Effects of counter cations on the water vapor sorption properties of alginic acid and alginates

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.