Confined Crystallization of Pigment Red 146 in Emulsion Droplets and Its Mechanism

In this work, the effect of confined space on crystallization processes of pigments was investigated by using C.I. Pigment Red 146 (PR 146) as a model compound. The colloidal system (i.e., emulsion droplets) was used as a nanoreactor to prepare nanoscale PR 146 for the inkjet printer. The effects of the space confinement were investigated by comparing the products of PR 146 prepared from bulk solution, macroemulsion, and miniemulsion. The results showed that PR 146 crystallized in mini-emulsion had the narrowest particle size distribution and the average particle size can be as small as 172.5 nm, one order of magnitude smaller than the one obtained from the bulk solution. X-ray diffraction (XRD) data revealed that PR 146 crystallized in all three solutions where the crystalline state and had similar crystallite sizes. The process mechanism of crystallization confined in the miniemulsion droplets was proposed and explained. The function mechanism of the co-stabilizer during the crystallization of PR 146 in emulsion was also explained. It was found that sodium chloride could counteract the pressure difference as an osmotic pressure agent and prevent the migrating of water from small droplets into big droplets. The influences of dosages of emulsifiers and co-stabilizers on droplet size and the size of the obtained PR 146 particles were evaluated and the optimal conditions were determined. Furthermore, the disparity of PR 146 products prepared by different methods was investigated by UV–Vis spectra. The aqueous dispersion of PR 146 crystallized in miniemulsion had the highest absorbance and darkest color.