(526f) Colloidal Dispersion Stability of CuPc Pigment Nanoparticles: Effects of Triton X-100 and NaNO3

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, steric, or other mechanism. Effects of a nonionic surfactant (Triton X-100) and an electrolyte (NaNO3) on the dispersion stability of CuPc particles were investigated. Two types of CuPc particles were examined: CuPc-U (unsulfonated) and CuPc-S (sulfonated). XPS surface analysis revealed that there is at least one sulfonate group per two CuPc molecules on the CuPc-S particle surface. The adsorption and desorption of Triton X-100 was determined with HPLC analysis. The experimental dispersion stability ratio W was determined from dynamic light scattering (DLS) data and the Rayleigh-Debye-Gans (RDG) light scattering theory. The dispersion stability of CuPc-S in NaNO3 solution has been studied (Langmuir, 2010, published in the web, DOI: 10.1021/la904224g). There was a large discrepancy in the W-values between the DLVO predictions and the values which were based on the experimental measurements. For CuPc-U and CuPc-S, the surfactant adsorption isotherms showed that the adsorption density increases with increasing concentration of surfactant up to the critical micelle concentration, or cmc, and then reaches a plateau. These results indicate that surfactant monomers adsorb on the particle-water interface and micelles do not adsorb. A Langmuir-like model fitted the isotherm data quite well. The maximum adsorption density was higher for CuPc-U than that for CuPc-S, implying there are more hydrophobic sites, or larger adsorption area, available on the CuPc-U particle surface. Desorption tests of the surfactant were conducted to determine the reversibility or irreversibility of adsorption. Preliminary results show that a portion of the surfactant adsorbs irreversibly. The average particle size of the stable CuPc-U dispersions was about two times larger than that of CuPc-S dispersions (160 vs 90 nm). The CuPc-U particles either floated, or settled, or dispersed, depending on the surfactant surface coverage. These particles were negatively charged in solution, possibly due to preferential adsorption of ions. The CuPc-S particles were negatively charged, mainly because of the attached sulfonate groups. For both types of particles, the zeta potential became less negative with increasing surfactant concentration, indicating that the ion adsorption is affected by the adsorbed nonionic surfactant. For CuPc-U particles , the W-values ranged from about 10 to 1000000 , and were affected by the surfactant surface coverage, and slightly by the surface charge estimated from the measured zeta potential. When the surfactant concentration increased up to the cmc, there was a three to five orders of magnitude jump in the W-values. The W-values increased further above the cmc. For CuPc-S, the W-values were 1000000 or higher, and correlated mainly with the surface charge for all dispersions tested, indicating a mostly electrostatic stabilization mechanism. For dispersions in surfactant solution in water with no added electrolyte, the W-values for CuPc-S were about one to two orders of magnitude higher than those for CuPc-U. When 50 or 100 mM NaNO3 was added to each of the particles dispersions, the maximum surfactant adsorption density was unchanged, while the zeta potential became less negative. The W-values in dispersions containing surfactant and NaNO3 suggest that the stabilization mechanism is steric for CuPc-U, and electrostatic and steric for CuPc-S.