2.2.7. Hydrogels Stability Studies

The stability of the as prepared hydrogels was studied as follows: a piece of each sample (100 mg) was immersed in 10 mL solutions, with different pH values: pH = 4 (Merck KGaA buffer solution citric acid/sodium hydroxide/hydrogen chloride), pH = 5.6 (distilled water), pH = 7 (Merck KGaA buffer solution potassium dihydrogen phosphate/di-sodium hydrogen phosphate) and pH = 9 (Merck KGaA buffer solution boric acid/potassium chloride/sodium hydroxide). All solutions containing the hydrogel samples were heated at 60 °C and maintain at this temperature for 2 h. Further, the solutions were cooled on an ice bath, and the samples were removed allowing drying for 2 h at ambient conditions, then dried in an oven at 60 °C for 2 h. Finally, the samples were weighed. The whole procedure was repeated 5 times. Independently, one piece of each sample has been subjected to the ultrasonic treatment. The sample was immersed in distilled water until it reached swelling equilibrium, then weighed, and placed in 100 ml of water, being subjected to ultrasound treatment for 10 min, using an Ultrasonic Processor GEX-500 sonicator, operating at 500 Watts and 20 kHz frequency, on an ice bath to prevent overheating.