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Ind. Eng. Chem. Res. 2005, 44, 3284-3288
GENERAL RESEARCH Solubility Measurements in the L-Isoleucine + L-Valine + Water System at 298 K Izumi Kurosawa, Amyn S. Teja,* and Ronald W. Rousseau School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0100
Solid-liquid equilibrium data are reported for L-isoleucine (L-ILE) + L-valine (L-VAL) + water at 298 K and ambient pressure, using experimental techniques that involve (a) cooling of an aqueous solution of the two amino acids until crystals are formed, or (b) the addition of one amino acid to an undersaturated aqueous solution of the other amino acid until any added crystals remain undissolved. The compositions of the phases in the two types of experiments were obtained by HPLC analysis. Solid phases were also analyzed by powder X-ray diffraction (XRD). The results show that, in the L-ILE + L-VAL + water system, homogeneous solid solutions are obtained in cooling crystallization experiments (method a), whereas isothermal experiments (method b) generally yield inhomogeneous solids. This suggests that literature data from isothermal experiments may not always represent true equilibrium values, especially when the solutes being crystallized are isomorphous or near-isomorphous, as in the case of the amino acids studied in this work. Introduction Solid-liquid equilibria in aqueous amino acid systems are of interest in processes for the separation and purification of amino acids from fermentation broths. However, few studies related to ternary or higher systems of amino acids have been published.1-6 This may be partly due to the fact that equilibrium is difficult to establish in solid-liquid systems, particularly when two or more of the solutes are similar in structure or functionality. Literature studies of solubilities of ternary and higher amino acid systems have generally employed the (isothermal) synthetic method, in which seed crystals of one amino acid are added to a solution containing two or more amino acids until the added crystals remain undissolved at constant temperature.1-5 The liquid composition, or solubility, is then calculated from the initial concentration of the solution and the mass of amino acid added. The assumption in these studies is that the solid phase consists of pure crystals of the added solute in equilibrium with a multicomponent solution. The solid phase is therefore seldom analyzed in these studies, and this may result in errors in the reported solubilities, particularly if solid solutions are formed. It is especially important to analyze the solid phase when the solutes have similar structures and/or functionality, because the two solutes may then substitute interchangeably in the crystal lattice to form solid solutions. This work reports solid-liquid equilibrium data in the system L-isoleucine (L-ILE) + L-valine (L-VAL) + water * To whom correspondence should be addressed. Tel.: 404-894-3098. Fax: 404-894-2866. E-mail: amyn.teja@ chbe.gatech.edu.
at 298 K and ambient pressure. Two experimental techniques were employed as follows: (a) the temperature of an aqueous solution of the two amino acids was decreased until a small amount of solid crystallized, or (b) crystals of one amino acid were added to an aqueous solution of the other amino acid until no more solid would dissolve. The two methods are compared in terms of their ability to yield reliable equilibrium data below. Messer et al5 employed the first method to obtain the solubility of several pairs of amino acids in water, including phenylalanine + L-leucine, methionine + L-leucine, methionine + norleucine, methionine + Lisoleucine, L-leucine + L-isoleucine, and aspartic acid + L-leucine. Pseudo-binary behavior was assumed in all cases, and the solid phases were not analyzed. The authors even noted that solid solutions were formed in the L-isoleucine + L-leucine + water system, but did not report any solid-phase compositions. Jin and Chao,4 on the other hand, employed the second method to obtain the solubility of pairs of amino acids in water. The amino acids studied included L-glutamic acid + glycine, L-glutamic acid + L-aspartic acid, L-glutamic acid + L-serine, and L-aspartic acid + L-serine, and their liquidphase compositions were calculated by assuming that the solid phases in their experiments consisted of pure crystals of one amino acid. A similar approach was also adopted by Kuramochi et al.3 in their studies of the DLalanine + DL-serine + water and DL-alanine + DL-valine + water systems. None of these studies, therefore, reported the compositions of the solid phases. Evidence for the formation of solid solutions in the L-isoleucine + L-leucine + water system has been provided by Koolman and Rousseau,7 who found that some peaks in the powder X-ray diffraction (XRD)
