Water interaction with hydrophobic and hydrophilic soot particles

Abstract

The interaction of water with laboratory soots possessing a range of properties relevant for atmospheric studies is examined by two complementary methods: gravimetrical measurement of water uptake coupled with chemical composition and porosity analysis and HTDMA (humidified tandem differential mobility analyzer) inference of water uptake accompanied by separate TEM (transmission electron microscopy) analysis of single particles. The first method clarifies the mechanism of water uptake for bulk soot and allows the classification of soot with respect to its hygroscopicity. The second method highlights the dependence of the soot aerosol growth factor on relative humidity (RH) for quasi-monodisperse particles. Hydrophobic and hydrophilic soot are qualitatively defined by their water uptake and surface polarity: laboratory soot particles are thus classified from very hydrophobic to very hydrophilic. Thermal soot particles produced from natural gas combustion are classified as hydrophobic with a surface of low polarity since water is found to cover only half of the surface. Graphitized thermal soot particles are proposed for comparison as extremely hydrophobic and of very low surface polarity. Soot particles produced from laboratory flame of TC1 aviation kerosene are less hydrophobic, with their entire surface being available for statistical monolayer water coverage at RH approximately 10%. Porosity measurements suggest that, initially, much of this surface water resides within micropores. Consequently, the growth factor increase of these particles to 1.07 at RH > 80% is attributed to irreversible swelling that accompanies water uptake. Hysteresis of adsorption/desorption cycles strongly supports this conclusion. In contrast, aircraft engine soot, produced from burning TC1 kerosene in a gas turbine engine combustor, has an extremely hydrophilic surface of high polarity. Due to the presence of water soluble organic and inorganic material it can be covered by many water layers even below water saturation conditions. This soot demonstrates a gradual diameter growth factor (D(wet)/D(dry)) increase up to 1.22 at 93% relative humidity, most likely due to the presence of single particles with water soluble material heterogeneously distributed over their surface.

Small-Scale Study of Siberian Biomass Burning: I. Smoke Microstructure

“…3.2). Such oxygen content is typical for soot produced by hydrocarbon and fossil fuel combustion (Kireeva et al, 2009;Bladt et al, 2012), and determinesthe hydrophobic character of soot interaction with water (Popovicheva et al, 2008). Bulk analysis confirmed the high EC fraction (68 and 63% in TC) with OC/EC ratios of 0.46 and 0.57 for pine and debris smoke, respectively.…”

Aging induced changes on NEXAFS fingerprints in individual combustion particles

“…After returning the RH to 0 %, the absorption intensity was observed in this case to not fully decrease to the initial level. This residual additional absorption could stem from either X-ray radiation damage or residual water trapped in the soot particle due to irreversible transformation of the soot particle during "swelling" (Popovicheva et al, 2008). We find an average water column of 15 nm in this soot particle at 90 % RH.…”

A molecular model for carbon black primary particles with internal nanoporosity

“…In particular, the large surface area can always facilitate the applications of templated carbon as catalyst supports and adsorbent agents. Carbon black has different chemical and physical properties from soot and black carbon that is normally generated from incomplete fuel burning [14][15][16][17]. In contrast to the low carbon composition (<60%) of soot or black carbon, carbon black material consists mostly of elemental carbon (>97%) in the form of nearly spherical particles of colloidal size less than 100 nm that are further fused together as aggregates in the size of hundreds nanometers.…”