Solvatochromic and Solubility Parameters of Solvents: Equivalence of

May 11, 2016 - The oil compatibility model is important for assessing the stability of crudes. The compatibility between maltenes and the correspondin...
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Solvatochromic and Solubility Parameters of Solvents: Equivalence of the Scales and Application to Probe the Solubilization of Asphaltenes Luzia P. Novaki,† Edgar O. Moraes,† André B. Gonçalves,† Raphael A. de Lira,† Vanessa N. Linhares,† Marcia C. K. de Oliveira,*,‡ Francis A. Meireles,‡ Gaspar Gonzalez,§ and Omar A. El Seoud*,† †

Institute of Chemistry, The University of São Paulo, SP P.O. Box 26077, 05513-970 São Paulo, São Paulo, Brazil Flow Assurance Laboratory, Petrobras Research Center (CENPES), Avenida Horácio Macedo 950, Cidade Universitária, 21941-915 Rio de Janeiro, Rio de Janeiro, Brazil § Institute of Macromolecules, Federal University of Rio de Janeiro, Avenida Horácio Macedo 950, Cidade Universitária, 21941-915 Rio de Janeiro, Rio de Janeiro, Brazil

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‡

S Supporting Information *

ABSTRACT: The oil compatibility model is important for assessing the stability of crudes. The compatibility between maltenes and the corresponding asphaltenes, Asphs, can be assessed from the solubility parameters (Hildebrand and Hansen) of both components of the crude. Solvatochromism is the effect of the medium on the UV/vis spectra of substances (solvatochromic probes) that are sensitive to the properties of the medium, namely, its empirical (or overall) polarity, Lewis acidity and basicity, dipolarity, and polarizability. Therefore, the solubility and solvatochromic parameters of solvents should be related. We synthesized a novel solvatochromic probe (E-2,6-di-tert-butyl-4-(2-(1-hexylquinolin-1-ium-4-yl)vinyl)phenolate, HxQMBu2) whose properties are convenient to study in nonpolar and polar solvents. The empirical solvent polarities measured with HxQMBu2 in 38 solvents correlated linearly with the corresponding Hildebrand solubility parameters. Likewise, the solvent Lewis acidity/basicity, dipolarity, and polarizability correlated linearly with the corresponding Hansen solubility parameters. To test the equivalence of the two scales (solvatochromic and solubility parameters), we determined the solubility of Asphs in 28 solvents, pertaining to different chemical classes. The dependence of Asph solubility on three solvent descriptors (Lewis acidity/ basicity, dipolarity, and polarizability) was tested. Our results indicated that alcohols and hydrocarbons are inefficient solvents; solvents of intermediate efficiency carry either a strongly dipolar group or polarized bonds. Aromatic and heterocyclic solvents are most efficient. The most relevant solvent descriptor (for the dissolution of Asphs) is its polarizability.

1. INTRODUCTION The problem of asphaltenes (Asphs) stability cannot be overemphasized due to its relevance to all phases of the petroleum oil industry, namely, extraction, transport, and refining. Of central importance is the flocculation followed by sedimentation of Asphs in reservoirs and surface facilities that leads inter alia to serious rheology problems during crude pumping and in extreme cases to clogging of pipes and other pieces of equipment. The Asph instability problem also arises when crude oils of very different origins (and hence compositions) are blended before fractional distillation.1,2 Therefore, several aspects of the chemistry of Asphs were intensely investigated, including the structure of Asphs and the associated resin3 and the mechanism of Asph stability.4−6 A parallel effort was the introduction of operational scales for evaluating Asph stability.7−14 The most employed scales are the colloidal instability index, CII, the Heithaus (P) parameter, the toluene equivalence (TE) parameter, and the related BMCI-TE (Bureau of Mines Correlation Index minus the toluene equivalence). Another approach is to use the oil compatibility model.14 Some advantages and limitations of these scales are discussed elsewhere.7 The CII index considers crude oil as a colloidal system made up of pseudophases (components); it relies on using the results © 2016 American Chemical Society

of SARA (saturates, aromatics, resins, and asphaltenes) analysis to calculate CII, according to eq 1, where the composition is SARA-analysis-based:15,16 CII =

(wt % asphaltene + wt % saturates) (wt % resins + wt % aromatics)

(1)

The stability of the crude oil is inferred from the value of CII: Unstable oils are associated with CII ≥ 0.9, stable ones have CII ≤ 0.7, and values between 0.7 and 0.9 are considered borderline cases. The scale developed by Heithaus relies on the determination of the state of peptization parameter, P, the values of which vary between 2.5 and 10 for bitumen asphalts and heavy oil residuum.17,18 Crudes with low values of P are designated as incompatible (with the corresponding asphalts), whereas those with high P values are designated as compatible.19,20 The TE index measures the capacity of crude oil dissolved in toluene to keep the Asph stable on addition of n-heptane. The BMCI index is calculated from the density and midboiling point of a given petroleum fraction. The crude oil is Received: February 26, 2016 Revised: May 10, 2016 Published: May 11, 2016 4644

