Kinetics of chitosan hydrogel formation in high internal phase oil-in-water emulsions (HIPEs) using viscoelastic measurements

High internal phase emulsions (HIPEs) have been used as templates for the preparation of low-density highly porous chitosan foams. The formation of hydrogels and porous foams, by crosslinking in the external phase of O/W HIPEs, has been studied. The stability of the emulsions, prepared using a nonionic surfactant (C13/C15 alkyl chains with 7 moles ethylene oxide), was investigated using droplet size vs. time measurements, in the presence and absence of chitosan. The stability was also assessed using dynamic (oscillating) measurements, where the storage modulus (G(LVR)') and cohesive energy density (E-c) were measured as a function of time. The effect of agitation was investigated by preparing the HIPEs at 700, 900 and 1200 rpm. The HIPEs prepared using nonionic surfactants in the absence of chitosan gave large droplet sizes but they were quite stable against coalescence. Addition of chitosan caused a significant reduction in droplet size and polydispersity, but the emulsions were less stable against coalescence. For emulsions prepared at low speeds of agitation (700 and 900 rpm), both G(LVR)' and E-c showed an initial increase of the modulus due to flocculation, but at times longer than 24 h, the flocculated emulsion showed coalescence. In the presence of chitosan, all HIPEs showed an exponential decrease in G(LVR)' and E-c with time, indicating coalescence of the emulsion. The crosslinking of chitosan with genipin was investigated using oscillatory and creep measurements. Both methods showed an increase in G(LVR)', E-c and compliance J with time, reaching a plateau value when t >= 24 h. This clearly shows completion of the crosslinking process after 24 h. Finally, crosslinked chitosan porous foams were obtained and characterized by SEM, which showed uniform porous textures.