Langmuir 1998, 14, 4169-4174
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Identification of Amino Acids Exhibiting the Ion-Exchange Isothermal Supersaturation Effect Dmitri Muraviev† Department of Physical Chemistry, Chemical Faculty, Lomonosov Moscow State University, 119899 Moscow, Russia Received October 28, 1997. In Final Form: March 3, 1998 This paper reports the results obtained by theoretical and experimental study of the phenomenon called ion-exchange isothermal supersaturation (IXISS). This effect is observed, e.g., by passing a monosodium salt of amino acids (AA) through the bed of a sulfonate cation exchanger in the H-form. Frontal separation of Na+ and AA anions (AA-) in this case is accompanied by conversion of AA- into AA zwitterions (AA() due to interaction of AA- with H+ released from the resin phase. Formation of the pure AA zone proceeds simultaneously with the concentration of AA up to and beyond the level exceeding its solubility at a given temperature. Moreover, this supersaturated solution remains stable within the column interstitial space for a period of 6-10 h. The mechanism of stabilization of AA supersaturated solutions in the interstitial space of the ion-exchange column has been shown to be attributed to the chainlike amine-carboxylate interaction of AA molecules. The validity of the mechanism proposed is confirmed by the results obtained by studying sorption isotherms of β-alanine and γ-aminobutyric acid on sulfonate (KU-2 × 8) and carboxylic (KB-4P2) resins in the H-forms. A number of physicochemical parameters of AA solutions such as enthalpies of crystallization, activity coefficients, and some others have been shown to be applicable for identification of AA exhibiting the IXISS effect. This identification can also be based on the structural features of AA molecules.
Introduction Amino acids (AA) manufactured by microbiological synthesis must be recovered from fermentation broths, containing AA, inorganic salts, and organic contaminants derived from microbial nutrients and metabolites. After removal of the biological matter by using centrifugal or membrane techniques, AA require further purification from mineral salt admixtures. This task can be successfully solved by ion-exchange (IX) methods. The general scheme of the majority of “standard” IX separation processes which are applied for recovery and purification of AA comprises several auxiliary operations such as (1) preparation of a stock solution, (2) concentration of the AA solution after the IX treatment, e.g., by evaporation, (3) recovery of the purified AA, e.g., by crystallization, (4) regeneration of ion-exchanger and auxiliary reagents for reuse, (5) neutralization of aggressive wastes prior to their disposal, and some others. Elimination of either of these auxiliary operations can improve the efficiency of the process due to significant saving of chemicals, energy, manpower, etc. One of the approaches applied for achieving this purpose is based on governing the separation process by modulation of some intensive thermodynamic parameters, such as temperature, pH, etc., which shift the equilibrium in the IX system.1,2 For example, application of dual-temperature IX separation methods allows for exclusion (at least partially) of two of the operations mentioned (4 and 5).3,4 Another possibility for † Present address: Department of Analytical Chemistry, Autonomous University of Barcelona, E-08193 Bellaterra (Barcelona), Spain.
(1) Gorshkov, V. I.; Ivanova, M. V.; Kurbanov, A. M.; Ivanov, V. A. Vestn. Mosk. Univ., Ser. 2: Khim. 1977, 5, 535 (Russian); Moscow Univ. Chem. Bull. (Engl. Transl.) 1977, 32, 23. (2) Muraviev, D.; Noguerol, J.; Valiente, M. React. Polym. 1996, 28, 111. (3) Khamizov, R.; Muraviev, D.; Warshawsky, A. In Ion Exchange and Solvent Extraction; Marinsky, J., Marcus, Y., Eds.; Marcel Dekker: New York, 1995; Vol. 12, p 93.
