Asphaltenes unit size

The GPC of a local crude (Bryan, Texas) sample spiked with a known mixture of n-alkanes and aromatics is shown in Figure 5 and the GPC of the crude is shown in Figure 6. The hydrocarbon mixture is used to calibrate the length of the species which separates as a function of retention volume. Ttie molecular length is expressed as n-alkane carboa units although n-alkanes represent only a fraction of the hydrocarbons in the crude. In addition to n-alkanes, petroleum crude is composed of major classes of hydrocarbons such as branched and cyclic alkanes, branched and cyclic olefins and various aromatics and nonvolatiles namely asphaltenes. Almost all of the known aromatics without side chains elute after n-hexane (Cg). If the aromatics have long side chains, the linear molecular size increases and the retention volume is reduced. Cyclic alkanes have retention volumes similar to those of aromatics. GPC separates crude on the basis of linear molecular size and the species are spread over 10 to 20 ml retention volume range and almost all of the species are smaller than the polystyrene standard (37A). In other words, the crude has very little asphaltenes. The linear... 

The pentane-soluble portion of asphaltene thermolyzed at 300° C has been found to contain a full complement of n-alkanes, acyclic isoprenoids, small ring saturated and small ring aromatic compounds, typical of unbiodegraded conventional crude oil (8). However, in all cases only a small fraction of the degraded asphaltene can be analyzed in this manner, and therefore the question of the size and homogeneity of the remaining hydrocarbon units remains unresolved. 

In order to limit filtration blank contamination, a clean PFA filter unit was developed to separate the oil fi-actions by precipitation of asphaltenes (heavy fi"action) in n-Heptane. Membrane filters of size 0.45 pm in regenerated cellulose were used with the filter unit. One gram of crude oil was dissolved in 50 ml of n-Heptane (in a 250 ml PFA balloon). The solution was then heated for 20 min at 60°C under magnetic stirring and finally filtered under vacuum. Asphaltenes were accumulated on the filter and stocked in 50 ml polypropylene tubes at room temperature in the dark for further analysis. Maltenes, which are soluble in n-Heptane, were collected in a polyethylene Erlenmeyer and evaporated under pure nitrogen flow in 50 ml polypropylene tubes. 

The individual film units increase in size upon addition of resins. This indicates interactions between asphaltenes and resins, providing aggregates of larger dimensions than observed for the pure fractions. Small and moderate amounts of resins give rise to a more poly disperse distribution of the film material, while a further increase in the resin content (i.e., 60 wt % resins) reduces the polydispersity, i.e., the monolayer becomes more uniform in component... 

From the Table n, it can be found that no matter what type of asphalt, all basic units of asphaltene cells are the same size. No matter what solvent is used, the asphaltene micelle is ca. 10 nm, and is consistent with the results of other methods. 

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