Interaction of glycine with a hydrogel based on a copolymer of acrylamide and sodium acrylate
To assess the possibility of absorption of amino acids by superabsorbents and the effect of the temperature of the sorption on the degree of absorption, we studied the interaction of glycine with a hydrogel based on a copolymer of acrylamide and sodium acrylate at 296 and 280 К in aqueous solutions with the concentration range of 0.005-0.1 mol/dm3. The study determined that the amount of absorbed glycine is smaller at 280 K than at 296 K. The dependences of the distribution coefficient on the equilibrium concentrations of glycine pass through maximums. The growth of the distribution coefficient is explained by the accumulation of the amino acid in the sorbent phase at low concentrations of the solution. The sorption centres are then filled and the internal solution is structured, which results in a decrease in the distribution coefficient. The amino acid is present in the aqueous solution in the form of a bipolar ion. This prevents the recharging of the system “functional group - counterion”. Therefore, the absorption proceeds according to the non-exchange sorption mechanism. Assuming that the sorption centre in the system is COOK, i.e. A group incapable of exchange under the said conditions, the thermodynamic description was based on the idea of a stoichiometric character of the process. We calculated the activity coefficients, thermodynamic constants, and Gibbs differential energies, which were negative, which indicates that the processes are spontaneous and energetically favourable. We should note that this value varied from -13 to -5 kJ/mol in all the systems during the sorption of the studied amino acid. This indicates the weak interaction of the absorbed amino acid with the hydrogel phase. Using the Gaussian 09W software by the DFT method at the B3LYP/6-31G (d, p) basis we performed computer modelling of the hydrogel swelling with the number of molecules of water in the functional group varying from 9 to 15. Quantum-chemical modelling demonstrated that absorption of glycine by hydrogel is accompanied by redistribution of molecules of water between the amino acid and the functional groups of the hydrogel.