Fluid Phase Equilibria 228–229 (2005) 83–87 Solid–liquid equilibria in l-leucine + l-valine + water  Izumi Kurosawa, Amyn S. Teja ∗ , Ronald W. Rousseau School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0100, USA Abstract Solid–liquid equilibria in the system l-leucine + l-valine + water have been measured at 298 K and ambient pressure. The system was shown to form solid solutions at equilibrium. The data also showed that the equilibrium composition of l-leucine in the liquid phase was only slightly affected by the presence of l-valine; whereas, the equilibrium composition of l-valine increased significantly with the addition of l-leucine. A simple thermodynamic model was used to correlate binary l-leucine + water and l-valine + water solubility data, and then to estimate nonidealities in the solid phase in the ternary system. Surprisingly, the solid solution exhibited significant nonidealities, emphasizing the need for analysis of the solid phase when ternary solubilities are measured. © 2004 Elsevier B.V. All rights reserved. Keywords: Solid–liquid Equilibria; Solid-phase activity coefficients; Recrystallization; Amino acids; l-Leucine; l-Valine; Water 1. Introduction The solubility of mixtures of amino acids in water is of interest in the design of crystallization processes for the re- covery of amino acids from fermentation broths [1]. However, there have been very few studies involving two or more amino acids in water [2–6]; although,a few studies of single amino acids in water [7–10] or of single amino acids in water + co- solvent mixtures [11–19] have been published. The studies involving amino acid pairs in water [2–5] have relied on the addition of an excess amount of crystals of one amino acid to a solution containing the other amino acid until no more of the first amino acid dissolved. Only the liquid phase was then sampled under the assumption that the solid phase consisted of the pure (added) amino acid in equilibrium with a solu- tion of the two amino acids. This assumption may not always DOI of original article:10.1016/j.fluid.2004.07.010.  Please note this article was also published in a recent regular article of Fluid Phase Equilibria Vol 224/2 pgs 245–249, while it should have been part of the PPEPPD 2004 proceedings special issue in which it is now pub- lished. Please use the original article for citation. The Publisher regrets any inconvenience this may cause. ∗ Corresponding author. Tel.: +1 4048943098; fax: +1 4048942866. E-mail address: amyn.teja@chbe.gatech.edu (A.S. Teja). be valid, especially when the solutes have similar structure and/or functionality. In this case, the two solutes may deposit simultaneously in the crystal lattice, and recrystallization can lead to the formation of solid solutions. Consequently, the compositions of both solid and liquid phases must be mea- sured when mixtures of amino acids are recrystallized from solution so that there is no ambiguity in the solubility deter- mination. This work reports data on the compositions of the solid and liquid phases in equilibrium in the system l-leucine (l- LEU) + l-valine (l-VAL) + water at 298 K and ambient pres- sures. l-LEU and l-VAL have similar structures, and our work demonstrates conclusively that solid–liquid equilibria in this system involve solid solutions. A simple thermody- namic model is then used to obtain solid-phase activity coef- ficients in order to evaluate the effect of nonidealities in the solid phase on the purity of the crystals. 2. Experimental l-LEU and l-VAL were obtained from Ajinomoto Co. Ltd. (Tokyo, Japan) and used as received. HPLC-grade wa- ter was obtained from Fisher Chemicals (Fair Lawn, NJ) and 0378-3812/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2005.03.001