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Figure 1. Solvatochromic dyes employed for determination of the empirical, i.e., total, solvent polarity. These include 2,6-diphenyl-4-(2,4,6triphenylpyridinium-1-yl)phenolate, RB, and 2,6-bis[4-(tert-butyl)phenyl]-4-{2,4,6-tris[4-(tert-butyl)phenyl]pyridinium-1-yl}phenolate, (t-Bu)5RB. The novel probe E-2,6-di-tert-butyl-4-(2-(1-hexylquinolin-1-ium-4-yl)vinyl)phenolate), HxQMBu2, was synthesized in the present work. We show the solvent-sensitive intramolecular charge transfer that occurs within HxQMBu2 upon light absorption.

Figure 2. Solvatochromic probes employed for determination of solvent descriptors, including acidity (SA, o-tert-butylstilbazolium betaine and o,o′di-tert-butylstilbazolium betaine, IA and IB, respectively), basicity (SB, 5-nitroindoline and 1-methyl-5-nitroindoline, IIA and IIB, respectively), dipolarity (SD, 2-(N,N-dimethylamino)-7-nitrofluorene, III), and polarizability (SD, β-carotene, IV).

considered stable if (BMCI-TE) is in the range of 7−14.21,22 The oil compatibility model is based on the assumption that Asphs are stable if they are “compatible” with the corresponding maltenes, i.e., it relies on determination of the solvent “power” or efficiency of maltenes. One approach to assess this compatibility is to compare the Hildebrand solubility parameters δ for both crude oil and the corresponding Asph. The value of δ is calculated from the cohesive energy density of the components of the crude oil (asphaltene and maltene). This parameter uses the unit of energy MPa0.5 (= 2.0455 cal1/2 cm−3/2); the assumption is that mixtures (e.g., maltenes and the corresponding Asphs) with similar cohesive energy densities within a certain range are mutually soluble.23−25 Hansen and co-workers proposed an extension of the Hildebrand solubility parameter concept to polar and hydrogen-bonding systems. As shown in eq 2, Hansen solubility parameter δt includes contribution from dispersion forces as well as specific interactions, e.g., dipolar ones and hydrogenbonding:26 δt 2 = δAB 2 + δ D2 + δ P 2

dispersion properties of the solvent, respectively. The symbols that we use in eq 2 are different from those originally suggested by Hansen. We use them for consistency with those employed in solvatochromism, vide infra. eq 2 has been applied to determine the contribution of these parameters to the stability of six crude oils.27 Equation 2 is a version of the general solvation free energy relationship (eq 3) that quantifies the relative contributions of specific solvent descriptors to a chemical phenomenon (reaction rate, position of equilibrium, solvation, spectroscopic transition, etc.): Effect of solvent (on a chemical phenomenon) = aSA + bSB + dSD + pSP

(3)

where the solvent’s Lewis acidity, Lewis basicity, dipolarity, and polarizability are given by SA, SB, SD, and SP, respectively. Therefore, eq 2 is a version of eq 3, where SA and SB are considered jointly in the δAB2 term. Likewise, some authors use the SD and SP terms of eq 3 jointly, as (d/p × SD/SP).28 When eq 3 is applied to a chemical phenomenon in a series of solvents, the medium effect is inferred from the sign and magnitude of the regression coefficients (a, b, d, and p). For

(2)

The terms AB, D, and P refer to hydrogen bonding donation (Lewis acidity) and acceptance (Lewis basicity), dipolar, and 4645

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Energy & Fuels Scheme 1. Schematic Representation of the Synthesis of HxQMBu2

Figure 8, were found to be similar. This corroborates our initial expectation that empirical solvent polarity scales can be used to probe Asph dissolution and hence the stability of crude oils.

example, the following regression coefficients were calculated for the reaction of 1-methylimidazole with 1-bromohexane in 10 solvents at 40 °C: a = −3.79; b = +20.89; d/p = +56.36. This means that solvent basicity and dipolarity/polarizability favor the SN2 reaction; the latter solvent descriptor is more important. However, the acidity disfavors the reaction, probably because of diazole solvation by hydrogen-bonding.29 In summary, eqs 2 and 3 are equivalent because they analyze the complex effects of solvent on chemical phenomena as a linear combination of individual contributions; eq 3 has the merit that it separates solvent Lewis acidity from its Lewis basicity. Equation 4 is an extensively employed version of eq 3. It uses solvatochromism (effect of the medium on the color of a dye or probe) to assess the relative importance of solvent descriptors to an electronic transition within the dye: E T(probe) = aSA + bSB + dSD + pSP

