Ind. Eng. Chem. Res. 1999, 38, 4843-4849
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Sorption of Neutral Components in Ion-Exchange Resins. 2. Sorption of D-Xylose in Sulfonated PS-DVB Resins from Water-Ethanol Mixtures Jari Tiihonen,* Markku Laatikainen, Ismo Markkanen, Erkki Paatero, and Juho Jumppanen† Laboratory of Industrial Chemistry, Lappeenranta University of Technology, P.O. Box 20, FIN-53851 Lappeenranta, Finland
Partition of D-xylose between ion-exchange resins and different water-ethanol mixtures at 298 K was studied. Gel-type sulfonated poly(styrene-co-divinylbenzene) resins cross-linked with 4 or 8 wt % divinylbenzene and carrying Na+, Ca2+, or La3+ as the counterion were used as the absorbent phases. Sorption of D-xylose increases with decreasing water mole fraction in the solvent mixture, irrespective of the resin. The increasing sorption is explained by the selective water uptake of the resin and the far better solubility of D-xylose in water than in ethanol. At high water contents, sorption of D-xylose increases with decreasing cross-link density, while the enhanced water selectivity of the densely cross-linked resin improves sorption at low water contents. The experimental data were compared with the literature data for D-glucose. Complexation of both sugars with the metal ions studied is shown to be negligible, and the sugars are absorbed more effectively by the resin loaded with the less solvated univalent counterion. The sorption model based on the UNIQUAC equation and the affine network theory of elasticity explains satisfactorily sorption of individual components from the ternary waterethanol-sugar solutions. 1. Introduction Liquid chromatographic (LC) separations of sugars using sulfonated ion exchangers are commonly carried out in aqueous eluents.1-3 Less attention has been paid to the use of mixed solvents to improve separation. For instance, the decreasing solubility of sugars with decreasing water content in alcohol-water solutions limits their use as eluents.4-6 However, chromatographic separations with low sugar solubility are still possible on an analytical scale, when normally dilute mixtures are used. Sorption of pure solvents and aqueous solvent mixtures into strong cation exchangers has been studied quite extensively, as noted in part 1 of this paper.7 There are also some reports on the distribution of sugars between ion-exchange resins and solvent mixtures, especially alcohol-water solutions. Ru¨ckert and Samuelson8,9 have studied sorption of glucose and Adachi and Matsuno10 sorption of a number of monosaccharides in strong cation exchangers from water-methanol and water-ethanol mixtures. According to Ru¨ckert and Samuelson8,9 the sugar sorption is greatly influenced by the interactions between the solvent components and the resin and also the interactions between the solvents and the solute. These effects include the selective sorption of water from water-ethanol mixtures and the solubility differences of sugars in ethanol and water. They also concluded that the interactions between the functional groups of the resin and sugars might have an impact on sorption. In fact, Adachi and Matsuno10 * Corresponding author. Phone: +358 5 6212259. Fax: +358 5 6212199. E-mail:
[email protected]. † Present address: Cultor Food Science Technology Development, FIN-02460 Kantvik, Finland.
