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POLY-THERMAL SOLUBILITY AND COMPLEX THERMAL ANALYSIS OF WATER-SOLUBLE TRIS-MALONATE OF LIGHT FULLERENE -
Ceo [= C(COOH)2]3
K.N. Semenov1, N. A. Charykov2,3, A. S. Kritchenkov1, I.A. Cherepkova2, O. S. Manyakina2, D.P. Tyurin2, A.A. Shestopalova2, V.A. Keskinov2, K.V. Ivanova2, N.M. Ivanova1, D.G. Letenko4, V.A. Nikitin5, E.L. Fokina1, I.A. Pestov2, A. O. Netrusov2
1St. Petersburg State University, Saint-Petersburg, Russia 2St. Petersburg State Technological Institute (Technical University), Saint-Petersburg, Russia 3St. Petersburg State Electro-Technical University (LETI), Saint-Petersburg, Russia 4St. Petersburg State University Architecture Academy, Saint-Petersburg, Russia 5St. Petersburg State Technical University, Saint-Petersburg, Russia
keskinov@mail.ru
PACS 61.48.+C
The poly-thermal solubility of the tris-malonate C6o[=C(COOH)2]3 - H2O binary system was investigated from 20 - 80 °C with the help of the method of isotherm saturation in ampoules. Concentration of tris-malonate C60[=C(COOH)2]3 in solutions was determined by light absorption at 330 nm. A diagram of the solubility is non-monotonic, consisting of 2 branches, which correspond to 2 different crystal-hydrates of C60[=C(COOH)2]3 and one non-variant point, corresponding to the saturation both crystal-hydrates. Complex thermal analysis of C60[=C(COOH)2]3 crystal hydrates, in equilibrium with a saturated aqueous solution at room temperature, was performed from 20 - 600 °C. Consecutive effects of the losses of C=O and C=O + H2O were determined.
Keywords: tris-malonate of light fullerene, solubility, density, complex thermal analysis.
Received: 17 March 2014
1. Introduction
This article continues investigations which were initiated in previous studies [1,2], which were devoted to the description of the synthesis and identification of tris-malonate
C6o[=C(COOH)2]3 [1] (the original synthesis of this water soluble derivative was described earlier in [3]) and the investigation of volume and refraction properties of its aqueous solutions at 25 °C [2]. This article is devoted to the investigation of poly-thermal solubility in binary system: tris-malonate C60[=C(COOH)2]3 - H2O. It is well-known that fullerenes them-
selves are practically insoluble in water and aqueous solutions. For example, the real solubility of C6o in water at 25 °C is 1.310-11 g/dm3 and C70 is 1.110-13 g/dm3 [4-10]. This fact
sufficiently limits the application of fullerenes in medicine, pharmacology, food industry etc., because fullerenes are incompatible with water and water based 'physiological liquids' such as blood, lymph, gastric juice etc. So, the synthesis and studying of the main properties, first of all solubility in water-based systems is very important. Such water soluble derivatives of light fullerenes as fullerenols, different malonates, complex esters of amino-acids etc have been investigated widely (see, for example [4,11-13]).
Table 1. Solubility in binary system: tris-malonate C6o[=C(COOH)2]3 - H2O from 20 - 80 °C
No. Temperature (°C) Solubility C (g/dm3) Density of saturated solutions p (g/sm3) Solid phase
1 20 254 1.112 C60[=C(COOH)2]3-3H2O
2 30 315 1.128 _n_
3 40 342 1.131 _n_
4 50 399 1.133 H
5 60 437 1.136 C60[=C(COOH)2]3-3H2O+ C60[=C(COOH)2]3
6 70 389 1.111 C60[=C(COOH)2]3
7 80 357 0.948 _n_
2. Poly-thermal solubility of tris-malonate C60[=C(COOH)2]3 in water
Poly-thermal solubility in binary system: tris-malonate C60[=C(COOH)2]3 - H2O from 20 - 80 °C is investigated with the help of the isotherm saturation method in ampoules (frequency v & 2 sec-1, temperature accuracy AT & 0.05 deg., time of saturation t & 6 h). Concentration of tris-malonate C60[=C(COOH)2]3 in saturated solutions was determined by light absorption at 330 nm (after the dilution and cooling of saturated solutions) see [1]:
Ctris-malonate(mg/dm3) & U6D330 (l = 1 Cm), (1)
where D330 - is optical density of the solution at A =330 nm and ditch width l = 1 cm.
Experimental solubility data are represented in the Table 1 and Fig. 1. One can see the following:
1. The solubility tris-malonate C60[=C(COOH)2]3 is very high thousands g/dm3, these values correspond to the solubility of such well-soluble phases as fullerenol-d [11-13] or, for example halite - NaCl.
2. Solubility against temperature changes non-monotonically, crossing through the maximum at 60 °C.
3. Diagram consists of 2 branches, which correspond to 2 different compounds: a tri-hydrate - C60[=C(COOH)2V3H2O and an anhydrous form - C60[=C(COOH)2]3 and one non-variant point (O in Fig. 1), corresponding to the saturation both compounds. Such parity at room temperature (one molecule of crystal-hydrate water per two car-boxyl groups of malonate is typical for malonates - for example for sodium malonate -Na-COO-CH2-COO-Na H2O [14].