2. EXPERIMENTAL SECTION 2.1. Solvents, Reagents, and Asphaltenes. We purchased solvents and reagents from Acros, Merck, or Synth (São Paulo) and purified them as recommended elsewhere.30 Mixtures of isomers of xylenes (ortho, meta, and para) and decalines (cis and trans) were employed without separation of the isomers. We obtained an asphaltene-rich solid residue from the deasphalting unit of Petrobras (REVAP, São José dos Campos, SP) and removed the nonasphaltic material by repeated extraction with n-heptane as follows: Solid (200 g) was suspended in 400 mL of n-heptane. The suspension was sonicated for 4 h (Fritsch, Laborette 17), and the heptane layer was separated by decantation. We repeated this procedure three more times. The remaining solid was extracted with n-heptane for 32 h (Soxhlet). The residue was dried in air, ground into powder, and further dried at 60 °C in a vacuum oven, until constant weight was achieved. Elemental analysis: C, 83.05%; H, 8.60%; N, 1.64%; and S, 5.35%. The H/C (atomic) ratio is 1.23, which is in the same range as that found for Asphs extracted from crude samples, e.g., from Brazil (1.18)31 and Venezuela (1.10−1.38).32 2.2. Synthesis of the Solvatochromic Probe HxQMBu2. We synthesized this probe according to Scheme 1.33−36 The halide 1-(nhexyl)-4-methylquinolinium iodide was synthesized by the reaction of 4-methylpyridine (4.8 mL, 0.036 mol) and 1-iodohexane (5.3 mL, 0.036 mol) in toluene (30 mL) in the presence of piperidine catalyst (0.5 mL, 5 mmol). The solution was kept under reflux for 36 h. The formation of the product (dark dense oil) was confirmed by 1H NMR spectroscopy (Varian Innova-300 spectrometer; CDCl3); it was employed directly in the next step. HxQMBu2 was obtained by condensation of the above-mentioned oil (9.2 g, 0.026 mol) and 3,5-di-tert-butyl-4-hydroxybenzaldeyde (6.0 g, 0.026 mol) in the presence of anhydrous ethanol (70 mL) and piperidine catalyst (500 μL, 5 mmol).34 We kept the reaction at 60 °C for 36 h and followed its progress by TLC (n-hexane/acetone 8:2, v/ v). An aqueous KOH solution (200 mL, 0.25 mol/L) was added; the precipitated solid was filtered, washed with water, and dried (yield = 5.94 g). A portion of this product (2 g) was further purified with flash column chromatography on silica gel using acetone solvent. The latter was evaporated to give HxQMBu2 as dark blue crystals (1.2 g; yield = 20%; mp 83−85 °C). Its purity was established by TLC analysis by using n-hexane/acetone eluent (8:2, by volume). 1H NMR data (Varian Innova-300 spectrometer; CDCl3) 0.86 (t, 3H, H14, JH13−H14 = 6.6 Hz), 1.34 (s, tBu, 18H, H15), 1.41 (m, CH2, 6H, H13), 1.74 (m, CH2, 2H, H12), 4.23 (t, NCH2, 2H, H11, JH11−H12 = 7.3 Hz), 7.14 (d, H9, JH8−H9 = 15.0 Hz);, 7.30 (d, H3, JH2−H3 = 8.1 Hz), 7.43 (br, H10, H10′), 7.52 (t, H6, JH5−H6 and JH6−H7, = 6.6 Hz), 7.67−7.79 (m, H4, H5, H8), 7.85 (d, H7, JH6−H7 = 6.6 Hz), 8.45 (d, H2, JH2−H3 = 8.1 Hz).37 2.3. Spectroscopic Determination of ET(HxQMBu2) in Pure Solvents. Aliquots of HxQMBu2 solution in acetone (30 μL, 10−3 mol/L) were pipetted into small glass vials, followed by evaporation of

(4)

where ET(probe) is the empirical (or total) solvent polarity parameter that refers to the energy of the intramolecular charge transfer transition (in kcal/mol) of the solvatochromic probe, calculated from eq 5: E T(probe) (kcal/mol) = 28591.5/ λmax (nm)

(5)

Figure 1 shows typical examples of solvatochromic probes employed for the determination of the empirical solvent polarity. In the present work, we introduced the novel probe HxQMBu2. In Figure 1, we depict the zwitterionic ground state (GS) and the exited state (ES, a diradical) of HxQMBu2 that occur upon light absorption by this dye. A simple way to arrive at the diradical state is to start from the quinonoid structure of the probe and perform a series of one-electron transfers that restore the aromaticity in the phenolate ring. Figure 2 shows solvatochromic probes for the determination of specific solvent parameters, namely, acidity (SA, probes IA and IB); basicity (SB, probes IIA and IIB); dipolarity (SD, probe III), and polarizability (SD, probe IV). The objective of the present work is to demonstrate the correlation between (Hildebrand and Hansen) solubility and solvatochromic parameters for representative classes of solvents and to test the use of these parameters to probe Asph solubilization. We investigated the solvatochromic response of HxQMBu2 in 38 solvents and found a linear correlation between ET(HxQMBu2) and the corresponding δ. Brazilian crude-based Asphs were solubilized in 28 solvents, and the corresponding weight percents (wt %) of dissolved Asphs were correlated with ET(probe) and δ. The resulting 3D plots between solvent properties and wt % dissolved Asphs, vide infra 4646

