Interaction and Structure of Surfaces Coated by Poly(vinyl amines) of Different Line Charge Densities † Luke J. Kirwan, ‡,§ Plinio Maroni, ‡ Sven H. Behrens, |,⊥ Georg Papastavrou,* ,‡ and Michal Borkovec ‡ Department of Inorganic, Analytical, and Applied Chemistry, UniVersity of GeneVa, Sciences II, 30, Quai Ernest-Ansermet, 1211 GeneVa 4, Switzerland, and Department of Polymer Physics, BASF SE, Ludwigshafen, Germany ReceiVed: March 18, 2008; ReVised Manuscript ReceiVed: June 25, 2008 Interactions between preadsorbed ﬁlms of poly(vinyl amine) (PVA) of two different line charge densities on silica substrates were studied with the colloidal probe technique based on the atomic force microscope (AFM). The preadsorbed ﬁlms were prepared by adsorption of PVA from a pH 4 solution without any added salt. The highly charged PVA adsorbs in a ﬂat conﬁguration and in laterally heterogeneous layers, while the more weakly charged PVA analog adsorbs in thicker and more homogeneous ﬁlms. As revealed by reﬂectivity measurements, such preadsorbed PVA ﬁlms are stable in polyelectrolyte-free solutions. However, force measurements with the colloidal probe reveal that their interactions depend strongly on the ionic strength. Upon approach, interactions are dominated by electrostatic diffuse layer overlap forces. Both PVA ﬁlms have very similar diffuse layer charge densities of about 1.5 mC/m 2 . Since these values are substantially lower than what would be expected from the total charge of the adsorbed polyelectrolytes measured by reﬂectivity, we infer that coadsorption of anions represents the principal mechanism in charge neutralization. Upon retraction, the adhesion between the ﬁlms is dominated by bridging forces due to single polymer chains. Such bridging adhesion becomes progressively important with increasing ionic strength, whereby their range and frequency increase. The work of adhesion due to bridging is about 0.3 mN/m. At low ionic strengths, the ﬁlms behave differently. While the highly charged PVA shows unspeciﬁc adhesion at small distances, the more weakly charged PVA analog shows few adhesion events occurring at long distances. 1. Introduction Poly(vinyl amine) (PVA) was recently suggested as an alternative to more traditional cationic polyelectrolytes used in industrial applications, especially as retention aids in paper making, stabilizers of colloidal suspensions, or ﬂocculants for water puriﬁcation. 1-6 In these applications, polyelectrolytes are added to the colloidal suspensions, whereby they adsorb to the respective particle surfaces. The adsorbed polyelectrolyte layers modify the interaction forces between the particles, and inﬂuence the suspension stability or its rheological properties. 7-12 The structure of such adsorbed layers depends on the properties of the polyelectrolytes and the surfaces, in particular, on the respective charge densities, solution composition, and polymer architecture. The highly ﬂexible PVA with its high line charge density was shown to be very effective to control the stability of colloidal suspensions. 4-6 Polyelectrolytes are further used as adhesion modiﬁers, coupling agents, or building blocks for multilayer ﬁlms. 13-22 These applications mostly rely on polymeric layers formed equally by adsorption from solution, but such adsorbed poly- electrolyte ﬁlms are subsequently exposed to polyelectrolyte- free solutions, possibly of a different composition from the solution used for adsorption. Upon changes of the solution composition, such preadsorbed ﬁlms may drastically change their properties, as recently demonstrated by the swelling of preadsorbed polyelectrolyte layers upon a decrease in the ionic strength. 23 PVA appears to be promising in this context, particularly for surface modiﬁcation, engineering of multilayers, or hybrid nanomaterials. 16-20 However, little information on preadsorbed PVA ﬁlms is available. Direct force measurements represent a powerful tool to study polyelectrolyte ﬁlms. The relevant techniques include the surface forces apparatus (SFA), 24,25 MASIF (i.e., measurement and analysis of surface interaction forces), 26,27 and the colloidal probe technique based on the atomic force microscope (AFM). 15,25,28,29 In the latter technique, one replaces the AFM tip at the end of the cantilever with a colloidal sphere. One important ﬁnding from direct force measurements between adsorbed polyelectro- lyte layers was to conﬁrm the importance of electrostatic interactions. 15,24,25,27,30-32 Poisson-Boltzmann (PB) theory has been commonly used to interpret such data, whereby diffuse layer potentials and the underlying surface charge densities can be estimated. In particular, the MASIF technique was success- fully used to investigate the interactions between PVA ﬁlms adsorbed on glass and cellulose. 27,31 Adhesion properties of single polyelectrolytes on surfaces have been equally investigated with the AFM. 33-36 In these studies, an AFM tip is put into contact with a layer of polymer chains, assuring that only few polymer segments bind to the tip. Upon retraction, the bound chains are stretched successively. † Part of the “Janos H. Fendler Memorial Issue”. * Corresponding author. Phone: +41 22 379 6429. Fax: +41 22 379 6069. E-mail: georg.papastavrou@unige.ch. ‡ University of Geneva. § Present address: Research and Development Department, Aughinish Alumina Limited, Askeaton, County Limerick, Ireland. | BASF SE. ⊥ Present address: School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0100. J. Phys. Chem. B 2008, 112, 14609–14619 14609 10.1021/jp802366p CCC: $40.75  2008 American Chemical Society Published on Web 08/22/2008