Optical characterization of asphaltenes at the air-water interface.

Asphaltenes extracted from Arabian light crude oil have been investigated at the air-water interface of a Langmuir trough by in situ optical techniques. Brewster angle microscopy (BAM) and reflection spectroscopy have been used to extract new information about the microscopic organization of the asphaltene films in terms of association phenomena and chromophore orientation, respectively. The use of different spreading concentrations in the range 0.1-15 mg mL-1 reveals significant changes in the behavior at the interface with more condensed isotherms above 2 mg mL-1. This break point is related to the nanoaggregate-to-cluster association threshold in organic solution widely accepted in the recent asphaltene literature. BAM images support this observation with very different morphologies for the two spreading concentrations employed, 0.1 and 4 mg mL-1, respectively. The study of intensity changes in the corresponding normalized reflection spectra also confirms the transition in the asphaltene interfacial behavior between these two spreading concentrations. Finally, this technique helps with understanding the changes observed in the asphaltene films during a set of compression-decompression cycles.

Laser desorption/ionization determination of molecular weight distributions of polyaromatic carbonaceous compounds and their aggregates.

Molecular weight distributions (MWDs) of model polyaromatic hydrocarbons (PAHs) and complex asphaltene samples have been investigated in laser desorption/ionization mass spectrometry (LDI-MS) experiments. Special efforts are devoted to the characterization of aggregation effects during the desorption process. It is found that non-covalent clusters of the PAHs and asphaltenes form readily in the desorbing plume. Aggregation is favoured in the experiments performed on dense samples at high laser energy and under continuous ion extraction conditions. In the absence of polar groups in the analyte molecules, the aggregation propensity correlates well with the size of the polycondensed system and with its degree of pericondensation, in qualitative agreement with previous theoretical predictions. For the polydispersed asphaltenes from two different crude oils, MWDs peaking at masses smaller than 500 amu with a high-mass tail extending up to about 3000 amu have been observed, yielding average weights around 900 amu. Such MWDs are in good agreement with previous mass spectrometric measurement, as well as with diffusion studies in solution. In addition, stable asphaltene aggregates have been detected giving rise to two broad bands in the mass spectrum corresponding to average molecular weights of 2200-3100 amu and 15,000-19,000 amu, respectively. It is concluded that the strong aggregation propensity of asphaltenes is likely to be responsible for the apparent inconsistency between the MWD for these compounds determined by different groups in independent LDI-MS experiments. The reliability of different sample preparation procedures, including solvent-free methods, is discussed, and strategies are outlined that serve to apply the potentiality of LDI mass spectrometry to the characterization of covalent and non-covalent compounds in complex carbonaceous systems.

On the determination of molecular weight distributions of asphaltenes and their aggregates in laser desorption ionization experiments.

Molecular weight distributions (MWD) of asphaltenes and their aggregates have been investigated in laser desorption ionization (LDI) mass spectrometric experiments. A systematic investigation of the dependence of the measured MWD on the asphaltene sample density and on the laser pulse energy allows the assignment of most probable molecular weights within 300-500 amu and average molecular weights of 800-1000 amu for the monomeric asphaltenes, as well as for the estimation of the contribution from asphaltene clusters in typical LDI measurements. The results serve to reconcile the existing controversy between earlier mass spectrometric characterizations of asphaltenes based on laser desorption techniques by different groups. Furthermore, the MWD measurements performed on particularly dense samples yield an additional differentiated broad band peaking around 9000-10,000 amu and extending over 20,000 amu, not observed previously in LDI experiments, thereby revealing a strong propensity of the asphaltenes to form clusters with specific aggregation numbers, which is in qualitative agreement with previous theoretical predictions and with the interpretation of measurements performed with other techniques.