Characterization of interactions in aqueous mixtures of hydrophobically modified alginate and different types of surfactant

Rheology, turbidity, and small-angle neutron scattering (SANS) measurements have been carried out on semidilute alginate and hydrophobically modified (HM)-alginate solutions in the presence of different surfactants. The turbidity and SANS result indicate that the cationic surfactant hexadecyltrimethylammonium bromide (CTAB) and Gemini induce the formation of large inhomogeneous structures in the alginate solutions, whereas their effects on the structure of HM-alginate is less marked. The addition of the nonionic surfactant Brij 35 leads to amended solvent conditions for both alginate and HM-alginate solutions. Compared to the changes induced by cationic and nonionic surfactant, the effect of SDS on the structure of the solutions is less marked. The rheological measurements show generally small viscosity changes upon addition of the surfactants to unmodified alginate. The HM-alginate system shows a different behavior, with a marked viscosity enhancement upon addition of ionic surfactants in the surfactant concentration range probed. Most pronounced viscosification is demonstrated when CTAB and Gemini are added into the solution. This is ascribed to the cooperative effects of hydrophobic association and electrostatic attractions. In contrast, the nonionic surfactant Brij 35 leads to a decrease in the solution viscosities, indicating the break-up of hydrophobic association due to the inclusion of the hydrophobic pendants by Brij 35 micelles.