Steric-Induced Effects on Stabilizing a Lamellar Structure Barbara B. Gerbelli, † Rafael L. Rubim, † Emerson R. Silva, †,§ Fre ́ de ́ ric Nallet, ‡ Laurence Navailles, ‡ Cristiano L. P. Oliveira, † and Elisabeth A. de Oliveira* ,† † Instituto de Física, Universidade de Sã o Paulo, Caixa Postal 66318, 05314-970 Sã o Paulo, Brazil ‡ Universite ́ de Bordeaux, Centre de Recherche Paul-Pascal - CNRS, 115 avenue du Docteur-Schweitzer, F-33600 Pessac, France ABSTRACT: We investigate the behavior of multilamellar phases composed of lecithin and a commercial cosurfactant (Simusol), which is a mixture of ethoxylated fatty acids. Using X-ray scattering and a new procedure to fit the data, relevant parameters characterizing the lamellar structure were deter- mined as a function of membrane composition, varying from 100% of lecithin to 100% of Simulsol. Scattering data illustrating the swelling of the lamellae for different amounts of cosurfactant are presented with the respective behavior of the Caille ́ parameter. With this experimental approach, we show that the incorporation of ethoxy brushes onto the lipid surface enhances repulsive interactions arising from membrane fluctuations and changes the interactions at the interface between bilayers. ■ INTRODUCTION Lamellar systems composed of regularly stacked bilayers have been used to investigate several biological processes, 1-4 providing important knowledge on fundamental aspects such as interactions between membranes, 5-8 and also on many biotechnological applications. 9-12 The structure is periodic only across the direction perpendicular to the membrane surface whereas in the other two dimensions the bilayers behave as a fluid. The fluidity of the membranes comes either from the orientational and translational disordering of surfactant molecules or from the flexibility of hydrocarbon chains. 13-15 Thermal fluctuations in membranes give rise to repulsive interactions as a result of the mutual steric hindrance when they are in close proximity. Such interaction plays a fundamental role for stabilizing the lamellar structure since its range and strength competes with van der Waals attraction for flexible enough bilayers. 16-19 X-ray and neutron scattering techniques have proved to be powerful tools to obtain a detailed picture of the lamellar structure. 20-28 Thermal fluctuations deeply affect the (quasi) long-range positional order, resulting in an enhanced diffuse scattering. The appropriate analysis of the scattering data brings information not only about the structure of the lamellar phase but also on its elastic properties. In this context, a major variable is the so-called Caille ́ parameter, which carries coupled information on both the bending constant of membranes and the compression modulus of the stack. 29 In previous reports, some of us demonstrated the ability of mixed lecithin/Simulsol phase to host DNA fragments and form lipoplexes from neutral lipids. 30-34 Our findings have shown that steric forces in lamellar system play a major role for complexation with DNA and hydration of the system (and consequently, its smectic periodicity) is able to drive confine- ment of nucleotides in-between bilayers. The confinement induces a rich polymorphism of supramolecular ordering of DNA-double-chains embedded in lamellar stacks. In the current work, we further investigate on steric interplay in these lecithin/Simulsol phases by using a recently developed model to analyze X-ray scattering data. 23 We investigate the stability of lamellar structures, when the range and strength of steric repulsive interactions are modulated by varying the amount of (short) amphiphilic polymers playing the role of a cosurfactant grafted at the membrane surface. The experimental approach consists in a systematic variation of the membrane composition, by adding the cosurfactant to the bilayers composed, initially, of lecithin, a common lipid. The cosurfactant, commercially known as Simusol, is a mixture of ethoxylated fatty acids with, typically, about 10 ethoxy groups. Because of its nonionic nature, the overall charge of the membrane remains neutral since lecithin is a zwitterionic molecule. The incorporation of single chain molecules to the bilayer is expected to increase its flexibility; however a second effect related to the hydrophilic part of the cosurfactant molecules is found: an extra repulsion of steric origin between membranes with an extended range depending on both grafting density and chain length, arises because water at room temperature is a good solvent for the ethoxylated part of the cosurfactant. Different domains of interactions between membranes can be experimentally accessed, by varying two experimental parameters. The first one is the hydration of the Received: August 6, 2013 Revised: October 11, 2013 Published: October 15, 2013 Article pubs.acs.org/Langmuir © 2013 American Chemical Society 13717 dx.doi.org/10.1021/la402962c | Langmuir 2013, 29, 13717-13722