Abstract
Model chemically heterogeneous surfaces containing different density of heterogeneous sites were used to study the adsorption of poly(acrylic acid) (PAA) from aqueous solutions. The heterogeneous surfaces were prepared by 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 (DT) and hydrophilic carboxylic acid sites (MPA), that interact differently with PAA. Microscopic imaging by scanning tunneling microscopy and macroscopic characterization by cyclic voltammetry were used to show that the domains of DT on the surface were of nanometer dimension and commensurate with the size of the polymer chain. Surface plasmon resonance was used to measure the adsorption kinetics of PAA at the solid–liquid interface. These experiments represent the first realization of systems modeled or simulated in recent theoretical studies and the results corroborate predictions of the theory. Nanometer scale heterogeneity within an uncharged surface had little effect on the kinetics of adsorption of neutral PAA during the early stages of adsorption. The initial rate of adsorption was unchanged from that measured on a homogeneous surface containing only one type of chemical site (DT or MPA). A threshold concentration of heterogeneous sites was necessary before their impact was significantly manifested in the number of polymer chains adsorbed on the surface. The adsorbed amount of PAA on the heterogeneous surface was found to lie in between the adsorbed amounts found on the two compositionally homogeneous surfaces (DT and MPA) and was dependent on the fraction of heterogeneous sites on the surface.