Self-assembly of sodium bis(2-ethylhexyl) sulfosuccinate in aqueous solutions: Modulation of micelle structure and interactions by cyclodextrins investigated by small-angle neutron scattering

The modulating effects of α-cyclodextrin (α-CD) and hydroxypropyl β-cyclodextrin (HP-β-CD) on micelles formed in water by the double tail surfactant sodium bis(2-ethylhexyl) sulfosuccinate (also known as dioctyl sulfosuccinate (DOSS) or Aerosol-OT (AOT)) have been investigated using small angle neutron scattering (SANS). SANS data were fitted to the core-shell oblate ellipsoid form factor and Hansen-Hayter-based structure factor in order to quantify the effect of cyclodextrins on the micelle structure and inter-micelle interactions. The results suggested that addition of cyclodextrins caused a sharp decrease in surfactant aggregation numbers, micelle dimensions and micelle charge. The micelle shape changed from elongated oblate to sphere upon addition of cyclodextrin at stoichiometric ratio of 1:1 CD:AOT for HP-β-CD and 2:1 for α-CD. HP-β-CD was found more effective than α-CD in influencing the micelle properties. For example, the decrease in surfactant aggregation number upon addition of 1 mM CD at constant total surfactant concentration was about double for HP-β-CD compared to α-CD. The results are consistent with the localization of small amount of cyclodextrins in the micelle, while the remainder stays in the solution pseudophase. The present study offers an improved understanding on the modulating effects of CDs on micelle structure and enables better design of emulsifiers or dispersants in formulations suitable for controlled release of actives.