Asphaltenes (Asps)
are operationally defined as the toluene-soluble
but n-pentane- or n-heptane-insoluble
fractions, e.g., of crude oils. Therefore, there is intense interest
in determining the concentration of n-heptane required
to precipitate Asps from their solutions in particular media (solvents,
solvent mixtures, and maltenes). Here, we report on the dependence
of Asp dissolution in binary mixtures of n-heptane
(solvent 1, S1)/organic solvent (solvent 2, S2) over the entire mole
fraction range of S2, χ
S2
, and in few
selected maltene models (n-heptane + S2 + benzothiazole
+ n-octyl-1-napthoate). The S2 employed were benzonitrile,
cyclohexanone, ethyl benzoate, 1-methylnaphthalene, tetrahydropyran,
and toluene. For all S2 and model maltenes, the dependence of wt %
dissolved Asp (determined by mass and UV/vis absorbance) on χ
S2
was nonlinear. We attribute this nonlinear,
i.e., nonideal dissolution behavior to “preferential solvation”
of the Asp by a component(s) of the medium (binary solvent mixtures
and maltenes). Although the occurrence of “solvent sorting”
during Asp dissolution was alluded to, this is the first direct and
unambiguous evidence for its occurrence. We used solvatochromism to
corroborate our rationale about the origin of the Asp nonideal dissolution
behavior. The term solvatochromism refers to the effect of the solvent
on the color of solvatochromic probes, substances whose spectra are
sensitive to the properties of the liquid medium, e.g., its empirical
polarity, E
T
(probe).
Recently, we used (E)-2,6-di-tert-butyl-4-[2-(1-hexylquinolin-1-ium-4-yl)vinyl]phenolate, HxQMBu2) to study Asp dissolution in pure solvents and model maltenes.
We showed that E
T
(HxQMBu2) correlates linearly with Hildebrand solubility parameters
of pure solvents as well as with lg (wt % dissolved Asps). In the
present work, we studied the solvatochromic response of HxQMBu2 in the above-mentioned n-heptane/S2 binary
mixtures. Except for toluene/n-heptane mixtures,
plots of E
T
(HxQMBu2) versus χS2 were nonlinear due to probe
preferential solvation. We successfully fitted a solvation model to
the solvatochromic and Asp dissolution data and extracted the enrichment
of the solvation layers in the more polar component(s) of the binary
mixture and model maltenes. Our results bear on the assessment of
additives employed to stabilize Asps: in the absence of adverse effects
of the additive on other properties (e.g., viscosity and water/oil
emulsion stability), efficient additives should accumulate in the
solvation layer of Asp particles where they displace the nonsolvents,
e.g., the saturates.