10.1021/ie049238c CCC: $30.25 © 2005 American Chemical Society Published on Web 03/10/2005
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pattern of crystals grown from aqueous solutions of the two amino acids could not be obtained by superposing the peaks in the diffraction patterns of crystals grown from binary solutions of the individual amino acids. They concluded that L-isoleucine + L-leucine crystals obtained from ternary solutions were not heterogeneous mixtures of crystals of the two pure solutes as a result of the substitution of one amino acid with the other in the crystal lattice during recrystallization. Their work emphasizes the need for analysis of the solid phases in crystallization experiments, particularly in systems in which substitution is known to occur during crystallization. The present work is concerned with solid-liquid equilibria in the L-isoleucine (L-ILE) + L-valine (L-VAL) + water system at 298 K and ambient pressure. According to Torii and Itaka,9,10 crystals of pure L-ILE and pure L-VAL are monoclinic and belong to the same space group, P21. Moreover, the structures of the two amino acids differ by only one CH2 group in their aliphatic side chains. It is therefore likely that L-ILE + L-VAL will form solid solutions when crystallized simultaneously from their aqueous solutions. We have therefore compared the XRD patterns of L-ILE + L-LEU crystals with the XRD patterns of pure L-ILE and pure L-VAL. In addition, we have analyzed both the phases in equilibrium and report the results below. Experimental Section Materials. L-Isoleucine (L-ILE) and L-valine (L-VAL) were obtained from Ajinomoto Company (Tokyo, Japan) and used as received. HPLC-grade water was purchased from Fisher Chemicals (Fair Lawn, NJ) and used to prepare solutions of the amino acids, as well as the mobile phases for high-performance liquid chromatography (HPLC) analysis. Cooling Method. Predetermined quantities of the two amino acids were added to 100 mL of water in a jacketed glass vessel, and the mixture was heated with constant stirring until all solids dissolved. A sample of the liquid was then withdrawn for analysis. After sampling, the temperature of the solution was decreased to 298 K at a rate of ∼0.8 K min-1 by circulating coolant through the jacket. The temperature was then maintained at 298 K for 48 h before sampling the phases in equilibrium. It should be added that, in a separate experiment, periodic sampling of the liquid showed that no change in concentration occurred after 24 h. An equilibration time of 48 h was therefore deemed adequate for obtaining solid-liquid equilibria. In addition, the initial concentration of the liquid was determined by trial and error to yield a very small amount of solid phase at 298 K. Further details and sampling procedure are given elsewhere.6 Isothermal Method. The isothermal method is similar to the method of Kuramochi et al,3 Jin and Chao,4 and Soto et al.2 An undersaturated solution of one of the amino acids was prepared by adding a known amount of that amino acid to 100 mL of water in a jacketed glass vessel. The concentration of the solution was determined by HPLC analysis when all crystals had dissolved at the set temperature. Small amounts of the other amino acid were then added to the solution with constant stirring, until it was no longer possible to dissolve any more of the added crystals. The system was then maintained at a constant temperature and the
liquid phase was sampled periodically until no change in its composition could be detected. Solid-Phase Analysis. After achievement of steadystate conditions in each experiment, the contents of the vessel were filtered and the solids were collected. It was decided the solids would not be washed with water in order to avoid preferential removal of one component. Instead, a correction was applied as follows: the wet solid sample was weighed and then dried overnight in an oven at 373 K, and then weighed again. The mass of water in the sample was obtained from the difference in weight before and after drying. The mass of each amino acid in the sample associated with the evaporated water was estimated from the concentration of the liquid phase (obtained by analysis) and the mass of evaporated water. It should be noted that the total amount of wet solids collected was always less than 3 g and the amount of mother liquor in the wet solids was always small. Furthermore, the solvent-free composition of the mother liquor was very close to the composition of the crystals in this system. Therefore, the correction to the composition of the crystal is small, and the error associated with this correction may be assumed to be negligible. The concentrations of the two amino acids in the solid phase were determined by HPLC analysis of a sample containing a small amount (