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Energy & Fuels acetone at room temperature under reduced pressure in the presence of P4O10. The solvent whose polarity is to be determined was added (2 mL), the probe was dissolved (final probe concentration = 1.0 × 10−3 mol/L), the solution transferred into 1 cm path cell, and its absorbance was recorded at 25 °C, 120 nm/min, using Shimadzu UV 2550 spectrophotometer. Values of λmax were calculated from the first derivative of the absorption spectrum; values of ET(HxQMBu2) in kcal/mol were calculated from eq 5. 2.4. Determination of Asphaltene Solubility in Pure Solvents. We determined the solubility of Asph as follows: A certain mass of the above-mentioned Asph powder (50−300 mg, depending on the solvent) was weighed into a 2 mL Eppendorf polypropylene tube; 1 mL of the solvent to be tested was added. The mixture was agitated first for 2 min using a vortex mixer, then for 5 h using a tube rotator (Glas-Col model 099A RD4512, 60 rpm), and then left overnight at room temperature. The tube was centrifuged for 1 h at 16 400 g (Himac CR20B2, RPR 20-2), and the supernatant was filtered through a PTFE filter (Milliuni, 0.45 μm). A certain mass of the filtered solution was dried in a vacuum oven until constant weight was achieved (T = 80 °C, ca. 36 h), and the mass of residual Asphs was determined. The solubility of dissolved Asph was calculated (wt %, i.e., g dissolved Asph/100 g filtered asphaltene solution).

Figure 3. Colors developed by HxQMBu2 in isooctane (A), decalines (B), and xylenes (C).

3. RESULTS AND DISCUSSION 3.1. Choice of the Solvatochromic Probe for Determining Solvent Empirical Polarity. A convenient solvatochromic probe to investigate complex systems such as petroleum fractions should fulfill the following requirements: (i) sufficient solubility in all classes of solvents, including lowpolarity hydrocarbons; (ii) strong absorption in the visible region, preferably above 500 nm, where petroleum fractions absorb moderately/weakly; and (iii) acceptable sensitivity to small variations in the polarity of solvent mixtures. HxQMBu2 conforms to the above-mentioned requirements. Its relatively elaborate structure mimics that of other model systems for resins;38,39 the presence of two tert-butyl groups flanking the phenoxide oxygen of HxQMBu2 made it soluble in all solvents employed in the present study. This enhanced solubility is akin to (t-Bu)5RB that is soluble in aliphatic hydrocarbons, whereas its precursor (RB) is not.28 The advantage of using HxQMBu2 is its favorable solvatochromism, relative to that of (t-Bu)5RB (for 11 solvents that we tested), as shown by the fact that λn‑hexane − λdecalines = 11.8 nm as compared with 7.3 nm for (t-Bu)5RB (the latter value was calculated from the equation ET(t-Bu)5RB = 0.9424 ET(RB) + 1.808).28 Figure 3 shows the colors of HxQMBu2 in three representative hydrocarbons (aliphatic, alicyclic, and aromatic). 3.2. Correlation between Solvatochromic Scales and Solubility Parameters. Table SI-1 lists the data of all solvents employed in the present study in the correlation analysis of pure solvents (e.g., ET(HxQMBu2 versus δ; Figure 4) and the correlation between solvent descriptors and Asph solubilization, e.g., Figure 8, vide infra. Table 1 below shows the data only for the solvents employed in the Asphs dissolution experiments. 3.3. Correlation between Empirical Solvent Polarity and Hildebrand Solubility Parameter. At the outset, we emphasize that Hildebrand/Hansen solubility parameters and the corresponding solvatochromic ones are calculated by entirely different approaches. Thus, the first two scales are based on physicochemical properties of the solvent. These are solvent enthalpy of vaporization (Hildebrand, δ), dipole moment and molar volume (δD), molar volume and surface tension (δP), and the difference (δt2 − (δD2 + δP2)0.5, δAB. However, the solvatochromic properties are based on solvent perturbation of the energy of the intramolecular charge transfer

Figure 4. Correlation between the solvatochromic scale ET(HxQMBu2) (in kcal mol−1) and Hildebrand solubility parameter δ (in MPa0.5) for 38 pure solvents. Values of ET(HxQMBu2) were determined in the present work; values of δ were taken from literature.41−43 The solvent numbering is that of Table SI-1.

