Effect of interlamellar interactions on shear induced multilamellar vesicle formation

Shear-induced multilamellar vesicle (MLV) formation has been studied by coupling the small-angle neutron scattering (SANS) technique with neutron spin echo (NSE) spectroscopy. A 10% mass fraction of the nonionic surfactant pentaethylene glycol dodecyl ether (C₁₂E₅) in water was selected as a model system for studying weak inter-lamellar interactions. These interactions are controlled either by adding an anionic surfactant, sodium dodecyl sulfate, or an antagonistic salt, rubidium tetraphenylborate. Increasing the charge density in the bilayer induces an enhanced ordering of the lamellar structure. The charge density dependence of the membrane bending modulus was determined by NSE and showed an increasing trend with charge. This behavior is well explained by a classical theoretical model. By considering the Caillé parameters calculated from the SANS data, the layer compressibility modulus B¯ is estimated and the nature of the dominant inter-lamellar interaction is determined. Shear flow induces MLV formation around a shear rate of 10 s^-1, when a small amount of charge is included in the membrane. The flow-induced layer undulations are in-phase between neighboring layers when the inter-lamellar interaction is sufficiently strong. Under these conditions, MLV formation can occur without significantly changing the inter-lamellar spacing. On the other hand, in the case of weak inter-lamellar interactions, the flow-induced undulations are not in-phase, and greater steric repulsion leads to an increase in the inter-lamellar spacing with shear rate. In this case, MLV formation occurs as the amplitude of the undulations gets larger and the steric interaction leads to in-phase undulations between neighboring membranes.