Flexibility of elongated sodium dodecyl sulfate micelles in aqueous sodiumchloride: A small-angle neutron scattering study

Aqueous salt solutions containing elongated micelles of sodium dodecyl sulf ate (SDS) have been studied at 45 degreesC using small-angle neutron scatte ring. At NaCl concentrations in the 1-2 M range, the scattering data are co nsistent with semiflexible rather than rigid rodlike micelles, and this con clusion is consistent with previous studies of SDS micelles using small-ang le scattering. However, until now quantitative determinations of micellar f lexibility, that is, of persistence lengths, especially as a function of io nic strength, have not been available. By making measurements in the range of scattering vectors appropriate for this length scale and by incorporatin g the effect of intermicellar interactions into the fitting protocol, it ha s been possible to determine persistence lengths accurately. Partitioning o f the values into intrinsic and electrostatic components is discussed, and the results are compared with data for the highly flexible anionic polyelec trolyte sodium polystyrenesulfonate. The one-dimensional bending moduli for the flexible SDS micelles are compared to the analogous two-dimensional be nding moduli for SDS bilayers.