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counted for by Kramers–Kronig relations (Jackson, 1999). It
should be noted that dispersion effects are neglected in simple RI
mixing rules.
2.6. Correlation of RI with Optical Absorptivity
RI of dilute solutions is known (Gray and Gubbins, 1984) to be
affected mainly by: 1) changes in molecular polarizability of a sol-
ute (due to variations in itermolecular interactions), 2) changes in
the number of solute molecules per unit volume (solute concentra-
tion). A trivial concentration factor may be excluded by calculation
of a “RI increment” dn/dc, which is directly proportional to the mo-
lecular polarizability α :
α = (M / 2πN
A
)(dn/dc) (2)
where M is the molar mass, N
A
is the Avogadro number and c is
the solute concentration.
The left part of Figure 3 shows the behavior of RI increment
in the studied solutions. At higher concentrations dn/dc is fairly
constant and negative (i.e. polarizability of asphaltenes is smaller
than that of toluene). In more dilute solutions dn/dc becomes posi-
tive and at 2–8 mg/l of asphaltenes its absolute value increases by
a factor of about 12. According to Eq. (2), this may have been in-
terpreted as a 12-fold polarizability increase of asphaltene mono-
mers as compared to molecules in asphaltene aggregates. However,
there is strong theoretical and experimental evidence (Jensen et
al., 2002) that electronic molecular polarizability (which deter-
mines a magnitude of RI for visible light wavelengths) is almost
unchanged by the intermolecular association. On the other hand,
the behavior of dn/dc appears to be qualitatively similar to the be-
havior of optical absorptivity at 670 nm (shown in the right part of
the Figure).
As stated above, we attribute the observed changes of the op-
tical properties to association/dissociation of asphaltene molecules
in the crude oil. This attribution is further supported by virtual co-
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