Effect of cationic size on gelation temperature and properties of gelatin hydrogels

Effect of Na+, K+ and Ca2+ on gel transition temperature (Tg) of gelatin hydrogels (5%, w/v) has been studied by oscillatory rheology in the salt concentration range I = 0.01-0.1 M, which showed increase in Tg with salt concentration with the trend for Tg showing Ca2+ > K+ > Na+. The dynamic light scattering (DLS) measurements in the sol state (T > Tg) showed two distinct relaxation modes whereas only a gel mode was observed in the gel state in all the samples which contained significant amount of heterodyne contribution. Low frequency (1.5 rad/s) isochronal storage modulus data revealed the formation of strong gel in presence of CaCl2 compared to that of NaCl and KCl situations. The slow mode relaxation and heterodyne parameter obtained from DLS data indicate the presence of larger clusters in Ca2+ gels.