Decanedioic Acid (C10H18O4)/Dodecanedioic Acid (C12H22O4) System: Polymorphism of the Components and Experimental Phase Diagram

The experimental temperature/composition phase diagram of the binary system decanedioic acid (C10H18O4)/dodecanedioic acid (C12H22O4) was established by combining X-ray powder diffraction (XRD), differential-scanning calorimetry (DSC), infrared spectroscopy (IR), scanning electron microscopy (SEM), and thermo-optical microscopy (TOM). Both compounds crystallize in the same ordered form, C (P21/c), which is the phase that melts in both cases. The C form melts in C12H22O4 earlier than in C10H18O4, in contrast to other unbranched-chain compounds (alkanes, alkanols, and alkanoic acids) in which the melting temperatures increase as the C-atom number rises. Contrary to what might be expected, total solid-state miscibility is not observed. The C10H18O4/C12H22O4 binary system shows a complex phase diagram. At low temperatures, a new monoclinic form, Ci (P21/c), stabilizes as a result of the disorder of composition in the mixed samples; two [C+Ci] domains appear. Upon heating, four solid–solid and seven solid–liquid domains appear related by eutectic and peritectic invariants. All the crystallographic forms observed are isostructural.

1. Introduction. -We present here part of a general study on organic-solid-state miscibility, undertaken a few years ago within our research group and the REALM 1 ) (¼ Réseau Européen sur les Alliages Moléculaires) collaborating laboratories.
We are interested in the preparation of molecular mixed crystals, the study of their crystallographic and thermodynamic properties and stability, the determination of their solid-state miscibility, and their practical applications [1 -6]. Therefore, our research is invariably focused on the study of different families of molecular substances, such as naphthalene derivatives, benzene derivatives, unbranched alkanes, alkanols, and alkanoic acids, and in this case unbranched alkanedioic acids.
Alkanedioic acids (HOOC(CH 2 ) n COOH) are unbranched-chain molecules with COOH groups at both ends. A very long one-dimensional chain can be formed through H-bond formation at both ends. Alkanedioic acids form a layered structure in the solid state, as do most polymorphic phases of long-chain compounds, such as unbranched alkanes, alkanols, and alkanoic acids [7 -12]. Owing to the presence of COOH groups at both ends of the chain, the density of H-bonds is significantly greater than in alkanoic acids. That could explain why their melting points are higher than those of analogous alkanoic acids: dodecanoic acid melts at 317.3 K, while dodecanedioic acid melts at 401 K [7].
Alkanedioic acids show a marked difference in the melting points of the shorter odd-and even-number diacids (i.e., consisting of an odd or even number of C-atoms, resp.), which converge as the chain length reaches 20 C-atoms. The odd-number diacids have a rather monotonous increase in melting points with increasing chain length, while the even-number diacids have sharply decreasing melting points as the chain lengthens [7].
Two polymorphic forms a (P2 1 /c) in the even-and odd-number series, and b (C2/c) only in the odd-number series have been described in the literature [9] [10] [13] [14]. The gross structural features are similar in the even-and (both forms of) odd-number series of alkanedioic acids [13]: i) COOH Groups form H-bonded dimers at both ends of the molecules, leading to infinite chains. ii) CH 2 Chains stack into columns through hydrophobic interactions. However, there are certain differences within these similar packing patterns that are important in the context of melting-point alternation: i) Molecules are offset along their length within the columnar stacks in the even-number series, whereas such an offset is absent in both forms of the odd-number series. ii) Molecules in both modifications of the odd-number series exhibit twisted molecular conformations with severe torsions as opposed to the nontwisted all-trans conformation in the even-number series.
The IR spectra showed that the solid states of unbranched alkanedioic acids bore a marked structural similarity to those of alkanoic acids. This is why in some reports [15 -19], the same nomenclature is used to describe the different forms 2 ): B, C, E (P2 1 /c), and A (P1). The hydrocarbon chains take the all-trans conformation in the A, C, and E forms, while the B form has a gauche conformation in the vicinity of the COOH group [15] [20]. The A super form has an unusual conformation, in which the COOH group rotates about the C(1)ÀC (2) bond. This structure is explained by the O,O-distance of the adjacent molecule [20].
The present paper deals with the study of the C 10 H 18 O 4 and C 12 H 22 O 4 polymorphism and the experimental determination of the binary phase diagram between these two compounds. We followed a complementary approach, including X-ray powder diffraction (XRD), differential-scanning calorimetry (DSC), infrared spectroscopy (IR), scanning electron microscopy (SEM), and thermo-optical microscopy (TOM).
To the best of our knowledge, the phase diagram of the C 10  Differential-Scanning Calorimetry (DSC). Perkin-Elmer DSC-7 calorimeter; conditions: sample weight between 3.9 and 4.1 mg and scanning rate 2 K min À1 . Six independent measurements were performed for each sample. The instrument was calibrated by reference to the enthalpy and m.p. of indium and decane standards. The random part of the uncertainties was estimated with Students method, with a 95% threshold of reliability. The characteristic temp. were determined from the DSC curves by the shape-factor method [21]. Enthalpy effects were evaluated by integration of the DSC signals.
X-Ray Powder Diffraction Measurements (XRD). Panalytical and Siemens D-500 diffractometers. Spectra were recorded on the Panalytical diffractometer at r.t. to improve resolution and minimize orientation effects. The Panalytical diffractometer was operated in transmission mode with CuK a radiation by means of a double Mo crystal as the primary monochromator, and the sample was mounted in a sealed capillary of 0.5 mm diameter which rotated perpendicularly to the X-ray radiation beam. Variable-temp. measurements were recorded with the Siemens D-500 diffractometer by means of Bragg -Brentano geometry, CuK a radiation, and a secondary monochromator. The data were collected with an Anton PAAR TTK system, with a heating rate of 0.02 K s À1 and 5 min of stabilization time. Samples were heated from 298 K to the melting point. The patterns were scanned with a step size of 0.0258 and step time of 5 s; the 2q range was 1.6 -608. In both cases, the cell parameters were refined with the Pawley profile-fitting procedure option [22] of the Materials Studio software [23].
IR Spectroscopy. Bomem-DA3-FTIR spectrometer. The samples were finely powdered and measured at r.t. by means of a diffuse reflection accessory (DRIFT) within 450 -4000 cm À1 . The resolution was 4 cm À1 , and all spectra were run in vacuum mode. The detector was an MCT (wide range), the beam splitter was KBr, and the source was a glow bar. Each spectrum was obtained by averaging 100 scans.
Scanning Electron Microscopy (SEM). Hitachi S-4100 (field emission). The polycrystalline samples were mounted on a support platform of 12 mm diameter with a conductor adhesive (Agar Scientific), and fixed with a thin carbon layer of ca. 20 nm. SEM analyses were done at r.t.
Thermo-Optical Microscopy (TOM). Linkam THMSG600 stage, mounted on a Carl-Zeiss microscope. The sample was placed on a 7 mm quartz cover slip and encased within a pure Ag lid so that it was heated from all sides, ensuring a uniform temp.  [14]. This crystalline form is observed in all unbranched even-number alkanedioic acids with chain lengths ranging from C 4 to C 16  The XRD results do not conclusively allow to identify the C form by analogy to the forms observed for the unbranched alkanoic acids. This is because the E, B, and C forms observed for the alkanoic acids are all monoclinic P2 1 /c [26 -33]. IR Spectroscopy, SEM, and TOM are needed to understand the C 10 H 18 O 4 /C 12 H 22 O 4 system. Crystal cell parameters of these two compounds are reported in Table 2.

