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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 Lira, Vanessa N Linhares, Marcia Cristina Khalil de Oliveira, Francis A Meireles, Gaspar Gonzalez, and Omar A. El Seoud Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00461 • Publication Date (Web): 11 May 2016 Downloaded from http://pubs.acs.org on May 14, 2016

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MS# ef-2016-00461u- Text, Revision 2 Solvatochromic and solubility parameters of solvents: Equivalence of the scales and application to probe the solubilization of asphaltenes

Luzia P. Novaki,a Edgar O. Moraes,a André B. Gonçalves,a Raphael Lira;a Vanessa N. Linhares,a Marcia C. K. de Oliveira,b,* Francis A. Meireles,b Gaspar Gonzalez,c and Omar A. El Seoud,a,* a- Institute of Chemistry, the University of São Paulo, SP P. O. Box 26077, 05513-970, São Paulo, SP; e-mail: [email protected] b- Flow assurance laboratory, Petrobras Research Center, (CENPES), Av. Horácio Macedo, 950, Cidade Universitária, 21941-915 Rio de Janeiro - RJ c- Institute of Macromolecules, Federal University of Rio de Janeiro, Av. Horácio Macedo, 950, Cidade Universitária, 21941-915 Rio de Janeiro – RJ

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


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[2]

corresponding Hansen solubility parameters. In order 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; 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 (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 resins,3 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: (i) the colloidal instability index, CII, (ii) Heithaus (P) parameter; (iii) 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 (iv).14 Some advantages and limitations of these scales are discussed elsewhere.7 The CII index considers a crude oil as a colloidal system made up of pseudo-phases (components); it relies on using the results of SARA analysis (Saturates, Aromatics, Resins, and Asphaltenes) to calculate CII, according to Eqn. 1, where the composition is SARA analysis-based:16


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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, whereas values between 0.7 and 0.9 are considered borderline cases. The scale developed by Heithaus (point ii) 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 (iii) 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 mid-boiling point of a given petroleum fraction. The crude oil is considered stable if (BMCI-TE) is in the range of 7-14.21, 22 The oil compatibility model (iv) 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 has the units 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 coworkers proposed extension of the Hildebrand solubility parameter concept to polar and hydrogen-bonding systems. As shown in Eqn 2, Hansen solubility parameter δt includes contribution from dispersion forces as well as specific interactions, e.g., dipolar ones and hydrogen-bonding:26

δt2 = δAB2 + δD2 + δP2

(2)


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[4]

The terms AB, D and P refer to hydrogen bonding donation (Lewis acidity) and acceptance (Lewis basicity), dipolar, and dispersion properties of the solvent, respectively. The symbols that we use in Eqn. 2 are different from those originally suggested by Hansen. We use them for consistency with those employed in solvatochromism, vide infra. Eqn. 2 has been applied to determine the contribution of these parameters to the stability of six crude oils.27 Eqn. 2 is a version of the general solvation free energy relationship, Eqn. 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) = a SA + b SB + d SD + p SP

(3)

Where S refers to solvent, whose Lewis acidity, Lewis basicity, dipolarity, and polarizability are given by A, B, D, and P, respectively. Therefore, Eqn. 2 is a version of Eqn. 3, where SA and SB are considered jointly in the (δAB)2 term. Likewise, some authors use the SD and SP terms of Eqn. 3 jointly, as (d/p x SD/SP).28 When Eqn. 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 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. On the other hand, the acidity disfavors the reaction, probably because of diazole solvation by hydrogenbonding.29 In summary, Eqns. 2 and 3 are equivalent because they analyze the complex effects of solvent on chemical phenomena as a linear combination of individual


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contributions; Eq. 3 has the merit that it separates solvent Lewis acidity from its Lewis basicity. Equation 4 is an extensively employed version of Eqn. 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:

ET(probe) = aSA + bSB + dSD + pSP

(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 Eqn. 5:

ET(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 occurs 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 restores the aromaticity in the phenolate ring.


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[6]

Figure 1: Solvatochromic dyes employed for determination of the empirical, i.e., total solvent polarity. These include 2,6-diphenyl-4-(2,4,6-triphenylpyridinium-1-yl)phenolate, RB;

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-4yl)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 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).


