Synthesis and modeling of calcium alginate nanoparticles in quaternary water-in-oil microemulsions

Calcium alginate nanoparticles were prepared using the water-in-oil microemulsion route. Sodium alginate and CaCl2 were solubilised in the aqueous phase of two microemulsions, and their mixing induced the crosslinking of the alginate polymer by the calcium ions. The microemulsions were based on the nonionic ethoxylated surfactant Brij30 and the nonionic poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock-copolymer L64. Dynamic light scattering analysis revealed that the microemulsion droplet size was not affected by the species in the aqueous phase and that the alginate polymer was strongly confined in the microemulsion water pool. Atomic force microscopy of the collected alginate nanoparticles indicated a polydisperse population with an average size of 54 nm and hinted at an active template role of the microemulsion droplets. A detailed description of the conformation adopted by the alginate polymer in the water droplet in the presence of mono- and divalent counterions was provided by Monte Carlo simulations. The binding of both counterions was nonspecific, with the chain configuration allowing the carboxylic, hydroxylic, and ether groups to coordinate the small ions. Addition of divalent counterions did not modify the radial extension of the alginate chain, yet it mediated the transient bridging between strands.