Salt Effect on Cationic Polyacrylamide Conformation on Mica Studied by Single Molecule “Pulling” with Scanning Probe Microscopy Brett Brotherson, † Lawrence A. Bottomley, ‡ Peter Ludovice, † and Yulin Deng* ,† School of Chemical & Biomolecular Engineering and School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0620 ReceiVed: July 5, 2008; ReVised Manuscript ReceiVed: August 18, 2008 The effect of salts on adsorbed polyelectrolyte conformations has been studied extensively over the past three decades. Previous researchers have proposed that increasing salt concentration results in larger loops and tails for weak polyelectrolytes adsorbed on a surface. However, no experimental verification of this theory has been published. In this work, we present experimental verification acquired by “pulling” single molecules of a polyelectrolyte from a mica surface using a scanning probe technique. We also present a new method for analyzing changes in adsorbed polymer tail lengths. We demonstrate that increasing solution salt concentration correlates with both loop and tail lengths of an adsorbed low charge density cationic polyacrylamide on a mica substrate. 1. Introduction The effects of electrolytes on polymer adsorption have been a study of great interest over the past two decades. 1-25 Much of this interest is due to the presence of salts in many industrial applications where polymers are used. Salts can affect the conformation and amount of adsorbed ionic and nonionic polymers. 1 According to Shubin and Linse, salts influence adsorbed polymer conformation by • screening of electrostatic attractions between the polymer and the surface, • screening of electrostatic repulsion between similarly charged groups, • competition between electrolyte ions and polymers for space near the charged surface, and • competition for adsorption sites between the electrolyte and the polymer. 2 It is expected that these effects would be the most pronounced on polyelectrolytes. 26 Similarly, Fleer et al. 1 suggested a weak salt effect for uncharged polymers and charged surfaces. They expected the addition of salt to increase the amount of charged polymers adsorbed on uncharged surfaces and to decrease the amount of charged polymer adsorbed onto an oppositely charged surface unless the polymer adsorbs via a nonelectrostatic mechanism. Electrolytes in solution have the ability to affect polyelec- trolytes in different ways depending on the polymer’s properties. They can screen electrostatic interactions and also compete for electrostatic adsorption sites. If a polyelectrolyte has a high charge density, there will be a large amount of repulsion between charged groups in the polymer chain. This will cause the polymer to occupy a large area upon adsorption and thereby reduce the number of polymer molecules which can adsorb on the surface. Adding salt to the solution screens these electrostatic forces allowing polymers to move closer to one another and increasing the adsorbed amount on the surface. 2 The competition for adsorption sites is not as important, in this situation, as the multiple charged sites on the polymer allow most groups on the polymer backbone to adsorb. Van de Steeg et al. showed, through simulations, that salt ions are also able to displace highly charged polymers. 27 This illustration of the opposite case points to the effect that slight differences in experimental settings can yield different results. For the case of low charge density polymers, adsorption to a surface is based on the interaction of a few charged groups with the oppositely charged surface. Here the competition for adsorption sites can interfere with polymer adsorption resulting in decreased adsorp- tion with increasing salt concentration. 2 Much of this effect has been shown through different modeling techniques. Dobrynin et al. used scaling theory to predict the effect of salt concentra- tion on adsorbed amount of different polyelectrolytes. 7 Hoda and Kumar used Brownian dynamics, which is an intermediate coarse grained simulation technique, to do this. 5 Carrillo and Dobrynin used molecular dynamics simulations to analyze this situation. 6 Scheutjens and Fleer’s self-consistent mean field theory was used by Shubin and Linse to again observe the salt effect on adsorbed polymers. 2 To date, modeling and experimental work has focused on how salt impacts the amount of polymer adsorbed. Very little work has addressed the impact of salt on adsorbed polymer conformation. This is because most of the techniques, including dynamic light scattering, neutron reflectivity, and surface force analysis with a force-balance apparatus, used to determine adsorbed polymer layer thickness are incapable of revealing details of a single adsorbed polymer’s conformation. 4,28-30 The impact of salt on hydrodynamic thickness is an open question. Both Wang and Audebert and Meadows et al. found that as the salt concentration was increased, the hydrodynamic thickness decreased 4,29 whereas Takahashi, Dahlgren et al., Rojas et al., and Shubin and Linse found the opposite. 2,11,12,25,28 Wang and Audebert and Meadows et al., suggested that the increase in salt concentration screens out the electrostatic interactions between the polymer chains and also the polymer and substrate surface. 4,29 Because the charged group interactions are screened the polymer can occupy a more compact structure than would be allowed if the electrostatic interactions were present. * To whom correspondence should be addressed. E-mail: yulin.deng@ chbe.gatech.edu. † School of Chemical & Biomolecular Engineering, Georgia Institute of Technology. ‡ School of Chemistry and Biochemistry, Georgia Institute of Technology. J. Phys. Chem. B 2008, 112, 12686–12691 12686 10.1021/jp805931b CCC: $40.75  2008 American Chemical Society Published on Web 09/17/2008