Dynamic Friction by Polymer/Surfactant Mixtures Adsorbed on Surfaces Linmao Qian, † Magali Charlot, and Eric Perez* Laboratoire de Physique Statistique de l’Ecole Normale Supe ´ rieure, UMR8550, 24 rue Lhomond, 75231 Paris Cedex 05, France Gustavo Luengo,* Anne Potter, and Colette Cazeneuve L’Oreal Recherche, 1 AVenue Euge ` ne Schueller, 93601 Aulnay sous Bois, France ReceiVed: June 3, 2004; In Final Form: September 23, 2004 A cationic polyelectrolyte was adsorbed on mica from highly concentrated solutions. The friction and surface force behaviors of the adsorbed layers in aqueous media were studied using a new homemade surface force apparatus (SFA). The long-range repulsions produced by the pure cationic polymer at low salt concentration indicate that the chains are in an extended conformation. The addition of anionic surfactant or of salt condenses the cationic polymer chains as evidenced by the much shorter range of the repulsions. These forces are, for both conformations, a combination of steric and double-layer forces. During sliding, the friction forces produced by the adsorbed layers increase monotonically with the load. A strong dependence of these forces on the sliding speed is noticeable for the extended conformations, while the dependence vanishes in the coiled conformations. This study shows the important role of the conformational state of adsorbed polymer chains on their tribological properties. Introduction Neutral or charged polymers display various adsorption properties depending on their flexibility, on the solvent, and on the surface characteristics. They play an important role in industrial applications. The presence of polymers on surfaces has been extensively researched in view of understanding and controlling the surface forces of steric nature and the tribological properties of surfaces. In general, the interest is focused on the protection of the underlying surfaces against wear and/or the improvement of their friction properties. 1,2 Therefore, the interaction of highly charged macromolecules (polyelectrolytes) adsorbed on a surface is of great interest from both fundamental and applied points of view. 3,4 Furthermore, in many industrial applications such as washing, emulsification, painting, and cosmetology, among others, these systems can be made more complex with the incorporation of free surfactants that can modify the polymer and compete for adsorption. It is nowadays accepted that the adsorption of these systems is governed by electrostatic forces that largely depend on the surface charge density and the ionic strength of the solution. 5 When a surfactant is added to a polyelectrolyte solution, their association is further complicated by the presence of a surface. The polymer conformation at the surface is a delicate balance of various interactions and yet far from being predictable. There is however an interest in understanding the molecular conforma- tions which the polymer and the surfactants can take near the surface in contrast to the bulk. The use of surface experimental techniques becomes neces- sary to probe the structure of the surface adsorbed layer and the extension of the polymer in the solution. Neutron reflec- tivity 6,7 has proved to be very adequate to measure the profile of adsorbed layers. The conformation of adsorbed molecules and the density of the layer determine the nature of the forces involved between such layers. In those situations, atomic force microscopy (AFM) 8 and measurements with a surface force apparatus (SFA) 9 were shown to be excellent experimental techniques. Much of the SFA work has been directed toward the understanding of the forces involved at the mica surface. 10-12 Brushes of polyelectrolytes could be formed on mica by adsorption of a copolymer onto hydrophobic surfaces 13 or from Langmuir-Blodgett deposition. 14 Numerous SFA studies in- volving adsorbed layers of polyelectrolytes involved low concentrations (0.001-0.01%) and long adsorption times (overnight). 15-19 These studies have concluded that, from a low concentration and a long adsorption time, highly charged polyelectrolytes adsorb flat on surfaces of opposite charge, and the resulting interaction is essentially double-layer forces. If the polyelectrolyte is adsorbed from a higher concentration, steric interactions set in. 17 Polyelectrolytes with a low charge density do not adsorb as flat layers and give rise to steric repulsions. 15,18 In the cosmetic industry, today’s shampoos do more than cleanse hair but are expected to provide also additional benefits, such us conditioning, ease of combing, a soft feel, and a nice appearance. A variety of cationic polymers are often formulated in association with surfactants to provide the conditioning effect. However, in shampoos, the adsorption process takes place during a limited amount of time (minutes) with highly concentrated solutions, and it is of interest to know the nanotribological behavior of such layers The aim of the present work is to study systems of industrial interest which involve highly charged polyelectrolyte layers resulting from adsorption for a limited amount of time (5 min) and from a highly concentrated solution (1%), conditions close to shampoo formulations used to promote conditioning of hair. With a homemade surface force ap- paratus 20,21 capable of measuring normal and shear forces * To whom correspondence should be addressed. E-mail: perez@lps.ens.fr (E.P.); gluengo@recherche.loreal.com (G.L.). † Present address: Tribology Research Institute, Southwest Jiaotong University, Chengdu 610031, Sichuan Province, China. 18608 J. Phys. Chem. B 2004, 108, 18608-18614 10.1021/jp047605s CCC: $27.50 © 2004 American Chemical Society Published on Web 11/09/2004