have suggested that the complex formation between the counterion and the sugar is the decisive factor in sorption of sugars from alcohol-water mixtures. The effect of ethanol-based eluents on the liquid chromatography of carbohydrates with strong cation-exchange resins has also been investigated by Samuelson and coworkers.11-16 All of these papers, however, focus only on cation exchangers carrying monovalent counterions and on concentrated ethanol solutions relevant in analytical applications. As can be seen from the studies referred to above, papers concerning the behavior of binary solvent mixtures and sugars in ion-exchange resins are rather scanty and the treatment in them is more or less qualitative. Therefore, a more quantitative approach is desirable for assessment of the applicability of a given solvent system for chromatographic use. For this purpose, experimental sorption data were used to test the equilibrium model presented in part 17 and extended to the quaternary water-ethanol-sugar-resin system. In particular, the effect of the water-ethanol solution composition on sorption of D-xylose was studied and compared with the literature data of D-glucose. These sugars were selected for their negligible ability to form complexes with metal ions,1,2 and therefore sorption can be considered to take place predominantly by partitioning. 2. Materials and Methods Sulfonated poly(styrene-co-divinylbenzene) (PS-DVB) cation exchangers were delivered by Finex Oy (Finland). The resins CS08G and CS16G were cross-linked with 4 (X4) and 8 (X8) wt % DVB, respectively. Properties and pretreatment of the resins have been explained in part 1.7 CS16G was used in Na+, Ca2+, and La3+ forms and
10.1021/ie990403b CCC: $18.00 © 1999 American Chemical Society Published on Web 11/05/1999
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Table 1. Size and Surface Parameters ri and qi, the Known Interaction Parameters aij, and the Elastic Parameter Kel for the Water (1)-Ethanol (2)-Sugar (3)-Resin (4) System j for aij, K i water (1) ethanol (2) water (1)a ethanol (2)a D-xylose (3) D-glucose (3) resin (4) Na+, X8 resin (4) Na+, X4 resin (4) Ca2+ resin (4) La3+ a
ethanol (2)
D-glucose
qi
water (1)
D-xylose
ri
(3)
(3)
Na+
resin (4) Ca2+
La3+
0.9200 2.576 0.9200 2.576 6.706 8.153 6000 6000 6000 6000
1.400 2.588 1.400 2.588 6.492 7.920 5000 5000 5000 5000
0 192.8 0 173.2 -47.44 -94.09 61.50 61.50 660.2 1030
56.36 0 112.6 0 320.9 298.1 229.6 229.6 155.7 138.7
71.51 0.7167
127.8 -2.937
-429.9 -237.1
-659.4 -298.2
-1030 -602.0
Kel, MPa
0 0 0 0 0 0
11 3.2 12 15
Used for the solubility data of D-glucose.
CS08G in Na+ form only. The reagents, with the exception of D-xylose, were the same as those in part 1.7 D-Xylose was obtained from Xyrofin Oy (Finland). Partial molar volumes of D-xylose and D-glucose in aqueous solutions are 95.4 and 112 mL/mol, respectively.17 These values are considered to be constant regardless of the solvent composition and the sugar concentration. Equilibrium Measurements. The initial solvent content of the resins was determined prior to the experiments using a known method.7 The equilibrium distribution of water, ethanol, and D-xylose between the resin phase and the liquid phase was measured using the following method. About 4-6 g of the water-swollen resin or 3 g of the ethanol-swollen resin was weighed accurately in a test tube, and a known amount of the desired water-ethanol-D-xylose solution was added. The test tube was tightly sealed, and the sample was agitated for 3 days at room temperature and equilibrated for 3 days at 298 ( 0.1 K. After equilibration, the solution was recovered and the resin was centrifuged in a sintered tube, to remove the excess water around the particles, and weighed. Next, the resin was washed with water to remove the absorbed sugar. The washed resin was dried at 383 K overnight and weighed. The ethanol and sugar contents of the recovered solution were determined by means of HPLC. A sulfonated PSDVB resin (Pb2+) column was used, and the eluent was ion-exchanged water. The deviation between duplicate measurements was less than (5%. Calculations. The model for the calculation of multicomponent sorption equilibria and the calculation procedures are presented in part 1.7 Briefly, the equilibrium condition for a given component is expressed as the sum of mixing effects and the “elastic pressure”. The mixing and elastic terms are evaluated by means of the quasi-chemical UNIQUAC equation and the affine network model, respectively. Symmetric activity coefficients were used for all components even though the reference state for the sugars is not practically attainable. Here the equilibrium system is composed of four components, water (1)-ethanol (2)-sugar (3)-resin (4), and the corresponding size and surface parameters and the known interaction and elastic parameters are shown in Table 1. The size and surface area parameters for water, ethanol, and D-glucose were taken from Peres and Macedo.18 For D-xylose, the values were estimated from the group contributions in the molecule,19 and for the resin, the parameters were the same as those in part 1.7
The UNIQUAC interaction parameters for the waterethanol, water-resin, and ethanol-resin pairs and the elastic parameters were calculated in part 1.7 For the water-sugar pairs, they were estimated from the activity and solubility data using the solid-liquid equilibrium model of Peres and Macedo.20 The fusion enthalpies of the sugars were taken from Raemy and Schweizer.21 For water-D-xylose 21 data points4,22 and for waterglucose 38 data points23,24 were fitted, and the absolute average deviations (AAD) of the solubility, osmotic coefficient, and activity coefficient data were 0.17%, 0.17%, and 0.18% for D-xylose and 0.31%, 0.16%, and 0.33% for D-glucose, respectively. The estimated interaction parameters of the waterethanol and water-sugar pairs together with the solubility data of sugars in water-ethanol mixtures were used to calculate the interaction parameters for the ethanol-sugar pairs.4,6,18 The referred18 solubility data for D-glucose were measured at 313-333 K instead of 298 K for which the parameters given in part 1 are valid. Therefore, new water-ethanol parameters within this temperature range were needed, and they were estimated from 143 vapor-liquid equilibria (VLE) data points given in refs 25 and 26. The AAD values were 1.62% for the vapor phase mole fraction and 1.45% for the vapor pressure. The number of data points for the D-xylose and D-glucose solubilities were 11 and 18, respectively, and the AAD values were 11.3% and 6.15%, respectively. One additional pair of interaction parameters, a34 and a43, was needed for the resin phase to describe the interactions between the sugar and the resin. These parameters were obtained by fitting the experimental sorption data to the equilibrium model. To test the ability of the model to predict the effect of increasing alcohol concentration and decreasing cross-link density, parameters a34 and a43 were estimated using only the data obtained for D-xylose sorption from pure water to the X8 resin. This procedure was repeated for the Na+, Ca2+, and La3+ resins. 3. Results and Discussion First, the influence of the solvent composition on the sorption of the individual components from the ternary solution was investigated. These data are presented in order to compare the distribution of the solvents in the presence and absence of the solute and also to demonstrate the ability of the sorption model to predict the experimental data. Sorption of the solute will be discussed in detail later in this section. As an example,
Ind. Eng. Chem. Res., Vol. 38, No. 12, 1999 4845
Figure 2. Sorption of D-xylose from water-ethanol-D-xylose mixtures in sulfonated PS-DVB resins (Na+) at 298 K. (b) X8 (CS16G); (1) X4 (CS08G). The open circles were calculated with the model presented in part 17 using the parameter values given in Tables 1 and 2.
Figure 1. Sorption of water and ethanol from water-ethanolD-xylose mixtures in sulfonated PS-DVB resins (Na+) at 298 K. A: water. B: ethanol. (b) X8 (CS16G); (1) X4 (CS08G). The open circles were calculated with the model presented in part 17 using the parameter values given in Tables 1 and 2.
