Significance: Life requires a delicate balance between cell membrane rigidity and fluidity to define cell boundaries yet enable cell deformation, growth, and division. Here, we bridge the molecular and macroscopic scales with neutron scattering techniques to demonstrate how the material properties of model lipid membranes are readily tuned by the constituent lipid composition. Mixing lipids leads to softer and less viscous membranes than predicted a priori from the pure component membrane properties. Instead, the dynamics are linked to the membrane structure and follow simple scaling relationships with the area per molecule. These results emphasize the importance of lipid packing and reveal the interwoven relationship between lipid membrane structure and dynamics.

The phospholipid lecithin (L) and the nonionic surfactant Tween 80 (T) are used together in various contexts, including in drug delivery and oil spill remediation. There is hence a need to elucidate the nanostructures in LT mixtures, which is the focus of this paper. We study these mixtures using cryogenic transmission electron microscopy (cryo-TEM), coupled with dynamic light scattering and small-angle neutron scattering. As the concentration of Tween 80 is increased, the vesicles formed by lecithin are transformed into spherical micelles. We identify bicelles (i.e., disc-like micelles) as well as cylindrical micelles as the key stable nanostructures formed at intermediate L/T ratios. The bicelles have diameters ∼13–26 nm, and the bicelle size decreases as the Tween 80 content increases. We propose that the lecithin lipids form the body of the discs, while the Tween 80 surfactants occupy the rims. This hypothesis is consistent with geometric arguments because lecithin is double-tailed and favors minimal curvature, whereas the single-tailed Tween 80 molecules prefer curved interfaces. In the case of cylindrical micelles, cryo-TEM reveals that the micelles are short (length < 22 nm) and flexible. We are able to directly visualize the microstructure of the aggregates formed by lecithin–Tween 80 mixtures, thereby enhancing the understanding of morphological changes in the lecithin–Tween 80 system.

A description and the performance of the very small angle neutron scattering diffractometer at the National Institute of Standards and Technology are presented. The measurement range of the instrument extends over three decades of momentum transfer q from 2 × 10⁻⁴ to 0.7 Å⁻¹. The entire scattering angle range from 8 × 10⁻⁵ to π/6 rad (30°) can be measured simultaneously using three separate detector carriages on rails holding nine 2D detector arrays. Versatile choices of collimation options and neutron wavelength selection allow the q resolution and beam intensity to be optimized for the needs of the experiment. High q resolution is achieved using multiple converging-beam collimation with circular pinholes combined with refractive lenses and prisms. Relaxed vertical resolution with much higher beam intensity can be achieved with narrow slit collimation and a broad wavelength range chosen by truncating the moderator source distribution below 4 Å with a Be crystalline filter and above 8 Å with a supermirror deflector. Polarized beam measurements with full polarization analysis are also provided by a high-performance supermirror polarizer and spin flipper, capable of producing flipping ratios of over 100, along with a high-efficiency ³He polarization analyzer.

Neutron spin-echo (NSE) spectroscopy to measure the bending fluctuations of large unilamellar vesicles having an asymmetric transbilayer distribution of high- and low-melting lipids. Image by Kayle Kathleen Marie Gorospe of the University of Windsor Science Meets Art program.