Effect of temperature on alpha olefin sulfonate induced softening of gelatin hydrogels

Dynamic light scattering (DLS) and oscillatory rheology experiments were performed to study temperature dependence (T = 10–25 °C) of the interactions in hydrogels of gelatin with AOS (alpha olefin sulfonate, anionic surfactant) for surfactant concentrations in the range 25–100 mM, chosen larger than cmc (∼8 mM). The network mesh size (ξ) values deduced from fastmode diffusivity (Df) data obtained from dynamic structure factor measurements, S(q, t) ∼ exp(−Dfq2t) (for t ≤ 1 ms and q being the scattering wave vector), of micelle-bound gelatin gels was analyzed within the framework of Flory–Rhener theory of cross-linking, which revealed a temperature dependence, ξ ∼ (0.5–χ)1/5 exp(−ΔGTotal/RT) where χ is the Flory–Huggins interaction parameter, the free-energy of the gel–surfactant complex is ΔGTotal and R is universal gas constant. The low-frequency isochronal storage, G′ and loss, G″ modulii revealed a transition from the rigid to a softened gel state occurring at surfactant concentration close to 55 mM, independent of temperature. The free-energy of interaction between gel and surfactant deduced from Arrhenius plots obtained from temperature dependent rheology, and light scattering data support this observation.