Stability of Brine Swollen Lamellar Phases

Abstract

Swollen lamellar phases show long-range smectic order and consist of thin lamellae with a wide lateral extension separated by very large average distances B. Correlatively their stability involves two successive requirements. First the individual membrane must be the optimum morphology for the aggregation of the surfactant in the given experimental conditions. And secondly, some minimum interaction potential (ΔF/A) between membranes must be present in order to induce the long-range smectic order. This second more collective aspect of the stability of the L_α swollen phase is presently the subject of intense theoretical and experimental investigations: in particular several contributions to this conference (W.Helfrich; E.Evans; D.Roux; D.Sornette; S.Leibler; R.Lipowsky) are devoted to the relevance of the Helfrich steric interaction in the effective intermembrane forces. Nevertheless, the first individual aspect is also important. It is related to the morphological transformations of the individual membrane when going into the phases that are adjacent to L_α in the diagrams. In general, the membrane morphology and the intermembrane interaction are both affected by the variations of any composition variable of the system. And it is usually difficult to define the adequate procedure to investigate separately these two intricate features of the stability.