Investigation of the stability of graphite particle dispersion and the hemimicelle formation process at graphite/solution interfaces using atomic force microscopy

The correlation between the dispersion stability of graphite particles suspended in aqueous solutions of dodecyldimethylhydroxylammonium chloride (C12DMAOH.Cl) and the structure of the adsorption layer on the graphite surface was investigated by a combination of data from AFM images, measurements of interaction forces, an adsorption isotherm, and turbidity measurements of graphite suspensions. A model for the two-step adsorption mechanism was successfully applied to describe the adsorption isotherm. (The first step was the formation of the horizontal adsorbed monolayer, and the second step was the formation of the hemicylindrical aggregate.) It was suggested that there is a transition regime where the horizontal adsorbed monolayer coexists with the hemicylindrical aggregate. A dispersion of graphite particles is unstable in the presence of the horizontal adsorbed monolayer. Surface force measurements suggest that the surface charges of the horizontal adsorbed monolayer are mostly neutralized by the bound counterions. The critical hemimicelle concentration (hmc) was about 0.8 mM, which was about (1)/(10) of the bulk critical micelle concentration (cmc approximate to 6 mM). The hmc value was close to the critical concentration at which the suspension stability started to increase dramatically with the surfactant concentration. We found that a stable dispersion of graphite particles can only be achieved once the hemicylindrical aggregates have formed on the surface at full surface coverage, leading to the development of the electric double layer forces from the positively charged aggregate.