Properties of surfactant formulations are regularly modified by cosurfactant addition, resulting in systematic control of structure and properties. The structural changes are supposed to proceed by changing continuously the packing parameter of the self-assembled aggregates. Accordingly, cosurfactant addition to spherical micelles transforms them into increasingly longer wormlike micelles and at still higher cosurfactant concentration to bilayers by proceeding through a first order phase transition.
In this work we show a substantially different self-assembly behaviour, seen for a classical nonionic surfactant (Tween 20) upon addition of 2-ethylhexylglycerol (EHG) as cosurfactant. Structural characterisation by light and neutron scattering (SLS, DLS, SANS), and especially cryo-TEM, shows that here elongated assemblies are formed. However, in contrast to normal expectation, they are composed of individual micelles that are locally ordered in a cylindrical fashion, which also explains the much lower viscosity observed in comparison to normally expected worm-like micelles (WLM). A first-order phase transition takes place at higher cosurfactant content, but here only a smaller fraction of amphiphile initially forms a bilayer structure and complete transformation to bilayers occurs within the single-phase region for EHG concentrations much higher than the ones of the phase transition.
This very uncommon structural evolution has not been reported before, but can be rationalised by the particular structure of the amphiphilic molecules involved. This work was then extended to other nonionic surfactants and cosurfactants with systematic variation of the head group size. As a result, the systematic variation of the phase behaviour can be assigned to the molecular architecture of the amphiphiles.