Measurements of interaction forces between polycations, between clay nanoplatelets, and between polycations and clay nanoplatelets by atomic force microscopy

Interaction forces have been measured between (i) apposing layers of 2.0 +/- 0.05 nm thick polydiallyl-dimethylammonium. chloride, PDDA, self-assembled onto a silicon wafer, silicon-substrate/PDDA, and onto a silicon microparticle, attached to a tipless AMF catilever, SMP-AFM-tip/PDDA; (ii) apposing layers of 2.0 +/- 0.05 nm thick montmorillonite, M, platelets, self-assembled onto the silicon-substrate/PDDA (silicon-substrate/PDDA/M) and onto the SMP-AFM-tip/PDDA (SMP-AFM-tip/PDDA/M); (iii) apposing layers of silicon-substrate/PDDA and SMP-AFM-tip/PDDA/M, and (iv) apposing layers of silicon-substrate/PDDA/M and SMP-AFM-tip/PDDA by scanning force microscopy. Interactions between the bare silicon substrate and the bare SMP-AFM-tip and those between the silicon-substrate/PDDA and the bare SNIP-AFM-tip have also been measured. The experimentally obtained force/radius vs probe-sample separation distance plots have been fitted to a simple exponential, taking advantage of the Derjaguin approximation for assessing the double-layer force between a charged sphere and a flat surface, and to the DLVO theory using the constant potential numerical approximation (with the exception of ii, where constant charge numerical approximation was used). Values for the adhesion (the pull back) force, Debye-length, and surface potential have also been evaluated.