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Wettability and Swelling Behavior of a Weak Polybasic Brush: Influence of Divalent Salts in the Environment

We have studied the response of surface properties and swelling behaviors of annealed poly(2-vinyl pyridine) (P2VP) brushes covalently tethered to solid planar surfaces to divalent salts in aqueous solutions at varied pH values. Results derived from the quartz crystal microbalance technique, atomic force microscope and contact angle goniometry indicate that annealed polybase brushes undergo conformational transitions upon addition of divalent salts over a wide range of pH values below pKa: at low ionic strength, polybase brushes swell upon salts addition; at high ionic strength, polybase brushes collapse with salts addition. The extent and sensitive range of brushes conformational transition induced by divalent ions are found to be grater and broader than that caused by monovalent ions at similar ionic strength, indicating stronger effects on screening, osmotic pressure and bridging interaction. In addition, wetting measurements indicate that polybase-divalent counterions interactions can be used to switch surface characteristics from hydrophilic to hydrophobic in a predictable manner. The immediate implications of these experimental results are related to design of "smart" surfaces with controllable charge distribution, membrane thickness and wettability.