Non-canonical self-assembly of surfactant/cosurfactant mixtures detected by combining the power of SANS and cryo-TEM

Abstract

Hypothesis: Addition of cosurfactants to surfactant micelles is normally assumed to lead, via a continuous change of the packing parameter, to a transition of spherical micelles to elongated ones, then to wormlike viscoelastic micelles and finally, via a phase transition, to planar lamellae. However, this conventional structural sequence may be different, if surfactants with variable head group size and cosurfactants that favour strongly planar structures are employed.

Experiments: A phase study was done on solutions of the nonionic surfactant Tween-20 (Tw20) with increasing amounts of added cosurfactant 2-ethylhexyl glycerine (EHG). This study was supported by a detailed structural characterisation of the aqueous solutions as a function of added amount of EHG by a combination of small-angle neutron scattering (SANS) and cryo-TEM, which allows uniquely to assign the structures present.

Findings: We found that the initially present small Tw20 micelles become somewhat larger and more oblate with increasing EHG concentration. However, they do not form wormlike micelles, but instead these small oblate micelles arrange themselves into wormlike aggregates of individual micelles, which only show low viscosity. For higher EHG concentration, a two-phase region is found, above which still such wormlike aggregates of small micelles are still observed, which are in equilibrium with bilayer structures. Only at much higher EHG concentrations pure bilayer structures are formed. The latter effect is ascribed to the intrinsic molecular polydispersity of the surfactant head group. However, the overall assembly behaviour arises from having a surfactant that favours to have a highly curved interface, while the cosurfactant favours a rather flat structure.

Keywords: Cryo-TEM; Micelle; Phase behaviour; SANS; Surfactant science.