Modification of Elastic Constants by Charge Addition to a Nonionic Lamellar Phase

We have investigated a lamellar phase (Lα) composed of a nonionic surfactant tetraethylene glycol monododecyl ether (C12E4), water, and decane, where a fraction of the nonionic surfactant has been replaced by an anionic one: sodium dodecyl sulfate (SDS). Changes induced by the addition of SDS have been determined by small-angle X-ray scattering (SAXS) and observations between crossed polarizers. Investigations have been carried out in both the water-rich and the oil-rich parts of the phase diagram. Changes in the Bragg distance, the Landau-Peierls order parameter η and the elastic constants κ and B̄ have been particularly examined. In the decane-swollen Lα phase, SAXS spectra did not present any changes. The Bragg distance d B, as well as η, remained constant as the nonionic surfactant was replaced by the anionic one. On the other hand, the water-swollen Lα phase was stiffened and η was decreasing dramatically with charge addition. For the same membrane volume fraction, d B decreased slightly as SDS concentration increased. The dependence of the membrane rigidity κ = Kd B and the smectic compressibility modulus B̄ with the SDS concentration have been determined and compared with recent theoretical predictions. It was found that κ increased with SDS concentration whereas B̄ presents a maximum when the Gouy−Chapman length is equal to the half of the water thickness.