Form-Factor Control of Alginate Aerogels Via Thixotropic Sols: From Monoliths to Fibers to Films

Abstract

Producing homogeneous alginate aerogel monoliths is challenging due to the uncontrollably fast gelation of sodium alginate (NaALG) solutions, which is typically induced with metal ions. This issue is overcome by decoupling molding from gelation. Aqueous NaALG solutions are first converted into thixotropic liquids via in situ acidification with acetic acid (AcOH) generated gradually through the hydrolysis of acetic anhydride (Ac₂O), thus, providing time for casting in molds. The resulting solid-like thixotropic liquids are rigidized into wet gels conforming to the molds by a membraneless dialysis process via nonsolvent-induced phase separation, treatment with strong acids (HCl), or aqueous metal ion solutions (Ca²⁺, Cu²⁺, Fe³⁺, Ni²⁺, Ag⁺, Au³⁺). The thixotropic nature of the NaALG/Ac₂O mixtures enables extrusion into fibers or spreading into films, followed by rigidization using the same methods. Wet gels in monolithic, fibrous, or film form were dried into aerogels with supercritical fluid (SCF) CO₂. All aerogels were characterized by SEM, N₂-sorption porosimetry, IR spectroscopy, solid-state CP MAS ¹³C NMR, and elemental analysis. Oscillatory rheology tracked the transition of NaALG to thixotropic liquids during Ac₂O and AcOH titrations run in parallel. Thixotropic liquids from both titrations were rigidized with the nonsolvent-induced phase separation method and were processed into aerogels establishing that AcOH from the hydrolysis of Ac₂O converts up to ∼30% mol/mol of NaALG to alginic acid (ALGH). Conversion of the remaining NaALG in thixotropic liquids to ALGH (>95% mol/mol) requires a strong acid (HCl), while rigidization with metal ions just replaces residual Na. Dynamic mechanical analysis (DMA) showed nearly identical storage moduli (E′) for wet gels and aerogels, confirming that the solid network is formed during the rigidization step and is unaffected by subsequent processing. Notably, elemental analysis, porosimetry, and DMA data indicated that gels rigidized with Fe³⁺ included a secondary network, which, based on literature Mössbauer reports, is assigned to sol–gel-derived iron oxide.