  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.
