Gelation of Na-alginate aqueous solution: A study of sodium ion dynamics via NMR relaxometry

Sodium alginate (SA) hydrogels have a wide range of applications including tissue engineering, drug delivery and formulations for preventing gastric reflux. The dynamics of sodium ions during the gelation process of SA solution is critical for clarification of the gelation procedure. In this work, nuclear magnetic resonance (NMR) relaxometry and pulsed-field-gradient (PFG) NMR diffusometry were used to investigate the dynamics of the sodium ions during the gelation of SA alginate. We find that sodium ions are in two different states with the addition of divalent calcium ions, corresponding to Ca²⁺ crosslinked and un-crosslinked regions in the hydrogels. The sodium ions within the un-crosslinked regions are those released from the alginate chains without Ca²⁺ crosslinking. The relative content of sodium ions within the Ca²⁺ crosslinked regions decreased with the increase in the content of calcium ions in the system. The relaxation time T₂ of sodium ions within the Ca²⁺ crosslinked and un-crosslinked regions shift to shorter and longer relaxation time with the increase in concentration of calcium ion, which indicates the closer package of SA chains and the larger space for the diffusion of free sodium ions. This work clarifies the dynamics of ²³Na⁺ in a calcium alginate gel at the equilibrium state.