Large water soluble macromolecules can be confined in all proportions into an electrostatically stabilized surfactant lamellar phase without change of the bilayer thickness. Upon progressive addition of salt, the sample phase separates into a polymer-rich and a surfactant-rich lamellar phase. Indeed, at some particular salt concentration, the intermembrane repulsive interaction is no longer sufficient to maintain the polymer coils squeezed into the lamellar structure. The phase separation is generally first order. But we could find a situation where it becomes second order. PACS numbers: 82.70.Dd, 36.20.Cw, 64.70.Md Amphiphilic molecules in aqueous solutions often selfassemble into very large bilayer membranes [1-3] regularly stacked parallel to each other. The resulting lamellar phase is birefringent and shows quasi-long-range smectic order. It has been shown experimentally [4] that for charged bilayers (ionic amphiphiles) in pure water the smectic order is stabilized by strong long-range electrostatic interaction, whereas in high salt brine (few 10 -, A/) the electrostatic interaction is screened beyond distances shorter than the smectic periodicity so that the stability of the phase only arises from the much weaker steric repulsion between membranes [5].On the other hand, the behavior of macromolecules confined into small pores or thin slits (of size much smaller than the natural radius of gyration of the unconfined coil) has been studied theoretically quite long ago [6]. However, experimental realizations of such situations that are convenient for the measurements of relevant quantities are not easy to achieve [7], So, at the present time, we are not aware of any convincing experimental checks of the theoretical conjecture. However, if some hydrosoluble polymer could be incorporated into the L a phase with suitable periodicity, one could have a chance to realize a situation somewhat similar to the confinement in infinite slits. But the only appropriate case would be one where (i) the polymer does not penetrate through the bilayers, so that the confinement is effective; (ii) the polymer shows no specific interaction with the membrane so that the strong adsorption regime is avoided. This idea is at the basis of the present study.We incorporate a large hydrosoluble polymer into an L a phase made of charged bilayers. If the polymer is effectively confined in between the bilayers, some additional contribution to the effective interaction between the bilayers will come into play. Two different physical situations may arise in the case of a semidilute solution of nonadsorbing polymers trapped in small slits [6].(i) In the three dimensional regime, the size of the slits is much larger than the three dimensional correlation length £; in this case the corresponding free energy of confined polymers consists of a "bulk" contribution and a term associated with the depletion of polymers near the surface.(ii) In the two dimensional regime, the size of the slits is much smaller than the three dimensional correlation lengt...
Small angle X-ray and neutron scattering data on an effective three-component lamellar phase composed of water, a non adsorbing water-soluble polymer (polyvynilpyrolidone), fluid membranes, made from a mixture of a cationic surfactant (cetylpiridiumchloride) and a cosurfactant (hexanol), are presented for various membrane as well as polymer concentrations. The data are fitted with a recently proposed model which takes into account the geometry and the fluctuations of these periodic structures. This allows a quantitative study of the polymer contribution to the smectic compression modulusB of the lamellar phase. Four different regimes of polymer confinement are expected. The associated variations inB are compared to a recent theoretical model, which predicts the polymer-mediated contribution to the smectic compression modulus.
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