the sorption data of water, ethanol, and D-xylose are shown in Figures 1 and 2 for the two Na+ resins of different cross-link density. To illustrate the effect of the solvent composition, the data are plotted against the water mole fraction in the sugar-free solvent. The apparent scattering in the experimental data is due to the fact that values measured using different sugar concentrations are depicted in the same figure. Therefore, the calculated values cannot be represented by smooth lines, and they are given as open circles instead. In general, the values calculated with the model follow the measured data with reasonable accuracy. Comparison of the water and ethanol data in Figure 1 with the data presented in part 17 (Figures 1 and 2 therein) shows that the addition of sugar to the sorption system does not markedly change the water-ethanol ratio in the resin. For example, at the water mole fraction of 0.8, the mole ratio of ethanol and water in the sodium resin (X8) was 0.12 in the binary waterethanol system and 0.11 in the ternary water-ethanolD-xylose system. Similar behavior was observed for all solvent compositions studied and also for the Ca2+ and La3+ resins. In other words, the solute affects sorption of the solvents only by decreasing their contents in both phases, but no selective displacement of either solvent from the resin phase seems to take place. This result
can be explained by the relatively small influence of the sugar on the activity coefficients of both solvent components. On the other hand, the activity coefficient of D-xylose increases markedly as the water content in the solvent mixture decreases and, consequently, the solubility decreases dramatically. For example, only 1.10 wt % of D-xylose dissolves in pure ethanol at 298 K, whereas its solubility in pure water is 55.0 wt %.4 The symmetric activity coefficients estimated at respective concentrations from the solubility data by means of the UNIQUAC model are 46 and 0.54. Therefore, the decrease in the absorbed amount of D-xylose in Figure 2 simply reflects the low solubility of the sugar in ethanol-rich mixtures, and possible variations in D-xylose affinity toward the resin are overshadowed by this effect. To study the dependence of solute sorption on solute concentration, the data measured for the aqueous D-xylose solutions are replotted as sorption isotherms in Figure 3. Furthermore, the effect of the solvent composition on the isotherm shape is illustrated by an example shown in Figure 4. The experimental isotherms of the X8 resins were used to fix the adjustable parameters a34 and a43 for each counterion. The fitted interaction parameters and the AAD values are given in Table 2. For clarity the data points given in Figures 2 and 3 are omitted from Figure 4. The reduced weight fraction of sugar in Figure 4 means the D-xylose concentration divided by its solubility in solvent composition under consideration. Figure 3 shows that the amount of D-xylose in the resin increases almost linearly with increasing sugar concentration of the solution. However, the isotherms appear to have a slight upward curvature, and this trend is most pronounced in the data of the densely cross-linked Na+ resin. The isotherm shapes obviously stem from the increase in the sugar activity coefficient with increasing solute concentration in the liquid phase. As will be shown later, the sugar mole fraction in the resin phase is smaller than that in the external liquid and, therefore, the activity difference between the phases tends to increase with increasing external sugar concentration. Figure 3 also clearly shows the lower
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Figure 3. Effect of sugar concentration on D-xylose sorption from water-D-xylose mixtures in sulfonated PS-DVB resins at 298 K. (1, ‚‚‚) Na+ (X4, CS08G); (b, s) Na+ (X8, CS16G); (9, - -) Ca2+ (X8, CS16G), ([, -‚‚-) La3+ (X8, CS16G). The lines were calculated with the model presented in part 17 using the parameter values given in Tables 1 and 2.
Figure 4. Effect of the solvent composition on the D-xylose isotherm shape calculated with the model presented in part 17 using parameter values given in Tables 1 and 2. Reduced weight fraction of sugar is the D-xylose concentration divided by its solubility in solvent composition under consideration.
