Eur. Phys. J. B 9, 123–136 (1999) T HE EUROPEAN P HYSICAL JOURNAL B c  EDP Sciences Societ` a Italiana di Fisica Springer-Verlag 1999 Softening of the interactions between surfactant bilayers in a lamellar phase due to the presence of a polymer E. Freyssingeas 1, a , D. Antelmi 1 , b , P. K´ ekicheff 1, c , P. Richetti 2 , d , and A.-M. Bellocq 2 1 Department of Applied Mathematics, Research School of Physical Sciences and Engineering, Australian National University, Canberra, A.C.T. 0200, Australia 2 Centre de Recherche Paul Pascal - CNRS, Avenue A. Schweitzer, 33600 Pessac, France Received 2 February 1998 Abstract. The compressibility modulus of a lamellar phase containing a neutral polymer guest molecule was measured directly using a surface force apparatus. The system studied consisted of sodium dodecyl sulphate (SDS), pentanol, water and polyethylene glycol (PEG) (Mw = 22 600 g/mol). The lamellar phase was induced from a micellar phase in situ via a confinement induced isotropic to lamellar phase transition. This avoided problems resulting from the viscosity and turbidity normally characteristic of these lamellar phase samples. Increasing the amount of PEG resulted in a marked decrease in the layer compressibility modulus ¯ B indicating a decrease in the repulsive forces between the lamellae. The origin of such a phenomenon is discussed in terms of different mechanisms including depletion interactions, bridging interactions and modification of the electrostatic interaction between the lamellae by the polymer. PACS. 82.70.-y Disperse systems – 64.70.Md Transitions in liquid crystals – 61.25.Hq Macromolecular and polymer solutions; polymer melts; swelling 1 Introduction Macromolecules such as polymers or proteins may alter significantly the properties of self- assembled structures of amphiphilic molecules in solution [1–6]. Examples in- clude the enhanced stability of red blood cell membranes by their cytoskeleton, the adhesion of vesicles induced by polymers and the rigidification of droplet interfaces in emulsions by proteins [3–6]. Many industrial processes take advantage of these modified properties in the pres- ence of macromolecules, e.g. to get colloidal stabilisation, to favour a particular state of surfactant organisation (e.g. fabric conditioners), to enhance the purification of pro- teins (by phase separation) or to affect the rheological re- sponse (lubricants) [1,2]. From the fundamental research point of view, most of the surfactant-polymer investiga- tions have focused on dilute systems of surfactant [1]. a Present address: Laboratoire de physique, ´ Ecole Normale Sup´ erieure de Lyon, 69364 Lyon Cedex 07, France. e-mail: efreyssi@physique.ens-lyon.fr b Present adress: ´ Equipe Mixte CEA-RhˆonePoulenc, Service de Chimie Mol´ eculaire, CEA Saclay, 91191 Gif/Yvette Cedex, France. c Present address: Institut Charles Sadron - CNRS, 6 rue Boussingault, 67083 Strasbourg Cedex, France. d Present address: Complex Fluids Laboratory - CNRS- Rhˆ one Poulenc, UMR 166, Prospect Plains Road, Cranbury, N.J. 08512, USA. In particular the conformation of polymeric chains in an isotropic environment of mixed micellar solutions is now firmly established. However, in recent years an interest has been increasingly directed towards the study of polymers effects on more concentrate surfactant solutions, especially on the lamellar phases (L α phase) [7–20]. The lamellar phases are Liquid-Crystal Smectic A phases which con- sist in planar stacks of bilayers, containing surfactant and cosurfactant molecules, separated by a solvent. The prop- erties of these phases are now fairly well-known [21]. Such surfactant lamellar phase-polymer systems are very inter- esting to study since a diverse set of physical situations can be realised. Different properties and behaviours can be achieved by playing with the inter-membrane inter- actions and the surfactants (and/or solvent)/polymer in- teractions. Depending on the polymer interactions with the surfactants (and/or the solvent), the polymer can be localised entirely in the membrane [9,20], both in the membrane and in the solvent [8], adsorbed onto the bi- layer surface [13,16,17], or localised entirely in the solvent [11, 12, 19]. Therefore, the addition of a polymer to a lamel- lar phase can influence the inter-membrane interactions by altering the structure of the bilayers (i.e. the bilayer thickness and the area per surfactant head group) and the elastic constants of the membranes. Moreover, the poly- mer can induce an additional inter-membrane interaction via depletion [11,12] or bridging [17]. The inter-membrane interactions can be tuned by the choice of the surfactant