The salt effect on the solubility of the amino acids L-aspartic acid, L-glutamic acid, L-tryptophan, and L-tyrosine,
seldomly found in the literature, was studied at 298.2 K, in aqueous solutions of KCl and (NH4)2SO4, for salt
concentrations up to 2.0 mol⋅kg -1. In this concentration range, both salts are salting-in agents for glutamic acid
and aspartic acid, with a stronger effect induced by (NH4)2SO4. Regarding the two aromatic amino acids, a slight
increase in the solubility was obtained at low salt concentrations, followed by a stronger salting-out effect, more
pronounced by (NH4)2SO4 than by KCl. The relative solubility data obtained in this work were compared to
literature data for other amino acids in the same electrolyte solutions to establish a relative solubility ranking
connected to their structure.
Finally, the solubility data were modeled using the electrolyte Perturbed-Chain Statistical Association Theory
(ePC-SAFT). The modeling requires parameters for the amino acids and ions as well as melting properties of the
amino acids. All these parameters and properties were obtained from previous works. To quantitatively describe
the solubility of amino acids upon salt addition, binary interaction parameters (kij) between any amino acid and
anions were determined, while between any amino acid and the cations were fixed to kij = 0.08. The kij parameters
between amino acid and the inorganic anions show very similar values for amino acids of the same
chemical class (e.g. kij between anion and amino acid with apolar side chains), which may be used to systematically
reduce the number of adjustable parameters in future work.
This work was developed within the scope of the project CIMOMountain
Research Center, UIDB/00690/2020 and LA/P/0007/2020,
and CICECO-Aveiro Institute of Materials, UIDB/50011/2020, UIDP/
50011/2020 and LA/P/0006/2020, financed by national funds through
the Portuguese Foundation for Science and Technology (FCT)/MCTES.
Mehriban Aliyeva thanks FCT and European Social Fund (ESF) for her
Ph.D. grant (SFRH/BD/139355/2018). The authors also thank Carina
Silva for participating in the measurement of part of the solubility data.
