Particle bridging in dispersions by small charged molecules: chain length and rigidity, architecture and functional...

The particle bridging behaviour of dicarboxylic acid bolaform compounds such as fumaric, oxalic,trans-beta-hydromuconic, trans,trans- and cis,cis-muconic acids were evaluated in terms of their effects on the yield stress of alpha-Al(2)O(3) dispersions. The adsorption behaviour of these additives and their effects on the particle zeta potential were also determined. This study aims to understand and identify molecular factors essential for particle bridging. Very rigid compounds like trans,trans- and cis,cis-muconic and fumaric acids were identified as excellent bridging compounds from the large increase in the maximum gel strength. This strength enhancement increases with chain length and is due to more bridging molecules located in the larger spherical cap bridging area and participating in bridging. Cis,cis-muconic acid with the same chain length as fumaric acid displayed a greater bridging capability as its bolaform carboxylate groups possessed a greater lateral reach. Trans-beta-hydromuconic acid with a more flexible backbone displayed a much diminished particle bridging capability. This study has revealed a number of new molecular structural factors essential for particle bridging attribute. These are (a) the degree of backbone rigidity, (b) chain length, (c) spatial position and (d) lateral displacement of the bolaform charged group. For fumaric, trans,trans- and cis,cis-muconic acids, the maximum gel strength was not located at the pH of zero zeta potential. A particle bridging model taking into account of electrostatic repulsive interactions between the interacting particles was proposed to explain the maximum gel strength enhancement by the bolaform compounds.