3. Poly-thermal densities of saturated tris-malonate C60[=C(COOH)2]3 aqueous solutions
To calculate the volume concentration of saturated tris-malonate C60[=C(COOH)2]3 aqueous solutions and also in order to have the possibility of recalculating the solubility diagram into the other concentration scales (for example mass % or mole fraction), one needs to investigate the concentration poly-thermal density. These data were obtained by the method of
Fig. 1. Solubility in binary system: tris-malonate C60[=C(COOH)2]3 - H2O
Fig. 2. Poly-thermal densities of saturated tris-malonate C60[=C(COOH)2]3 aqueous solutions
pycnometry with the help of quartz pycnometers [2]. Data are also represented in the Table 1 and Fig. 2.
Table 2. The results of complex thermal analysis of crystal-hydrates of C6o[=C(COOH)2]3
No. of thermal effect (i) T m (Tb - Te) (°C) Ami / Am0 calculation (%) Ami / Am0 experiment (%) Process Product of decomposition
0 — 0.0 0.0 — Cec(= C(COOH)2)3 • 3H2O
1 97 (60 - 130) 5.2 5.0 -3HOH C6o(= c (COOH )2)3
2 150 (140 - 180) 2.6 2.5 -C=O C60(= C (COOH)2)2COH (COOH)
3 208 (195 - 240) 2.6 2.8 -C=O C60 = C (COOH )2(COH (COOH ))2
4 271 (255 - 295) 2.6 2.7 -C=O C60(COH (COOH ))3
5 337 (320 - 385) 4.3 4.1 -C=O-HOH C60 = CO(COH (COOH )2
6 420 (400 - 440) 4.3 4.3 -C=O-HOH C60(= CO)2COHCOOH
7 488 (480 - 520) 4.3 4.2 -C=O-HOH C60(= CO)3
Sum effect 25 - 560 25.9 25.6 -3HOH--6C=O--3HOH C60(= CO)3
where: Tm — temperature maximum of thermal effect, Tb and Te — temperatures of the
beginning and end of the effect, Amj/Am0 — the mass loss, m0 — initial mass.
One can see the following:
1. Diagram has one singular point (O in Fig. 2), where the type of crystal-hydrates and course of the curve are changing psat(T).
2. Before point O (T = 20 ^ 60oC), the density is practically constant, and after point O (T = 60 ^ 80°C), the density starts to decrease comparatively quickly. The last fact is connected with two reasons: the solubility of 'more heavy component' - tris-malonate decreases (see Fig. 1), and the density of the solvent decreases while temperature is also increasing.
4. Complex thermal analysis of crystal-hydrates of C60[=C(COOH)2]3
Complex thermal analysis of C60[=C(COOH)2]3 hydrates, in equilibrium with saturated aqueous solution at room temperature, was performed from 20 - 600 °C. A NETZSCH STA 449F3 thermo-gravimeter was used (velocity of the analysis v & 5 K/min, atmosphere - air, sample mass m 27.3 mg). Results are represented in the Table 2 and Fig. 3. One can see the following:
1. The first effect of losing 3 molecules of H2O from the trihydrate proves the solubility data (the start of the effect Tb & 60 °C corresponds to the singular points in the Fig. 1, 2).
2. The subsequent three effects correspond to 'decarbonylation' (C=O removal) from the three different malonate groups, thus each removal stabilizes residual groups. 'Rigid
Fig. 3. The results of complex thermal analysis of crystal-hydrates of C60[=C(COOH)2]3 (curves in the left axis Thermo-Gravimetry (TG) — top; Differential Thermo-Gravimetry (DTG) — bottom; Differential Thermal Analysis (DTA) — middle)
decarboxylation' (CO2 removal) did not occur because of the tertiary nature of the carbon atom with geminally-substituted carboxyl groups.
3. The subsequent three effects also correspond to 'decarbonylation with dehydration' (C=O and H2OH loss) from the three different malonate groups, thus against each allocation stabilizes residual groups. In these cases such process cannot occur without dehydrogenation because ketone hydrates (one carbon atom with two hydroxyl groups, OH - (R2)C(Ri) - OH) are usually unstable.
4. One can see that (according to TG curve) mass effect of the first three allocations is nearly 60 relative % from the mass effect of second three allocations, which also proves the complex mechanism of malonate decomposition.
Thus, poly-thermal solubility of water soluble tris-malonate of light fullerene -C60[=C(COOH)2]3 from 20 - 80 °C and complex thermal analysis of the last one in the temperature range 25 - 600 °C were investigated. One can see that diagram of solubility in the binary system consists of two branches, which correspond to the crystallization of the C60 -tris-malonate trihydrate and tris-malonate without water, correspondingly. Complex thermal analysis demonstrates six-stage soft and crude decarbonylation processes, with the formation of gaseous CO and CO + H2O, correspondingly.
Acknowledgment
Research was performed using equipment of the Resource Center 'GeoModel' of Saint-
Petersburg State University.
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