Pressure dependence of the solubility of light fullerenes in 1-hexanol from 298.15 K to 363.15 K

Journal of Molecular Liquids 209 (2015) 71–76

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Pressure dependence of the solubility of light fullerenes in 1-hexanol from 298.15 K to 363.15 K Konstantin N. Semenov a,⁎, Teresa Regueira b,d, Josefa Fernández b, Nikolay A. Charykov c, Igor V. Murin a a

Institute of Chemistry, Saint-Petersburg State University, Saint-Petersburg 198504, Universitetskii pr. 26, Russia Departamento de Física Aplicada, Universidade de Santiago de Compostela, Santiago de Compostela E-15782, Spain Saint-Petersburg State Technological Institute (Technical University), Saint-Petersburg 190013, Moskovskii pr. 26, Russia d Department of Chemistry, Center for Energy Resources Engineering (CERE), Technical University of Denmark (DTU), DK 2800 Kgs. Lyngby, Denmark b c

a r t i c l e

i n f o

Available online xxxx Keywords: Light fullerenes C60 C70 Solubility High pressure 1-Hexanol

a b s t r a c t The solubility of light fullerenes (C60 and C70) in 1-hexanol was investigated in the range of pressures of 0.1–100 MPa and in the range of temperatures of 298.15–363.15 K. In all of the studied temperatures, solubility increases monotonously with increasing pressure. At ambient pressure, we have found that the temperature dependence of solubility in the binary system C60–1-hexanol is non-monotonic: the solubility diagram consists of two branches corresponding to the crystallization of different solid phases and one invariant point corresponding to the simultaneous saturation of both phases (monosolvated fullerene C60 and non-solvated C60). The composition of the solid crystallosolvate was determined by thermogravimetric analysis. The solubility diagram of the binary system C70–1-hexanol in the temperature range of 298.15–328.15 K at 0.1 MPa consists of only one branch corresponding to the crystallization of non-solvated C70. © 2015 Published by Elsevier B.V.

1. Introduction The knowledge of the solubility of light fullerenes in various solvents is important in many of its practical applications, but it is also an essential tool for verifying the established regularities of the thermodynamics of nonelectrolytes. Today there is a large number of articles on experimental solubility data of the individual light fullerenes (C60 and C70) and industrial fullerene mixtures of different compositions, which were recently reviewed by Semenov et al. [1] and McHedlovPetrossyan [2]. This last author concludes that molecular solutions of C60 may possess a dual nature: a non-electrolyte solution and a lyophobic colloidal system, and that for this kind of mixture, the theory of molecular solutions and the concepts of colloid chemistry are colliding. Much experimental data have been reported concerning the solubility of light fullerenes for binary systems composed by an individual fullerene–pure solvent or pseudo-binary systems (fullerene mixtures of constant composition–pure solvent) at atmospheric pressure [3–14]. Data on solubility on the ternary systems fullerene С60–fullerene С70– pure solvent or fullerene С60 (or fullerene С70)–solvent (1)–solvent (2) are much more scarce and incomplete [15–18]. Additionally, the solubility of light fullerenes was measured for multicomponent systems composed by individual light fullerenes or industrial fullerene mixtures and biocompatible solvents (vegetable oils, essences, animal fats, fatty ⁎ Corresponding author. E-mail address: [email protected] (K.N. Semenov).

http://dx.doi.org/10.1016/j.molliq.2015.04.004 0167-7322/© 2015 Published by Elsevier B.V.