within the probe; see Figure 1. Therefore, the quality of correlation between the two scales (Hildebrand/Hansen and solvatochromic) depends on how much solvent−solvent interactions (Hildebrand/Hansen) and solvent−probe interactions (solvatochromism) are affected by solvent properties. Another factor that bears on the correlation between the two scales is that we employed solvents of very different classes, i.e., solvents that interact with each other (Hildebrand/Hansen) and with the probe (solvatochromism) by different mechanisms. Examples are formamide that is involved in extensive hydrogen bonded networks,44 the strongly dipolar DMSO,45,46 the strongly dipolar bidendate sulfolane,47 and weakly interacting solvents such as halogenated hydrocarbons and nonpolar ones such as hydrocarbons. Figure 4 shows the correlation of ET(HxQMBu2) (in kcal mol−1) and Hildebrand solubility parameter δ (in MPa0.5) for 38 pure solvents, including several classes of protic and aprotic solvents, e.g., esters, halogenated hydrocarbons, ketones, nitro compounds, alcohols, ethers, hydrocarbons, and nitriles. The 4647

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Table 1. Solvents Employed in the Present Work for the Dissolution of Asphaltenes, Their Hildebrand and Hansen Solubility Parameters, Their Solvatochromic Parameters, and Asphaltene Solubility Thereina,b,c Hildebrand parameters

Hansen parameters

solvatochromic parameters ET (HxQMBu2) (kcal/mol)

number

solvent

δ

δAB

δD

δP

3 5 6 13 14 15 16 19 20 21 22 24 25 38 47 48 51 53 56 59 62 64 65 68 69 76 79 81

benzene bromobenzene 1-bromobutane chlorobenzene 1-chlorobutane chloroform (trichloromethane) cyanobenzene cyclohexane cyclohexanol cyclohexanone decalines (cis) 1,2-dichlorobenzene 1,2-dichloroethane ethyl benzoate 1-hexanol methoxybenzene methyl phenyl ketone 1-methylnaphthalene nitrobenzene 1-octanol (2-propyl)benzene pyridine quinoline tetrahydrofuran 1,2,3,4-tetrahydronaphthalene 1,2,4-trimethylbenzene toluene o-, m-, p-xylenes

18.8 20.0 17.8 19.4 17.2 18.9 22.7 16.8 23.3 20.2 17.8 20.5 20.3 16.2 20.8 20.2 20.8 20.3 22.6 21.0 17.4 21.6 21.5 19.0 19.6 18.0 18.2 18.1

2.0 4.1 4.4 2.0 2.0 5.7 3.3 0.2 13.5 5.1 0 3.3 4.1 6 12.5 6.7 3.7 4.7 4.1 11.9 1.2 5.9 5.7 8.0 2.9 1.0 2.0 3.1

0.0 5.5 7.7 4.3 5.5 3.1 9.0 0 4.1 6.3 0 6.3 7.4 6.2 5.8 4.1 8.6 0.8 8.6 3.3 1.2 8.8 5.6 5.7 2.0 1.0 1.4 1.0

18.4 20.5 16.3 19.0 16.2 17.8 17.4 16.8 17.4 17.8 18.0 19.2 19.0 17.9 15.9 17.8 19.6 20.6 20.0 17.0 18.1 19.0 19.8 16.8 19.6 18.0 18.0 17.6

47.2 46.1 47.1 46.3 47.1 46.3 45.5 49.1 45.5 46.1 48.7 48.0 45.9 46.3 46.4 45.5 46.0 45.4 45.5 46.1 45.4 45.1 46.5 47.0 47.4 47.3 47.4

SA

SB

SD

SP

dissolved asphaltene (wt %)d

0.000 0.000 0.000 0.000 0.000 0.047 0.047 0.000 0.246 0.000 0.000 0.033 0.030 0.000 0.315 0.084 0.044 0.000 0.056 0.299 0.000 0.033 0.052 0.000 0.000 0.000 0.000 0.000

0.124 0.192 0.176 0.182 0.138 0.071 0.281 0.073 0.793 0.482 0.056 0.144 0.126 0.389 0.879 0.299 0.365 0.156 0.240 0.923 0.144 0.581 0.482 0.591 0.180 0.190 0.128 0.157

0.27 0.497 0.430 0.537 0.529 0.614 0.852 0.000 0.605 0.745 0.000 0.676 0.742 0.613 0.552 0.543 0.808 0.510 0.873 0.454 0.209 0.761 0.740 0.634 0.182 0.155 0.284 0.266

0.793 0.875 0.735 0.833 0.693 0.783 0.851 0.683 0.736 0.766 0.744 0.869 0.771 0.793 0.698 0.82 0.848 0.908 0.891 0.713 0.767 0.842 0.931 0.714 0.838 0.775 0.782 0.791

