CONCENTRATION EFFECTS IN HPLC-SEC ANALYSIS OF PETROLEUM ASPHALTENES

The effect of concentration on the HPLC-SEC elution profile of petrole um 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 c onstant in shape. TC was found to be in the range of previously report ed CMC values by calorimetric titration. However, according to micelli zation theory the profile should start changing beyond the threshold c oncentration. Hence the association process of asphaltenes may even co mmence below the previously reported CMC possibly through a step-wise mechanism. A large dependence of the observed concentration effect upo n wavelength of the UV-vis diode array detector is also reported. Ther efore caution should be exercised when applying HPLC-SEC in deriving p hysicochemical information on heavy self-associating petroleum fractio ns.