X-Ray Study Of Freely Suspended Films Of A Multilamellar Lipid System

Abstract

We report the first x-ray scattering measurements from freely suspended lipid (DMPC)-water multilayer films. By combining several techniques, we are able to grow stable, oriented films of various thicknesses (∼10² to 10⁴ Å) in a controlled temperature (0°C to 80°C) and humidity (0% to 100%) environment. A specially designed film holder allows both transmission and reflection x-ray measurements. Very importantly, the humidity control system allows us to construct the temperature-chemical potential phase diagram in addition to the normally studied temperature-concentration phase diagram. Our most important results are as follows. First, we find a metastable L_α phase in bulk DMPC-water mixtures when the water concentration is between 30 and 40 weight percent, and second, from the variation of the interplanar peak position as a function of concentration and chemical potential, we are able to deduce the functional form of the interaction force between layers in the L_α. phase. This form is found to be consistent with a repulsive short range hydration force. Independently, x-ray reflectivity measurements of eight harmonics of the structure factor in the L_α phase are consistent with the existence of strong short range interactions between the layers.