M. Nagao, S. Chawang, and T. Hawa, Soft Matter 7, 6598 (2011).

1. Surfactants are one of the basic building blocks of nano-scale self-assembling structures, and several applications have been identified, from household and pharmaceutical to biological and industrial products. Collective movements of surfactants are key to understanding the physics of membranes. Bending motions and thickness fluctuations are the subjects of this investigation. Small-angle neutron scattering (SANS) and neutron spin echo (NSE) experiments using CHRNS-supported instruments, together with molecular dynamics (MD) simulations, have been used to study the dependence of membrane thickness fluctuations on the inter-monolayer distance dm. The MD and SANS data confirm effective changes in dm with changes in the volume fraction of oil to surfactant, R, as shown in the top right panel. The dynamics measurement with NSE reveals anomalous behavior of the bending modulus k with R, shown in the bottom left panel. The increase in k, at R≈0.3, suggests an effective increase of the membrane rigidity due to an increase in the membrane thickness fluctuations. Further increases in dm cause a decrease in k, which corresponds to a reduction in the synchronization between two surfactant monolayers. The thickness fluctuation amplitude ATF, estimated in two different ways, is shown in the bottom right panel. The good agreement suggests that the method of estimating ATF is reasonable. Interlayer distance dependence of thickness fluctuations in a swollen lamellar phase M. Nagao1,2, S. Chawang3, and T. Hawa3 1NIST, 2Indiana University, 3The University of Oklahoma Dan Neumann, National Institute of Standards and Technology, DMR 0944772 M. Nagao, S. Chawang, and T. Hawa, Soft Matter 7, 6598 (2011).