Molecular Dynamics Simulation Study of the Structure of Poly(ethylene oxide) Brushes on Nonpolar Surfaces in Aqueous Solution Dmitry Bedrov* and Grant D. Smith Department of Material Science & Engineering and Department of Chemical Engineering, UniVersity of Utah, 122 S. Central Campus Dr., Room 304, Salt Lake City, Utah 84112 ReceiVed February 24, 2006. In Final Form: May 1, 2006 The structure of poly(ethylene oxide) (PEO, M w ) 526) brushes of various grafting density (σ) on nonpolar graphite and hydrophobic (oily) surfaces in aqueous solution has been studied using atomistic molecular dynamics simulations. Additionally, the influence of PEO-surface interactions on the brush structure was investigated by systematically reducing the strength of the (dispersion) attraction between PEO and the surfaces. PEO chains were found to adsorb strongly to the graphite surface due primarily to the relative strength of dispersion interactions between PEO and the atomically dense graphite compared to those between water and graphite. For the oily surface, PEO-surface and water-surface dispersion interactions are much weaker, greatly reducing the energetic driving force for PEO adsorption. This reduction is mediated to some extent by a hydrophobic driving force for PEO adsorption on the oily surface. Reduction in the strength of PEO-surface attraction results in reduced adsorption of PEO for both surfaces, with the effect being much greater for the graphite surface where the strong PEO-surface dispersion interactions dominate. At high grafting density (σ ≈ 1/R g 2 ), the PEO density profiles exhibited classical brush behavior and were largely independent of the strength of the PEO-surface interaction. With decreasing grafting density (σ < 1/R g 2 ), coverage of the surface by PEO requires an increasingly large fraction of PEO segments resulting in a strong dependence of the PEO density profile on the nature of the PEO-surface interaction. I. Introduction Poly(ethylene oxide) (PEO) is used extensively to alter the interfacial properties of surfaces in aqueous solutions in ap- plications ranging from controlling particle aggregation in solutions 1 to improving biocompatibility by preventing protein and microbial adsorption on organic and inorganic surfaces. 2 It has been observed that the ability of PEO (adsorbed or chemically grafted to a substrate) to exert a repulsive force on another surface, particle, or biomolecule depends on surface coverage and the nature of the interactions between PEO and the substrate. It has been suggested that at relatively low surface coverages, corre- sponding to σ < 1/R g 2 , where σ is the grafting density (polymer chains/unit area) and R g is the root-mean-square radius of gyration of the chain, conventional ideal brush theories are no longer valid and the interaction of polymer chains with the surface determines the brush structure and hence defines the repulsive property of the brush. 3 Particularly puzzling is the interaction of PEO chains with nonpolar surfaces in aqueous solution. Systematic study of the interaction between PEO brushes and self-assembled alkanethiol monolayers has revealed an increased adhesion of PEO chains to the self-assembled monolayers with increasing fraction of terminal hydrophobic methyl groups, indicating attraction between PEO chains and the nonpolar interface, 4 whereas other studies reported repulsive interactions between PEO brushes and an alkane-modified AFM tip. 5 PEO has been also reported to adsorb on polystyrene latexes 6 and hydrophobic (methylated) silica, 7 and there are also reports indicating that some proteins have a tendency to adsorb on PEO brushes supposedly due to attractive interaction between PEO chains and hydrophobic patches of proteins. 8 Clearly, an improved understanding of PEO interaction with nonpolar surfaces in aqueous media is important for a wide variety of PEO brush applications. II. Simulation Details A. System Description. In this work, we utilize atomistic molecular dynamics (MD) simulations to better understand the structure of PEO brushes on solid nonpolar surfaces in aqueous solution. In all systems, methyl terminated PEO chains (M w ) 526, 12 repeat units) were grafted to a single surface (described below). A fully atomistic, quantum chemistry based force field for inter- and intramolecular interactions, PEO and PEO-water interactions, 9 and the TIP4P water model 10 were used. These force fields have been extensively validated in our previous studies of PEO melts and PEO in aqueous solution. 11 All systems consisted of 3000 water molecules and 9, 25, or 64 PEO chains * To whom correspondence should be addressed. (1) Harris, J. M. In Poly(Ethylene Glycol) Chemistry: Biotechnical and Biomedical Applications; Harris, J. M., Ed.; Plenum Press: New York, 1992. Andrade, J. D.; Hlady, V.; Jeon, S. I. Polym. Mater.: Sci. Eng. 1993, 69, 60. Abo-El-Enein, S. A.; Hanafi, S.; El-Hosiny, F. 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