Tuning of the aggregation number of Platonic micelles with a binary mixture of calix[4]arene surfactants

The mixture ratios were used as a factor for aggregation number (N_agg) determination in binary mixtures of calix[4]arene surfactants.

Monitoring the Discontinuous Dodecamer-Icosamer Transition of a Calix[4]arene-Derived Surfactant by Time-Resolved Small-Angle X-ray Scattering

Calix[4]arene-derived surfactants form monodisperse micelles with a well-defined aggregation number (N_agg ) of 4, 6, 8, 12, or 20, corresponding to the Platonic solids. This feature is in strong contrast to conventional micelles. In this study, a transition from a dodecamer (N_agg =12) to an icosamer (N_agg =20) was induced by a rapid increase in the NaCl concentration (C_NaCl ) using a stopped-flow device and directly observed by time-resolved small-angle X-ray scattering. The N_agg remained unchanged during the first 60 s after the increase in C_NaCl  , and then abruptly increased to 20. This feature resembles phase transitions in supersaturated or supercooled states, or highly cooperative phenomena. We surmise that this finding may be due to the fact that only a few N_agg values are thermodynamically allowed when N_agg is sufficiently small. This is the first observation of such an induction time in micellar aggregation.