Results and
The IR spectra, by analogy to the unbranched alkanoic acids [34 -36] and other unbranched alkanedioic acids [16], indicate that both pure compounds adopt the C form at room temperature: i) The r(CH 2 ) band splits into a doublet at 730 and 720 cm À1 due to the O ? subcell. ii) The d(OCO) band appears at 688 cm À1 , and the peak of the s(OÀH) band is at ca. 945 cm À1 .
The SEM results show regular prismatic plates for the two alkanedioic acids (Fig. 1), with an acute angle of 548, characteristic of the C form, as observed for unbranched alkanoic acids [37] [38].
Binary Phase Diagram. To understand the experimental phase diagram (Fig. 2), DSC, XRD, and TOM techniques were used. Twenty-three binary mixed samples (1-x) were analyzed under isothermal and isoplethic conditions. Thermo-energetic data deduced from thermal analyses are given in Table 3.

The experimental phase diagram is characterized at room temperature by two solid -solid fields. One of these fields is larger in intermediate compositions, and the other is narrower in compositions rich in C 12 H 22 O 4 . The [C], [C þ C i ], [C i ], [C i þ C] and
[C] sequence is observed when increasing the percentage of C 12 H 22 O 4 . The C phase is the stable form of two pure compounds, and the C i phase appears as a consequence of compositional disorder. The C and C i forms belong to the same space group (P2 1 /c), so it is difficult to differentiate between them. By XRD, when the C and C i forms coexist, the FWHM (¼ full width at half maximum) of the h00 Bragg reflections increases (Fig. 3). No differences between C and C i are observed by IR spectroscopy. By SEM and optical microscopy, all the prismatic plates show an acute angle of 548.  Mol-% Upon heating, a eutectic invariant is observed, covering a large part of the composition range (from ca. 30 to 80 mol-% of C 12 H 22 O 4 ) and located around T E1 382.2 K (Fig. 4, a). Also upon heating, for compositions rich in C 10 (Fig. 4, b). As a result of the overlapping of different phenomena in the DSC signal, XRD isoplethic analysis for different compositions was required to establish the stability limits of the different phase domains as a function of temperature. As an example, Fig. 5 shows the XRD measurements for a binary mixture containing 10 mol-% of C 12 H 22 O 4 at different temperatures; the results establish the following sequence:  Fig. 6; the results establish the following sequence:   Our conclusions are somewhat different from those proposed in previous publications. In those publications, a complete range of miscibility at high temperature and an only solid -solid domain for compositions rich in C 12 H 22 O 4 at low temperatures was pointed out. However, in another system of the same family studied by our group, the tetradecanedioic acid/hexadecanedioic acid binary system [16], certain similarities are observed, i.e., at room temperature, also two [C þ C i ] domains are present, and upon heating, solid -solid and solid -liquid domains are related by peritectic and eutectic invariants, with the common point that all the phases are C (P21/c).
From a practical point of view, only alloys with a composition close to the components, or near the eutectic composition (ca. 50 mol-% of C 12 H 22 O 4 ) can be considered as suitable MAPCMs (molecular alloys as phase-change materials) for applications in the fields of energy storage and thermal protection. These can provide solutions at around 405, 399, and 382 K with a latent heat around 45 kJ/mol. This allows us to provide MAPCMs at temperatures where many of the molecular substances used as PCMs (unbranched alkanes and alkanoic acids) may not offer a solution.