Single-Molecule Study Reveals that Sodium Alginate is Hydrophobic

Sodium alginate (SA) is generally considered highly hydrophilic due to two hydroxyl groups and a carboxylate group on each pyranose ring. However, SA will form a gel after dissolving in water for a certain period. The two properties of SA, hydrophilicity and gelation, seem to be paradoxical. In this study, to explore the mechanism behind these paradoxical properties, the single-chain behaviors of SA in various liquid environments have been investigated by using single-molecule force spectroscopy (SMFS). In nonpolar solvents such as nonane, SA exhibits its single-chain inherent elasticity consistent with the theoretical elasticity derived from quantum mechanical (QM) calculations. Notably, the experimental curve of SA obtained in water shows a long plateau in the low force region. Further research reveals that this phenomenon is driven by the hydrophobic effect. Additionally, SA shows greater rigidity than its inherent elasticity in the middle and high force regions due to electrostatic repulsion between carboxylate groups on adjacent sugar rings. Comparative single-molecule studies suggest that SA exhibits considerable hydrophobicity, offering new insights into the gelation process in water.