J. Phys. II France
Volume 4, Number 12, December 1994
Page(s) 2197 - 2214
DOI: 10.1051/jp2:1994256
J. Phys. II France 4 (1994) 2197-2214

Flexibility of molecular films as determined by deuterium solid state NMR

F. Auguste, P. Barois, L. Fredon, B. Clin, E. J. Dufourc and A. M. Bellocq

Centre de Recherche Paul Pascal, CNRS, Av. Albert Schweitzer, 33600 Pessac, France

(Received 9 June 1994, revised 1 August 1994, accepted 12 August 1994)

A new technique is proposed for the measurement of the bilayer bending modulus $\kappa$, in the lamellar phase L $_{\alpha}$ of lyotropic systems. It resides in the measurement of quadrupolar splittings by deuterium solid state NMR. By separating the different motional contributions accounting for the reorientation of the C-D bond with respect to the magnetic field, we have calculated within a simple Gaussian theory the effect of thermal fluctuations of the film upon the recorded quadrupolar splitting. This has been applied to the study of the effects of membrane composition on the mean bending modulus $\kappa$ in ternary and pseudo ternary systems of water-(salt)-sodium dodecyl sulfate-alcohol. It is found that $\kappa$ is sensitive both to the amount of alcohol in the membrane and to the alcohol chain length, with typical values increasing between 1.3 $k_{\rm B} \, T$ and 13.0 $k_{\rm B} \, T$ from bexanol to decanol systems. Our results allow to attribute the reduction in $\kappa$ obtained by replacing surfactant by alcohol to the thinning of the membrane and to the increase of the area occupied by the surfactant at the interface.

© Les Editions de Physique 1994