Structure–Property Relationship in Cationic Surfactant/Hydroxypropyl Methylcellulose Hydrogels and Cryogels: Role of Headgroup and Counterion Dissociation

Understanding the factors governing the rheological properties of hydrogels and the mechanical properties of the corresponding cryogels obtained by freeze-drying is crucial for diverse applications. In this study, we investigated how incorporating three cationic surfactants, namely, cetyltrimethylammonium bromide (CTAB), cetyltrimethylammonium chloride (CTAC), and cetylpyridinium chloride (CPC), each at 20 mM, affects the rheological behavior of hydroxypropyl methylcellulose (HPMC) aqueous solutions (30 g L^-1) and the mechanical properties of the resulting cryogels. Among the systems studied, CTAC-containing hydrogels showed the highest storage modulus of <i>G</i>' and the longest relaxation time (τ = 0.685 s), indicating a denser network favored by enhanced counterion dissociation. For comparison, pure HPMC hydrogels presented τ = 0.167 s. Small-angle X-ray scattering (SAXS) data indicated that CTAC micelles possessed the largest intermicellar spacing (238 Å), a result attributable to high ion dissociation, which enhances the electrostatic repulsion among micelles. Cryogels derived from systems containing HPMC and CTAC exhibited a Young's modulus (<i>E</i>) of ∼600 kPa, nearly 3-fold higher than that obtained for pure HPMC cryogels (228 kPa). In contrast, HPMC solutions containing CPC showed intermediate values for τ (0.461 s) and <i>E</i> (∼176 kPa), reflecting the lower degree of ion dissociation. The weak dissociation of bromide ions resulted in solutions of HPMC and CTAB with the shortest relaxation time (τ = 0.305 s); these systems crystallized upon freezing, impairing interactions with HPMC chains and leading to brittle cryogels. These findings demonstrated, for the first time, a direct correlation between <i>G</i>' values of HPMC and ionic surfactant solutions and the <i>E</i> values of the resulting cryogels; surfactants with higher ion dissociation led to higher <i>G</i>' and <i>E</i> values.