Asphaltene Adsorption onto an Iron Surface: Combined Near-Infrared (NIR), Raman, and AFM Study of the Kinetics, Thermodynamics, and Layer Structure

A combination of near-infrared (NIR) spectroscopy, Raman microscopy, and atomic force microscopy (AFM) was used to analyze the adsorption behavior of petroleum macromolecules (asphaltenes) on an iron (Fe) surface. A partial least-squares (PLS/PLSR) regression method was used for determining the concentration from NIR spectroscopic data. A Langmuir model was used to fit the experimental data. Effective kinetic and thermodynamic parameters (Gibbs energy of adsorption, adsorption/desorption rate constant, maximal adsorbed mass density) of asphaltenes adsorbed from benzene solution were evaluated: the maximum adsorbed mass density (Gamma(max)) was found to be 4.90 +/- 0.07 mg m(-2); the adsorption constant (K) was found to be 0.084 +/- 0.007 L mg(-1); and a value of 34.3 +/- 0.2 kJ mol(-1) was calculated for the effective Gibbs energy of absorption (-Delta G(ads)). The structure of the adsorbed layer was also analyzed by AFM. Asphaltenes were found to form aggregates on Fe surface with an average size of a few hundred nanometers. The notion of a nonuniform distribution of crude oil macromolecules adsorbed on metal surfaces was experimentally confirmed.