Synthesis and characterization of starch sulfates obtained by the sulfamic acid-urea complex

Abstract

The process of sulfation of starch with sulfamic acid in the presence of urea was studied. The sulfur content of starch sulfate increases with increasing sulfation temperature from 70 to 100 °C. To obtain starch sulfates with a high sulfur content (about 10 wt%), It is advisable to carry out the sulfation process at a temperature of 100 °C for 120 min. Mathematical modeling of the process of sulfation of starch with sulfamic acid in the presence of urea was carried out. The introduction of a sulfate group in the structure of starch is confirmed by FTIR spectroscopy. In the FTIR spectra of sulfated starch, in contrast to the original starch, there are absorption bands at 1255 cm−1, 808-817 cm−1 and 859-867 cm−1, corresponding to sulfate groups. Sulfation of starch with a complex of sulfamic acid - urea can improve the environmental safety and efficiency of the process in comparison with the known methods of sulfation. Initial starch and sulfated starch were analyzed by gel permeation chromatography. The initial starch was shown to have a main peak with Mn 139 kDa and Mw 382 kDa, and sulfated starch has a peak in the chromatogram relating to Mn 44 kDa and Mw 60 kDa. Besides, the quantum chemical calculations of the starch and sulfated starches (in dimer forms) formed when different number of hydroxyl groups in starch were replaced with sulfate groups were performed using the DFT/B3LYP method with 6-31+G (d, p) basis set in the ground state. Firstly, these molecules were optimized. Then, from the optimized geometry of the starch and sulfated starches, frontier molecular orbitals (FMOs), molecular electrostatic potential (MEP) surfaces, bond parameters, chemical reactivity descriptors such as chemical potential, electron affinity, electronegativity, ionization energy, electrophilicity index and chemical hardness have been calculated theoretically. In addition, spectroscopic analyzes of starch and sulfated starches such as FTIR and 1H NMR were theoretically performed using the same method and compared with each other.