The role of secondary interactions in selective electrostatic multilayer deposition

Electrostatic layer-by-layer thin films maybe directed to specific regions of a surface based on electrostatics, secondary interactions, and steric repulsion. In this study, a series of polyamines ranging from hydrophobic polyallylammonium hydrochloride (PAH) to hydrophilic linear polyethyleneimine (LPEI) were coadsorbed with poly(acrylic acid) (PAA) onto patterned self-assembled monolayer (SAM) surfaces. When an acid (COOH) and oligoethylene glycol (EG) SAMs are used as the surface template, it was found that the region of preferred deposition for the more hydrophobic polyamine species was generally the EG region at pH 4.8, rather than the partially charged COOH region. The more hydrophilic LPEI species preferred adsorption on the COOH surface. It is proposed that PAH undergoes complexation with the EG surface based on hydrophobic and hydrogen bonding interactions at moderate pH. On the other hand, the LPEI backbone is hydrated and is thought to undergo large steric repulsion with the EG surface, thus resulting in deposition only on the COOH surface. The region of preferred adsorption also varied with pH, as the hydrogen-bonding interaction of PAA became important at low pH, and electrostatic interactions became the controlling element for selectivity at high pH, when all systems were highly charged. By tuning polyion-surface interactions, the region of polyion deposition can be carefully controlled on patterned chemical surfaces.