The electrostatic charging of non-aqueous dispersions involves the interaction
of basic dispersants or basic charge-control agents with the acidic surface sites of
the particles to provide negative charges, or the interaction of acidic dispersants
or acidic charge-control agents with the basic surface sites of the particles to
provide positive charges . The mechanism has three steps : 1) , adsorption of the
dispersant or charge-control molecules by acid-base interaction with the surface
sites; 2), proton-transfer or electron-transfer between some of the adsorbed species
and surface sites; and 3), desorption of some charge-carrying adsorbed dispersant or
charge-control molecules due to a dynamic adsorption-desorption process.
In these studies the importance of the last step is illustrated with comparisons
of zeta-potentials and heats of adsorption determined with various pigments
interacting with several charge-control agents. It is found that although acid-base
interactions are necessary for electrostatic charging, stronger acid-base interactions
result in lower zeta-potentials, probably because of less desorption in the
last step. Additives used to enhance charging of acidic particles are believed to
be effective by diminishing surface acidity, so as to weaken the adsorption of
charge-control agents, and favor their easier desorption in the last step of the
charging process.
|