27.3 12.2 12.8 23.3 9.0 24.4 18.4 3.0 0.5 24.7 3.3 14.1 7.0 16.7 0.5 17.3 14.5 21.4 11.6 0.3 26.2 17.5 25.7 29.0 16.7 22.8 29.2 20.4

Abbreviations: δ = Hildebrand solubility parameter; δAB, δD, and δP refer to Hansen hydrogen bonding, dipolarity, and polarizability parameters, respectively. ET(HxQMBu2), SA, SB, SD, and SP refer to solvent empirical polarity, Lewis acidity, Lewis basicity, dipolarity, and polarizability, respectively. We use here the same solvent numbering as that employed in Table SI-1. bThe solvatochromic parameters of quinoline, 1bromobutane, cyclohexanol, and ethyl benzoate were determined in the present work. The solvatochromic data of the other solvents were taken from literature.40 The values that we employed for 1-bromobutane, o-, m-, p-xylenes, and 1,2,4-trimethylbenzene are those for 2-bromobutane, oxylene, and 1,3,5-trimethylbenzene, respectively. cHildebrand and Hansen parameters were taken from literature.26,41−43 dThese results are given as wt %, i.e., g dissolved Asph/100 g filtered asphaltene solution (Experimental Section). The Asph dissolution experiments were carried out four times by two independent workers. The uncertainty in the mass of dissolved Asphs was ≤10%. a

Table 2. Results of the Correlations between Hildebrand/Hansen Solubility Parameters and the Corresponding Solvatochromic Parameters entry

descriptors correlated

1 2 3 4

δ and ET(HxQMBu2) δAB and (SA + SB) δD and SD δP and SP

intercept 99.94 0.45 −1.75 5.05

(±5.73) (±0.60) (±0.75) (±0.55)

slope −1.72 14.05 14.20 16.10

(±0.12) (±1.05) (±1.08) (±0.74)

number of solvents

correlation coefficient

38 44 63 63

0.830 0.805 0.736 0.883

3.4. Correlation between Individual Solvatochromic Parameters and Hansen Solubility Parameters. In principle, the correlation shown in Figure 4 for δ and ET scales is expected to hold for the individual solvent descriptors, namely, SA, SB, and so on. These correlations are shown in Figures 5−7; the results of correlations are listed in the Table 2. Regarding these results, the following notes are relevant: Hansen uses a single parameter, δAB, to describe the hydrogen-bonding ability of the solvent, i.e., its Lewis acidity/ basicity. Although solvatochromic parameters are available separately for acidity (SA) and basicity (SB), we correlated

result of this correlation is listed in Table 2 and is associated with satisfactory correlation coefficients (entry 1), showing that the different mechanisms of interactions within the solvent (Hildebrand) are also reflected in the solvent−HxQMBu2 interactions. Unlike Figures 5−7 (vide infra), the linear correlation in Figure 4 has a negative slope. The reason is that the solvatochromic response of HxQMBu2 is “positive”, i.e., λmax increases as a function of increasing solvent polarity. Consequently, polar solvents are associated with large value of δ but small values of ET(HxQMBu2). 4648

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Figures 4−7 corroborate our initial expectation that the Hildebrand/Hansen parameters should correlate with the

Figure 5. Correlation between Hansen hydrogen bonding parameter δAB and the sum of the solvatochromic acidity plus basicity (SA + SB) for 44 solvents. The solvent numbering is that of Table SI-1. Figure 7. Correlation between Hansen polarizability parameter δP, and solvatochromic polarizability parameter SP for 63 solvents. The solvent numbering is that of Table SI-1.

several combinations of these (SA × SB, SA − SB, SA + SB, and SA/SB) with δAB; the best correction was found with (SA + SB) as shown in Figure 5. The correlation is satisfactory if we consider that the worst offenders are solvents that may be involved in networks of hydrogen bonding, i.e., by acting as both acid and base (entries 2, 10, 37, 41, 49, and 59 of Table SI1). Figure 6 shows the correlation for solvent dipolarity in both scales. Interestingly, the solvents that deviate most from the

corresponding solvatochromic ones, indicating that they are similarly susceptible to the properties of a large population of protic, dipolar aprotic, weakly polar, and nonpolar solvents. 3.5. Application to the Solubility of Asphaltenes in Organic Solvent. The general equation for the solubility of a solute in a series of solvents is given by49 log(solubility) = aSA + bSB + d SD + pSP

(6)

As indicated above (see Figure 5) the best correlation of δAB was with (SA + SB), so the versions of eq 6 that we employed for correlating the dependence of the solubility of Asphs on solvent properties are log(% wt dissolved Asphs) = a /b(SA + SB) + dSD + pSP log(% wt dissolved Asphs) = a/bδAB + dδ D + pδ P

(7) (8)