sorption capacity of resins having higher cross-link density or higher counterion valence. It should be stressed that the isotherm of the X4 sodium resin is predicted using the interaction parameter determined for the X8 resin and the elastic parameter obtained in part 1.7 The fairly good agreement with the experimental data further supports the idea of incorporation of resin elasticity in the sorption process. As shown by the calculated isotherms in Figure 4, the slope of the D-xylose isotherm becomes steeper and the slightly concave shape is gradually transformed to a markedly convex curve when the water content in the external liquid decreases. Similar trends have been observed by Ru¨ckert and Samuelson9 for D-glucose at high ethanol concentrations. Both changes are readily understandable from the discussion above: the increasing initial slope reflects the effect of ethanol on the sugar activity coefficient in the liquid phase and the convex shape indicates gradual saturation of the resin phase. The primary sorption data shown in Figure 2 are not particularly helpful for assessment of behavior in prac-
tical applications and a more convenient measure for the sorption equilibrium of the sugar is the distribution coefficient K. In chromatography, knowledge of the K values of the solutes is required in order to predict the selectivity of the separation system. Following Samuelson and co-workers,9 the distribution coefficient is defined here as the ratio of the sugar mole fractions in the solution inside the resin and in the external solution. A somewhat different definition is normally used in chromatography. For example, the volume distribution coefficient is generally calculated as the ratio of the amount of sorbed solute per unit volume of exchanger bed and the amount per unit volume of external solution.27 The effect of solvent composition on the experimental and calculated K values is shown in Figures 5 and 6 for resins having different degrees of cross-linkage and different counterions. Obviously, the general trend observed in the K values is independent of cross-linkage and ionic form. The solvent composition has only a negligible influence on the distribution coefficients at water mole fractions above 0.8, but at low water contents K increases nearly exponentially. The same trend was observed in the selectivity curves of water-ethanol mixtures earlier in part 1.7 The selectivity of the resins for water is small at water mole fractions over 0.8 and increases rapidly toward lower values. The selective uptake of water and the variation of the D-xylose solubility with the solvent composition give a qualitative explanation for the behavior of the distribution coefficient.27 When expressed in terms of the activity coefficients used in the present model, this effect can be explained as follows. High water selectivity at low external water contents provides in the resin an environment of low sugar activity, while in the liquid phase the activity coefficient becomes very high as discussed earlier. Moreover, it was shown in part 17 that the magnitude of the shear modulus representing the “elastic pressure” term in a given resin is nearly independent of the external solvent composition. Therefore, it can be assumed that the shape of the K curves is predominantly due to the activity effects. The relative importance of the activity and elastic contributions was further assessed by a model calculation. The magnitude of the elastic term was halved by using a molar volume of 47.7 mL/mol for D-xylose while keeping the interaction parameters unchanged. The resulting change in the K values was on average only 15% even in the most densely crosslinked (8 wt % DVB) Na+ resin. Therefore, it can be assumed that both the shape of the K curves and the absolute K values are predominantly determined by the activity effects. Adachi and Matsuno10 have put forward a different explanation. They concluded that complex formation between the sugar molecules and the Na+ ions in the sulfonated PS-DVB resin (X4) plays a major role in sorption of carbohydrates from water-ethanol mixtures. However, their results are at variance with the fact that, at least in aqueous solutions, the complex stability between Na+ ion and sugars is very low.1,2 The complex formation has been exploited in chromatographic separations of carbohydrates, but counterions with higher valence are normally required.1,2 The higher ion valence enhances the stability of sugar complexes,1 and this should also result in an increasing K value in Figure 6 when moving from sodium to lanthanum. However, as can be seen, the opposite is the case, and in the present
Ind. Eng. Chem. Res., Vol. 38, No. 12, 1999 4847 Table 2. Estimated Interaction Parameters a34 and a43 for the Sulfonated PS-DVB Resins at 298 K in the Water-Ethanol-Sugar Solutionsa AAD, %
a
resin
a34, K
a43, K
water
ethanol
sugar
N
CS08G, Na+, D-xylose CS16G, Na+, D-xylose CS16G, Ca2+, D-xylose CS16G, La3+, D-xylose Dowex 50 × 8, Na+, D-glucoseb
-496.7 -496.7 -524.2 -769.8 -496.7
366.0 366.0 362.4 405.7 366.0
2.2 8.1 7.6 10 17
4.6 15 8.5 12 32
8.1 17 25 9.2 23
13 27 26 31 10
AAD is the absolute average deviation, and N is the number of data points. b Data from Ru¨ckert and Samuelson.9
Figure 5. Effect of cross-linkage and sugar on the sugar distribution coefficient K in water-ethanol-sugar mixtures in sulfonated PS-DVB resins (Na+) at 298 K. (b, s) X8, D-xylose (CS16G); (1, ‚‚‚) X4, D-xylose (CS08G); (9, - -) X8, D-glucose (Dowex 509). The lines were calculated with the model presented in part 17 using the parameter values given in Tables 1 and 2.