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Figure 2: Solvatochromic probes employed for determination of solvent descriptors, including acidity (SA, o-tert-butylstilbazolium betaine, o,o’-di-tert-butylstilbazolium betaine, IA and IB, respectively); basicity (SB, 5-nitroindoline and 1-methyl-5nitroindoline, IIA and IIB, respectively); dipolarity (SD, 2-(N,N-dimethylamino)-7nitrofluorene, III), and polarizability (SD, β-carotene, 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 wt% of dissolved Asphs were correlated with ET(probe) and δ. The resulting 3D plots between solvent properties and wt% dissolved Asphs, vide infra Figure 8, were found to be similar. This corroborates our initial expectation that empirical solvent polarity scales can be used to probe Asph dissolution, hence the stability of crude oils.

2- Experimental Section 2.1-Solvents, reagents, and the asphaltenes We purchased the 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 Petrobras de-asphalting unit (REVAP, São José dos Campos, SP), and removed the non-asphaltic material by repeated extraction with n-heptane, as follows: 200g solid were suspended in 400 mL n-heptane. The suspension was sonicated for 4 hs (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 hours (Soxhlet). The residue was dried in air, ACS Paragon Plus Environment

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[8]

grinded into powder, and further dried at 60 °C in a vacuum oven, until constant weight. 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 found for Asphs extracted from crude samples, e.g., from Brazil (1.18),31 and Venezuela (1.10 to 1.38).32

2.2- Synthesis of the solvatochromic probe HxQMBu2: We synthesized this probe according to the following scheme:33-36

Scheme 1: Schematic representation of the synthesis of HxQMBu2

The halide 1-(n-hexyl)-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 (30mL), in the presence of piperidine catalyst (0.5 mL, 5 mmol). The solution was kept under reflux for 36 hours. The formation of the product (dark dense oil) was confirmed by 1

H 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 presence of anhydrous ethanol (70 mL) and piperidine catalyst (500 µL, 5 mmol).34 We kept the reaction at 60°C for 36 hours, and followed its progress by TLC (n-hexane/acetone 8:2,


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v/v). An aqueous KOH solution (200 mL, 0.25 mol/L) was added, the precipitated solid filtered, washed with water and dried; .yield 5.94g. A portion of this product (2g) 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 %; m.p. 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 =6.6Hz); 8.45 (d, H2, JH2-H3=8.1Hz).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 acetone at room temperature, under reduced pressure, in the presence of P4O10. The solvent (2 mL) whose polarity is to be determined was added, the probe was dissolved (final probe concentration = 1.0 x 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; ET(HxQMBu2) in kcal/mol were calculated from Eqn. 5.

2.4-Determination of asphaltene solubility in pure solvents: We determined the solubility of Asph as follows: A certain mass of the abovementioned Asph powder (50 to 300 mg, depending on the solvent) was weighted into 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 hours using a tube rotator (Glas-Col model 099A RD4512, 60 rpm), and then left overnight at room temperature. The tube was centrifuged for one hour at 16400 g (Himac CR20B2, RPR 20-


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[10]

2) and the supernatant was filtered through a PTFE filter (Milliuni, 0.45 micron). A certain mass of the filtered solution was dried in a vacuum oven until constant weight (T= 80°C, ca. 36 hours), and the mass of residual Asphs was determined. The solubility of dissolved Asph was calculated in wt%, i.e., g dissolved Asph/100 g filtered asphaltene solution.


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3-Results and Discussion

3.1-Choice of the solvatochromic probe for determining solvent empirical polarity A convenient solvatochromic probe to investigate complex systems, e.g., petroleum fractions should fulfil the following requirements: (i) sufficient solubility in all classes of solvents, including low polarity hydrocarbons; (ii) strong absorption in the visible region, preferably above 500 nm, where petroleum fractions absorb moderately/weakly; (iii) acceptable sensitivity to small variation 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 tertbutyl 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 (t-Bu)5RB (for eleven 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).

