(642a) 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.