acid esters, etc.) [19–23]. These data may be easily used in food industry, pharmacology or perfumery, medicinal chemistry and cosmetics [24–27]. Analysis of literature shows that experimental values on pressure dependence of the light fullerene solubility are few [28,29]. This fact is due to the considerable difficulty of such experimental investigations. Up to now, solubility of fullerenes at high pressures was investigated only for two binary systems: fullerene C60–toluene and fullerene C60–n-hexane [28,29]. Let us describe briefly the main results obtained in these articles. Sawamura and Fujita [28] investigated the pressure dependence of solubility for the binary (C60–n-hexane) system at 298.15 K up to 400 MPa. The solubility of C60 strongly increases when the pressure increases, although for molecular solids the solubility usually decreases. This unusual pressure dependence of C60 solubility was explained by the authors to be due to the higher molar volume of the solid fullerene as compared with its partial molar volume in hexane, i.e., the molar volume of C60 in the crystal decreases with solvation. The decreasing of the C60 molar volumes causes a typical effect on the increase of solubility with the increasing of pressure according to Planck–van Laar equation: 0 1 ðlÞ d lnxC ΔV 60 A @ ¼− ð1Þ dp RT T

where ΔV is the change of molar volumes of C60 in the process of transition from solid crystalline C60 to saturated solution (this equation is absolutely correct in the cases of ideal or infinitely diluted solution).

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On the other hand, the article of Sawamura and Fujita [29] should be noted especially because it is the first time that a three-dimensional pTx diagram for a binary system containing fullerene appears. Thus, these authors investigated the binary C60–toluene system in a wide range of temperatures (T) and pressures (p), and found a maximum on the solubility within the range of 43–111 MPa, depending on temperature. These results were explained assuming the existence of “crystalline solvates” (i.e., solids with intercalated solvent molecules). Adamenko et al. [30,31] carried out pVT measurements in the binary (C60–toluene) and (C60–water) systems in the temperature range of 313–371 K and in the pressure range of 0.1–103.1 MPa [30,31]. In this work, the solubility of light fullerenes (C60 and C70) in 1-hexanol was measured from ambient pressure to 100 MPa and from 298.15 K to 363.15 K in order to analyze how the pressure affects the solubility at several temperatures. It should be marked that temperature dependences of the individual light fullerenes and industrial fullerene mixture solubility in 1-alkanols CnH2n + 1OH (n = 1–11) were extensively studied previously only at atmospheric pressure [1,12–14, 32–35]. Semenov et al. [32,33] have studied the temperature dependences of solubility of the individual light fullerenes in alkanols (C1–C11) in the temperature range (293 to 353) K as well as the solubility of industrial fullerene mixture (39% C70; 60% C60; 1% Сn (n N 70)). Heyman [34,35] has studied the solubility of individual light fullerenes at 293.15 K in alkanols.

(335 and 472) nm corresponding to the maximum absorbance. The accuracy of wavelength maintenance was 0.5 nm, the photometric accuracy (ΔD) was equal to 0.005, and the thickness of the absorption layer was 1 cm. The overall uncertainty of the concentrations of light fullerenes (C 60 and C70 ) in a saturated solution was nearly 5%. The temperature dependence of the solubility of light fullerenes (C60 and C70) in 1-hexanol from 293.15 K to 363.15 K at atmospheric pressure was determined using a temperature-controlled magnetic stirrer. The saturation time was 8 h; the temperature was maintained with accuracy equal to ± 0.1 K. The following experimental method was used for the determination of the solvent content in solid crystal solutes. The solid phase deposited from 1-hexanol solution was filtered on a Schott filter (porosity factor 10), rinsed quickly with ethanol, and then dried for (10 to 15) min at 293 K. The solid phase was then weighted, repeatedly washed with ethanol in a Soxhlet apparatus at 351 K and 1 atm, dried for 1 h under vacuum (13.3 Pa) at 473 K, and weighed again. The weight change corresponded to the solvent content in the initial crystal solutes. The described experimental method for the determination of solid phase compositions was verified by thermogravimetric analysis under nitrogen atmosphere on Hungarian thermo-gravimeter Q-1500, from 298.15 to 523.15 K, and the heating rate of 5 K/min.

