Effects of Gum Arabic macromolecules on surface forces in oxide dispersions

The effects of Gum Arabic (GA) on the zeta potential-pH and yield stress-pH behaviour of α-Al2O3, TiO2 and ZrO2 dispersions were evaluated. GA is amphoteric in character as it contains protein fractions. Itʹs macromolecule has a molecular weight ranging from 50 000 to several million. In aqueous solution, the GA molecule assumed a very compact structure. Upon adsorption it will form a relatively thick protrusion from the surface that may act as a steric layer or participate in bridging. At charge neutral condition, GA increased the maximum yield stress, τymax, at pH >5.0 by as much as 60–80% for both α-Al2O3 and ZrO2 dispersions. This strengthening of the network structure was attributed to bridging. In contrast GA decreased the τymax of TiO2 dispersions at pH <5.0. Here the prevailing interaction was steric. Bridging effect was not important because the free carboxylate group, responsible for bridging, was uncharged at pH <5.0.