(40e) The Effect Of Image Forces Upon Polyelectrolyte Adsorption

Adsorption of charged macromolecules, i.e. polyelectrolytes
(PE), is of interest for numerous applications including layer-by-layer assembly
of thin films, membranes, controlled adhesion of biomolecules to surfaces, and
photoresist dissolution.  Oftentimes the solution pH and ionic strength are
studied to understand how electrostatics affect PE adsorption; however, the
relative dielectric properties of the solvent and surface have received far
less attention.  Recent theoretical work predicts that image-charge-induced
image forces should play a role in determining adsorption kinetics, adsorbed
amount, and adsorbate structure when there is a large dielectric discontinuity between
solvent and substrate.

This talk presents the experimentally measured effects of
image forces on the adsorption of PE onto oxide dielectric surfaces.  Surfaces
ranging in dielectric constant of 4-40 (SiO₂ and TiO₂)
are used with solvent mixtures of dielectric constant 80-20 (water and alcohol
mixtures).  Polyelectrolyte adsorption amount and kinetics are determined in
situ and in real time using a liquid phase quartz crystal microbalance cell
with optical reflectivity probe.  The chemical and physical nature of
substrates and adsorbed polymer layers are studied with x-ray photoelectron
spectroscopy (XPS) and x-ray reflectivity (XRR).  Conformation and coverage of
the adsorbed polyelectrolyte layer are determined in situ using neutron
reflectivity with deuterated solvents in a liquid cell.  Experimental results
scale well with theoretical expectations for image charge effects and suggest that
image forces can be used to direct assembly of polyelectrolytes to patterned areas
of a surface based on the dielectric properties of the solvent and substrate. 
The effects of buried regions of dielectric contrast are investigated to
pattern adsorption based on image forces.