Solubility Measurements in the L-Isoleucine + LValine + Water System at 298 K

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.