Effect of Solvent Composition on Porosity, Surface Morphology and Thermal Behavior of Metal Alginate Prepared from Algae (Undaria pinnatifida)

Abstract

Alginates, extracted from algae are linear unbranched polymers containing b-(1?4)-linked D-man- nuronic acid (M) and a-(1?4)-linked L-guluronic acid (G) residues. The conversion of alginic acid into the metal alginate is confirmed using FTIR spectroscopy. Asym- metric and symmetric stretching of free carboxyl group present in metal alginate occurs almost at the same position in various solvent compositions. Total intrusion volume of metal alginate prepared in propanol (0.0742 mL/g) is greater compared to those in ethanol (0.0648 mL/g) and methanol (0.0393 mL/g) as solvent. Surface morphology as well as porosity and pore size distribution of metal alginate are greatly influenced by solvent. It can be seen from thermal analysis results that calcium alginate prepared using different solvent compositions started decomposing at 100 C, but rapid degradation started around 200 C. The results showed a stepwise weight loss during thermal sweep, indicating different types of reactions during deg- radation. First and second step of rapid degradation was situated around 200-300 and 300-550 C, respectively; whereas the final step is situated around 550-650 C. The trend of degradation was similar for all the solvents, although the amount of final residue varied from one sol- vent to another. At the same time, lower thermal stability was also observed with higher heating rates. Additionally, a kinetic analysis was performed to fit with TGA data, where the entire degradation process has been considered as three consecutive first order reactions.