The influence of hydrophobically modified cyclodextrins (HM-CD) on the aggregation behaviour of surfactants

Cyclodextrins and hydrophobically modified β-cyclodextrins (HM-CD) form inclusion complexes with hydrophobic molecules, such as alkylcarboxylic acids or hydrocarbons. HM-CD are also able to form micelles above a critical concentration. In this work, an effect is described which is not based on this known behaviour. This work shows the influence of HM-CD on the aggregation behaviour of viscoelastic surfactant solutions in the L1-phase. For the measurements, several surfactants were used which show very strong viscoelastic properties. The zero shear viscosity of these solutions decreases by adding HM-CD at a molar ratio of surfactant/HM-CD of 100:2 from a very high value (103 Pa·s) nearly to the viscosity of water. The high ratio of surfactant/HM-CD shows that this effect cannot depend on the formation of inclusion complexes. Light scattering and small angle neutron scattering experiments showed that the addition of HM-CD causes a transition of worm-like micelles to spherical micelles in these solutions. The HM-CD is solubilized in the interior of the micelles. With the solubilization into the micelles, a single CD-molecule is able to influence the aggregation behaviour of many surfactant molecules and thereby the shape of the micelles. HM-CD have therefore the same effect on micellar solutions, like solubilized hydrocarbons.