Surface forces between adsorbed quaternarized poly(2-vinylpyridine) layers: Molecular rearrangements and bridging interactions

The surface forces between quaternarized poly(2-vinylpyridine) (QP2VP) layers adsorbed onto mica have been measured over a range of solution pH conditions. It was found that QP2VP overcompensates the mica's negative charge, resulting in a positively charged interface. The surface potential at the mica/polyelectrolyte interface is a balance of the charge on the adsorbed polyelectrolyte and the charge on the underlying mica surface. It was also found that, below and near the pK(a) of the polyelectrolyte, segment-segment repulsion results in the polyelectrolyte adopting a conformation with significant extension into solution. Interpenetration of the extended chains leads to a bridging attraction. At high pH a decrease iri the polyelectrolyte charge density decreases segment-segment repulsion, resulting in a compact adsorbed conformation where no bridging attractive jump is observed. An adhesion is measured between adjacent layers under all pH conditions, regardless of the presence of an attractive jump in. Here the important distinction is made between an attractive force caused by the overlap of anchored polyelectrolyte chains extending into solution and chain entanglement that occurs due to molecular rearrangements when polyelectrolyte layers are under compression.