Biopolymer Produced by Rhizobium Tropici—Chemistry, Water Retention and Interactions with Clays

This study aims to explore the interaction between microbial polysaccharide, water and clay particles. Extracellular polymeric substances (EPS) from Rhizobium tropici (RT-EPS) were extracted by ethanol precipitate method (RT-EPM). Different molecular level analysis was performed to understand the chemical, physical and thermal properties of RT-EPM. Liquid Chromatography-Mass Spectrometry (LC-MS) results show that RT-EPM is composed of glucose and galactose. Size Exclusion Chromatography shows RT-EPM extracted has a high M_w~1.3*10⁶ Da and narrow polydispersity, M_w/M_n~1.013. DSC were performed from which the portion of free and bound water are calculated and estimated to be 79% and 21% at 200mg/mL of RT-EPM. This was consistent with drying experiments from TGA which showed that EPS retained water much longer. Addition of clay to biopolymer solutions formed a strong gel, depressing the melting point of water by 8 degrees C, accelerating drying, and decreasing the bound water content. Contact angle goniometry is consistent with blocking penetration of water to the polymer chains.