Unary and Binary Heterogeneous Nucleation of Organic Vapors on Monodisperse WOx Seed Particles with Diameters Down to 1.4 nm

Unary and binary heterogeneous nucleation has been studied experimentally using monodisperse tungsten oxide (WOx) particles in the size range from 1.4 nm up to 7 nm. WOx particles were generated using a commercially available WOx generator in combination with a nanoDMA for size selection. n-Propanol and n-nonane as well as mixtures thereof were used as the nucleating substances. The experiments were performed with an expansion-type condensation particle counter in combination with a constant-angle Mie scattering detector providing counting efficiency of 100% down to at least 3 nm particle diameter. The binary n-propanol-n-nonane system was found to co-nucleate only reluctantly which is in agreement with homogeneous nucleation experiments as well as composition of critical clusters as determined by the heterogeneous nucleation theorem. The size dependent measurements for the pure systems revealed that n-nonane leads to contamination of the chamber at particle sizes below 4 nm. In contrast, n-propanol is suitable to detect particles as small as 1.4 nm.