the elimination of additional auxiliary operations (such as, e.g., 2 and 3) from the flowsheet of the standard IX separation process deals with a combination of separation and concentration processes in one stage.5,6 Two IX fractionation techniques such as frontal and reversefrontal separation7-9 can be applied for this purpose. In certain cases, e.g., when the purification process is accompanied by the formation of low-solubility substances, both methods allow for achievement of the concentration of the target substance, which exceeds its solubility at a given temperature. Furthermore, this supersaturated solution may remain stable within the resin bed in the column for a long period. After leaving the column, a supersaturated solution crystallizes spontaneously, which allows for obtainment of a crystalline product right after the IX treatment cycle. This phenomenon, known as ionexchange isothermal supersaturation (IXISS), has been discovered for the first time by Muraviev10 for lowsolubility AA such as glutamic10 and aspartic acids.11 Later Selemenev et al. found the IXISS effect for tyrosine.12,13 Khamizov et al. observed this effect for some low-solubility calcium and magnesium salts.14,15 Several practical applications of the IXISS phenomenon have been reported (4) Muraviev, D.; Noguerol, J.; Valiente, M. Environ. Sci. Technol. 1997, 31, 379. (5) Bonn, G.; Bobleter, O. In Ion Exchangers; Dorfner, K., Ed.; Walter de Gruyter: Berlin, Germany, 1991; p 1176. (6) Beckmann, K.; Wuensch, G. Fresenius’ Z. Anal. Chem. 1992, 342 (6), 469. (7) Bobleter, O.; Bonn, G. In Ion Exchangers; Dorfner, K., Ed.; Walter de Gruyter: Berlin, Germany, 1991; p 1208. (8) Muraviev, D.; Chanov, A. V.; Denisov, A. M.; Omarova, F.; Tuikina, S. R. React. Polym. 1992, 17, 29. (9) Gorshkov, V. I.; Muraviev, D.; Warshawsky, A. Solvent Extr. Ion Exch. 1998, 16(1), 1. (10) Muraviev, D. Zh. Fiz. Khim. 1979, 53 (2), 438 (Russian). (11) Muraviev, D.; Saurin, A. D. Zh. Fiz. Khim. 1980, 54, 1271 (Russian). (12) Selemenev, V. F.; Zagorodny, A. A.; Polupanov, N. A.; Ogneva, L. A. Zh. Fiz. Khim. 1986, 60 (6), 1461 (Russian). (13) Selemenev, V. F.; Kotova, D. L.; Amelin, A. N.; Zagorodny, A. A. Zh. Fiz. Khim. 1991, 65 (4), 996 (Russian).
S0743-7463(97)01174-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/25/1998
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Muraviev
Figure 1. Schematic diagram of column loading (a) and distribution of stock solution (NaAA) components (b) in experiments on the study of the stability of supersaturated AA solutions (see text).
by Muraviev et al.16,17 and Khamizov et al.14,15 Nevertheless, the tailored application of the IXISS effect requires a deeper physical insight of this phenomenon. Another important problem which also needs to be solved refers to identification of IXISS-active substances, i.e., substances which can form stable supersaturated solutions in IX columns. The present study was addressed (1) to study the stabilities of supersaturated solutions of different AA obtained by the IXISS technique and the mechanism of their stabilization and (2) to develop a theoretical approach for identification of IXISS-active AA. Experimental Section Materials, Ion Exchangers, and Analytical Methods. L-Glutamic acid, DL-aspartic acid, β-alanine, and γ-aminobutyric acid of p.a. grade were purchased from Reanal (Hungary) and used as received. Carboxylic (KB-4P2) and sulfonate (KU-2 × 8) resins were commercial products (Russian production). The total IX capacity of the resins equaled to 4.5 (KU-2 × 8) and 9.6 (KB-4P2) mequiv/g. The concentration of H+ was determined by potentiometric titration using a Radiometer pH-meter with a combined glass electrode. The concentration of AA was determined by the formaldehyde method. Procedure. The study of stabilities of AA supersaturated solutions was carried out by applying the experimental procedure shown schematically in Figure 1. The bottom part of a thermostated column (i.d. ) 1.2 cm) was loaded with a granular solid phase under study (bed height ≈ 40 cm) and then with KU-2 × 8 resin in the H-form (bed height ≈ 50 cm). A monosodium salt of AA under study at pH ) 8.0 and T ) 295 K was passed through the column at a constant flow rate until the appearance of a AA supersaturated solution of a constant concentration. At this point the solution flow was stopped for a certain period of time, then resumed for collection of a sample of the supersaturated solution exposed in the interstitial space within a given time interval, and stopped again for the next period. The volume of each sample exceeded a little bit the void volume of the lower bed; hence, during the sample collection this space was refilled with the fresh supersaturated solution formed in the upper resin bed. The sorption isotherms of AA on KU-2 × 8 and KB-4P2 resins from unsaturated solutions were obtained under dynamic conditions by using a standard technique. The isotherm of L-glutamic acid (L-Glu) from a supersaturated solution was obtained as follows: a monosodium salt of L-Glu (NaGlu) with a known concentration was passed through the column (i.d. of 1.2 cm; resin bed height ≈ 50 cm) containing KU-2 × 8 resin in the H-form which served for yielding a supersaturated solution of L-Glu of a given supersaturation degree γ. This solution was passed through two small columns loaded with portions of KU-2 (14) Khamizov, R.; Mironova, L. A.; Tikhonov, N. A.; Bychkov, A. V.; Poezd, A. D. Sep. Sci. Technol. 1996, 31, 1. (15) Khamizov, R.; Novitsky, E. G.; Mironova, L. A.; Fokina, O. V.; Zhiguleva, T. I.; Krachak, A. N. Tekhnol. Machin. 1996, 4, 112 (Russian). (16) Muraviev, D.; Gorshkov, V. I.; Medvedev, G. A.; Ferapontov, N. B.; Kovalenko, Ju. A. Zh. Prikl. Khim. 1979, 52, 1183 (Russian). (17) Muraviev, D.; Gorshkov, V. I. Zh. Fiz. Khim. 1982, 56 (4), 1560 (Russian).
Figure 2. Concentration-time histories (stabilities) of L-Glu solutions with different supersaturation degrees γ (curves 1-4) in contact in the interstitial space of the column with an equivalent mixture (mixed bed) of strong acid and strong base ion exchangers in H- and OH-forms, respectively (1); sulfonate cation exchanger KU-2 × 8 in H-form (2, 4), and carboxylic resin KB-4P2 in H-form (3) in comparison with a L-Glu supersaturated solution removed from the column (5). T ) 293 K. The dotted line (6) denotes the solubility of L-Glu at T ) 293 K. × 8 resin in the H-form of known total IX capacity (Q0) until equilibration of the resin. Then the solution phase was evacuated from the column and L-Glu was stripped from the resin with 0.5 M HCl. The capacity of resins toward AA (QAA) was recalculated and expressed in terms of NAA values (mmol of AA per one functional group of the resin) as follows:
NAA ) QAA/Q0
(1)
The relative uncertainty on NAA determination was 373 K results in the formation of respectively lactame, pyrolidone carboxylic acid. Hence, the purified product appears to be recontaminated. Since IXISS combines purification and concentration in one stage and is ap(19) Khamsky, E. V. Supersaturated Solutions; Nauka: Leningrad, Russia, 1975 (Russian).
