Measurement of the Membrane Flexibility in Lamellar and “Sponge” Phases of the C12E5/Hexanol/Water System

The present work is primarily concerned with the measurement, by several different methods (using X-ray and static light scattering experiments or dynamic light scattering experiments), of the elasticity of bilayers made up of n -dodecyl pentaethylene glycol monoether (C 12 E 5 ) and hexanol molecules. The membrane bending elasticity modulus, κ, has been measured for both the L α (lamellar) and the L 3 (“sponge”) phase of the C 12 E 5 /hexanol/water system. For the lamellar phase, κ was measured using both the excess area and the dynamics of the fluctuations, while for the sponge phase κ was measured using only the excess area. The values of κ measured seem to be of the same order (approximately k B T ) for both the lamellar phase and the sponge phase, although the molar ratio of hexanol to C 12 E 5 is 4 times bigger in the sponge phase. The effect of temperature on κ was also studied: the slight increase in κ found for this system with temperature seems to be related to a decreasing polar head area of the C 12 E 5 molecules. For the sponge phase, κ cannot be measured from the dynamics of the fluctuations; however, some results can be extracted from the dynamic light scattering experiments. The most interesting one was that the collective diffusion coefficient D is proportional to the membrane volume fraction, φ m, and is a function of the membrane elastic constants, κ and κ, as well as of the thermal energy k B T, leading to an estimate of κ.