From Vesicle Size Distributions to Bilayer Elasticity via Cryo-Transmission and Freeze-Fracture Electron Microscopy

Three methods of evaluating vesicle mean radii and polydispersity, quasi-elastic light scattering (QLS), freeze-fracture electron microscopy (FF-TEM), and cryo-transmission electron microscopy (cryo-TEM), were used to determine the size distributions of spontaneous vesicles made from mixtures of cetyltrimethylammonium tosylate (CTAT) and sodium dodecylbenzene sulfonate (SDBS). While QLS is probably the most commonly used method to size vesicles, it is limited to measures of the mean hydrodynamic radius and an estimate of the polydispersity, both of which are heavily weighted toward the largest structures in the solution. Cryo-TEM can provide the entire size distribution of the outer diameters of spherical vesicles, from which the sum of the Helfrich bilayer elastic parameters, K = κ + κ/2 and the spontaneous curvature radius, R 0, can be determined. FF-TEM can provide the number-average mean diameter and polydispersity once the influence of the fracture plane has been factored into the distribution, thereby confirming the cryo-TEM size distribution. For 7:3 wt CTAT/SDBS at 1% total surfactant in water, K = κ + κ/2 = 0.15 ± 0.03 k B T and R 0 = 55 nm ± 10 nm. For CTAT/SDBS, w/w, at 2% total surfactant, K = 0.54 kT ± 0.05 k B T and R 0 = 36 nm ± 1 nm. We find that surfactant mixing is likely the origin of the low bilayer elasticity in catanionic vesicles. However, the lower value of K in the CTAT-rich sample is likely due to the hydrophobic tosylate counterion increasing the area per headgroup.