Handbook of Size Exclusion Chromatography and Related ...

associate. Girdler (67) and Speight et al. (68) report large ranges of molecular
weights measured by various methods. SEC molecular weight curves must be
calibrated by some external standard, such as against vapor pressure osmometry
(VPO) measurements of preparative SEC fractions of the asphalt (1,12,69–76).
The results are thus limited by the accuracy of the standard, and these methods are
very dependent on the solvent and concentration. Markedly different retention
times for molecules of different structure but the same molecular weight are a
major complicating factor (9,12,68,69,71–73), and data of Bergman and Duffy
(77) with model compounds indicate that this is very solvent dependent. A number
of researchers have used intrinsic viscosity data in an attempt to eliminate the
effect of structurally dependent elution volumes (12,69,71,73,78), but it has been
demonstrated (79) that the assumption of a constant relation between molecular
volumes and elution volumes does not apply to the differing structural types in
asphalt. Domin et al. (80) compared SEC measured MWs using N-methyl
pyrrolidinone to VPO and mass spectrophotometric values.
Because of the tendency of asphalts to associate and also to be adsorbed on
the column (7,10,42,69,81–83), the choice of solvent is very important. Jennings
et al. (42) reported that the relative percentage of LMS between asphalts could be
reversed by using chloroform instead of tetrahydrofuran (THF). Altgelt and Gouw
(81) report that 5% methanol in chloroform or benzene is an excellent solvent.
Bishara and McReynolds (84) added 5% pyridine to THF to reduce adsorption
of polar materials. Brulé (12) compared several solvents: the greater the polarity,
the smaller the LMS region. Although increasing polarity tends to decrease the
percentage LMS, this is not automatic and depends on the specific interactions.
Jennings et al. (85) showed 5% methanol (MeOH) in THF increasing the
percentage of LMS. Done and Reid (82) and Donaldson et al. (86) compared THF
and toluene. Higher concentrations, higher flow rates, as well as a poorer solvent
can cause an increase in the LMS region (12,41,83,87,88). A lengthy residence
time of asphalt in a solvent also causes a growth in the LMS region (12,35,89).
There is an increasing use of polymers in asphalt and these are easily
detected by SEC. One of the most common uses is to detect the changes in
polymer molecular size as it is mixed with asphalt at high temperature (66,90,91).
SEC is also used to detect the changes in polymer molecular size as oxidation
occurs (92–96).
2 ASPHALT CHEMISTRY
Asphalt is probably the most complex material routinely studied by SEC. Asphalt
is the residual left when practically everything that can be recovered from crude oil
by high-vacuum, high-temperature distillation has been vaporized. Alternatively,
the residuum may be propane extracted to remove even more material and the
© 2004 by Marcel Dekker, Inc.