Synthesis and characterization of ionic guar gum derivatives

Abstract

Polysaccharides are unique biopolymers with a significant structural diversity. Many organisms, including plants, animals, fungi, algae, and microorganisms, form large amounts of polysaccharides through biosynthesis due to their structure-forming ability as storage polymers and as structure-forming macromolecules. Thanks to their ability to form structures by supramolecular interactions of variable types, polysaccharides are exceptional. Furthermore, polysaccharides are increasingly recognized as essential substances in biotransformation processes, for example, in terms of activity and selectivity. While naturally occurring polysaccharides are already remarkable, chemical modification can improve given properties and can even be used to tailor-engineer advanced materials. Polysaccharides are used in food and other industries as stabilizers, thickeners, gelling agents and inhibitors of crystal formation. Polysaccharides such as cellulose, xylan and chitosan appear in nature as structural materials. They form the cell walls of plants and the shells of crustaceans and insects. Other polysaccharides such as glycogen, amylose and amylopectin are essential for storing sugar in many animals and plants(Seidi et al., 2018). Gums are also categorized under polysaccharides. Guar gum, cashew gum, xanthan gum, gum arabic, and chitin are some examples of gums. Guar gum is a galactomannan, a naturally occurring polymer, obtained from the soil endosperm of the plant Cyamopsis tetragonolobus or Cyamopsis psoraloides. Guar gum belongs to the legume family. When dissolved in water, the presence of a large number of hydroxyl groups increases the ability of guar gum to form hydrogen bonds. Thanks to this ability, it increases the viscosity and enhances the gelling properties. Based on these properties, guar gum has a variety of applications in various industries, such as textiles, food, petrochemicals, mining, and paper. With the increasing supply in the market in recent years, there has been increased work on the development of various compounds of guar gum. Combining guar gum with other substances not only enhances its properties but also increases its versatility for different applications in various fields such as water treatment, drug delivery, pharmaceutical, cosmetic and food industries. The aim of this study was to investigate the rheological properties of pure guar gum and cationic and anionic modifications of guar gum. Guar gum was preferred in this study due to its water solubility, thickening properties, and industrial use as a stabilizing agent. 3-Chloro-2hydroxypropyltrimethylammonium chloride (CHPTAC) was used as an etherifying agent in different ratios for the cationic modification of guar gum. The reaction medium was determined as water. After the guar gum was completely dissolved in water, CHPTAC was added to the system using a dropping funnel. At the end of the addition, the temperature was adjusted to 50°C and the reaction was carried out at this temperature for 24 hours in a basic environment. The new product obtained at the end of the reaction was collected by precipitation with ethanol. A freeze-drying system was preferred to remove the water in the medium. Anionic modification of guar gum was obtained from the reaction of guar gum with different ratios of monochloroacetic acid (MCA) aqueous solutions. The reaction medium was determined as water, and the reaction was carried out at basic pH as in the cationic modification. MCA aqueous solution was added to the system with a dropping funnel of 1 mL/min. After adding the MCA aqueous solution, the reaction was carried out at 50°C for 4 hours. After the reaction time, precipitation and freeze-drying processes were applied, respectively, to collect the synthesized product. Ethanol was used in the precipitation step as in the cationic modification.FTIR analyses were performed to determine the spectral properties of the modifications obtained from the reactions. The 1642 cm-1 peak in the FTIR analyses of the products obtained as a result of cationic modification proves that the reaction was successful. In the anionic modification, 1600 cm-1, 1391 cm-1, and 1253 cm-1 belong to the asymmetric and symmetric stretching vibrations of the added -COO- group, respectively. The presence of these peaks proves that the anionic modification was successful.1H NMR analysis was performed to understand the difference in modified guar gums. The characteristic signals were observed in the 1H NMR. TGA analyses were performed to understand how the thermal properties of unmodified guar gum were affected as a result of the modifications. In TGA analyses, 25-600°C and 20 °C/min heating rate were used. According to the TGA results, the thermal stability of anionic and cationic modification decreased compared to guar gum as expected. This shows that the added groups were successfully reacted. In order to examine the rheological behaviour of the obtained modifications and natural guar gum in water, viscosity-shear rate graphs were plotted. In order to examine the rheological properties, solutions were formed by dissolving each modification in pure water at the same ratio. The rheological properties of anionic, cationic modifications, and guar gum were investigated under increasing shear rate. As a result of the measurements, it was observed that aqueous solutions of the cationic modification showed Newtonian flow properties. The initial viscosity values of the modifications are lower than those of natural guar gum. It was observed that anionic modifications showed shear thinning flow properties like guar gum. The rheological difference of anionic modifications from guar gum is that the viscosity value at each shear rate is higher than guar gum. In order to examine the rheological effects of anionic and cationic modifications on each other, aqueous solutions of cationic and anionic modifications at the same concentration were mixed 1:1 by weight and viscosity values were measured against increasing shear rate. In the mixture of the cationic modification containing a low percentage of CHPTAC (AK-6) with the modification of the anionic modification containing a high percentage of MCA (AK-A2), the mixture showed shear thinning flow characteristics as in the anionic modification. In the mixture of AK-6 with low MCA containing modification (AK-A1), the mixture showed shear thinning flow characteristics. In the mixture of the modification of the cationic modification (AK-9) with high etherifying agent (CHPTAC) with the anionic modification (AK-A1) containing low MCA, the mixture showed shear thinning flow behaviour at low shear rates and Newtonian flow at high shear rates. The modification of AK-9 with high MCA content (AK-A2) showed Newtonian flow as in the cationic modification.