Dissolution of Asphaltene in Binary Mixtures of Organic Solvents and Model Maltenes: Unambiguous Evidence for Asphaltene Preferential Solvation and Relevance to Assessing the Efficiency of Additives for Asphaltene 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, SO/organic solvent (solvent 2, S2) over the entire mole fraction range of S2, xs,, 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 xs, 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-buty1-4-(1-hexylquinolin-1-ium-4-Avinyl]phenolate, HxQMBu(2)) to study Asp dissolution in pure solvents and model maltenes. We showed that E-T(HxQMBu(2)) 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 HxQMBu(2) in the above -mentioned n-heptane/S2 binary mixtures. Except for toluene/n-heptane mixtures, plots of E-T(HxQMBu(2)) versus X-s2 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.