Average molecular properties asphaltenes

Table I summarizes comparisons between asphaltenes derived from bituminous coal liquefaction and those derived from petroleum crudes. The molecular size and atomic H/C ratios suggest a molecular profile quite different for the two asphaltenes. The ranges represent, as best as could be found, reasonable extremes for each of the properties. We are well aware that the number-average molecular weight of petroleum asphaltenes has been influenced by aggregate formation. To overcome this effect, molecular weight determinations should be made in dilute noninteracting solvents (e.g., methylene chloride), and solutions should be filtered or ultracentrifuged in helium-degassed solvents.

The effect of these two distributions, namely the partitioning of species between the two phases, and the overlap of the two phases with respect to molecular weight and functionality implies that the separation between maltenes and asphaltenes is nondefinitive in terms of chemical functionalities and structures. While properties measurements commonly detect differences in the average properties, these differences are small when compared with the width of the distribution about that average. 

The effort must be multidimensional, taking into account all of the properties and characteristics of the material. Extreme caution is advised when average structures are used. For example, models that could better explain the asphaltene behavior should be selected from the various parts of the molecular weight-polarity diagram. The easiest would be to select the two extremes of this diagram and design highly polar and neutral species. This would be the amphoteric and substantially less polar (or near neutral) constituents of asphaltenes (IS, 19, 93). 

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