In this study, the water vapor sorption of four types of alginate, namely, alginic acid (Alg), Na‐Alg, Ca‐Alg, and NH4‐Alg, were measured, and the effect of differences in counter cations on water vapor solubility and crystallization mechanism associated with the arrangement of hydrogen bonds generated during humidification was investigated. The sorption amounts in the high‐humidity region can be arranged as follows: Na‐Alg > NH4‐Alg > Ca‐Alg > Alg. All samples formed a (C)‐type double helix structure before and after the sorption based on the results of Fourier transform infrared spectroscopy and wide‐angle X‐ray diffraction. The counter cation affects the arrangement of the hydrogen bonds in this structure. Only Ca‐Alg possessed a pseudocrosslinking among the (C)‐type double helix structures due to the divalent Ca2+ ions, which induced variations in the crystal arrangement behavior of the other three samples before and after sorption. Cases I–III show a case of how the orientation is aligned with a new hydrogen bond and classified for each sample.

In this study, we investigated the influence of the difference in the chemical structures of hyaluronic acid (HA), sodium hyaluronate (Na-HA), and starch water-soluble polysaccharides and the difference in sample shape (as-received samples and membranes) on water vapor solubility. The sorption at this time was characterized by a Fickian diffusion increase at one stage, but HA and starch had a non-Fickian diffusion increase at the two stages in the high humidity region of 52% or more. In addition, all sorption isotherms of HA, Na-HA, and starch did not depend on the sample shape and had an extended dual-mode sorption type behavior consisting of Flory-Huggins and dual-mode sorption types. Furthermore, the infinite dilution solubility coefficient for water molecules increased with increasing surface free energy and correlates with the surface free energy obtained from the contact angle of water of HA, Na-HA, and starch membranes.