Static and Dynamic Forces between Adsorbed Polyelectrolyte Layers (Quaternized Poly-4-vinylpyridine) Marina Ruths,* ,² Svetlana A. Sukhishvili, ‡ and Steve Granick* Department of Materials Science and Engineering, UniVersity of Illinois, Urbana-Champaign, Urbana, Illinois 61801 ReceiVed: January 17, 2001; In Final Form: April 26, 2001 Static forces (a combination of steric and electrostatic interactions) and dynamic shear forces (resulting from interactions between polymer chains) were measured between quaternized poly-4-vinylpyridine layers (QPVP) adsorbed onto mica from 1 mM borate buffer with or without added 0.25 M NaCl. At the polyelectrolyte concentration chosen, 0.4 mg mL -1 , extended layers were formed at all adsorption conditions. The adsorbed amount of 98% quaternized poly-4-vinylpyridine was lower at low ionic strength than of 14% quaternized, and increased with added salt. Layers formed by one-step adsorption showed a predominantly elastic response to small-amplitude oscillatory shear in the frequency range 0.13-130 Hz. In the case of two-step adsorption, the confined layers allowed viscous dissipation of similar magnitude as the elastic, which suggests that additional chains adsorbing at high salt concentration onto a preformed layer had fewer segments in contact with the solid surface. Introduction Polyelectrolytes (charged polymers) are commonly used to control the stability of colloidal dispersions in aqueous systems, for example in paper-making and in water-based paints. The adsorption of polyelectrolytes on surfaces, which has been studied extensively both experimentally 1-20 and theoretically, 21-31 is primarily the result of electrostatic attraction that leads to an entropically favored replacement of small, adsorbed counterions with charged segments of the macromolecule. It is also influenced by van der Waals forces. In this study, we will not be concerned with the specific lock-and-key interactions that are prominent in some biological systems. Due to desorption of hydrated small cations from many solid surfaces into aqueous solution, 32 negatively charged surfaces are abundant in nature. The adsorption of cationic polyelectrolytes is therefore of large practical importance. The different conformations of polyelectrolyte chains in solution and on a surface result from a balance between elastic (stretching) energy and conformational entropy. In addition to these usual considerations for neutral polymer chains, there are additional electrostatic interactions between segments. 21-31 The details of how to describe this contribution have proven difficult and controversial to quantify. Typically, highly charged poly- electrolytes adsorbing at low concentration from low ionic strength solution form very flat layers on oppositely charged surfaces, 3,5-12,15,17-31 and the adsorbed amount is not strongly sensitive to the chain length. In the absence of other interactions, an increased concentration of counterions would compete with the polyelectrolyte for adsorption sites and cause a decrease in the adsorbed amount. 25 However, in the presence of van der Waals or specific surface attractions, the amount of highly charged polyelectrolyte adsorbed from low solution concentra- tion initially increases with increasing salt concentration due to screening of the repulsion between segments, and forms a progressively more extended layer. 3-7,9,11,12,18,21,24,25,29-33 Ex- perimentally, it has been found 11,12,19,33 that the time needed for equilibration after changing the adsorption conditions can be very long due to the slow rearrangement of long chains with many strongly adsorbed segments. This leads to difficulties in determining the equilibrium conformation at different conditions. One distinguishes between weak polyelectrolytes, where the charge density depends on the pH 1,6-8,12,15,20,24,27,28 (dissociation of polyacids and polybases), and strong polyelectrolytes 2-5,9-21,24,25 that contain permanent charges in aqueous solution. We have studied a strong polyelectrolyte system, 14% and 98% quater- nized poly-4-vinylpyridine, adsorbed on mica from 0.4 mg mL -1 aqueous solution at high pH and varying electrolyte concentra- tion. Our emphasis was (i) to compare (see (ii) later) layer thickness, extension, and interfacial rheology at different charge density, and (ii) to study effects of adsorption history when the layers were adsorbed from a higher solution concentration than in most previous studies of polyelectrolytes on mica. 5-11,14,17,20 The adsorbed amount is compared to recent experiments on the adsorption of these polymers on oxidized silicon at similar conditions, 18,19,33 and to a study on 98% quaternized poly-4- vinylpyridine adsorbed on mica from 0.005 and 1.0 mg mL -1 solution. 33 The interaction forces measured on approach and separation of two adsorbed layers are compared to studies on other strong cationic polyelectrolytes with high charge density at low concentration of added salt and significantly lower polyelectrolyte concentration than in our systems, 5,9-11,17,20 and to studies of partially charged ones (1%, 14 10%, 13 and 30% 16 ) at concentrations lower than or comparable to in our systems. Measurements of dynamic (shear) interactions as a function of separation distance were used to detect the extension of the adsorbed polyelectrolyte layers. The total electrolyte concentra- tion and the order of adsorption from different solution conditions were found to strongly affect the adsorbed amount * Corresponding authors. ² Present address: Department of Physical Chemistry, Åbo Akademi University, Porthansgatan 3-5, FIN-20500 Åbo, Finland. ‡ Present address: Department of Chemistry and Chemical Biology, Stevens Institute of Technology, Castle Point on Hudson, Hoboken, NJ 07030. 6202 J. Phys. Chem. B 2001, 105, 6202-6210 10.1021/jp010182z CCC: $20.00 © 2001 American Chemical Society Published on Web 06/12/2001