Effects of surfactant and temperature on rheological and structural properties of semidilute aqueous solutions of unmodified and hydrophobically modified alginate

The dynamic and structural perturbations that result from the interactions between the anionic surfactant sodium dodecyl sulfate (SDS) and the hydrophobically modified biopolymer alginate (HM-alginate) have been studied with the aid of rheological methods, turbidimetry, and small-angle neutron scattering (SANS). The rheological results for a semidilute HM-alginate solution in the presence of SDS disclose strong interactions between HM-alginate and SDS at a low level of surfactant addition, and this feature is accompanied by enhanced turbidity. At higher surfactant concentrations the association complexes are disrupted. A strong temperature effect of the viscosity is observed in HM-alginate solutions at moderate SDS concentrations, where an elevated temperature leads to enhanced chain mobility, which promotes a breakup of the association complexes. The SANS results reveal a pronounced peak in the plot of scattered intensity versus wavevector q at intermediate q values for SDS concentrations above the critical micelle concentration (cmc). With contrast-matching conditions, using deuterated SDS instead of SDS, no interaction peak appears but an "upturn" in the scattered intensity is observed at small q value. The magnitude of this effect decreases with increasing surfactant concentration, showing clearly that SDS is capable of breaking up the large aggregates created.