On the hydrophilic/hydrophobic character of carbonaceous nanoparticles formed in laminar premixed flames

Carbon nanoparticles in laminar premixed flames are broadly divided into two classes based on the bimodal shape of the particle size distribution and on the different chemical and physical properties that these particles present depending on the combustion conditions, such as residence time, equivalence ratio, and fuel chemical composition. The chemical and structural characteristics of carbon nanoparticles have been the subject of numerous works because these properties might be of relevance for particle reactivity and optical properties. Few information are available on their hydrophilic properties although these are of relevance for the human health, the climate change, in addition to the technological implementation of condensation nuclei particle counters in aerosol science, water scrubbers and electrostatic precipitators. The aim of this work is to investigate the hydrophilic/hydrophobic behavior of carbon nanoparticles formed in different flame conditions. Static contact angle measurements in addition to chemical and physical characterization of the carbon nanoparticles have been implemented. Results show that nanoparticles formed in richer flame conditions, are the most hydrophobic, whereas nanoparticles of organic carbon, formed in relatively leaner flame condition, appeared to be the most hydrophilic. The reason for a different water affinity of particles, and especially of the smaller organic carbon nanoparticles, has been discussed by analyzing the different material in terms of their chemical/structural composition and in terms of surface functionalities. While no significant differences have been found by Raman spectroscopy in terms of their carbon structure, the different hydrophilicity is explained in terms of the different amount of surface oxygen detected by X-ray photoelectron spectroscopy (XPS). Combustion conditions are therefore very important in outlining the hydrophilic/hydrophobic tendency of the carbon particles.