Role of Impurity Nanoparticles in Laser-Induced Nucleation of Ammonium Chloride

Abstract

Results of experiments on laser-induced nucleation (LIN) in supersaturated (120%) aqueous ammonium chloride solutions are presented. Measurement of the particle-size distribution in unfiltered solutions near saturation (95%) indicates a population of nanometer-scale species with a mean hydrodynamic diameter of 750 nm, which is almost entirely removed by single-pass filtration through a poly(ether sulfone) membrane (0.2 μm pores). Analysis of filter residues reveals iron and phosphate as major impurities in the solute. Experiments show that the number of nuclei induced by LIN can be reduced substantially by preprocessing (filtering or long-term exposure to laser pulses) and that this reduction can be reversed by intentional doping with iron-oxide (Fe3O4) nanoparticles. The use of surfactant to assist dispersion of the nanoparticles was found to increase the number of laser-induced nuclei. We discuss the results with reference to mechanisms of non-photochemical laser-induced nucleation.