  Salt is a very common molecule in aqueous environments but the question of
whether the interactions of monovalent ions Na^+ and Cl^- ,with the neutral
heads of phospholipids are impactful enough to change the membrane rigidity is
still a mystery. To provide a resolution to this long simmering debate, we
investigated the dynamics of DOPC vesicles in the fluid phase with increasing
external salt concentration. At higher salt concentrations, we observe an
increase in bending rigidity from neutron spin echo spectroscopy (NSE) and an
increase in bilayer thickness from small-angle X-ray scattering (SAXS). We
compared different models to distinguish membrane undulations, lipid tail
motions and the translational diffusion of the vesicles. All the models
indicate an increase in bending rigidity by a factor of 1.3 to 3.6. We
demonstrate that even for t > 10 ns, and for Q > 0.07 1/{\AA} the observed NSE
relaxation spectra is clearly influenced by the translational diffusion of the
vesicles. For t < 5 ns, the lipid tail motions dominate the intermediate
dynamic structure factor. As the salt concentration increases this contribution
diminishes. We introduced a new time-dependent analysis for the bending
rigidity that highlights only a limited Zilman-Granek time window where the
rigidity is physically meaningful.
