Influence of temperature on non-exchange sorption of aromatic amino acids by anion exchanger AV-17-8
Abstract
The equilibrium in thearomatic amino acid (tryptophan, phenylalanine, tyrosine and histidine) – anion exchanger AV-17-8 (Cl-) systems at different temperatures was investigated In order to identify the influence of both increased and decreased temperatures on the sorption equilibrium the temperature range was 277-343 K was studied. Sorption isotherms were obtained at 277, 293, and 343 K. The thermodynamic description of sorption systems was carried out using an approach based on the idea that the sorption process is stoichiometric. The integral and differential energy characteristics and the compensation effect between the enthalpy and entropy components for the studied sorption systems were calculated.
A change in temperature led to a change in the physicochemical properties of the studied sorbates and sorbent. In the temperature range 277-243 K, the solvent content in the sorbent phase remained unchanged. A low solvent content at a temperature of 333 K and an increase in its amount at 343 K were experimentally established. It should be considered that at different temperatures the ionization constants of functional groups and the regions of existence of ionic forms of amino acids change, but the sorption mechanism does not change, since the recharging of amino acids did not occur.
It was established that the process is exothermic for tryptophan and phenylalanine. With increasing temperature, the amount of sorbed substance decreased. The absorption energy was low due to the significant compensatory effect of enthalpy and entropy factors, the differential Gibbs energy was practically independent of temperature due to the stabilizing effect of hydrophobic forces in the sorbent phase. For the poorly soluble amino acid tyrosine, when the temperature increased from 277 to 293 K, a slight increase in sorption was observed, and with a further increase, a decrease to trace amounts was revealed. In the temperature range of 277-293 K, the sorption process was characterized by the formation of a supersaturated solution in the sorbent phase and was endothermic. The sorption of hydrophilic histidine increased with increasing temperature, which correlates with the solvent content in the sorbent phase, including low content at 333 K. The process of histidine sorption is endothermic, characterized by a strong dependence of the differential Gibbs energy on temperature and the absence of a compensation effect between the enthalpic and entropic components.
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References
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