Figure 6. Effect of ionic form on the sugar distribution coefficient K in water-ethanol-sugar mixtures in sulfonated PS-DVB resins (X8, CS16G) at 298 K. (b, s) Na+; (1, ‚‚‚) Ca2+; (9, - -) La3+. The lines were calculated with the model presented in part 17 using the parameter values given in Tables 1 and 2.
case a more obvious explanation for the increasing K values is based on nonspecific partition which is mainly caused by the sugar solubility differences between water and ethanol. Despite the general similarity of the K curves, there are some differences between the resins which deserve closer inspection. The K values of the less densely cross-
linked Na+ resin (Figure 5) are markedly larger than those of the X8 resin at water mole fractions above 0.8, while the order appears to be the reverse at low water contents. As can be seen in Figure 1, the solvent content of the X4 resin is much higher and, therefore, the steric hindrance of the resin matrix for the sugar molecules is smaller and the sugar sorption easier. Also, all resins are relatively nonselective for water at high water contents and, consequently, the influence of the solvent distribution is negligible (part 1, Figure 47). On the other hand, the higher water selectivity of the densely crosslinked resin at low water contents (part 1, Figure 47) turns the situation over, and the K values of this resin become larger. In other words, because of the higher water selectivity, the activity difference between the phases increases more steeply for the resins of higher cross-link density. As was shown in Figure 3, the calculated values were in good agreement with the experimental data in aqueous solutions. Moreover, the values predicted for the X8 resin at high water mole fractions agree with the measured data within experimental error. At low water contents the agreement is appreciably worse. For the X4 resin the agreement between measured and predicted values is good and, above all, the intersection of the two lines is correctly predicted. The discrepancy in measured and predicted values for the X8 resin is due to underestimated ethanol concentrations (Figure 1B) which result in too small values for the activity coefficients of the sugar in the resin phase. The influence of the counterion on the distribution coefficient is less straightforward (Figure 6). As a general trend, the K values decrease in the order Na+ > Ca2+ > La3+. This order has also been observed earlier for the sodium and calcium ions.27 The K values of the sodium and the calcium resins are approximately equal, whereas the values of the lanthanum resin are distinctly smaller at water mole fractions over 0.8. These findings cannot be fully explained by means of the corresponding water and ethanol isotherms given in part 17 because the differences between, for example, sodium and lanthanum resins are too small to account for the marked drop in the K values. For instance, in the sodium X4 resin, the solvent content is more than twice the value of the X8 resin and the ratio of the K values is on the average 1.3. On the other hand, the difference in the solvent contents of the sodium X8 and lanthanum X8 resins is only 10-30% and, nevertheless, the ratio of the K values is still about 1.4. Therefore, an explanation probably lies in the specific interactions between the solvents and the counterions. As discussed in part 1,7 the more strongly solvated calcium and lanthanum resins contain less free solvent available for dissolution of the sugar.2 Consequently, the K values should decrease in the sequence of increasing hydration tendency of the ions, i.e., Na+ > Ca2+ > La3+. Although
