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UV/Vis absorptivity of crude oils is known to be governed by
π–π* transitions in the conjugated aromatic chromophores of as-
phaltenes (cf. references in Evdokimov et al., 2003a,b). By analogy
with other aromatic compounds (Birks, 1970) we may attribute a
resonance band (285 nm) to the S
0
(
1
A
g
) → S
2
(
1
B
3u
) transition. The
lower energy transition S
0
(
1
A
g
) → S
1
(
1
B
1u
) may be assigned to a
low-intensity broad band centered at ∼580–600 nm and clearly re-
vealed only in derivative spectra.
Numerical evaluation of K–K formula was performed for both
spectra of Figure 4. Even if only this limited frequency range is
considered, evaluation shows that K–K effects may account for a
large increase of RI
oil
– from ≈1.47 in concentrated solutions to
≈1.72 at high dilutions. It may be expected that with inclusion of
short-UV and IR spectral ranges, K–K effects would be found fully
responsible for a “surprisingly high” experimental value of
RI
oil
= 2.0121 (cf. Section 2.4).
3. Conclusions
Dilute asphaltene-containing solutions in toluene (asphaltene con-
centrations below 150–200 mg/l) show peculiar non-ideal behavior
of refractive index and UV/Vis absorptivity. Sharp inflections in
experimental concentration dependencies are indicative of changes
in association states of asphaltenes.
Apparently, asphaltenes may exist in a monomeric state only
at concentrations below 1–2 mg/l. A transition from monomeric to
associated asphaltenes is accompanied by a notable increase of a
resonant absorption band at
∼285 nm. Since the emerging absorp-
tion band is located in a wavelength region shorter than the probe
wavelength for RI, it creates a positive refractive index change, ac-
cording to the Kramers–Kronig relations.
We believe that the present study may provide further insight
into the formation, stability and optical properties of molecular ag-
gregation in crude oils, as well as in other asphaltene-containing
industrial fluids.
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