Polar and dispersion interactions at carbon surfaces: further development of the XPS-based model Original Research Article

Enthalpy of immersion (ΔHi) in water has been measured for a series of ozone oxidised non-porous carbon blacks and, as in our previous studies been found to correlate directly with the total surface oxygen level [O]T measured by X-ray photoelectron spectroscopy. An equation that allows calculation of either parameter from the other is given and shown to describe behaviour for a wide range of carbon black surfaces which contain ozone-generated or native oxygen functional groups. Using this approach, the surface polarity and the relative hydrophilic character of such surfaces can be predicted. A molar enthalpy for the polar interaction between water and surface oxygen atoms of 17 kJ mol−1 is obtained by assuming a 1:1 co-ordination between water molecules and carbon surface oxygen atoms. The data lead to a predicted value of 37.5 mJ m−2 for the immersion of oxygen-free carbon black external surface into water. This equates to a value of 2.5 kJ mol−1 for the non-specific dispersion interaction between water and an oxygen-free carbon black surface when a molecular area of 10.5×10−20 m2 for water is assumed. The same carbon black when oxidised using nitric acid gives a different enthalpy of immersion to the ozone-treated and native oxide materials, this is attributed to differing chemistry of the two surface types, this aspect is discussed. The nitric acid treated carbons do, however, give the same value as the ozonated and native oxide carbons (37.5 mJ m−2) for the immersion of an oxygen-free carbon surface into water. A correlation between the point of zero charge (pHPZC) of the carbons and ΔHi or [O]T is also presented. The results from these measurements show extremely good agreement with data from other groups who have used TPD to assess surface oxygen concentration. This gives a firm basis for confident prediction of the thermodynamic properties of carbon surfaces from single measurement techniques.