Pore-size-dependent structure of intermediate water in hydrogels of cellulose and agarose

Abstract To investigate the effects of pore size on the water structure in polysaccharide hydrogels, differential scanning calorimetric measurements were performed on a freeze cross-linked hydrogel of carboxymethyl cellulose nanofiber (CMCF) and conventional agarose hydrogel. The melting points and relative amounts of 4 types of water were analyzed as functions of water content. In addition to the 3 well-known types of water (i.e. bound, intermediate, and free water) in hydrogels, intermediate water was classified into 2 types: 1st and 2nd intermediate water (IW), depending on the structure around amphiphilic polymers. The melting point of 1st IW, which forms hydrogen bonds with bound water, decreases as the water content decreases for both hydrogels. Similar trends observed for both hydrogels suggest that the effects of pore size on 1st IW are negligible. In contrast, significant differences between the CMCF and agarose hydrogels were observed in the relative amount of 2nd IW, which exists at the interface between free water and the hydrophobic regions of polymers. The results indicate that the relative amount of 2nd IW, which depends on the specific surface area of the pore walls determined by pore size, is a key factor in the physicochemical properties of hydrogel materials.