Supramolecular gels of cholesterol-modified gellan gum with disc-like and worm-like micelles

Abstract

The association between a hydrophobically modified polysaccharide, gellan gum, with micelles based on a surfactant bearing the same hydrophobic tail as pendant groups was investigated by rheology and small-angle neutron scattering (SANS). Gellan gum grafted with cholesterol groups (20% mol/mol tetrasaccharide unit), GeCh, was mixed with polyoxyethylene cholesteryl ether (ChEO10), which comprises a cholesterol group as the tail linked to a small polyoxyethylene headgroup, and self-assembles into micelles with an unusual disc-like morphology. The addition of 0.5% polymer to solutions of ChEO10 induced a remarkable transition from a Newtonian fluid to a predominantly solid-like viscoelastic behaviour, leading to a ×105 increase in zero-shear viscosity (with 5% ChEO10). Increasing surfactant concentration led to an enhancement of the viscoelasticity, but the elastic modulus G′ reached a plateau around 15% surfactant, attributed to a saturation of the sticker groups. The effect of micellar morphology on the network was studied by adding a small headgroup co-surfactant, triethylene glycol monododecyl ether, to ChEO10 micelles, which drives their elongation into wormlike micelles. Networks obtained with the long, flexible micelles displayed enhanced solid-like behaviour, with no cross-over between G′ and G″ over the measured range of frequencies, reflecting relaxation times of the order of minutes or hours. The morphology of the gels studied by SANS revealed a scattering dominated by strongly interacting micelles (described by discs of 140 Å diameter and a hydrated ∼38 Å PEO corona) and the presence of micellar clusters induced by the presence of the polymer. The scattering data therefore confirm that the onset of gelation is due to surfactant micelles acting as junction points for the network.