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Handbook of Size Exclusion Chromatography and Related ...

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The SEC elution times are dependent on molecular hydrodynamic volume<br />

rather than molecular weight, M, as is the intrinsic viscosity, [h]. Thus the idea <strong>of</strong> a<br />

universal calibration curve is proposed (78) in which log[h]M is plotted vs. the<br />

elution volume. Brulé (12) shows a single curve for a number <strong>of</strong> asphalts, although<br />

it still deviates from the universal curve established for polystyrene or other<br />

polymers (71). In fact, there is considerable deviation from the universal curve for<br />

aromatic <strong>and</strong> highly condensed compounds (79,135). Lafleur <strong>and</strong> Nakagawa<br />

(136), using N-methyl pyrrolidone as the carrier solvent, investigated molecular<br />

weight vs. retention times for a variety <strong>of</strong> molecules in the 100 to 300 MW range.<br />

For polar molecules the retention was largely independent <strong>of</strong> size effects. Most<br />

impressive were results for 19 naphthalene derivatives for which retention volumes<br />

varied from 19 to 30 mL for the same MW.<br />

There are a variety <strong>of</strong> limitations for any SEC asphalt calibration procedure.<br />

First, it is no better than the method used to establish the fraction molecular<br />

weights. This in turn is affected by the solvent, the concentration, <strong>and</strong> the<br />

temperature, with no certainty that complete dissociation has been attained. The<br />

SEC chromatogram is also affected by all these conditions plus others imposed by<br />

the column <strong>and</strong> detector.<br />

Both Girdler (67) <strong>and</strong> Speight et al. (68) published data showing an enormous<br />

range <strong>of</strong> asphalt molecular weights determined by various methods. Table 4<br />

shows a summary <strong>of</strong> some <strong>of</strong> these data in which the entries are average molecular<br />

weights for 14 asphaltenes measured by VPO. Molecular weights so determined<br />

usually decrease with decreasing concentration; elution times for large, associating<br />

material tend to increase with greater dilution. However, Moschopedis et al. (137)<br />

show that even if the molecular weight does not decrease with dilution in one<br />

solvent, it may still show a much lower molecular weight in another.<br />

Noting that VPO molecular weights become relatively constant in hot<br />

nitrobenzene, Moschopedis assumed that these molecular weights corresponded to the<br />

individual asphaltene particles. Based on this assumption, Nali <strong>and</strong> Manclossi (75)<br />

Table 4 VPO Molecular Weight Variations with Solvent Properties<br />

Solvent Temperature (8C) Molecular weight<br />

C6H6 37 5047<br />

CH 2Br 2 37 4015<br />

C2H5N 37 2766<br />

C6H5NO2 100 1900<br />

C 6H 5NO 2 115 1857<br />

C 6H 5NO 2 130 1798<br />

Source: Ref. 137.<br />

© 2004 by Marcel Dekker, Inc.

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