2.3. Solubility measurement technique at high pressures 2. Experimental section 2.1. Materials We have used samples of C60 fullerene (99.9 wt.%) and C70 fullerene (99.5 wt.%) purchased from ILIP, St. Petersburg, with controllable principal admixtures of C70 in C60 and C60 in C70 of (0.1 and 0.5) wt.%, correspondingly. 1-Hexanol sample was anhydrous (N99 wt.%), purchased from Sigma Aldrich. 2.2. Solubility measurement techniques at atmospheric pressure The concentrations of C60 and C70 fullerenes in liquid 1-hexanol at atmospheric pressure at saturated conditions in the temperature range of 298.3–363.3 K were measured with a spectrophotometric technique (using the double-beam spectrophotometer Specord M40 made in Germany, Karl Zeiss) at characteristic wavelengths of

Phase equilibria measurements have been performed in a pressure cell recently used for the investigation of the phase equilibria of two carbon dioxide–biodegradable oil systems [36]. The apparatus was slightly modified to perform the measurements of this work. The new scheme is shown in Fig. 1. The cell is constructed in stainless steel, and supports working pressures and temperatures of up to 100 MPa and 423 K, respectively. A video acquisition system is located in front of a sapphire window for viewing inside the measuring cell. A pressure transducer (Kulite, model HEM375), directly connected to minimize the dead volumes, allows to measure the pressure with a typical uncertainty less than ± 0.03 MPa. The temperature is kept constant by circulating a fluid from a thermostatic bath through three internal lines in the cell wall, and it is measured with an uncertainty of ±0.02 K by means of a Pt100 probe. The operating procedure was as follows: initially the cell was charged with a known amount of the light fullerene (C60 and C70) solution in 1-hexanol precisely measured with a Sartorius MC210P balance (the light fullerene concentration in the initial solution was

Fig. 1. Schematic experimental set-up of the phase equilibrium equipment. (E1) Thermostatic bath, (E2) high pressure cell, (E3) magnetic stirrer, (E4) endoscope and video camera, (E5) computer and (I3) data acquisition unit.

K.N. Semenov et al. / Journal of Molecular Liquids 209 (2015) 71–76

73

Fig. 2. Images of the cell (a) immediately after the addition of solid fullerenes (one can see solid particles) and b) after complete solution of solid fullerenes (no solid particles).

determined using spectrophotometric method), after which weighted portions of the fullerene powder were added to the cell. Under isothermal conditions, the mixture of known composition was compressed to achieve a single phase under continuous stirring. After dissolution of the added solid phase, a new portion of fullerene (C60 or C70) was added. Subsequently, the disappearance of a solid phase was determined visually by slowly increasing the pressure. Fig. 2 shows cell images before and after dissolution of the solid phase in the solvent.

corresponding spectra in a series of one-component aromatic solvents (o-xylene, benzene, toluene, and o-dichlorobenzene), so that the use of empirical relationships (2) and (3) is quite reasonable in our case. Eqs. (2) and (3) are a result of the Firordt method application to o-xylene solutions of fullerenes [1,2]:

3. Results and discussion

where D335 and D472 are the optical densities of the mixtures referred to the absorption layer of 1 cm width, and C(C70) and C(C60) represent the corresponding fullerene concentration (g · l−1). 1-Hexanol mixtures were preliminarily diluted, the reference system being pure 1-hexanol. Negligible admixtures of the heavy fullerenes with carbon numbers higher than 70 were ignored. Table 1 shows the new solubility values determined with the spectrophotometer together with literature data of individual light fullerenes (C60 and C70) as well as the industrial fullerene mixture in 1-hexanol. Analysis of new and literature data reveals that: (i) the experimental data on solubility of individual light fullerenes obtained previously in our scientific group [32,33] agree with the new values. The deviations of the solubility values are due to the recalculation of the experimental data obtained in volume concentration

3.1. Experimental solubility at atmospheric pressure It should be pointed out that the electronic spectra of individual light fullerenes in 1-hexanol (Fig. 3) demonstrate that there are no solvatochromic effects, i.e., no strong change in the spectrum induced by the variation of the solution concentration or solvent composition (in the case of using binary solvent mixtures) [1,2]. Therefore, there is a full analogy among the spectra we recorded in 1-hexanol and the

0.30

0.25

D / a.u.

