Oil/water “hand-bag like structures”: how interfacial rheology can help to understand their formation?

Bicompartmental structures, named “hand-bag like structures” (HBS), where phospholipid membranes stabilise both an oily and a water phase, were firstly observed in cationic emulsions thanks to cryomicroscopy. The emulsion formulation was found to play a crucial role: no HBS were observed neither in the absence of cationic lipid nor in the presence of DOTAP, while oleylamine and DSPE-PEG were found to increase the proportion of HBS compared to stearylamine. The HLB value and the unsaturation of the lipids acyl chains then appeared as some key parameters for HBS formation. Interfacial rheology measurements have given additional information on the interfacial film such as its cohesion (Ee), interactions with the adjacent phases (Ene) and relaxation time τ which was found to be the most discriminating parameter.τ was indeed significantly longer for the samples containing no HBS. This suggests that the ability of the interface to rearrange could play a role in HBS formation.