Identification of Amino Acids Exhibiting the Ion-Exchange Isothermal Supersaturation Effect

Abstract

This paper reports the results obtained by theoretical and experimental study of the phenomenon called ion-exchange isothermal supersaturation (IXISS). This effect is observed, e.g., by passing a monosodium salt of amino acids (AA) through the bed of a sulfonate cation exchanger in the H-form. Frontal separation of Na + and AA anions (AA-) in this case is accompanied by conversion of AA- into AA zwitterions (AA ± ) due to interaction ofAA- with H + released from the resin phase. Formation of the pure AA zone proceeds simultaneously with the concentration of AA up to and beyond the level exceeding its solubility at a given temperature. Moreover, this supersaturated solution remains stable within the column interstitial space for a period of 6-10 h. The mechanism of stabilization of AA supersaturated solutions in the interstitial space of the ion-exchange column has been shown to be attributed to the chainlike amine-carboxylate interaction of AA molecules. The validity of the mechanism proposed is confirmed by the results obtained by studying sorption isotherms of β-alanine and γ-aminobutyric acid on sulfonate (KU-2 × 8) and carboxylic (KB-4P2) resins in the H-forms. A number of physicochemical parameters of AA solutions such as enthalpies of crystallization, activity coefficients, and some others have been shown to be applicable for identification of AA exhibiting the IXISS effect. This identification can also be based on the structural features of AA molecules.