Adsorption of Myrj 45 on copper phthalocyanine pigment nanoparticles and effect on their dispersion stability in aqueous solution

The composition of a commercial nonionic surfactant, Myrj 45, was analyzed with mass spectrometry (MS). Six classes of components, which are polyglycols, monoesters (monostearates and monopalmitates), and diesters (distearates, dipalmitates, and mixed diesters which are the diesters with stearoyl residues at one end and palmitoyl residues at the other) were detected. Each class of components shows a wide distribution of homologues. The monoesters are the dominant components in Myrj 45. The adsorption isotherm of each class of components was obtained using HPLC with a MS detector. The adsorption densities increase with increasing concentrations of each class of components up to a plateau. The monoesters adsorb the most at the same total surfactant concentration. The diesters and polyglycols also adsorb, but to a minor extent. The overall adsorption densities of Myrj 45 by taking into account the contributions from these six classes of components, are much lower than those of Triton X-100 at the same total molar concentration. The effect of Myrj 45 on the dispersion stability of the CuPc particles in pure water was studied and compared to that of Triton X-100. Upon adsorption of Myrj 45, the CuPc particles become quite hydrophilic, dispersible, and colloidally stable. Freezing/thawing and heating/cooling cycles tests suggest that the dispersions of CuPc particles with adsorbed Myrj 45 are not as stable as those with adsorbed Triton X-100. Particle aggregation primarily occurred during the freezing/thawing process. The experimental Fuchs–Smoluchowski dispersion stability ratios Wexp of CuPc particles in surfactant solutions were determined from dynamic light scattering (DLS) data, for the Rayleigh–Debye–Gans (RDG) light scattering regime. The Wexp-values with Myrj 45 are found to be about one order of magnitude smaller than those with Triton X-100. The zeta potentials are substantial, in the range of −16 to −46 mV, while the resulting surface charge densities and the total particle charges are quite low. Hence, electrostatic interactions are unlikely to contribute to the dispersion stabilization mechanism, which appears to be primarily steric.

The total adsorbed densities as a function of the total molar concentration of Myrj 45: 1 (■) total; 2, () polyglycols; 3, () monostearates; 4, () monopalmitates; 5, () distearates; 6, () dipalmitates; 7, () mixed diesters. The adsorbed monolayer is dominated by the monoesters 83 mol%, and contains about 12 mol% diesters and 5% polyglycols.Figure optionsDownload as PowerPoint slideFigure optionsDownload as PowerPoint slideFigure optionsDownload as PowerPoint slideFigure optionsDownload as PowerPoint slideFigure optionsDownload as PowerPoint slideFigure optionsDownload as PowerPoint slideFigure optionsDownload as PowerPoint slideHighlights
► Myrj 45 contains six major classes of components with 10–14 homologues per class.
► The monoesters adsorb the most and the diesters and polyglycols adsorb to a minor extent.
► The stabilization mechanism for hydrophobic CuPc particles with adsorbed Myrj 45 is steric.
► The zeta potential for dispersions in pure water is not a good predictor of dispersion stability.