Insight into polymorphism of the ethosuximide (ETX)
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Rich polymorphism of ethosuximide compound (ETX) is described in detail using complementary thermal analysis methods. The paper shows as well the results of differential scanning calorimetry (DSC), of polarizing microscope observations (POM) and of X-ray diffraction (XRD) measurements taken using horizontal rotating capillary method. Molecular structure of ethosuximide favors appearance of a conformationally disordered (CONDIS) crystal phase CrI in its polymorphism. Ethosuximide is a good glass former, and glass of the CrI phase was observed even for 5 °C min−1 rate of cooling. Moreover, monotropic plastic crystal CrII phase was observed during heating above the CrI temperature range.
KeywordsEthosuximide Polarizing microscopy Phase transition Thermooptical analysis DSC CONDIS crystal vitrification
Macroscopically, matter can exist in solid, melt and gas states. The large amplitude motions as translations and rotations of molecules, internal rotational and/or conformational changes determine the differences between fluid (IL) and solid states. In fact, some solid-state phases are not fully ordered crystal and not all large amplitude motions are frozen there. In liquid crystal (LC) phases, molecules act as orientationally (and in some cases even positionally) ordered but they are mobile to various extents. In plastic crystal phase (ODIC), molecules exhibit orientational mobility and local rotational disorder, but their centers of masses are positionally ordered. In conformationally disordered (CONDIS) crystal phase, molecules are on average positionally and orientationally ordered, but they have partial (or full) conformational freedom [1, 2, 3]. It is well known that any disordered phase may be supercooled to form glass, and in case of LC, ODIC and CONDIS phases partially ordered glassy phases are formed . When a compound exists in various solid-state forms, the following important questions should be asked: 1/ what is their thermodynamic stability, 2/ what are thermodynamic conditions in which any transformation can occur, and 3/ how long it lasts to have new phase in equilibrium state. Answers for those questions are given by thermal analysis methods.
Studying solid-state polymorphism of pharmaceutical compounds is a crucial issue, as each polymorph may have different bioactivities. In amorphous state, pharmaceuticals are more advantageous in therapy, so to gain knowledge of complexity of phase diagram and its evolution during storage may help in usage of smaller dose of medicine. Ethosuximide or 3-ethyl-3-methylpyrrolidine-2,5-dione (ETX) is a well-known substance used in epilepsy disease treatment . In the literature, one can find publications on its bioactivity studies [6, 7], while there is less information about ETX polymorphism and physicochemical properties. Most of physicochemical parameters were obtained at room temperature [8, 9, 10]. In this paper, results of differential scanning calorimetry (DSC), polarizing microscope observation (POM) and TOA thermooptical analysis are presented. This approach allowed to show rich solid-state polymorphism of ETX compound. To define structure of ETX polymorphs, XRD method was applied.
As we are going to present, complementary methods should be used in polymorphism investigation even for the material of such simple molecule as ETX. DSC method is one of the most commonly used techniques allowing to estimate the thermodynamic functions of the phase transition and its temperature. Microscopic texture observations together with thermooptical analysis (TOA) help in identification of liquid-like and solid-like phases found. Usually, DSC results are in good agreement with TOA [12, 13, 14]. By DSC alone, it is difficult to detect phase transitions characterized by small changes of heat capacity. TOA is not only very sensitive to any changes of phase structure but it does not suffer from thermal relaxation behavior after cooling/heating, what permits using fast rates of temperature changes during experiment . The XRD results show difference in detail of ETX crystal structures.
ETX sample was purchased in Sigma-Aldrich Company and studied on cooling and heating the samples with various rates of temperature changes in 0.2–50 °C min−1 range.
Polarizing microscope textures were observed using Biolar PI polarized microscope (PZO Warsaw) with the scanning rates 5, 10, 20 and 50 °C min−1. The temperature was stabilized by Linkam THM 600 silver heating/cooling stage and TMS 90 temperature controller. Substance was placed between two glass plates at the temperature above melting point. Temperature was measured by platinum resistance thermometer with 0.1 °C accuracy.
Thermooptical analysis (TOA) was performed by TOApy program  based on digitalized images of ETX microscopic textures observed on cooling and heating experiments.
