DMTMM-mediated amidation of sodium alginate in aqueous solutions: pH-dependent efficiency of conjugation

Abstract

DMTMM-mediated amidation of sodium alginate is one of the methods used for the chemical modification of alginate with amines. However, there is a limited understanding of how the reaction conditions, particularly the pH value, influence the conjugation efficiency (CE) and the resulting degree of substitution (DS). In this study, we investigated the effect of the pH during the reaction, focusing on both neutral and weakly basic conditions, using water and buffer as solvents. Two model amines with high pK_aH values were selected, furfurylamine (FFA, pK_aH = 9.12) and 4-(2-aminoethyl)morpholine (AEM, pK_aH = 9.93). Sodium alginate with a high mannuronate content (60 mol%) and molar mass of 168 kg·mol⁻¹ was used for amidation. Our results show that both FFA and AEM effectively conjugate to sodium alginate under the selected reaction conditions. We found that pH significantly affects both CE and DS, which varied between 2 % to 40 % and 3 % to 53 %, respectively, depending on the specific reaction conditions. Optimal conditions were observed at neutral pH in water, whereas weak basic pH led to lower CE. Our findings thus offer a recommendation for optimizing the DMTMM-mediated amidation of sodium alginate, emphasizing the importance of pH values during the reaction.