plicable even at low temperatures, it can be considered as a “mild”concentration technique, which avoids the above and similar complications and may be recommended for purification and recovery of temperature-labile compounds. 4. A supersaturated AA solution formed is in contact with the granular resin until it leaves the column. The time during which this solution remains in contact with the resin phase depends on the solution flow rate and the height of the resin bed. This time may vary from several tens to several hundreds of minutes. The stabilization of the supersaturated solution which is observed (see Figure 2) within this period is of particular importance in designing IXISS-based purification processes. After formation of a pure AA zone (see Figure 1), AA( interact with, e.g., sulfonate cation exchanger in the H-form, according to following reaction:
R - SO3-H+ + +NH3 - R′ - COO- / R - SO3- + NH3 - R′ - COO-H+ (2) Protonated carboxylic groups of the adsorbed AA cations are known to dissociate in the resin phase;20-22 i.e., sulfonate cation exchanger in the AA-form may be considered to be similar to carboxylic cation exchanger (see below). This means that reaction 2 can be continued due to the further amine-carboxylate interaction of AA molecules according to the following scheme:10
R - SO3- + NH3 - R′ - COO-H+ + n+NH3 - R′ COO- / R - SO3- + NH3 - R′ - COO- + NH3 R′ - COO- H+ + (n - 1)+ NH3 - R′ - COO- / etc. (3) The first unit of the AA chain is fixed on the resin phase while the rest of the chain may spread into the solution film surrounding the resin bead. Stabilization of AA supersaturated solutions by the resin or by any other granulated phase bearing charges on the surface (see Figure 2) may also be explained by sorption of the AA precrystalline associates which are formed in the interbed space. Hence, the second version of the stabilization mechanism can be described as follows:23
m(+NH3 - R′ - COO-) a (+NH3 - R′ - COO-)m (4) R - SO3-H+ + (+NH3 - R′ - COO-)m / R - SO3- (+NH3 - R′ - COO-)mH+ (5) According to this version of the stabilization mechanism, formation of the precrystalline associates, representing, in fact, polymolecular zwitterions (see eq 4), is followed by their sorption on a cation exchanger (eq 5) and is accompanied by the conversion of polymolecular zwitterions into cations. The second version of the proposed mechanism helps also to explain some other features of AA uptake by ion exchangers such as, for example, the superequivalent sorption (SES). (20) Nys, P. S.; Savitskaja, E. M.; Bruns, B. P. In Theory of Ion Exchange and Chromatography; Chmutov, K. V., Ed.; Nauka: Moscow, 1968; p 90. (21) Savitskaja, E. M.; Nys, P. S.; Bulycheva, M. S. Khim. Farm. Zh. 1969, 7, 32 (Russian). (22) Nys, P. S.; Savitskaja, E. M. Zh. Fiz. Khim. 1969, 43 (6), 1536 (Russian). (23) Muraviev, D.; Obrezkov, O. N. Zh. Fiz. Khim. 1986, 60 (2), 396 (Russian).
4172 Langmuir, Vol. 14, No. 15, 1998
Muraviev Table 2. Log fa of Amino Acids and Peptides in Aqueous Solutions at 298 K molality
Figure 3. Isotherm of the sorption of L-Glu on the KU-2 × 8 resin in H-form at 298 K from unsaturated (A) and supersaturated (B) solutions.
The SES of zwitterlytes on ion exchangers of different types was reported by many authors.20,24,25 For lowsolubility AA such as, e.g., Glu, the SES on a sulfonate cation exchanger in the H-form from unsaturated solutions has not been observed;26,27 nevertheless, this effect is manifested from the supersaturated Glu solutions. This is seen in Figure 3, where the isotherm of L-Glu sorption on KU-2 × 8 resin in the H-form from unsaturated and supersaturated solutions at T ) 295 K is shown.10 As seen, the Glu isotherm develops a Langmuir-like pattern with a well-defined plateau followed by a linear branch, which corresponds to the SES. This allows one to establish an analogy between the mechanisms of the SES of AA and the IXISS phenomenon and to interpret both effects in terms of the amine-carboxylate interaction of adsorbed AA molecules.23 The confirmation of this hypothesis is provided by the analysis of the literature data28-31 on the molal activity coefficients fa of AA and peptides. The logarithms of fa of zwitterlytes in aqueous solutions at 298 K are collected in Table 2. As follows from Table 2, the sign of log fa is determined by the presence (+) or absence (-) of a hydrophobic radical in the zwitterlyte molecule. Introduction of a hydrophilic group into this radical (e.g., OHin serine and threonine) causes an alteration of the log fa sign to the opposite in comparison with that of the next homologue (cf., serine and R-alanine). The data given in Table 2 reflect, in fact, the features of the interaction between water and zwitterlyte molecules and can be interpreted in terms of the stabilities of water-zwitterlyte associates.32 (24) Greenland, D. J.; Laby, R. H.; Quirk, J. P. Trans. Faraday Soc. 1965, 61, 2013, 2024. (25) Vorobieva, V. Ja.; Naumova, L. V.; Samsonov, G. V. Zh. Fiz. Khim. 1981, 55 (7), 1679 (Russian). (26) Oros, G. Ju.; Selemenev, V. F. In Theory and Practice of Sorption Processes; VGU: Voronezh, 1976; Vol. 11, p 22 (Russian). (27) Selemenev, V. F.; Miroshnikova, Z. P.; Ogneva, L. A.; Ermakova, I. I.; Kotova, D. L.; Oros, G. Ju. Zh. Fiz. Khim. 1985, 59 (8), 1992 (Russian). (28) Smith, E. R. B.; Smith, P. K. J. Biol. Chem. 1937, 117, 209. (29) Smith, P. K.; Smith, E. R. B. J. Biol. Chem. 1937, 121, 607. (30) Smith, E. R. B.; Smith, P. K. J. Biol. Chem. 1940, 132, 47. (31) Smith, E. R. B.; Smith, P. K. J. Biol. Chem. 1940, 135, 273.