DSC measurements were taken using TA Instruments, Q2500. The mass of sample was equal to 13.56 mg. The sample was placed in aluminum TA Tzero pan and TA Tzero hermetic lid. During DSC experiment, the nitrogen purge was on the level 1.3 bar. The cooling/heating rate was 0.2, 2, 5, 8, 10, 15 and 20 °C min−1.
XRD measurements were taken in horizontal rotating capillaries made by borosilicate glass, with outside diameter 0.3 mm on Empyrean 2 (PANalytical) diffractometer with CuKα anode, parabolic mirror on the incident beam, slit for capillaries and PIXcel detector working in 1D scanning mode. The temperature was controlled with the help of Cryostream 700 Plus (Oxford Cryosystems). The data were collected in temperature range between − 90 °C and 50 °C at several chosen temperature points during heating and subsequent cooling the sample. The XRD data were analyzed using XCell program from Material Studio software package. Fitting the XRD data was made using the Pawley refinement. The Rwp uncertainty parameter was on level 12% and Rp vary around 9%.
Results and discussion
During heating glass of CrI of the ETX compound, the first metastable CrI was identified at the TOA curve and then evidence of additional crystal CrII phase appearance was found. Softening of glass gCrI is observed until 18 °C as a slightly growing light intensity due to a process of cracks shrinkage. The CrI crystal phase is illustrated by a plateau in the TOA plot, and then, increase in intensity due to a solid–solid CrI–CrII transition is visible (see Fig. 4). Just below the isotropization point, a maximum is visible corresponding to a new crystal CrII phase with narrow temperature range of 48 °C–50 °C.
Comparing plots in Figs. 2 and 4, one can see that ETX compound exhibits tendency to supercooling IL and CrI phases, which we found to be dependent on the thermal history in POM observations. It was established that crystallization temperature decreases with increasing of the cooling rate. Moreover, for the sample cooled with slow temperature change rates (i.e., 2–0.2 °C min−1) crystallization was observed after 1–2 min. For higher cooling rates (i.e., 10–50 °C min−1), crystallization was not observed at, for example, 30 °C, when the waiting time was below 2 h.
The peak separation corresponding to two phase transitions, which overlap to each other, may be observed, if the DSC experiment is performed with smaller temperature change. Figure 7 shows result of DSC plot obtained for ETX compound with heating range of 0.2 °C min−1. As one can see the DSC peak is asymmetric and derivative of DSC plot points clearly two phase transitions near 48°C.
The DSC experiment did not show evidence of glass transition, but the POM observation clearly shows cracks on the texture, which are signature of glass transition. During heating, cracks start to shrink until they disappear, what is a sign of glass softening. In Fig. 8, the temperature of glass transition Tg registered on cooling during POM observation is presented. Vitrification of phases with some degrees of disorder, i.e., of plastic crystals and conformationally disordered CONDIS crystals, is a well-known behavior. On changing the cooling rate from 50 °C min−1 to 5 °C min−1, the Tg temperature shifts toward lower values (see Fig. 8). Usually, substances lose ability to vitrification if the cooling rate is lower than 8 °C min−1. ETX seems to be good glass former, as it shows good vitrification tendency even for 5 °C min−1 cooling rate.
ETX molecule compound occurs to show interesting solid-state polymorphism with the CrI, gCrI and the CrII phases. POM observations and the TOA analysis have allowed to obtain isotropic CrI phase transition and vitrification of CrI/ softening of glass of CrI and have clearly shown monotropic crystal CrII phase on heating. Based on the results of thermooptical methods, free energy Gibbs diagram of ETX has been proposed. The XRD measurements confirm the presence of two crystal phases and vitrification of CrI phase. Analysis of entropy changes at phase transitions given by DSC results points to conclusion that both crystalline phases are conformationally disordered. Thanks to analysis of derivative of heat flow vs temperature, the montropic CrII phase has been detected also by DSC method. The unusual DSC result found during crystallization of CrI on cooling was explained as accompanied by a weak process of crystallization of the CrII phase. The DSC experiment alone could not evidence of vitrification/softening phenomenon in ETX compound.
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