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1. Introduction
Properties of crude oil asphaltenes in refining and processing de-
pend on their association state in a hydrocarbon environment. In-
creased association may result in lower transfer rates in catalyst’s
porous networks, in higher levels of entrapment of light fractions
within asphaltene aggregates. For various industrial applications
it is necessary to have a detailed knowledge of complex asphaltene
colloids in order to be able to master their behavior. Refractive in-
dex (RI) represents various important properties of multicompo-
nent crude oils as well of the respective components. Values of RI
can be measured very accurately and are used to correlate density,
parachor, and other properties of hydrocarbons with high reliabil-
ity) (Touba et al., 1997). Information obtained from RI measure-
ments can be applied for various reservoir engineering calcula-
tions.
The RI of light crude oils is directly measured in conventional
refractometers (Yang et al., 2002; Buckley and Wang, 2002; Buck-
ley, 1999). However, direct measurements of the RI of heavy cru-
des are unattainable since these liquids are too opaque. In these
cases RI is determined for a series of oil/solvent mixtures and the
data are extrapolated (in an assumption of a certain mixing rule)
to determine the RI of the crude oil (Yang et al., 2002; Goual and
Firoozabadi, 2002; Taylor et al., 2001). It is usually assumed that a
solution of a crude oil behaves as an ideal binary mixture of the
components (Taylor et al., 2001; Buckley, 1999; Buckley et al.,
1998).
It should be noted that there are no universally accepted mix-
ing rules for interpreting the refractive indices of such presumably
ideal mixtures. Most frequent are mixing rules based on the Lor-
entz–Lorenz additivity rule for specific polarizabilities/refractions
of the components (Shoemaker et al., 1996). In notations of
Buckley and Wang, 2002; Buckley et al., 1998 and Wattana et al.,
2005 :
F(RI)
mixture
= F(RI)
oil
φ
oil
+ F(RI)
solvent
(1 – φ
oil
) (1)
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