Rheological properties of carboxymethyl cellulose–fucoidan mixture: effect of fucoidan concentration and salt

In this study, we investigated the effect of fucoidan concentration and salts (NaCl, KCl, and CaCl₂) on the rheological properties of carboxymethyl cellulose (CMC)–fucoidan mixtures. All mixtures exhibited shear-thinning behavior, with the apparent viscosity (η_a) of CMC–fucoidan mixtures at shear rates < 3.0 s⁻¹ being higher than that of CMC alone. However, as the shear rate increased to ≤ 30 s⁻¹, a more significant decrease in η_a was observed in CMC–fucoidan mixtures than in CMC alone. Consequently, the η_a,100 value of the mixtures decreased in a fucoidan concentration-dependent manner. In contrast, viscoelastic moduli increased with a higher fucoidan concentration, with a more pronounced increase observed in the elastic modulus than in the viscous modulus. Upon the addition of monovalent salts, the η_a value of CMC–fucoidan mixtures decreased due to the charge screening effect of cations. Conversely, the opposite result was observed with CaCl₂ addition due to Ca²⁺-induced crosslinking between both anionic polymers. Moreover, regardless of the salt type, CMC–fucoidan mixtures with salt showed higher viscoelastic moduli than those without salt, with a noticeable increase observed when CaCl₂ was added. This was likely due to the indirect/direct crosslinking effect of mono- and divalent cations. Our findings demonstrate that fucoidan and CMC exhibit a viscoelastic synergistic interaction, which is sensitive to shearing and influenced by the type of salt.