Tuning polyelectrolyte adsorption on surfaces via image forces


Author: Scott K. Stanley

Advisor: Wen-li Wu

Division 854, MSEL, Bldg. 224, Rm. B312, Mail Stop 8541

Ph: (301) 975-6219 Fax (301) 975-3928

Non-member in Sigma Xi.

Category: Materials



The adsorption of polyelectrolytes (PE) to surfaces is relevant to numerous applications including layer-by-layer assembly of thin films, membranes, and controlled adhesion of biomolecules to surfaces.  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 poster 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 studied in situ and in real time using a liquid phase quartz crystal microbalance cell with optical reflectivity probe.  The adsorbate structure is characterized via in situ neutron reflectivity (NR) using a liquid cell and adsorbed amount is determined by X-ray photoelectron spectroscopy (XPS).  These results 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 changing pattern size, depth, and geometry on the magnitude of image force will be discussed.