Handbook of Size Exclusion Chromatography and Related ...

parameterisbasedontheinternalpressure,definedasthesquarerootofthemolar
internalenergyofvaporizationdividedbythemolarvolume.Strictlyspeaking,the
formulationappliesonlytosolutionshavinganidealentropyofmixing,butinfact
itisalsoremarkablygoodforawiderangeofnonpolarandweaklypolarmixtures.
In the modification of Hansen it is assumed that the effective solubility parameter
can be divided into three factors resulting from dispersion forces, polarity,and
hydrogen bonding. The dispersion forces were estimated from the hydrocarbon
homomorph. The polar factor was calculated from theoretical considerations
based on measurements of dielectric constant, dipole moment, and refractive
index.Itisthenassumedthatthemeasuredparameteristhesumofthedispersive,
polar, and hydrogen bonding components, and the latter is calculated from the
difference. The parameter has found many applications and was applied to
asphalt by Hagen et al. (145). In this treatment the polar and hydrogen bonding
components were combined and solubility correlated on atwo-dimensional scale.
They found that asphalt solubility could be represented as contours on this twodimensional
plot. The maximum solubility occurred in aregion occupied by such
solvents as THF,chloroform, and toluene. That these solvents are far from equal
shows the imperfections in the system, but they also found that as the asphalts
aged, the maximum solubility moved in the direction of an increasing hydrogen
bonding parameter.
Thesignificanceisthatthematerial exhibitingmaximumassociationisalso
the most oxidized material, and the solvent should be chosen for this material,
not the whole asphalt. Thus with increasing oxidation, asolvent of increasing
hydrogen bonding should be chosen. This is seen in the data of Cipione et al.
(146), in which the highly oxidized material, which is most tightly bound to the
aggregate in aged asphalt concrete, is much better extracted if ethanol is added to
the solvent.
Asecond useful treatment is that of Snyder (147), in which solvents are
evaluated on the basis of apolarity index calculated from the solvent interaction
with three test solutes: dioxane, ethanol, and nitromethane. Figure 16 (12) shows
an SEC chromatogram of an asphalt for the four solvents indicated. The results
show significant decrease in association at 800 A ˚ as one goes from tetraline to
benzonitrile. Although tetraline has the lowest dielectric constant and benzonitrile
the highest, the order is reversed for THF (E ¼ 7.25) and chloroform (E ¼ 4.806).
On the basis of Snyder’s polarity parameter P 0 , however, the order is THF
ðP 0 ¼ 4:2Þ, chloroform ðP 0 ¼ 4:4Þ, and benzonitrile ðP 0 ¼ 4:6Þ, which agrees with
the 800 A ˚ order.
As with any system, the effect of sample size depends on the response
characteristics of the detector, but with asphalt this is complicated by the greater
association in more concentrated solutions and the dissociation kinetics following
injection. There is usually a decrease in the percentage of LMS as lower
concentrations are injected.
© 2004 by Marcel Dekker, Inc.