Synthesis and characterisation of hydrogels based on starch and citric acid

In this study, two series of hydrogels were synthesised with different molar ratio of glucose units of starch and citric acid. The advantage of using natural raw materials (starch and citric acid) for the synthesis of hydrogels lies in the fact that these raw materials are synthesised in nature, beneficial to human beings, easily degradable after use and harmless. The generated hydrogels and starting reactants were structurally characterised by applying the method of Fourier transform infrared spectroscopy (FTIR). FTIR analysis shows that the synthesis reaction of hydrogels based on starch and citric acid rests upon the esterification reaction of carboxyl groups in citric acid with hydroxyl groups in starch. The residual amount of citric acid in synthesised hydrogels was determined under high pressure with high-performance liquid chromatography (HPLC). The values of unreacted citric acid range between 9.85 and 16.08%.The scanning electron microscopy was applied in the analysis of hydrogel morphology. The swelling of hydrogel was monitored at 25 °C in the solutions of pH values of 5, 7 and 9. The highest swelling degree (a=8.55) was achieved in water at pH 7 for the hydrogel synthesised with the smallest amount of citric acid as the cross-linker at the ratio of glucose units of starch and citric acid being 72. Hydrogels synthesised from such monomers are environmentally friendly and fully biodegradable.