Self-dispersed (SDP) carbon black inks have been successfully increased optical density by adding different multivalent metal ions. These selected metal ions cause SDP carbon black particles flocculating on paper surface after printing,, but not affected the stability and jetability of inks.
Chien-Wen Lee, Hsiao-San Chen, "Self-Dispersed Carbon Black for Inkjet Printing Application" in Proc. IS&T Int'l Conf. on Digital Printing Technologies and Digital Fabrication (NIP30), 2014, pp 194 - 197, https://doi.org/10.2352/ISSN.2169-4451.2014.30.1.art00048_1