Department of Inorganic and Analytical Chemistry, University of Debrecen, Egyetem tér 1, Debrecen H-4032, Hungary.
MTA-DE Redox and Homogeneous Catalytic Reaction Mechanisms Research Group, Egyetem tér 1, Debrecen H-4032, Hungary.
ACS Appl Mater Interfaces. 2021 Jan 20;13(2):2997-3010. doi: 10.1021/acsami.0c17012. Epub 2021 Jan 6.
The most relevant properties of polysaccharide aerogels in practical applications are determined by their microstructures. Hydration has a dominant role in altering the microstructures of these hydrophilic porous materials. To understand the hydration induced structural changes of monolithic Ca-alginate aerogel, produced by drying fully cross-linked gels with supercritical CO, the aerogel was gradually hydrated and characterized at different states of hydration by small-angle neutron scattering (SANS), liquid-state nuclear magnetic resonance (NMR) spectroscopy, and magic angle spinning (MAS) NMR spectroscopy. First, the incorporation of structural water and the formation of an extensive hydration sphere mobilize the Ca-alginate macromolecules and induce the rearrangement of the dry-state tertiary and quaternary structures. The primary fibrils of the original aerogel backbone form hydrated fibers and fascicles, resulting in the significant increase of pore size, the smoothing of the nanostructured surface, and the increase of the fractal dimension of the matrix. Because of the formation of these new superstructures in the hydrated backbone, the stiffness and the compressive strength of the aerogel significantly increase compared to its dry-state properties. Further elevation of the water content of the aerogel results in a critical hydration state. The Ca-alginate fibers of the backbone disintegrate into well-hydrated chains, which eventually form a quasi-homogeneous hydrogel-like network. Consequently, the porous structure collapses and the well-defined solid backbone ceases to exist. Even in this hydrogel-like state, the macroscopic integrity of the Ca-alginate monolith is intact. The postulated mechanism accounts for the modification of the macroscopic properties of Ca-alginate aerogel in relation to both humid and aqueous environments.
多糖气凝胶在实际应用中最相关的性质取决于其微观结构。水合作用在改变这些亲水多孔材料的微观结构方面起着主导作用。为了了解通过超临界 CO2 干燥完全交联凝胶制备的整块 Ca-藻酸盐气凝胶在水合作用下的结构变化，通过小角中子散射（SANS）、液态核磁共振（NMR）光谱和魔角旋转（MAS）NMR 光谱对气凝胶在不同水合状态下进行了逐步水合和特性研究。首先，结构水的掺入和广泛水合球的形成会使 Ca-藻酸盐大分子发生运动，并引发干态三级和四级结构的重新排列。原始气凝胶骨架的初级原纤维形成水合纤维和束，导致孔径显著增大、纳米结构表面变平滑以及基质分形维数增加。由于在水合骨架中形成了这些新的超结构，气凝胶的刚性和抗压强度与干燥状态的性质相比显著提高。气凝胶的含水量进一步升高会导致临界水合状态。骨架中的 Ca-藻酸盐纤维会崩解成充分水合的链，最终形成准均匀的水凝胶状网络。因此，多孔结构会坍塌，明确的固态骨架不再存在。即使在这种水凝胶状状态下，Ca-藻酸盐单体的宏观完整性也保持完整。所提出的机制解释了 Ca-藻酸盐气凝胶在潮湿和水相环境中宏观性质的变化。
