Effect of Nonionic Surfactants On the Dispersion Stability of Copper Phthalocyanine Pigment Nanoparticles In Aqueous Solution

Aqueous dispersions of copper phthalocyanine (CuPc) pigment nanoparticles are used as major ingredients of commercial blue inks. The dispersion stability of hydrophobic CuPc nanoparticles is quite poor, unless the particles are stabilized by an electrostatic or steric mechanism. Using surfactants helps improve dispersibility and dispersion stability by increasing the particle hydrophilicity. Moreover, such surfactants may be used for deinking by froth flotation for paper recycling applications, for which the particles should retain some hydrophobicity.

The effect of a nonionic surfactant, Triton X-100 on the colloidal dispersion stability of CuPc-U (unsulfonated and hydrophobic) and CuPc-S (surface sulfonated and hydrophilic) particles in water or aqueous NaNO₃ solutions was investigated. Its adsorption density was determined from surfactant concentrations analyzed using an HPLC method with a UV detector. The experimental Fuchs-Smoluchowski dispersion stability ratios W of the particles were determined from dynamic light scattering (DLS) data, for the Rayleigh-Debye-Gans (RDG) light scattering regime. The adsorption densities of Triton X-100 on both types of particles increase with increasing concentration of surfactant up to the critical micelle concentration (cmc), and then reach a plateau. The maximum adsorption density Γ_m is higher for the CuPc-U than that for the CuPc-S. The hydrophobic chains are inferred to be adsorbed onto the surfaces, and the hydrophilic ethylene oxide chains are in a coil conformation. The W-values for the CuPc-U dispersions are affected mainly by the surfactant fractional surface coverage θ. Adding NaNO₃ has no significant effect on the dispersion stability. The stabilization mechanism for the CuPc-U is inferred to be primarily steric, as expected. The W-values for the CuPc-S in solutions with NaNO₃ are higher than those for CuPc-U, and decrease with increasing concentration of NaNO₃, indicating that the stabilization is affected by the screening of electrostatic repulsive forces. The zeta potential is reported but it is found not to be a good predictor of the electrostatic stabilization in pure water, pointing to the need for new and improved theory for such system.

The effect of a commercial nonionic surfactant, Myrj 45 on the dispersion stability of the CuPc-U particles in water was also studied and compared to that of Triton X-100. The composition of Myrj 45 was analyzed with mass spectrometry (MS). Six classes of components, which are polyglycols, monostearates or monopalmitates monoesters, and distearates or dipalmitates or mixed diesters, were detected. Each class of components shows a quite wide distribution of homologues. The adsorption isotherm of each class of component was obtained using an HPLC with a MS detector. The adsorption densities increase with increasing concentrations of each class of components up to a plateau. The monoesters adsorb most strongly, and the diesters adsorb slightly more than the polyglycols. The overall adsorption isotherm of Myrj 45 was estimated by taking into account the contributions from six classes of components. The overall adsorption densities of Myrj 45 are much smaller than those of Triton X-100 at the same molar concentration. Freeze-thaw tests suggested that the dispersions of CuPc-U particles with adsorbed Myrj 45 are not as stable as those with adsorbed Triton X-100. Particle aggregation primarily occurred during the freezing process. The stability ratios for CuPc-U particles in Myrj 45 solutions were also determined. The W-values are found to be about one order of magnitude smaller than those with Triton X-100.