Figure 3: The colors developed by HxQMBu2 in isooctane (A), decalines (B) and xylenes (C)

3.2-Correlation between solvatochromic scales and solubility parameters


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[12]

Table SI-1 (Table 1 of Supplementary Information) 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.


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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 therein.a, b, c

ET

Hildebrand Hansen parameters Number

Solvent

parameters

(HxQMBu2),

Solvatochromic parameters

asphaltene,

kcal/mol

δ

δAB

δD

δP

Dissolved

SA

SB

SD

SP

wt% d

3

Benzene

18.6

2.0

0.0

18.4

47.2

0.000

0.124

0.27

0.793

27.3

5

Bromobenzene

20.0

4.1

5.5

20.5

46.1

0.000

0.192

0.497

0.875

12.2

6

1-Bromobutane

17.8

4.4

7.7

16.3

47.1

0.000

0.176

0.430

0.735

12.8

13

Chlorobenzene

19.6

2.0

4.3

19.0

46.3

0.000

0.182

0.537

0.833

23.3

14

1-Chlorobutane

17.2

2.0

5.5

16.2

47.1

0.000

0.138

0.529

0.693

9.0

19.0

5.7

3.1

17.8

46.3

0.047

0.071

0.614

0.783

24.4

Chloroform 15 (Trichloromethane) 16

Cyanobenzene

22.7

3.3

9.0

17.4

45.5

0.047

0.281

0.852

0.851

18.4

19

Cyclohexane

16.8

0.2

0

16.8

49.1

0.000

0.073

0.000

0.683

3.0

20

Cyclohexanol

22.4

13.5

4.1

17.4

45.5

0.246

0.793

0.605

0.736

0.5

21

Cyclohexanone

20.2

5.1

6.3

17.8

46.1

0.000

0.482

0.745

0.766

24.7

22

Decalines (cis)

18.0

0

0

18.0

48.7

0.000

0.056

0.000

0.744

3.3


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[14]

24

1,2-Dichlorobenzene

20.5

3.3

6.3

19.2

48.0

0.033

0.144

0.676

0.869

14.1

25

1,2-Dichloroethane

20.3

4.1

7.4

19.0

45.9

0.030

0.126

0.742

0.771

7.0

38

Ethyl benzoate

19.9

6

6.2

17.9

46.3

0.000

0.389

0.613

0.793

16.7

47

1-Hexanol

20.8

12.5

5.8

15.9

0.315

0.879

0.552

0.698

0.5

48

Methoxybenzene

20.2

6.7

4.1

17.8

46.4

0.084

0.299

0.543

0.82

17.3

51

Methyl phenyl ketone

20.8

3.7

8.6

19.6

45.5

0.044

0.365

0.808

0.848

14.5

53

1-Methylnaphthalene

20.3

4.7

0.8

20.6

46.0

0.000

0.156

0.510

0.908

21.4

56

Nitrobenzene

22.2

4.1

8.6

20.0

45.4

0.056

0.240

0.873

0.891

11.6

59

1-Octanol

21.0

11.9

3.3

17.0

45.5

0.299

0.923

0.454

0.713

0.3

62

(2-Propyl)benzene

17.4

1.2

1.2

18.1

46.1

0.000

0.144

0.209

0.767

26.2

64

Pyridine

21.6

5.9

8.8

19.0

45.4

0.033

0.581

0.761

0.842

17.5

65

Quinoline

21.5

5.7

5.6

19.8

45.1

0.052

0.482

0.740

0.931

25.7

68

Tetrahydrofuran

19.0

8.0

5.7

16.8

46.5

0.000

0.591

0.634

0.714

29.0

1,2,3,4-

19.6

2.9

2.0

19.6

0.000

0.180

0.182

0.838

1.0

1.0

18.0

0.000

0.190

0.155

0.775

69

Tetrahydronaphthalene

1,2,4-

18.0

76

47.0 47.4

16.7 22.8

Trimethylbenzene 79

Toluene

18.2

2.0

1.4

18.0

47.3

0.000

0.128

0.284

0.782

29.2

81

o-, m-, p-Xylenes

18.0

3.1

1.0

17.6

47.4

0.000

0.157

0.266

0.791

20.4


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[15]