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the model does not take into account the specific solvation interactions, the calculated distribution coefficient curves follow the experimental data for all ionic forms. Especially for the La3+ resin, the prediction is distinctly good. Inspection of the interaction parameters a34 and a43 given in Table 2 shows that the values of the Na+ and Ca2+ resins are similar, while those of the La3+ resin are much larger in absolute magnitude. This seems to indicate specific interactions in the latter resin, but in view of the shapes of the experimental K curves in Figure 6 a more probable explanation is that the difference merely reflects the problems found in representing sorption in the La3+ resin. As discussed in part 1,7 the strong solvation tendency of this ion cannot be described adequately with the present model. Therefore, more experimental data with uni- and divalent counterions are needed in the future in order to find out whether a34 and a43 remain constant. Finally, the K values obtained for D-xylose are compared with those of D-glucose given by Ru¨ckert and Samuelson9 (see Figure 5). Because both sugars are weakly complexing and comparable in size and the Finex CS16 resin used in this study and the Dowex 50 resin used by Ru¨ckert and Samuelson9 behave similarly,7 the distribution coefficients should be similar. Moreover, the solubilities of D-glucose and D-xylose in water-ethanol solutions at 298 K differ only a little.4,6,28 The experimental data in Figure 5 show that D-glucose has a somewhat smaller distribution coefficient when absorbed from aqueous solutions in the Na-Dowex 509 resin than D-xylose when absorbed in the corresponding Na-Finex CS resin. It should be noted, however, that the sugar concentrations investigated by Ru¨ckert and Samuelson9 were much smaller than those used in this study. In fact, model calculations show that the difference in the sugar concentrations largely explains the smaller K values for D-glucose. At intermediate and low water contents, the K values are equal within the experimental error. Because of the similarity of the two systems, the K values for D-glucose were predicted by using the parameter values obtained for D-xylose and the Na-Finex CS resin. The calculated curve is shown in Figure 5. The calculated D-glucose curve follows the experimental data with good accuracy, further proving the similar behavior of D-glucose and D-xylose in the waterethanol-Na-resin system. 4. Conclusions Sorption of D-xylose from water-ethanol solutions in strong cation exchangers of different cross-link densities and counterions has been examined. The literature data about D-glucose were also used. These noncomplexing sugars were selected in order to study the partition effect and to exclude the influence of complex formation. Using water-ethanol solvents instead of pure water, partition of sugars in the resins can be enhanced by making use of the better sugar solubility in water than in ethanol. At low water contents, where the water selectivity of the resins is highest, the concentration of sugars in the resin exceeds several times the concentration in the external solution. The influence of crosslinkage on D-xylose sorption is dependent on the water mole fraction in the external solution. Sugar sorption on the resin of low cross-link density is pronounced at high water contents, whereas the enhanced water
selectivity of the densely cross-linked resin turns the situation over at low water contents. Sorption of D-xylose is more pronounced in the resin with univalent counterions than in the resins with multivalent counterions because of higher free water content. Adding sugar to the water-ethanol-resin system does not affect the ratio of ethanol and water sorption within the whole experimental range. The model, based on the UNIQUAC equation and the affine network theory of elasticity, is able to explain the sorption equilibrium not only in binary solvent-resin systems but also in ternary solution-resin systems. Within the model applicability range, the calculated results of each component are at least satisfactory and in some cases even excellent. At lower water contents, the model can explain sorption of components remarkably well despite the failure of one of its basic assumptions. From the chromatographic point of view, the addition of ethanol to water eluent can be exploited to enhance sugar separation, as it increases the sorption differences between sugars. This is possible because