«Green» synthesis and characterization of galactomannan sulfates obtained using sulfamic acid

Abstract

The “green” synthesis of galactomannan sulfates using a sulfamic acid–urea mixture has been studied for the first time. The effect of the time and temperature of the galactomannan sulfation process on the degree of substitution of galactomannan sulfates has been investigated. It is shown that, at a temperature of 70 °C with an increase in the process time up to 120 min, the degree of substitution increases up to 0.70. An increase in the process temperature up to 80 °C leads to the production of galactomannan sulfates with a degree of substitution of 1.67. With a further increase in the process temperature to 90 °C, the galactomannan structure is partially destructed, and the degree of substitution decreases. Embedding of the sulfate groups into the galactomannan structure has been confirmed by elemental analysis and Fourier-transform infrared spectroscopy. In addition, the initial and sulfated galactomannans have been characterized by X-ray diffraction, scanning electron microscopy, atomic force microscopy, and gel permeation chromatography. The thermal analysis shows that the initial galactomannan exhibits endothermic peaks at 254 and 294 °C and an exothermic peak at 315 °C, while sulfated galactomannan exhibits endothermic peaks at 209 and 275 °C and an exothermic peak at 281 °C. Using atomic force microscopy, it has been shown that the sulfated galactomannan film consists of spherical particles with an average diameter of 200–300 nm; according to the phase contrast data, it has the uniform composition without extraneous impurities.