LAYER COMPRESSION MODULUS OF ELECTROSTATICALLY STABILIZED LYOTROPIC LAMELLAR PHASES REVISITED - APPLICATION TO THE QUANTITATIVE-ANALYSIS OFA POLYMER-INDUCED CRITICAL SMA-SMA PHASE-SEPARATION

We present a calculation of the smectic compression modulus (B) over b ar of a lyotropic lamellar phase stabilized by electrostatic interacti ons. This calculation, based on a numerical solution of the Poisson-Bo ltzmann equation, shows that the classical analytical expressions of ( B) over bar given in the literature in the two asymptotic limits (abse nce of added salt, or large added salt concentration) are valid under very restrictive conditions only. The motivation of this study lies in the understanding of the physical behavior of a doped amphiphilic lam ellar phase formed from a mixture of non ionic surfactants (Triton X10 0/Triton X35), cationic surfactant (cetylpyridinium chloride), water, decane and water-soluble polymer (polyvynilpyrolidone). A critical lam ellar/lamellar phase separation observed in our system is quantitative ly well-explained by using both the present calculations and a recent model proposed by Ligoure et al. which predicts the polymer-mediated i ntermembrane interaction contribution to the smectic compression modul us.