Relationship between carbon black surface chemistry and energy

Abstract

Nitrogen adsorption data are interpreted in terms of surface area and porosity, for a range of carbon blacks and porous carbons, using the BET, α_s and Dubinin–Radushkevich methods. The relative merits of these approaches for identifying and isolating adsorption in micropores, from adsorption on external surface, are considered. Heat of immersion (Δ_immH) data, for non-polar (n-heptane) and polar (water) liquids are related to surface structure and chemistry, respectively.

An average value of Δ_immH≈ 107 mJ m^–2 is obtained for the dispersion interaction of non-porous carbon blacks by immersion into n-heptane. Where nitrogen adsorption data indicate the presence of microporosity, higher values of Δ_immH are observed which are thought to contain a contribution due to enhanced adsorption forces within these pores. This indicates that the adsorption mechanism within the porosity is one of either primary or secondary micropore filling. Calculation of surface energy (γ^d_s) data from nitrogen spreading pressures also supports this conclusion.

An approximately linear relationship is observed between the surface oxygen levels of several commercially available carbon blacks and their polar interaction with water. This linear relationship is confirmed by Δ_immH data for samples of one specific black which had been oxidised to varying levels, measured by X-ray photoelectron spectroscopy, (XPS), using an oxygen–ozone fluidised bed apparatus. This oxidation process is shown to lead to the development of carboxylic acid groups on the carbon surface; these have been identified using XPS chemical shift data, IR spectroscopy and pH measurement.