Colloid probe investigation of the stabilization mechanism in aqueous 1,2-propanediol nano-zirconia dispersions

Surface forces controlling colloidal stability of nano-zirconia particles with dispersant [2-(2-(2-methoxy ethoxy) ethoxy) acedic acid-TODA] and without dispersant were investigated using the colloid probe technique in an atomic force microscope. 1, 2-propanediol /water mixtures (containing 0, 25, 50, and 75 wt.% propanediol) were used as dispersing media. Hamaker constants for zirconia in these mixtures were calculated and the surface-zeta-potential determined for nano-zirconia powder in these media. Quantitative analysis of surface forces for zirconia without dispersants suggests an electrostatic stabilization mechanism. The size of the electrostatic barrier was found to be a function of the propanediol content in the medium and decreased with increasing propanediol level. Since adding 1,2-propanediol reduced the Hamaker constant A(131), all investigated samples remained colloidally stable (no attraction was detected). Application of Derjaguin-Landau Verweij-Overbeek theory (DLVO) to fitting curves strongly indicates that TODA acts mainly as a steric dispersant. An additional depletion effect is being discussed, which may contribute to the stabilization process. Further, it is suggested that TODA does not preferably adsorb onto zirconia with the carboxylic head group. Instead, a "flat" adsorption may be indicated, where the molecule forms a layer surrounding the zirconia and the oxygen groups in the backbone may interact with the OH surface sites. However, TODA may only act successfully as a dispersant, if the dispersing medium possesses a suitably small Hamaker constant. A minimum of 50% propanediol was needed to give a significant dispersing force.