Experimental and modeling studies of colloidal dispersion stability of CuPc pigment nanoparticles in aqueous solution
Stability is important for producing stable inks and other dispersions, and for developing simpler, less costly, and environmentally safer inks. Understanding and controlling the stability is also important in deinking and paper recycling. This thesis deals with (a) experimental investigation of dispersion stability and (b) modeling of aggregation process with Brownian dynamics simulations for understanding energetic and hydrodynamic interactions. Aqueous copper phthalocyanine (CuPc) cyan pigment dispersions are commonly used in inkjet printing, and in paint and varnish industries. For many of these applications, it is essential to induce hydrophilicity and dispersibility leading to good dispersion stability. Two types of CuPc particles were examined: (i) CuPc-U particles, which are unsulfonated and initially hydrophobic; and (ii) CuPc-S particles, which have chemically attached sulfonate groups on the surface. CuPc nanoparticles can be stabilized mainly by electrostatic and steric mechanisms. The Fuchs-Smoluchowski dispersion stability ratio W, as determined from DLS data and the Rayleigh-Debye-Gans (RDG) scattering theory, is used as a quantitative measure of stability. Electrostatic stabilization was investigated using a model system of CuPc-S particles in aqueous NaNO3 solutions. The particles effective non-retarded Hamaker constant, was determined from the London dispersion coefficient, which was computed with a time-dependent density functional theory method. The estimated Hamaker constant allowed the predictions of W-values from the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, which ignores short range forces in solution and hydrodynamic interactions. Large discrepancies were found between the experimental and the predicted W-values, indicating that either significant particle shape effects are important, or that there are additional short-range repulsive forces, not predicted by the macroscopic DLVO theory. Nonetheless, electrostatic repulsive forces play a substantial role in the dispersion stability, as revealed qualitatively by the effect of the ionic strength. Steric stabilization is usually achieved by adsorption of surfactants or polymers. The effect of a nonionic surfactant Triton X-100 on the dispersion stability of CuPc-U and CuPc-S in water and in NaNO3 solution was studied. The adsorption isotherms showed that the adsorption density for both types of particles increased with increasing concentration of surfactant and then reached a plateau above the cmc. The results also imply that micelles do not adsorb. Desorption tests showed that some surfactant molecules adsorbed irreversibly, suggesting the presence of some strong and some weak adsorption sites. The maximum molar adsorption density was higher for the CuPc-U than that for the CuPc-S. The values of minimum areas per molecule indicated coil or "mushroom" conformations of the ethylene oxide chains. At a quite high surface coverage, CuPc-U particles became quite stable, because of a steric mechanism. Adsorption of Triton X-100 on CuPc-S particles in NaNO3 solutions, affected the stability, suggesting that there is a minor steric effect in addition to electrostatic stabilization. The effect of a commercial nonionic surfactant Myrj 45 on stabilizing CuPc-U particles was also studied. A mass spectrometry (MS) analysis revealed that Myrj 45 contains at least 78 components of 6 classes. The adsorption of each component class on CuPc-U particles was determined using an HPLC-MS method. The overall adsorption density of Myrj 45 was much lower than that of Triton X-100. Accordingly the W-values with Myrj 45 were about one order of magnitude smaller. For both systems, the zeta potentials were substantial while the resulting total particle charges were quite low. Hence, electrostatic interactions are unlikely to contribute to the dispersion stabilization mechanism, which appears to be primarily steric. Brownian dynamics (BD) simulations were performed to provide a better understanding of particle aggregation process and information not easily obtained from experiments. Using this computational method, the effects of ionic strength, particle volume fraction, and initial particle size polydispersity were studied. In addition, the average aggregation number and distribution of particle aggregates were examined. A more accurate description of stability could be obtained by BD simulations when the actual interaction potential energies of particles are known or can be estimated.
Corti, Purdue University.
Chemical engineering|Materials science
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