Influence of pH Value and Ionic Liquids on the Solubility of l-Alanine and l-Glutamic Acid in Aqueous Solutions at 30 °C

The solubility of the amino acids l-alanine and l-glutamic acid and its sodium salt (sodium l-glutamate monohydrate) in aqueous solutions at 30 °C and atmospheric pressure was investigated in the pH range between 3 and 9 and in the presence of the ionic liquids (ILs) 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([bmim][OTf]) and choline dihydrogencitrate ([ch][dhcit]) at pH 7. The solubility of l-alanine and l-glutamic acid in the solutions without IL was measured by UV spectroscopy and with a gravimetrical method. In the presence of an IL HPLC-analysis was applied. The solid phases were characterized using Raman spectroscopy and powder X-ray diffraction to distinguish the amino acids from their salts. While the solubility of l-alanine did not depend on pH within the considered pH range, the solubility of l-glutamic acid strongly increased with increasing pH. Below pH 6.2 the solid phase was characterized to be l-glutamic acid, while sodium l-glutamate monohydrate was found to be the solid at pH higher than 6.2. It could be observed that the solubility of sodium l-glutamate monohydrate was comparatively high, and increased with increasing pH. Upon addition of the ILs under investigation ([bmim][OTf]) and [ch][dhcit]) the solubility of l-alanine and l-glutamic acid was decreased. Original PC-SAFT was applied to predict the solubility of l-alanine and l-glutamic acid (and its sodium salt) in water, with and without the ILs under consideration, at the experimental conditions with quantitative agreement to the experimental data.