a-Abbreviations: δ = Hildebrand solubility parameter; δAB , δD, δP refer to Hansen hydrogen bonding, dipolarity, and polarizability parameter, respectively. ET(HxQMBu2), SA, SB, SD, SP refer to solvent empirical polarity, Lewis acidity, Lewis basicity, dipolarity, and polarizability, respectively. We kept the same solvent numbering employed in Table SI-1. b- The solvatochromic parameters of quinoline, 1-bromobutane, 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 1bromobutane, o-, m-, p-Xylenes, and 1,2,4-trimethylbenzene are those for 2-bromobutane, o-xylene and 1,3,5trimethylbenzene, respectively. c- Hildebrand and Hansen parameters were taken from literature.26, 41-43 d- These results are given as wt%, i.e., g dissolved Asph /100 g filtered asphaltene solution, vide Experimental. The Asph dissolution experiments were carried out four times by two independent workers. The uncertainty in the mass of dissolved Asphs was ≤ 10%.


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[16]

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 physico-chemical 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. On the other hand, the solvatochromic properties are based on solvent perturbation of the energy of the intramolecular charge transfer 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., 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 as well as weakly interacting solvents (halogenated hydrocarbons) and nonpolar ones, 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 result of this correlation is listed in Table 2, and is associated with satisfactory correlation coefficients (see 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).


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[17]

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.

Table 2- Results of the correlations between Hildebrand/Hansen solubility parameters and the corresponding solvatochromic parameters

Entry

Descriptors

Intercept

Slope

correlated

Number of

Correlation

solvents

Coefficient

1

δ and ET(HxQMBu2)

99.94 (±5.73)

-1.72 (±0.12)

38

0.830

2

δAB and (SA+SB)

0.45 (±0.60)

14.05 (±1.05)

44

0.805

3

δD and SD

-1.75 (±0.75)

14.20 (±1.08)

63

0.736

4

δP and SP

5.05 (±0.55)

16.10 (±0.74)

63

0.883


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[18]

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, etc. These correlations are shown in Figures 5 to 7; the results of correlations are listed in the Table 2.

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 1 SI-1

Regarding these results, the following is relevant: 1- 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 several combinations of these (SA x SB; SA - SB; SA + SB; 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 SI-1).


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[19]

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.

2- Figure 6 shows the correlation for solvent dipolarity in both scales. Interestingly, the solvents that deviate most from the linear correlation include water, strongly dipolar ones, e.g., acetonitrile, nitromethane, sulfolane, DMF, 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


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[20]

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.

3- 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.

In summary, Figures 4 to 7 corroborate our initial expectation that the Hildebrand/Hansen parameters should correlate with 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 by:49 log (solubility) = a SA + b SB + d SD + p SP


(6)

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[21]

As indicated above (see Fig. 5) the best correlation of δAB was with (SA + SB), so that 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) + d SD + p SP log (%wt dissolved Asphs) = a/b δAB + d δD +p δP

(7) (8)

Correlating Asph solubilization with solvent parameters has been previously employed by 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 oneterm 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, cycloalkanes (see Table 1). As the latter Table shows, the best solvents for Asphs are the aromatics and halogenated aromatics, trichloromethane, THF, quinoline, and cyclohexanone. The efficiency of these solvent, especially the heterocyclic ones, agree with previous data on the solubilization of Asphs in organic solvents.52-53 The correlations equations shown below (Eqn. 8 and 9) are based on reduced values of the solvent parameters (both solvatochromic and Hansen), so that the regression coefficients can be compared directly. Here each reduced value is designated by the sub


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[22]

index (r) calculated, e.g., for SD by Eqn. 10 where (max) and (min) refer to the values of the SD for the more dipolar and less dipolar solvent, respectively; (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)

(8)

log (%wt dissolved Asphs)= – 0.52±0.41 δAB,r + 0.68±0.35 δD,r + 1.52±0.30 δP,r ; 0.754 (n=28)

R2 =

(9)

SDr = SDS – SDS, min/ SDS, max – SDS, min

(10)

