Concentration Effects in HPLC-SEC Analysis of Petroleum Asphaltenes

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Abstract
The effect of concentration on the HPLC-SEC elution profile of petroleum asphaltenes has been examined in order to investigate the possible determination of critical micelle or microstructure concentrations as observed by other more laborious methods. It was observed in one case that above a certain threshold concentration (TC) the profile became constant in shape. TC was found to be in the range of previously reported CMC values by calorimetric titration. However, according to micellization theory the profile should start changing beyond the threshold concentration. Hence the association process of asphaltenes may even commence below the previously reported CMC possibly through a step-wise mechanism. A large dependence of the observed concentration effect upon wavelength of the UV-vis diode array detector is also reported. Therefore caution should be exercised when applying HPLC-SEC in deriving physicochemical information on heavy self-associating petroleum fractions.