Interlayer distance dependence of thickness fluctuations in a swollen lamellar phase

Thickness fluctuations in a swollen lamellar structure, composed of a non-ionic surfactant, water and oil have been characterized by means of small-angle neutron scattering (SANS) and neutron spin echo (NSE) experiments, and coarse-grained molecular dynamics (CGMD) simulation. The static and dynamic structures of the membranes are measured as a function of the interlayer distance (membrane thickness), d(m). The oil to surfactant volume ratio is changed at a constant surfactant volume fraction, so that d(m) is changed while maintaining the inter-lamellar repeat distance constant in the experiments. Two relaxation modes are observed from the NSE data, which are the bending motion and the thickness fluctuations. The bending rigidity of the membranes is a function of the membrane thickness. At low d(m) the membranes become rigid due to the enhancement of the thickness fluctuations, while at large d(m) the membranes tend to be flexible because of the decrease in the synchronization between the two interface layers. The thickness fluctuations are measured by NSE as an excess dynamics from the bending motion around the length scales of the membrane thickness, and a similar excess dynamics is observed in the CGMD simulation. Moreover, a method to estimate the thickness fluctuation amplitude in the experiment is proposed, and the validity of the method is verified by the simulation. An excellent agreement between the experiments and the simulations shows that the amplitude is about 12% of the membrane thickness and almost linearly increases with d(m). The present result shows the importance of the intra-membrane dynamics to determine the elastic properties of membranes, and the feasibility of the measurement of thickness fluctuations in surfactant membranes using NSE experiments and MD simulations.