IMAGING OF PETROLEUM ASPHALTENES USING SCANNING-TUNNELING-MICROSCOPY

Scanning tunneling microscopy (STM) was used to image Ratawi (neutral zone) asphaltenes deposited on highly oriented pyrolytic graphite (HOPG). STM images revealed the formation of ordered arrays covering hundreds of angstroms of the surface. Samples deposited from pyridine solutions above and below the critical micelle concentration were quite similar. The individual features imaged in these arrays were attributed to the termini of the pendant aliphatic chains known to be a part of the asphaltene molecular structure. The ordered arrays imaged were extremely flat and uniform, in contrast to the assumed molecular complexity and the absence of unique structure of asphaltenes. The formation of these ordered surface arrays is further evidence of the tendency of asphaltenes to self-associate.