Controlling the permeability of multilayered polyelectrolyte films through derivatization, cross-linking, and hydrolysis

Partial Fischer esterification of poly(acrylic acid) allows tailoring of the hydrophobicity and charge density of multilayered films containing poly(allylamine hydrochloride) (PAH) and derivatized poly(acrylic acid) (d-PAA). As hydrophobicity and charge density strongly affect film permeability, control over these properties is vital for possible applications of PAH/d-PAA films as ion-separation membranes and sensors. The hydrophobicity of these films depends on both the extent of esterification and the nature of the derivatizing alcohol. Even though PAH/d-PAA films are composed of polyelectrolytes, the presence of hydrophobic ester groups results in advancing water contact angles as high as 101°. The hydrophobicity of these coatings allows them to effectively passivate underlying electrodes as shown by minimal peak currents in cyclic voltammograms (CVs) of Ru(NH₃)₆³⁺ and Fe(CN)₆^3-. Cross-linking of hydrophobic PAH/d-PAA films via heat-induced amidation stabilizes coatings over a wide pH range but does not significantly decrease the already low film permeability to Ru(NH₃)₆³⁺ and Fe(CN)₆^3-. Stabilization due to cross-linking does, however, allow base-promoted hydrolysis of the ester groups of PAH/d-PAA coatings. After hydrolysis, films are extremely hydrophilic and selectively permeable to Ru(NH₃)₆³⁺ over Fe(CN)₆^3- due to the high density of newly formed -COO- groups. In the case of some hydrolyzed films, the presence of small concentrations of Ca²⁺ results in dramatic current decreases in CVs of Ru(NH₃)₆³⁺, suggesting possible use of these films in sensing applications.