Adsorption of Sodium Poly(styrenesulfonate) to the Air Surface of Water by Neutron and X-ray Reflectivity and Surface Tension Measurements: Polymer Concentration Dependence H. Yim, M. S. Kent,* A. Matheson, and M. J. Stevens Sandia National Laboratories, Albuquerque, New Mexico 87185-1411 R. Ivkov and S. Satija National Institute of Standards and Technology, Gaithersburg, Maryland J. Majewski and G. S. Smith LANSCE, Los Alamos National Laboratories, Los Alamos, New Mexico Received January 10, 2002 ABSTRACT: The adsorption of the strong polyelectrolyte sodium poly(styrenesulfonate) (NaPSS) to the air surface of water was investigated as a function of polymer concentration from the dilute regime to the beginning of the semidilute regime. Detailed segment profiles of the deuterated polymer were determined by neutron reflection (NR). Data were obtained for 0.67 and 2.50 M KCl. For two samples differing widely in molecular weight (1150 and 56.1 kg/mol), we find that with increasing polymer concentration the adsorbed amount first increases, reaches a maximum, and then decreases strongly. The PSS concentration at which the maximum is reached is dependent on both the molecular weight and the salt concentration in a manner that correlates with the chain overlap concentration. Regarding the segment profiles, at low polymer concentration the profiles are composed of a thin layer of high concentration at the air surface (trains), followed by a distinct second layer of much lower segment concentration that extends to larger depths into the liquid (loops and tails). Complementary X-ray reflection (XR) revealed a localization of ions about 10 Å below the surface for dilute PSS concentration. This layer becomes more diffuse at higher PSS concentration, in conjunction with the decrease in PSS adsorbed amount measured by NR. This surprising behavior of the adsorbed amount with polymer concentration is not explained by current SCF theory treating the adsorption of strong polyelectrolytes to neutral surfaces. We discuss a few possible explanations for this desorption transition. Finally, we observe that the surface tension decreases monotonically with increasing concentration of PSS in bulk solution but is not correlated with the adsorbed amount of PSS at the surface. Introduction The adsorption of polyelectrolytes onto surfaces is important in many technologies. Polyelectrolyte adsorp- tion onto colloid surfaces plays a central role in the stabilization of emulsions in food, photographic, and pharmaceutical industries. 1,2 The adsorption of biologi- cal polyelectrolytes is a key step in many biochemical processes. Understanding such processes is also impor- tant for controlling fouling in biotechnological operations and for controlling biocompatibility in biomedical ap- plications. 3 From a fundamental point of view, it is important to investigate how the conformation of a polyelectrolyte is changed near an interface and the factors which control the conformation, the adsorbed amount, and the distribution of counterions. 4 The present work focuses on the variation in the adsorption of strong polyelectrolytes with bulk polyelec- trolyte concentration. We are concerned with variations in the adsorbed amount and the conformations of the adsorbed chains. In bulk solution, experimental studies of polyelectrolyte concentration effects have focused on variations in correlation lengths by neutron 5-9 and light 10-13 scattering, along with rheological proper- ties. 9,14,15 Recently, Takahashi et al. reported radii of gyration for sodium poly(styrenesulfonate) (NaPSS) in pure water over a range of NaPSS concentration in the semidilute and concentrated regimes by neutron scat- tering. 16 Polyelectrolyte chain conformations have not been directly studied in the dilute regime due to the limited sensitivity of the experimental techniques. Theoretical understanding of polyelectrolyte systems has been hampered due to the complication of the long- range nature of the Coulomb interaction, which cannot be handled presently beyond the linear Debye-Hu¨ ckel approximation, which surely breaks down for strong polyelectrolytes. In particular, for salt-free solutions the regime in which neither Coulomb interactions nor entropy dominates cannot be accurately treated at present. In this case, the counterions are neither entirely condensed on the chains nor entirely free but are distributed between the two states. Recently, Stevens and Kremer 17 reported molecular dynamics simulations of linear polyelectrolytes in solution. They covered a range of concentrations from dilute to semidilute. In contrast to the predicted dilute limit of rodlike chains, they found that the polyelectrolyte chains have signifi- cant bending even at very low polymer concentrations. Furthermore, the chains begin to contract at concentra- tions roughly an order of magnitude lower than the chain overlap concentration. This is due to the overlap of the counterion clouds. In the presence of high salt concentration, the counterions exchange rapidly with free ions, and the concept of a counterion cloud is no longer valid. Several experimental studies have been reported on the adsorption of polyelectrolytes to interfaces as a 9737 Macromolecules 2002, 35, 9737-9747 10.1021/ma0200468 CCC: $22.00 © 2002 American Chemical Society Published on Web 11/14/2002