Adsorption of a Polyelectrolyte on Surfaces with Nanometer Sized Chemical Patchiness

Abstract

The adsorption of a cationic polymer, poly(2-vinylpyridine) (P2VP), from aqueous solutions was investigated on model chemically heterogeneous surfaces. The heterogeneous surfaces were prepared using self-assembly of ω-functionalized alkanethiols on gold substrates such that the surfaces consisted of a patchwise distribution of two types of sites, hydrophobic methyl (A) and hydrophilic carboxylic acid sites (B), that interact differently with P2VP. Microscopic imaging by scanning tunneling microscopy and macroscopic characterization by cyclic voltammetry was used to demonstrate that the domains of A on the surface were of nanometer dimensions and commensurate with the dimensions of the polymer chain. Surface plasmon resonance was used to measure adsorption kinetics of P2VP at the solid-liquid interface under two pH conditions such that the polymer chain was either protonated or uncharged. Under these conditions adsorption of P2VP occurred on a heterogeneous surface containing a distribution of patchy sites that are either more attractive or more repulsive than their surroundings. These experiments represent the first realization of a system modeled or simulated in recent theoretical studies, and the results corroborate predictions of the theory. It is found that the surface heterogeneity does not alter the kinetics during the early stages of adsorption and that the rate of adsorption is same as that measured on a homogeneous surface containing only one type of chemical site (A or B). Presence of the heterogeneity influences the number of polymer chains adsorbed on the surface, and the final adsorption amount on the heterogeneous surface lies between the adsorbed amounts found on the compositionally homogeneous surfaces ofA and B.