ð2Þ

CðC70 Þ ¼ 0:0425ðD472 −0:0081D335 Þ

ð3Þ

Table 1 Solubility of individual light fullerenes (C60, C70) and the industrial fullerene mixture (39% C70, 60% C60 and 1% Сn (n N 70)) in 1-hexanol. w is the mass fraction of fullerenes in the solution; ⁎ corresponds to the solubility of the industrial fullerene mixture components.

0.20

0.15

0.10

0.05

0.00 300

C ðC60 Þ ¼ 0:0131ðD335 −1:808D472 Þ

400

500

λ / nm

600

700

Fig. 3. Optical spectra of the solutions of fullerenes in 1-hexanol: C60 (dashed line) and C70 (solid line). D, optical density and λ, wavelength. Fullerene concentrations (wt.%): C60, 0.009; C70, 0.015.

T/K

w (C60)/%

w (C70)/%

w (C60)/%⁎

w (C70)/%⁎

293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15 353.15 358.15 363.15

0.0076; 0.008 [27] 0.0051 [12,28,29] 0.0033; 0.004 [27] 0.0044 0.0057; 0.007 [27] 0.0058 0.0059; 0.007 [27] 0.0062 0.0066; 0.008 [27] 0.0071 0.0072; 0.008 [27] 0.0079 0.008; 0.01 [27] 0.009 0.0098

0.015 [26] 0.0141; 0.004 [12,28,29] 0.0147;0.015 [26] 0.0147 0.0148; 0.015 [26] 0.0148 0.0149; 0.015 [26] 0.0157 0.018 [26]

0.017[26]

0.021[26]

0.016 [26]

0.019 [26]

0.016 [26]

0.015 [26]

0.014 [26]

0.015 [26]

0.017 [26]

0.015 [26]

0.021 [26]

0.018 [26]

0.014 [26]

0.026 [26]

0.019 [26]

0.013 [26]

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0.040 0.035 0.030

w /%

0.025 0.020 0.015 0.010

O

0.005 0.000 290

300

310

320

330

340

350

360

370

T/K Fig. 4. Experimental (w-T) data of the binary systems fullerene C70–1-hexanol (►-[27], ▼ – this study, ◄ – [12,28,29]) and fullerene C60–1-hexanol (▲-[27], ● – this study, ■ – [12,28,29]) at 0.1 MPa. «О» is an invariant point corresponding to dissociation of the solid crystal solvate C60·C6H13OH; the dashed line corresponds to the crystallization of non-solvated fullerene C60; the dotted line corresponds to crystallization of the mono-solvated C60 (C60·C6H13OH); w is the mass fraction of the solute (С60, C70).

(see ref. [33]) into mass fractions (we did not take into account the densities of the saturated solutions, but those of the pure 1-hexanol) as well as to the different conditions of saturation — the procedure described in ref. [32,33] included saturation in ampoules using thermostatic shaker, whereas in this study we have used a magnetic stirrer; moreover, the swept volume was different, as in the present study we used small quantities (up to 5 ml) of the solvent. The latter fact leads to the decreasing of the solubility determination accuracy; (ii) we can see some deviation between the solubility values for the binary system C70–1-hexanol at 298.15 K measured in the current study and the one obtained by Heyman [12,35]. We suppose that this is due to different conditions of experiment; in the last case the authors did not use the constant shaking of the heterogeneous mixtures, and added new portions of the solvent to the system. Thus, the values presented in ref. [12,35] are not equilibrium solubility values of individual light fullerenes. Several independent conditions must be satisfied in obtaining correct solubility values: (1) the solid phase should reach an equilibrium by itself (it is necessary to have sufficient time for the recrystallization and redistribution of fullerene components between surface and volume layers of fullerene solid solution crystals); (2) the solid phase should be an equilibrium fullerene crystal solvate (experimental duration should be sufficient for the solvation of the solid phase); (3) the liquid

