EXPERIMENTAL-STUDY OF UNDULATION FORCES IN A NONIONIC LAMELLAR PHASE

The osmotic stress technique was used to measure the force between the surfactant bilayers in a nonionic lamellar phase. The bilayer repeat distance, d, was varied in the range 90 < d < 250 Angstrom, correspond ing to volume fractions between 0.12 and 0.33. Since the bilayers are uncharged and sufficiently far apart, the only relevant force is the s teric undulation force. The obtained relation between force and bilaye r separation was compared to Helfrich's expression for fluctuating mem branes, and a good agreement was found in the whole concentration rang e. Together with the qualitative proof of Helfrich's expression we als o obtained a realistic value of the bending modulus kappa = (2.5 +/- 0 .5)k(B)T.