Chapter Six Surfactant induced softening in gelatin hydrogels

6.1 Abstract Effect of binding of three surfactants, alpha olefin sulfonate (AOS, anionic), TritonXIOO (TX-100, non-ionic) and cetyl trimethyl ammonium bromide (CTAB, cationic) to the hydrogels of gelatin was studied at room temperature (25°C) by dynamic light scattering and oscillatory rheology with surfactant concentrations (20-1 OOmM) much larger than the critical micellar concentrations (erne) of these surfactants. The measured intensity auto-correlation function of light scattered from gels revealed the presence of finite heterodyne contribution ~0.11 ±0.0 1 that increased to ~0.25±0.02 after transition to the soft gel state indicating a softening process for surfactant concentrations exceeding 50mM. The dynamic structure factor S(q,t) of micelle bound gelatin gels revealed two clearly identifiable relaxation modes namely; the fast mode, S{q,t)exp.{-DtGt) for t ~ lms and a stretched exponential mode, S(q,t)exp.-(thc) 13 for lms ~ t ~ Is. This behavior was universal with 13~ 0.85± 0.04 independent of the surfactant type. The low-frequency (1.5 rad/s) storage modulus G', loss modulus G" and tan8 behavior revealed a gradual softening of the gel independent of the surfactant type. The exponent ((3) fast mode diffusivity (Dr) and stretched exponential mode relaxation time were found to be less sensitive to this softening transition.