of different solubilities of sugars in the water-ethanol eluents. In addition, interactions such as complex formation between resin and sugar influence the separation when adding ethanol to eluent. However, complexing does not play any role in the present case. On the other hand, with higher ethanol mole fractions, the solubility of several sugars, e.g., D-xylose and glucose, and the total solution content of the resin drop steeply. That is why the ethanol fraction of the water-ethanol eluent should be kept at a level where the solubility and diffusional problems are avoided and, on the other hand, the positive effect on separation is achieved. Acknowledgment The authors gratefully acknowledge financial support from TEKES (Technology Development Centre, Finland) and from Cultor Oyj. The CS resins were kindly supplied by Finex Oy (Finland). Literature Cited (1) Angyal, S. Complexes of Metal Cations with Carbohydrates in Solution. Adv. Carbohydr. Chem. Biochem. 1989, 47, 1. (2) Davankov, V. A.; Navratil, J. D.; Walton, H. F. Ligand Exchange Chromatography; CRC Press: Boca Raton, FL, 1988. (3) Davankov, V. A. Packings in Ligand Exchange Chromatography. In Packings and Stationary Phases in Chromatographic Techniques; Unger, K. K., Ed.; Chromatographic Science Series, vol. 47; Marcel Dekker: New York, 1990. (4) Gabas, N.; Carillon, T.; Hiquily, N. Solubilities of D-Xylose and D-Mannose in Water-Ethanol Mixtures at 25 °C. J. Chem. Eng. Data 1988, 33, 128. (5) Moye, C. J. Non-Aqueous Solvents for Carbohydrates. Adv. Carbohydr. Chem. Biochem. 1972, 27, 96. (6) Saarnio, J.; Kuusisto, R. The Solubility of Some Monosaccharides. Pap. Puu 1971, 195. (7) Tiihonen, J.; Laatikainen, M.; Markkanen, I.; Paatero, E. Sorption of Neutral Components in Ion-Exchange Resins. 1. The Effect of Cross-Link Density and Counterion on Selective Sorption of Water-Ethanol Mixtures in Sulfonated PS-DVB Resins. Ind. Eng. Chem. Res. 1999, 38, 4832. (8) Ru¨ckert, H.; Samuelson, O. Adsorption of Sugars on Ion Exchangers from Ethyl Alcohol-Water Solutions. Sven. Kem. Tidskr. 1954, 66, 337. (9) Ru¨ckert, H.; Samuelson, O. Die Verteilung von Glukose bei Ionenaustauschern auf Harzbasis in A ¨ thylalkohol-Wassergemischen. Acta Chem. Scand. 1957, 11, 315.
Ind. Eng. Chem. Res., Vol. 38, No. 12, 1999 4849 (10) Adachi, S.; Matsuno, R. Effect of Eluent Composition on the Distribution Coefficient of Saccharides onto a Cation-exchange Resin in Sodium-ion Form. Biosci. Biotechnol. Biochem. 1997, 61, 1296. (11) Jonsson, P.; Samuelson, O. Automated Chromatography of Sugars on Cation Exchange Resin. Anal. Chem. 1967, 39, 1156. (12) Samuelson, O.; Stro¨mberg, H. Partition Chromatography of Mixtures Containting Polyols and Carbonyl Compounds (Including Sugars) on Ion Exchange Resins. Acta Chem. Scand. 1968, 22, 1252. (13) Martinsson, E.; Samuelson, O. Partition Chromatography of Sugars on Ion-Exchange Resins. J. Chromatogr. 1970, 50, 429. (14) Pa¨a¨r, E.; Samuelson, O. Partition Chromatography of Cyclitols on Ion-Exchange Resins. J. Chromatogr. 1973, 85, 101. (15) Havlicek, J.; Samuelson, O. Separation of Oligosaccharides by Partition Chromatography on Ion Exchange Resins. Anal. Chem. 1975, 47, 1845. (16) Samuelson, O. Partition Chromatography on Ion-Exchange Resins. Methods Carbohydr. Chem. 1972, 65. (17) Goldberg, R. N.; Tewari, Y. B. Thermodynamic and Transport Properties of Carbohydrates and their Monophosphates: The Pentoses and Hexoses. J. Phys. Chem. Ref. Data 1989, 18, 809. (18) Peres, A. M.; Macedo, E. A. Solid-Liquid Equilibrium of Sugars in Mixed Solvents. Entropie 1997, 202-203, 71. (19) Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Properties of Gases and Liquids, 4th ed.; McGraw-Hill: New York, 1987. (20) Peres, A. M.; Macedo, E. A. A Modified UNIFAC Model for the Calculation of Thermodynamic Properties of Aqueous and Non-Aqueous Solutions Containing Sugars. Fluid Phase Equilib. 1997, 139, 47.
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Received for review June 7, 1999 Revised manuscript received September 22, 1999 Accepted September 22, 1999 IE990403B