zwitterlyte
m ) 0.2
m ) 0.3
m ) 0.5
glycine R-alanine R-aminobutyric acid R-aminoisobutyric acid valine leucine sarcosine (N-methylgylcine) serine threonine proline glytamic acid aspartic acid β-alanine β-aminobutyric acid β-aminopentanoic acid γ-aminobutyric acid γ-aminopentanoic acid -aminohexanoic acid diglycine triglycine
-0.0168 0.002 0.009 0.011 0.013 0.029a 0.002 -0.016 -0.005 0.008 -0.444a -0.444a -0.003 0.003 0.007 -0.007 0.000 -0.013 -0.040 -0.041
-0.0241 0.003 0.012 0.016 0.019
-0.0386 0.005 0.020 0.026 0.032
0.003 -0.025 -0.007 0.012
0.005 -0.042 -0.011 0.020
-0.056 -0.070
-0.005 0.008 0.027 -0.017 0.008 -0.024 -0.082 -0.095
a
The data refer to the solutions saturated at 298 K.
Figure 4. Isotherms of the sorption of γ-aminobutiric acid, β-alanine, and R-alanine on KU-2 × 8 (upper curves) and KB-4P2 (bottom curves) resins in H-form at pH ) pI and T ) 298 K.
On the other hand, the zwitterlytes manifesting SES, such as glycine,20 its peptides,24 and Glu10 are also characterized by fa < 1 (log fa < 0). From this, one can presume that this feature is characteristic of all zwitterlytes with log fa < 0. The above assumption is confirmed by the results shown in Figure 4, where the sorption isotherms of R-alanine, β-alanine, and γ-aminobutyric acid on a sulfonate cation exchanger KU-2 × 8 and carboxylic resin KB-4P-2 in the H-forms at pH ) pI (pI is the isoelectric point of AA) are presented.23 As seen, β-alanine and γ-aminobutyric acid manifest the SES sorption on the KU-2 × 8 resin, while for R-alanine this effect is not observed. The sorption isotherms of AA on carboxylic resins are similar to those of their SES on a sulfonate ion exchanger. This substantiates the above assumption (32) Selemenev, V. F.; Zagorodny, A. A.; Uglyanskaya, V. A.; Zavialova, T. A.; Chikin, G. A. Zh. Fiz. Khim. 1992, 66 (6), 1555 (Russian).
