Acidic and basic nature of ferric oxide surfaces. Adsorption, adhesion, zeta potentials and dispersibility in magnetic inks for hard disks

The surface FeOH sites of ferric oxides are amphoteric in that they form interfacial acid-base complexes with either acids or bases. Adsorption of acidic or basic test molecules from neutral organic solvents was used to characterize the acid-base properties of y-Fe2O3, a magnetic ferric oxide of importance in magnetic tapes and disks for computer memories. Gafac RE-610, a dispersant commonly used in the dispersion of ferric oxide powders for fabrication of the magnetic polymer composites used in computer tapes and disks, is an amphoteric diester of phosphoric acid with a polyoxyethylene adduct of an alkylphenol. The POH group of RE-610 adsorbs as a strong acid onto basic ferric oxide sites, and the 10-12 ether groups adsorb as bases onto acidic ferric oxide sites. When RE-610 adsorbs onto ferric oxide, the first monolayer is spread out broadly over the surface, occupying 2.2 nm2 per molecule, because several ether groups as well as the POH group are complexed to surface FeOH sites. A second monolayer adsorbs, a rare phenomenon in adsorption from non-aqueous solutions, and this second layer can be attached only through its POH acid group, as evidenced by an area per molecule of only 0.72 nm2. RE-610 is used as a dispersant for y-Fe2O3- Such dispersions have a strong tendency to flocculate, for the magnetic inter-particle attractions add appreciably to the normal London-Lifshitz attractions, illustrated by comparing magnetic with non-magnetic ferric oxide dispersions. Such dispersions are much more deflocculated after adsorption of acidic polymers.