We extended the oil compatibility model to the dissolution of asphaltenes (Asps) in maltenes from 10 crude oils (COs). As scales for the power of solvents of interest, vide infra, we used solvatochromic parameters, calculated from the UV–vis spectra of solvatochromic compounds (probes), Hildebrand/Hansen solubility parameters, and the colloidal instability index of COs. As the colors of maltenes or asphaltene-free crude oils CO(Asp‑free) were too dark to permit recording the absorption spectra of the probes, we formulated models for these fractions (MCO(Asp‑free)). They were composed of low molar mass hydrocarbons, namely, cis and trans decalines, isooctane, 1-methylnaphthalene and, as model for resins, benzothiazole/n-octyl-1-naphthoate. We based formulations of these MCO(Asp‑free) on SARA analysis of the COs and elemental analysis of the corresponding resins. We validated MCO(Asp‑free) as models for the corresponding CO(Asp‑free) by showing that the correlation between Hildebrand solubility parameter (δt) of (COs) and δt for MCO(Asp‑free) is linear with a slope close to unity. Regarding Asp dissolution, we show that the correlations between log­(dissolved Asp, mass %) and each of the following solvent descriptors is linear: empirical polarity of MCOs(Asp‑free); δt of COs; colloidal instability index of COs. Furthermore, the multiple correlation between log­(dissolved Asp, mass %) and other solvatochromic parameters showed that solvent dipolarity and polarizability are important factors for Asp dissolution, in agreement with our previous results on Asp dissolution in pure solvents. The formulation of a model that successfully mimics maltenes is potentially very useful, e.g., in rationalizing the efficiency of certain classes of additives employed for Asp stabilization.

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.