Correlating Asph solubilization with solvent parameters has been previously employed by a few authors, with varying degree of success. Masliyah et al. employed a one-term equation (Hildebrand) and a three-term equation (Hansen) to correlate the solubility of Asph in binary mixtures of aliphatic, alicyclic, and aromatic hydrocarbons. Although the three-term equation gave better predictions of the solubilization of Asphs than the one-term version, its success was only marginal for predicting the solubility in highly polar solvents.50 Rogalski et al. correlated Hildebrand solubility parameters for six crude oils with inverse gas chromatography based parameters (of these crudes). The signs of the regression coefficients were negative (Lewis acidity and Lewis basicity) and positive (dipolarity/ polarizability and dispersion interactions).27,51 We determined the solubility of Asphs in 28 solvents including aromatic hydrocarbons, heterocyclic compounds, halogenated hydrocarbons, ketones, esters, ethers, alcohols, nitriles, nitrobenzene,and cycloalkanes (Table 1). As the latter data shows, the best solvents for Asphs are the aromatics and halogenated aromatics, trichloromethane, THF, quinoline, and cyclohexanone. The efficiency of these solvents, especially the

Figure 6. Correlation between Hansen dipolarity parameter δD and the solvatochromic dipolarity parameter SD for 63 solvents. The solvent numbering is that of Table SI-1.

linear correlation include water, and strongly dipolar ones, e.g., acetonitrile, nitromethane, sulfolane, DMF, and propylene carbonate (entries 17, 12, 80, 63, 70, 57 of Table SI-1). These are precisely the solvents that form extensive networks by dipolar interactions and hydrogen bonding. The same trend was observed for the solubility of supercritical CO2 in organic solvents. The data of solvents that form hydrogen bonds and strongly dipolar ones were excluded for the correlation between δ and SD/SP in order to get a better correlation.48 The best correlation observed (cc = 0.883 for 63 solvents) is that between δP and SP because this solvent descriptor depends essentially on van der Waals interactions, dipolar interactions and hydrogen-bonding play no role. 4649

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Figure 8. Dependence of the solubility of Asphs on solvent parameters. The Asph solubility power of the solvents were arbitrarily classified, as inefficient (red color; < 5 wt % dissolved Asphs), intermediate (green color; between 5 and 15 wt % dissolved Asphs), and good (blue color; > 15 wt % dissolved Asphs). The 3D plots are for solvatochromic parameters (left) and Hansen solubility parameters (right), respectively.

the determination of the log P values (solute partitioning between water and 1-octanol).56 The signs of each term in eqs 9 and 10 are the same, indicating that the susceptibility of both scales to Asph−solvent interactions is correlated. There is agreement that Asphs are composed of polynuclear aromatic “core” with aliphatic chains attached. They have some polar character due to the presence of sulfur, nitrogen, and oxygen.39 In principle, therefore, one expects dependence of their solubility in organic solvents on all terms of eqs 9 and 10. The negative sign of the Lewis acidity/basicity term is interesting in view of (i) the opinion that hydrogen bonding is important for asphaltene stability,57 (ii) the demonstrated efficiency of (relatively basic) THF as Asph solvent (see entry 69 of Table 1), and (iii) the reported favorable effect of the basicity of additives on stability of Asphs.58 The negative sign of the hydrogen-bonding terms maybe because the solvent descriptors that we employed (SA + SB or δAB) are composite ones and hence reflect the effects of solvent Lewis acidity and Lewis basicity simultaneously. Because isolation of the effect of each solvent descriptor is not possible with Hansen parameters, further discussion of the sign of the acidity/basicity term is not warranted here. It is relevant, however, that asphaltenes are only slightly soluble in alcohol even relatively hydrophobic ones such as 1-hexanol and 1-octanol (entries 47 and 59, respectively, of Table 1). This may indicate that the autoassociation of alcohols is stronger than alcohol−Asph interactions.59 Consequently, high solvent acidity and basicity is a required but not sufficient condition for high Asph solubility.60 The signs of the dipolarity (SD and δD) and polarizability terms (SP and δP) are positive, i.e, both solvent descriptors enhance Asph dissolution and contribute more than Lewis acidity/basicity. The signs of both solvent descriptors agree with the results of Rogalski et al;27,51 dipolar and dispersion interactions are listed by most authors as dominant factors for Asph dissolution and hence stability.14,27,51 The two parts of Figure 8 demonstrate the requirements for Asph dissolution. Inefficient solvents (shown in red) are of two

heterocyclic ones, agree with previous data on the solubilization of Asphs in organic solvents.52,53 The correlations equations shown below (eq 9 and 10) are based on reduced values of the solvent parameters (both solvatochromic and Hansen), so the regression coefficients can be compared directly. Here the subindex r designates reduced value, calculated for SD by eq 11, max and min refer to the values of SD for the more dipolar and less dipolar solvent, respectively, and S is the solvent: log(% wt dissolved Asphs) = −1.05 ± 0.41(SA + SB)r + 1.08 ± 0.41(SD)r + 1.18 ± 0.40(SP)r ; R2 = 0.809 (n = 28)

