The Hofmeister effect on protein hydrogels with stranded and particulate microstructures

The infiltration of ions into hydrogel matrix could significantly affect the microstructure and macroscopic mechanics of the gels. Here, the Hofmeister effect of various salts on the whey protein isolate hydrogels with fine-stranded and particulate microstructures is investigated by soaking the preformed hydrogels in the sodium salts of different anions. The infiltration of kosmotropic anions yield stiffer hydrogels, whereas the chaotropic anions soften the hydrogels. The hydrogels with fine-stranded microstructures are more sensitive to the salts comparing to the particulate ones due to the microscopic phase transitions and enhanced hydrophobic interactions between polymer chains occurred in fine-stranded hydrogels. Besides, despite the significant difference in water binding ability of different salts, the water holding capacity of the salt-treated hydrogels was mainly determined by the gel stiffness instead of the salt types. Similar mechanical responses of BSA and egg white protein hydrogels to the Hofmeister series was also demonstrated, suggesting that the results shown here could potentially be generalized for other globular protein hydrogels.