Rupture of Pluronic Micelles by Di-Methylated β-Cyclodextrin Is Not Due to Polypseudorotaxane Formation

Spectroscopic measurements (uv/vis absorbance and fluorescence) and time-resolved small-angle neutron scattering experiments (TR-SANS) were used to follow the breakdown of Pluronic micelles by heptakis(2,6-di-<i>O</i>-methyl)-β-cyclodextrin (DIMEB) over time in order to elucidate the mechanism of micellar rupture, generally attributed to polypseudotorotaxane (PR) formation between the cyclodextrin and the central hydrophobic PPO block. The spectroscopic measurements with two different probes (methyl orange and nile red) suggest that very rapid changes (on the order of seconds) take place when mixing DIMEB with F127 Pluronic and that no displacement of the probe from the cyclodextrin cavity occurs, which is in disagreement with PR formation. TR-SANS measurements demonstrate for the first time that the micelles are broken down in less than 100 ms, which categorically rules out PR formation as the mechanism of rupture. In addition, the same mechanism is demonstrated with other Pluronics, P85 and P123. In the latter case, after micellar rupture, lamellar structures are seen to form over a longer period of time, thus suggesting that after the instantaneous micellar disruption, further, longer-scale rearrangements are not excluded

Remarkable Viscoelasticity in Mixtures of Cyclodextrins and Nonionic Surfactants

We report the effect of native cyclodextrins (α, β, and γ) and selected derivatives in modulating the self-assembly of the nonionic surfactant polyoxyethylene cholesteryl ether (ChEO₁₀) and its mixtures with triethylene glycol monododecyl ether (C₁₂EO₃), which form wormlike micelles. Cyclodextrins (CDs) generally induce micellar breakup through a host–guest interaction with surfactants; instead, we show that a constructive effect, leading to gel formation, is obtained with specific CDs and that the widely invoked host–guest interaction may not be the only key to the association. When added to wormlike micelles of ChEO₁₀ and C₁₂EO₃, native β-CD, 2-hydroxyethyl-β-CD (HEBCD), and a sulfated sodium salt of β-CD (SULFBCD) induce a substantial increase of the viscoelasticity, while methylated CDs rupture the micelles, leading to a loss of the viscosity, and the other CDs studied (native α- and γ- and hydroxypropylated CDs) show a weak interaction. Most remarkably, the addition of HEBCD or SULFBCD to pure ChEO₁₀ solutions (which are low-viscosity, Newtonian fluids of small, ellipsoidal micelles) induces the formation of transparent gels. The combination of small-angle neutron scattering, dynamic light scattering, and cryo-TEM reveals that both CDs drive the elongation of ChEO₁₀ aggregates into an entangled network of wormlike micelles. ¹H NMR and fluorescence spectroscopy demonstrate the formation of inclusion complexes between ChEO₁₀ and methylated CDs, consistent with the demicellization observed. Instead, HEBCD forms a weak complex with ChEO₁₀, while no complex is detected with SULFBCD. This shows that inclusion complex formation is not the determinant event leading to micellar growth. HEBCD:ChEO₁₀ complex, which coexists with the aggregated surfactant, could act as a cosurfactant with a different headgroup area. For SULFBCD, intermolecular interactions via the external surface of the CD may be more relevant