Identification of Amino Acids
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about the similarity of the sorption properties of sulfonate resins in the AA form with those of carboxylic ion exchangers. The estimation of the dissociation constant of the adsorbed β-alanine, Ka*, provides additional confirmation of this hypothesis. The sorption isotherm of AA at pH ) pI can be described by the following equations:
F(CH+) 1 )1+ NAA+ KnKaCAA CAA( )
Kn )
CAAKa F(CH+)
QAA+ QH+CAA(
CH+2 + KaCH+ + KaKb F(CH+) ) CH+
Ka*KaKnCAA Q0F(CH+)
(6)
(7)
(8)
(9)
(10)
where Q0 is the same as that in eq 1. As seen from eq 10, the plot of NAA( vs CAA is a straight line with a slope ) tga:
tga )
Ka+KaKn Q0F(CH+)
Wc, kJ/mol amino acid L-glutamic
∆H, kJ/mol
γ ) 2.3
γ ) 3.2
25.3 23.9 23.4 27.2 33.5 21.1 24.9 11.6 10.6 8.7 6.7 7.4 3.5
31.5 31.0 31.2 32.8 35.9 33.4 37.0 25.1 23.7 20.1 19.0 17.3 10.8
30.3 29.6 29.2 32.7 35.9 32.7 35.1 20.7 19.2 14.7 14.4 13.3 9.1
acid
DL-glutamic L-aspartic
where NAA is defined by eq 1; Q and C are the concentrations in the resin and solution phases, respectively, and Kb is the dissociation constant of the amino group of AA. Equation 9 was derived using an approach similar to that proposed by Seno and Yamabe.33,34 At NAA+ ) 1, the SES can be defined as NAA( ) NAA 1, and expressed as follows:
NAA( )
Table 3. Wc and ∆H of Spontaneous Isothermal Crystallization of Amino Acids from Aqueous Supersaturated Solutions with γ ) 2.3 and 3.2 at 298 K
(11)
An estimation of Ka* from the isotherm shown in Figure 4 gives a value of pKa* ) 3.67, which correlates well with pKa of β-alanine ) 3.60.35 Since the sorption of β-alanine is observed on a carboxylic resin of lower acidity (pKa of -COOH groups ≈ 536), the same can be expected to proceed on a sulfonate resin in the β-alanine form according to eq 3. It is interesting to note that the tga value (see eq 11) determined for Glu from the isotherm, shown in Figure 3, exceeds more than 10 times those obtained for β-alanine and γ-aminobutyric acid. This can be ascribed to the sorption of Glu associates appearing in the supersaturated solutions (see eq 5). The interpretation of the superequivalent zwitterlyte sorption and IXISS phenomenon in terms of the unified amine-carboxylate interaction mechanism allows one to establish the general nature of these two effects. This generalization makes it also possible to identify IXISSactive zwitterlytes from, for example, the respective ionexchange equilibrium data. Another route for identifying IXISS-active compounds has been proposed by Muraviev and Fesenko37 and is based (33) Seno, M.; Yamabe, T. Bull. Chem. Soc. Jpn. 1960, 33 (11), 1532. (34) Seno, M.; Yamabe, T. Bull. Chem. Soc. Jpn. 1961, 34 (7), 1021. (35) Greenstein, J. P.; Winitz, M. Chemistry of Amino Acids; John Wiley & Sons: New York, 1961; Vol. 1. (36) Dorfner, K. In Ion Exchangers; Dorfner, K., Ed.; Walter de Gruyter: Berlin, Germany, 1991; p 99. (37) Muraviev, D.; Fesenko, S. A. Zh. Fiz. Khim. 1982, 56, 1960 (Russian).
DL-aspartic asparagine DL-serine L-tyrosine DL-phenylalanine DL-norleucine DL-leucine DL-valine DL-isoleucine L-isoleucine
on the calculation of the nucleation energy for spontaneous isothermal crystallization of AA from supersaturated solutions by the homogeneous mechanism.38 The work on the formation of 1 mol of a monocomponent crystallization nucleus from a supersaturated solution can be described as follows:39
Wc )
16πσ3M3 3(FRT ln γ)2
(12)
where σ is the surface tension; F and M are the density and the molecular weight of the substance, and γ has the same meaning as above. The only unknown parameter in eq 12 is σ. Since the experimental determination of σ for a solid-liquid interface (in a supersaturated solution) represents a difficult task, this value has been estimated by using the calculation method proposed by Rudik.40 The method is based on the statistical theory for crystallizing liquid developed by Frenkel.41-43 The results on estimation of Wc for different AA at γ ) 2.3 and 3.2 and at T ) 293 K are collected in Table 3,37 where the values of the differential enthalpies of AA crystallization (see ref 35, p 532) are also shown. As seen from Table 3, all AA can be divided into two groups. The first group includes L- and DL-Glu and Asp; asparagine; DL-serine and