Gelation,Network Structure and Properties of Physically Crosslinked Gelatin Gels: Effect of Salt Cation Valence

A series of physically cross-linked gelatin networks with and without salts (NaCl, CaCl₂ and CrCl₃) were prepared. The effect of the valence of the cations at the same concentration on the gelation of the gelatin was investigated by a rheometer. It was found that both the gelation temperature and the distance between crosslinks of the networks increased with an increase in valence of the cations. However, the opposite trend was observed for the storage modulus of the gels. Moreover, the influences of the valence and concentration of cations on the swelling and thermal behaviors of the gelatin composite films were studied in detail. It was generally observed that the equilibrium swelling ratio (ESR) decreased with salt concentration for all three cations following the trend ESR for Cr³⁺ <Ca²⁺ <Na⁺. However, the glass transition temperature (T_g) showed an opposite dependence on the valence and concentration of the cations. The relationships between the network structure and the properties of the gelatin composite films are discussed based on two effects (shielding and crosslinking effect), caused by the introduction of the salts.