use in the separation of Rb+ and Cs+ from other alkali metal cations is evident (Figure 8a). The last few fractions contained mostly Rb+ and Cs+, very small amounts of K+, and undetectable Li+ and Na+. The elution profile for 5, with 24C8 rings (Figure 8b), is similar to that for 4. Surprisingly, the elution profile for functionalized silica gel 6, which has 14C4 rings (Figure 8c), closely resembles that for 5. In fact, 6 shows a greater potential for the separation of Cs+ from Rb+ and the other alkali metal cations than does either 4 or 5. Since Cs+ is far too large to fit within the cavity of the 14-crown-4 units in 6, it appears that the greater lipophilicity of Cs+ in a very open perching32 complex is the causative factor for the preferred sorption of Cs+ by 6 (Figure 4b) and its retarded elution (Figure 8c).
The functionalized silica gels 7 and 9 are distinguished from 1-6 in that the latter have less flexible crown ether units and a proton-ionizable side arm, which is expected to be oriented over the crown ether cavity.23-27 For 7 (Figure 9a) and 9 (Figure 9c), which contain DB14C4 and DB16C5 rings, respectively, Na+ is more strongly retained during the stripping step than the other alkali metal cations. Although the sorption values given in Table 1 show that 9 has higher sorption selectivity for Na+ than 7, the elution profiles reveal a better separation of Na+ from the other alkali metal cations by 7. The uncapped functionalized silica gel 8 with DB16C5 rings exhibits retarded elution of Cs+ as well as Na+ (Figure 9b). Comparison of the elution profiles for 8 (Figure 9b) and 9 (Figure 9c) shows the advantage of functionalized silica gels in which the residual silanol groups are deactivated by a capping step. Reproducibility of Elution Profiles. For sorption of alkali metal cations by silica gel-bound crown ether carboxylic acid groups, the sample solution must be slightly basic. In this study, the pH values for the sample solutions were 9.87-9.94. Triplicate runs were performed with a single column for each functionalized silica gel. No deterioration in the sorption capacity of the packings after multiple uses at this pH was evident. Figure 10 shows elution profiles obtained in three consecutive runs conducted with 5. Although the profiles are not identical, curve shapes and peak heights are approximately the same for all three runs. One noticeable difference between elution profiles for the three runs in some volume displacement of peaks from one run to the next. This is attributed to small residual amounts of water left in the column from the washing step, which precedes the elution step.
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CONCLUSIONS For the first time, proton-ionizable crown ethers have been immobilized on silica gel, and their sorption and selective column concentration of alkali metal cations from aqueous solutions have been investigated. Four of the eight functionalized silica gels exhibit selectivities that may be useful in alkali metal cation separations. For 3, which contains 18C6 units, a potential for separation of K+ from other alkali metal cations is evident. It appears that 4, which has 21C7 rings, could be useful for separation of Rb+ and Cs+ from the other three alkali metal cations. For 6, which bears 14C4 rings, a pronounced preference for complexation of Cs+ is noted. Finally, 8, which contains a
DB14C4 unit with a preorganized, proton-ionizable side arm, shows promise for application in Na+ separations. ACKNOWLEDGMENT This research was supported by the Division of Chemical Sciences of the Office of Basic Energy Sciences of the U.S. Department of Energy (Grant DE-FG03-94ER14416). Received for review November 29, 1995. Accepted May 28, 1996.X AC951159A X
Abstract published in Advance ACS Abstracts, July 1, 1996.
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