L-tyrosine and is characterized by high ∆H values and a slight Wc vs γ dependence. The Wc values for AA of this group remain at a practically constant and sufficiently high level when the supersaturation degree rises. AA of this group, such as L- and DL-Glu,10 L- and DL-Asp,11 asparagine,44 and tyrosine,12,13 are known to manifest the IXISS effect. Note that the molecules of AA of this group do not contain any hydrophobic radical. The second group involves DL-valine, DL-leucine, DLnorleucine, L- and DL-isoleucine, and DL-phenylalanine. As seen in Table 3, the ∆H values of AA of this group are substantially lower and Wc decreases remarkably when γ rises. For AA of the second group the IXISS effect has not been observed that can be attributed by far lower Wc values at equal γ in comparison with AA of the first group. (38) Muraviev, D.; Khamizov, R.; Tikhonov, N. A.; Kirshin, V. V. Langmuir 1997, 13 (26), 7186. (39) Adamson, A. Physical Chemistry of Surfaces, 3d ed.; WileyInterscience Publ.: New York, 1976; Chapter 8. (40) Rudik, A. V. Zh. Fiz. Khim. 1975, 49, 1525 (Russian). (41) Frenkel, J. P. Zh. Eksp. Teor. Fiz. 1939, 9, 199, 952 (Russian). (42) Frenkel, J. P. Kinetic Theory of Liquid; Izd. ANSSSR: Leningrad, Russia, 1945; p 409 (Russian). (43) Landau, L. D.; Lifshits, E. M. Theoretical Physics; Nauka: Moscow, Russia, 1965; Vol. 5, Chapter 3 (Russian). (44) Muraviev, D. Unpublished results, Lomonosov Moscow State University, Moscow, Russia, 1979.
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The Wc value characterizes the energetic barrier of spontaneous crystallization from the unseeded (free from any heterogeneous nucleation centers) supersaturated solution;45 hence, less stable supersaturated solutions are associated with a lower Wc value. As seen in Table 3, Wc values decrease in the sequence norleucine > leucine > valine > isoleucine. The hydrophobicity of the aliphatic radicals of these AA follows the same trend, i.e., grows from norleucine (CH3-CH2-CH2-CH2-) to isoleucine (CH 3-(C2H5)-CH-). A comparison of the data presented in Table 3 with those given in Table 2 shows that for AA of the first group log fa < 0, while AA of the second are characterized by log fa > 0. This substantiates the above conclusion about the general nature of the SES and IXISS effects. Another feature, which differentiates AA of the first group from those of the second, refers to the temperature dependence of their solubility, Cs, which can be described in a general form as follows:
ln Cs ) a + bT ) dT2
(13)
where a, b, and d are coefficients, which are constants for each AA (see ref 29, p 532). For AA of the first group d ) 0, while for those of the second d * 0; i.e., in the first case Cs vs T dependence is linear, whereas in the second it is not.46 (45) Kidyarov, B. N. Kinetics of Crystals Formation from Liquid Phase; Nauka: Novosibirsk, Russia, 1979; p 6 (Russian).
Conclusions In conclusion it seems important to emphasize the following: (1) The formation of stable supersaturated AA solutions in the interstitial space of ion-exchange columns is observed by passing AA through the bed of a strong acid cation exchanger in the H-form. The stabilization of a AA supersaturated solution can be attributed to aminecarboxylate interaction of AA molecules. The superequivalent sorption of AA can also be explained by the same mechanism. (2) The general nature of superequivalent sorption and IXISS effects allows for use of a number of physicochemical parameters such as activity coefficients in solutions, enthalpies of crystallization, and others for identifying AA (and some other zwitterlytes), which manifest the IXISS effect. This identification can also be based on the structural features of AA molecules. However, nevertheless, this point requires a stricter theoretical interpretation and experimental confirmation, and we intend to continue our investigation on the subject. Acknowledgment. The author thanks all co-workers cited in the various references for their assistance and efforts in making this publication possible. Catalonian Government is acknowledged with thanks for financial support of the author during his Visiting Professorship at Universitat Auto´noma de Barcelona. LA9711746 (46) Muraviev, D. Ph.D. Thesis, Moscow State University, Moscow, 1979.