Concerning Eqns 8 and 9, the relevant points are: 4- 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 On the other hand, Ashs have very complex and heterogeneous structures.55 Additionally, Asphs form aggregates, e.g., 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. 8 and 9 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 the determination of the log P values (solute partitioning between water and 1-octanol).56 5- The sign of each term in Eqns 8 and 9 is 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 presence of sulphur, nitrogen and oxygen.39. In principle,


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[23]

therefore, one expects dependence of their solubility in organic solvents on all terms of Eqns. 8 and 9. 6- 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); (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, 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 warrant now. It is relevant however, that asphaltenes are only slightly soluble in alcohols, even relatively hydrophobic ones, e.g., 1-hexanol and 1-octanol (entries 47 and 59, respectively of Table 1). This may indicate that the auto association 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 7- 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, hence stability.14, 27, 51 8- The two parts of Figure 8 demonstrate the requirements for Asph dissolution. Inefficient solvents (shown in red color) are of two sub-groups: (i) the alcohols, cyclohexanol, 1-hexanol and 1-octanol. These have high acidity/basicity, and dipolarity, but most certainly auto-associate. As indicated above, it is possible that the auto association 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


Energy & Fuels

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[24]

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 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, based on 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.

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 5-15 wt% dissolved Asphs);


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[25]

good (blue color; > 15 wt % dissolved Asphs). The 3D plots are for solvatochromic parameters (left) and Hansen solubility parameters (right), respectively.

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-parameters Eqns. 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.

Acknowledgements This project was carried out with a Petrobras grant No. TC-81884-13-9. L. P. Novaki, E. O. Moraes, A. B. Gonçalves, R. Lira, and V. N. Linhares thank Petrobras Research Center (CENPES) for research fellowships. O. A. El Seoud thanks the Institute of Chemistry, USP, FAPESP (State of São Paulo Research Foundation; 2014/22136-4) for support and CNPq


Energy & Fuels

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[26]

(National Council for Scientific and Technological Research; 307022/2014-5) for research productivity fellowship.

Supporting Information. The data for all solvents employed in the present work are listed in Table SI-1. These include Hildebrand and Hansen solubility parameters; the corresponding solvatochromic parameters, and the wt% dissolved asphaltenes.

5-References (1) Asomaning, S., Watkinson, A. P. In: Bott, T. R., Melo, L. F., Panchal, C. B., Somerscales, E. F. C., eds. Understanding Heat Exchanger Fouling and its Mitigation. New York: Begell House, 1999. (2) Wiehe, I. A., Kennedy, R. J. The Oil Compatibility Model and Crude Oil Incompatibility. Energy Fuels 2000, 14, 56–59. (3) Merdrignac, I., Espinat, D. Physicochemical Characterization of Petroleum Fractions: the State of the Art. Oil Gas Sci. Technol. – Rev. IFP, 2007, 62(1), 7-32. (4) Anisimov, M. A.; Ganeeva, Y. M.; Gorodetskii, E. E.; Deshabo, V. A.; Kosov, V. I.; Kuryakov, V. N.; Yudin, D. I.; Yudin, I. K. Effects of Resins on Aggregation and Stability of Asphaltenes. Energy Fuels 2014, 28, 6200−6209; (5) Maqbool, T.; Raha S.; Hoepfner M.P.; Fogler, H. S. Modeling the Aggregation of Asphaltene Nanoaggregates in Crude Oil-Precipitant Systems. Energy Fuels 2011, 25, 1585–1596. (6) Hosseini-Dastgerdi, Z.; Tabatabaei-Nejad, S.A.R.; Khodapanah, E.; Sahraei, E., A comprehensive study on mechanism of formation and techniques to diagnose asphaltene structure; molecular and aggregates: a review. Asia-Pac. J. Chem. Eng. 2014, 10, 1-14. (7) Moura, L.G.M.; Santos, M.F.P.; Zilio, E.L.; Rolemberg, M.P.; Ramos, A.C.S. Evaluation of indices and of models applied to the prediction of the stability of crude oils. J. Pet. Sci. Eng. 2010, 74, 77-87.


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