Tuning the viscoelasticity of nonionic wormlike micelles with β-cyclodextrin derivatives: a highly discriminative...

We report the influence of five β-cyclodextrin (β-CD) derivatives, namely: randomly methylated β-cyclodextrin (MBCD), heptakis (2,6-di-O-methyl)-β-cyclodextrin (DIMEB), heptakis (2,3,6-tri-O-methyl)-β-cyclodextrin (TRIMEB), 2-hydroxyethyl-β-cyclodextrin (HEBCD) and 2-hydroxypropyl-β-cyclodextrin (HPBCD), on the self-assembly of mixtures of nonionic surfactants: polyoxyethylene cholesteryl ether (ChEO10) and monocaprylin (MCL). Mixtures of ChEO10/MCL in water form highly viscoelastic wormlike micelle solutions (WLM) over a range of concentrations; herein, the composition was fixed at 10 wt % ChEO10/3 wt % MCL. The addition of methylated β-CDs (MBCD, DIMEB, TRIMEB) induced a substantial disruption of the solid-like viscoelastic behavior, as shown from a loss of the Maxwell behavior, a large reduction in G' and G″ in oscillatory frequency-sweep measurements, and a drop of the viscosity. The disruption increased with the degree of substitution, following: MBCD < DIMEB < TRIMEB. Cryo-TEM images confirmed a loss of the WLM networks, revealing short rods and disc-like aggregates, which were corroborated by small-angle neutron scattering (SANS) measurements. Critical aggregation concentrations (CAC), measured by fluorescence spectroscopy, increased in the presence of DIMEB for both ChEO10 and MCL, suggesting the existence of interactions between methylated β-CDs and both surfactants involved in WLM formation. Instead, hydroxyl-β-CDs had a very different effect on the WLM. HPBCD only slightly reduced the solid-like behavior, without suppressing it. Quite remarkably, the addition of HEBCD reinforced the solid-like characteristics and increased the viscosity 10-fold. Cryo-TEM images confirmed the subsistence of WLM in ChEO10/MCL/HEBCD solutions, while SANS data revealed a slight elongation and thickening of the worms, and an increase of associated water molecules. CAC data showed that HPBCD had little effect on either surfactant, while HEBCD strongly affected the CAC of MCL and only slightly affected the ChEO10. For both DIMEB and HEBCD, time-resolved SANS measurements showed that morphology changes underlying these macroscopic changes occur in less than 100 ms.