Processes of Obtaining Nanostructured Materials with a Hierarchical Porous Structure on the Example of Alginate Aerogels

Currently, materials with specific, strictly defined functional properties are becoming increasingly important. A promising strategy for achieving these properties involves developing methods that facilitate the formation of hierarchical porous materials. Aerogels represent a promising class of materials for implementing this approach. The synthesis of aerogels based on biopolymers exhibiting a hierarchical porous structure—combining micropores, mesopores, and macropores—enables the development of active substance delivery systems with controlled release profiles and highly efficient platforms for cell cultivation. The formation of hierarchical porous structures in aerogels can be achieved using soft and hard templating methods or by foaming techniques. This paper presents a comprehensive study of three methods for forming hierarchical porous structures in alginate aerogels: (1) employing surfactants (Pluronic F-68), (2) using zein as a pore-forming component, and (3) foaming in a carbon dioxide environment. The results of computed X-ray microtomography (micro-CT), scanning electron microscopy (SEM) and nitrogen porosimetry are presented. For each method of forming hierarchical porous structures, we investigated the effects of various parameters on the structural characteristics of the resulting material: for method (1), the surfactant concentration (0.25-1.00 wt.%); for method (2) – alginate-to-zein ratio (20:1, 10:1, 5:1); and for method (3) – pressure (50 and 100 bar) and holding time (1 and 3 hours). The results of micro-CT showed that each of the methods contributes to the formation of macropores within the structure of the resulting aerogels. Size distribution curves of the detected macropores were obtained, showing the presence of macropores ranging from 16 to 323 micrometers in size for samples obtained using surfactants, from 5 to 195 micrometers for samples obtained using zein, and from 20 micrometers to 3 mm for samples obtained by foaming in a carbon dioxide medium. The SEM images demonstrated the macro- and mesoporous fibrous structure of the obtained materials. The nitrogen porosimetry results indicated that samples obtained using surfactants and zein are characterized by a high specific surface area (592-673 m2/g), comparable to the specific surface area for an alginate-based aerogel obtained without the use of pore-forming components. However, the use of the developed methods for the formation of a hierarchical porous structure contributes to an increase in the specific mesopores volume (up to 17.7 cm3/g). The materials obtained by foaming in a carbon dioxide medium are characterized by lower specific surface areas (112-239 m2/g) and specific mesopores volumes (0.6-2.1 cm3/g). Thus, this paper presents a set of methods for forming hierarchical porous structures, wherein macropores facilitate effective mass transfer of substances to meso- and micropores, where subsequent adsorption or reaction processes occur.