(9)

log(% wt dissolved Asphs) = −0.52 ± 0.41δAB,r + 0.68 ± 0.35δ D,r + 1.52 ± 0.30δ P,r ; R2 = 0.754 (n = 28)

SDr = SDS − SDS,min /SDS,max − SDS,min

(10) (11)

Concerning eqs 9 and 10, the relevant points are as follows: The sign of the regression parameters indicates the effect (favorable or unfavorable) of the solvent descriptor on the solubility of Asphs, whereas its magnitude indicates the relative importance (or contribution) of this solvent property. This reasoning was employed to explain the solubilization of organic compounds in pure solvents, in particular, where solute aggregation plays no role.49,54 However, Asphs have very complex and heterogeneous structures.55 Additionally, Asphs form aggregates according to the continental model, where π−π interactions are important, or the archipelago model where π−π interactions and hydrogen bounding should be taken into account.6 This complexity should be born in mind when the results of eqs 9 and 10 are analyzed. In other words, the regression coefficients in case of Asphs are expected to be lower than those observed for the dissolution of simple organic compounds, or their partitioning in two-phase systems, e.g., in 4650

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Energy & Fuels subgroups: (i) The alcohols, cyclohexanol, 1-hexanol, and 1octanol. These have high acidity/basicity and dipolarity but most certainly autoassociate. As indicated above, it is possible that the autoassociation of alcohols via hydrogen bonding is stronger than the corresponding hydrogen bonding with Asph.59,60 (ii) The alicyclic hydrocarbons, cyclohexane, and decalines. These have no acidity/basicity or dipolarity and intermediate polarizability. The results for these hydrocarbons show that high solvent polarizability is a necessary but not sufficient condition for the dissolution of Asphs. Solvents of intermediate efficiency either carry a strongly dipolar group (acetophenone and nitrobenzene) or polarized bonds (1bromobutane, 1-chlorobutane, bromobenzene, and 1,2-dichlorobenzene). Except for one example, trichloromethane, all good solvents are aromatic or heterocyclic. The favorable effect of the aromatic ring on Asph solubilization can be inferred by comparing the data of two good solvents (tetraline and benzene) with those of the structurally related saturated ones (cyclohexane and decalines, inefficient solvents). The effect of heteroatoms on solvent efficiency is clearly shown by the data of anisol, THF, and quinoline. The observed order of efficiency of aromatics (toluene > benzene >1,2,4-trimethylbenzene > xylenes) shows that solvent molar volume is important.60 It is tempting to suggest that we use the solvatochromic characteristic of solvents to classify their efficiency for Asph dissolution. For example, on the basis of the data of Table 1, we predict that 3- and 4-methylpyridines and cyclopentanone should be efficient solvents. The generality of this argument is under study at the moment.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was carried out with a Petrobras grant No. TC81884-13-9. L.P.N., E.O.M., A.B.G., R.L., and V.N.L. thank Petrobras Research Center (CENPES) for research fellowships. O.A.E. thanks the Institute of Chemistry, USP, FAPESP (State of São Paulo Research Foundation; 2014/22136-4) for support and CNPq (National Council for Scientific and Technological Research; 307022/2014-5) for research productivity fellowship.

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REFERENCES

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4. CONCLUSIONS The compatibility between maltenes and the corresponding Asphs can be assessed by using the solubility parameters (Hildebrand and Hansen) of both crude oil components (Asphs and maltenes). We show that the solvent “power” or efficiency can be conveniently evaluated from the solvatochromism of certain probes. In addition to being a model for resins, the novel solvatochromic probe that we synthesized (HxQMBu2) proved to be convenient in terms of solubility and sensitivity to small variations in medium polarity. The empirical medium polarity calculated for 38 pure solvents, pertaining to several classes of organic compounds, correlated linearly with the corresponding Hildebrand solubility parameters. Likewise, the solvent acidity/basicity, dipolarity, and polarizability correlated linearly with the corresponding Hansen solubility parameters. Therefore, the solvent descriptors can be conveniently expressed either by the solubility or the solvatochromic parameters. We tested this equivalence of scales by correlating the dissolution of Asphs (wt %) with solvent descriptors based on the two scales. The results for 28 solvents were correlated with three-parameter equations. The results indicated that alcohols and hydrocarbons are inefficient solvents; solvents of intermediate efficiency carry either a strongly dipolar group or polarized bonds. Aromatic solvents and especially heterocyclic ones are efficient solvents for Asphs. The most relevant solvent descriptor (for the dissolution of Asphs) is its polarizability.

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Hildebrand and Hansen solubility parameters, the corresponding solvatochromic parameters, and the wt % dissolved asphaltenes. (PDF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b00461. 4651

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