phase should be a liquid solution of fullerenes at equilibrium (the duration of saturation should be sufficient for the complete organization of the fullerene liquid solution including the formation of the superstructure); (iii) analysis of experimental data in Table 1 shows that solubility of C70 is always higher than solubility of the C60 fullerene. The latter fact is caused by higher polarizability of C70 due to the lower symmetry of the molecule structure [1]; (iv) analysis of the experimental data devoted to the solubility of the industrial fullerene mixture (39% C70, 60% C60 and 1% Сn (n N 70)) (Table 1) shows that in the temperature range of 293.15–303.15 K the liquid phase is enriched by the C70 fullerene (the enrichment is equal to 10–15% in comparison with the composition of the industrial fullerene mixture). A method of pre-chromatographic separation of the industrial fullerene mixtures can be developed based on such a difference in the content of the fullerene components in liquid and solid phases. We can also mention that the solubility of the fullerene mixture in 1-hexanol is higher than solubility of individual fullerenes due to the salting-in effect [1]. Fig. 4 shows the temperature dependence of the experimental solubility data of light fullerenes (C60 and C70) in 1-hexanol at 0.1 MPa. The solubility of the C70 in 1-hexanol increases monotonously with increasing temperature; the solubility diagram consists of a monovariant line of the crystallization of individual C70. The solubility diagram for the system C60–1-hexanol can be characterized by the non-monotonic temperature dependence of solubility and consists of two monovariant lines of the crystallization of individual C60 at high temperatures and monosolvated crystal C60·C6H13OH at low temperatures. The C60–1-hexanol diagram consists of an invariant point (point «O») corresponding to three phase equilibrium: solid C60–solid C60·C6H13OH–saturated binary solution. Thus, the positive definiteness over the invariant   ðlÞ  ðlÞ  d ln xC d ln xC 60 70 and derivatives take place [37]: point of dT dT P

0 @

ðl Þ

d ln xC dT

60

1

P

sol

A ¼ ΔH i RT 2 P

0 1 ðlÞ sol d lnxC ΔH i 70 A @ N0 and ¼ N0 dT RT 2

ð4Þ

P

where ΔHsol i is the heat of the dissolution of the i-th solid phase. Thus, the enthalpies of the fullerenes dissolution are positive in the case of the studied systems [1,2]. The fact of dissociation of the crystal-solvate C60·C6H13OH according to the scheme: C60  C6 H13 OH ðsolidÞ→C60 ðsolidÞ þ C6 H13 OH ðliquidÞ

ð5Þ

at the normal pressure can be also proved by thermogravimetric analysis of the solid sample C60·C6H13OH (Fig. 5). One can easily see the start

Fig. 5. TG and DTG curves for the sample C60·C6H13OH (m = 51 mg).

K.N. Semenov et al. / Journal of Molecular Liquids 209 (2015) 71–76

In all studied temperatures solubility increases monotonously with h . i h . i increasing pressure, i.e., dCC60 N0; dCC70 N0. Thus, pressure de-

Table 2 Experimental (pTw) data in the binary systems C60–1-hexanol and C70–1-hexanol. p/MPa C60–1-hexanol 303.3 K 0.1 41.8 102.2 323.3 K 0.1 7.6 74.1 – 338.3 K 0.1 3.2 27.7 86.6 353.3 K 0.1 7.4 68.0 C70–1-hexanol 298.3 K 0.1 52.0 – 313.3 K 0.1 27.5 62.4 73.2 328.3 K 0.1 23.1 55.3 92.4

w/%

0.0033 0.0038 0.0043 0.0059 0.0074 0.0128 – 0.0071 0.0074 0.0128 0.0176 0.0080 0.0128 0.0176

0.0141 0.0220 – 0.0148 0.0337 0.0523 0.0571

p/MPa

w/%

313.3 K 0.1 40.0 101.6 328.3 K 0.1 10.9 43.3 97.9 343.3 K 0.1 1.3 18.2 80.0 358.3 K 0.1 4.7 58.2

303.3 K 0.1 42.7 98.0 318.3 K 0.1 21.2 45.9 61.1

p/MPa

0.0057 0.0067 0.0074 0.0062 0.0074 0.0128 0.0176 0.0072 0.0074 0.0128 0.0176 0.0090 0.0128 0.0176

