FLUID MEMBRANE THERMAL FLUCTUATIONS AND BENDING CURVATURE MODULUS DETERMINATION

Extending the current theory, a new formula, concerning the average ti lt angle for a thermally undulating fluid membrane in an, on average, equally spaced stack, has been developed. For large interlayer spacing s, this new formula resembles the existing one-the tilt angle varies l ogarithmically with the interlayer spacing. When the interlayer spacin g is relatively small, the new formula differs from the existing theor y. This new formula is then used to extract the membrane bending curva ture modulus kappa for membranes in a lamellar phase made from sodium dodecyl sulfate, water, pentanol, and dodecane. The fit of the water l ayer thickness to the formula shows that the membrane has a bending cu rvature modulus of 2.1 +/- 0.5 b(B)T, with T = 300 K. The water layer thicknesses are accurately determined by combining X-ray scattering de termined interlayer spacings with sample specific gravities.