Forces between nitrogen-containing self-assembled monolayers (SAMs) and zirconia particles in aqueous solutions

Abstract

An atomic force microscope was used to measure the interaction forces between siloxane-anchored organic self-assembled monolayers (SAMs) with nitrogen-containing surface groups on silicon and micron-sized zirconia spheres in aqueous electrolyte solutions. The surface groups studied were alkylammonium salt and amines (primary, secondary, and tertiary). The pH of the solutions ranged from 2.5 to 9 (controlled by adding NaOH or HCl), and their ionic strength was held constant at 0.022 m (by adding NaCl). The results indicated that the alkylammonium salt surface exhibits a standard Debye double layer, whereas all of the amine surfaces exhibited a Stern layer, i.e. a charge inversion layer. The observed interactions were similar for SAMs formed from surfactants of two different alkyl chain lengths (11 and 16). The corresponding electrostatic interactions comprise the dominant part of the DLVO interactions under the conditions studied. The observed forces were consistent with film deposition results: vanadia films, but not titania films, form on alkylammonium salt surfaces; whereas both titania and vanadia films deposit on amine surfaces.