0.0147 0.0279 0.0389 0.0148 0.0337 0.0523 0.0571

318.3 K 0.1 41.1 96.9 333.3 K 0.1 7.6 33.6 91.5 348.3 K 0.1 10.5 76.3 – 363.3 K 0.1 3.2 42.5

308.3 K 0.1 37.9 99.1 323.3 K 0.1 14.2 38.3 49.6

75

dP T

w/%

0.0058 0.0101 0.0109 0.0066 0.0074 0.0128 0.0176 0.0079 0.0128 0.0176 – 0.0098 0.0128 0.0176

0.0147 0.0337 0.0523 0.0149 0.0337 0.0523 0.0571

0.0157 0.0526 0.0751 0.0916

dP T

pendences of solubility in both systems C60–1-hexanol and C70–1hexanol are similar to that corresponding to the solubility data reported for the C60–n-hexane system [28]. The slope of the changing of isothermal h2 . i h2 . i solubility falls when pressure increases; d CC60 b0; d CC70 b0, 2 2 dP

T

dP

T

therefore the functions CC60 ðP ÞT and CC70 ðP ÞT are both concave. Opposite pressure dependences of solubility take place in the case of the binary system C60–toluene. In the latter case the pressure dependences of solubility are non-monotonic, or the decreasing of the solubility values takes place with increasing pressure [29].

4. Conclusions The isotherms of the solubility of light fullerenes (C60 and C70) in 1-hexanol were investigated in the range of pressures of 0.1–100 MPa and in the range of temperatures of 298.15–363.15 K. In all the studied temperatures solubility increases monotonously with increasing pressure. In the case of temperature dependence of solubility (at ambient pressure) we have observed that in the case of C60–1-hexanol binary system the temperature dependence of solubility is non-monotonic (in comparison with C70–n-hexanol system). The latter fact is connected with desolvation of the solid phase (the solubility diagram consists of an invariant point corresponding to the phase transition of the monosolvated C60 fullerene–C60·C6H13OH). The composition of the solid phase was determined using the thermogravimetric method.

Acknowledgments

of the dissociation process, Eq. (5), at Tdiss, which corresponds to the invariant point O in Fig. 4. 3.2. Experimental solubilities at high pressures Experimental pTw data in the binary systems fullerene C60–1-hexanol and fullerene C70–1-hexanol in the range of pressures of 0.1–100 MPa and in the range of temperatures of 298.15–363.15 K are represented in Table 2 and in Figs. 6 and 7. All data are represented in isothermal cuts.

This work was supported by the interuniversity exchange program between Saint-Petersburg State University (Russia) and University of Santiago de Compostela (Spain) and by the Grant of the President of Russian Federation for Supporting Young Scientists MK-4657.2015.3. Support was received from Saint Petersburg State University: “Development and in vitro study of the interactions of receptor-ligand pairs required for the invention of new treatment strategies of socially significant diseases, and the development of methods of biological surface functionalization of materials for medicine (“alive” and artificial implants)”.

0.018 - 303.3 K - 313.3 K - 323.3 K - 328.3 K - 333.3 K - 338.3 K - 343.3 K - 348.3 K - 353.3 K - 358.3 K - 363.3 K

0.016

0.012

60

wC / %

0.014

0.010 0.008 0.006 0.004 0

20

40

60

80

100

p /MPa Fig. 6. Experimental (pTw) data of the binary system fullerene C60–1-hexanol.

120

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K.N. Semenov et al. / Journal of Molecular Liquids 209 (2015) 71–76

0.10 0.09

- 298.3 K - 303.3 K - 308.3 K - 313.3 K - 318.3 K - 323.3 K - 328.3 K

0.08

70

wC / %

0.07 0.06 0.05 0.04 0.03 0.02 0.01 0

20

40

60

80

100

120

p /MPa Fig. 7. Experimental (pTw) data of the binary system fullerene C70–1-hexanol.

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