Aggregation of inkjet ink components by Ca and Mg ions in relation to colorant pigment distribution in paper

Papers coated with salts containing divalent cations exhibit superior inkjet print quality, which has been suggested to be due to fast aggregation of the colorant pigments close to, or even on, the surface of the paper. In this work we show the pivotal role of the carboxylic acid containing dispersing polymer. We report a series of aggregation and sedimentation experiments with commercial inks, generic ink formulations and specific ingredients comprising these formulations, and find differences in their response to the presence of MgCl2 or CaCl2. In particular, flocs and sediments formed in the presence of MgCl2 are denser than those formed in the presence of CaCl2. These differences are not predicted by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. We suggest that ion specific interactions occurring between Mg2+ or Ca2+, and charged carboxylate groups residing on the dispersing polymers, provoke the observed behavior.